NIMP| Alina CRISAN, Author at NIMP

Publications

1. High Magnetic Performance in MnGa Nanocomposite Magnets

Authors: Crisan, O; Crisan, AD

Published: AUG 2024, NANOMATERIALS, 14, 1245, DOI: 10.3390/nano14151245

In view of their potential applicability in technology fields where magnets are required to operate at higher temperatures, the class of nanocomposite magnets with little or no rare earth (RE) content has been widely researched in the last two decades. Among these nanocomposite magnets, the subclass of magnetic binary systems exhibiting the formation of L10 tetragonal phases is the most illustrious. Some of the most interesting systems are represented by the Mn-based alloys, with addition of Al, Bi, Ga, Ge. Such alloys are interesting as they are less costly than RE magnets and they show promising magnetic properties. The paper tackles the case of MnGa binary alloys with various compositions around the Mn3Ga stoichiometry. Four MnGa magnetic alloys, with Mn content ranging from 70 at% to 75 at% were produced using rapid solidification to form the melt. By combining structural information arising from X-ray diffractometry and transmission electron microscopy with magnetic properties determined by vibrating sample magnetometry, we are able to document the nature and properties of the structural phases formed in the alloys in their as-cast state and upon annealing, the evolution of the phase structure after annealing and its influence on the magnetic behavior of the MnGa alloys. After annealing at 400 degrees C and 500 degrees C, MnGa alloys are showing a multiple-phase microstructure, consisting of co-existing crystallites of L10 and D022 tetragonal phase. As a consequence of these structurally and magnetically different phases, co-existing within the microstructure, promising magnetic features are obtained, with both coercive fields and saturation magnetization exceeding values previously reported for both alloys and layers of MnGa.

2. Lithographically Ordered FePt L1₀ Dots with High Coercivity for Logic-Conditioned Magnetic Nanostructures

Authors: Crisan, O; Crisan, AD

Published: JAN 2024, CRYSTALS, 14, 58, DOI: 10.3390/cryst14010058

In order to develop the building blocks for future biosensing and spintronic applications, an engraving technique using electron beam lithography is employed in order to develop nanomagnetic pre-patterned structures with logic potential. The paper describes the realization and morphological and magnetic characterization of potentially logic-conditioned substrates, a building block to be further used as an integration platform upon which nanodevices, such as magnetic wires, or various geometrical shapes, circles, triangles, can be considered as pre-requisite for full integration into logic devices. As a proof of concept, regular arrays of FePt circles or magnetic dots were devised and structural characterization by X-ray diffraction and transmission electron microscopy proved the occurrence of the tetragonal L1(0) phase. Moreover, the magnetic characterization provided more insight into the potential of such arrays of magnetic devices as the hysteresis provided good values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of potential applications in various applications.

3. Magnetic Memory Effects in BaFe₂(As_0.68P_0.32)₂ Superconducting Single Crystal

Authors: Badea, AM; Ivan, I; Miclea, CF; Crisan, DN; Galluzzi, A; Polichetti, M; Crisan, A

Published: NOV 2024, MATERIALS, 17, 5340, DOI: 10.3390/ma17215340

Among many iron-based superconductors, isovalently substituted BaFe2(As1-xPx)2 displays, for x approximate to 0.3, apart from the quite usual Second Magnetization Peak (SMP) in the field dependence of the critical current density, an unusual peak effect in the temperature dependence of the critical current density in the constant field, which is related to the rhombic-to-square (RST) structural transition of the Bragg vortex glass (BVG). By using multi-harmonic AC susceptibility investigations in three different cooling regimes-field cooling, zero-field cooling, and field cooling with measurements during warming up-we have discovered the existence of a temperature region in which there is a pronounced magnetic memory effect, which we attributed to the direction of the structural transition. The observed huge differences in the third harmonic susceptibility at low and high AC frequencies indicates the difference in the time-scale of the structural transition in comparison with the timescale of the vortex excitations. Our findings show that the RST influence on the vortex dynamics goes beyond the previously observed influence on the onset of the SMP.

4. Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa₂Cu₃O_7-x Superconducting Film

Authors: Galluzzi, A; Crisan, A; Ionescu, AM; Ivan, I; Leo, A; Grimaldi, G; Polichetti, M

Published: JUN 2024, APPLIED SCIENCES-BASEL, 14, 4379, DOI: 10.3390/app14114379

The study of granularity in superconducting films by using AC susceptibility has a crucial role in the development of and improvement in the ReBCO-coated conductors, which are a constantly evolving reality in the modern power applications of superconductivity. Specifically, the study of the granularity is essential because the ReBCO superconducting wires and tapes are far from the regularity of a single crystal while they often present an inter- and intragranular contribution to the critical current density. On the other hand, the AC susceptibility is a key part of the characterization of a granular sample because this technique is very sensitive to the presence of granularity in the superconductors and, moreover, the study of its first harmonic allows for determining pivotal properties such as the pinning energy as well as the dissipation processes acting in the sample. The pinning energy values and the granularity of an YBCO thin film have been studied by means of AC susceptibility measurements as a function of the AC amplitude, temperature, and DC field. In particular, the first harmonic imaginary component of the AC susceptibility chi 1 '' related to the dissipation processes of the sample has been studied. First, starting from the Brandt approach, the critical current density J(c) and the pinning energy U of the sample have been extracted at 77 K by using the chi 1 '' measurements as a function of the AC amplitude at different AC frequencies and DC fields. From these measurements, a first signal of granularity appears. In order to confirm it, the temperature dependence of the chi 1 '' at different DC fields has been studied and a contribution deriving from the inter- and intragranular part of the sample has emerged. By taking the temperature corresponding to the crossover between the two contributions at the different DC fields, the intergranular and intragranular response has been separated. Successively, the temperature has been fixed to 77 K, together with an AC frequency equal to 1597.9 Hz, and the chi 1 '' as a function of the DC field at different AC amplitudes has been analyzed showing a clear presence of granularity in all the curves. By drawing the contour plot of the chi 1 '' with the DC and AC values, it was possible to determine the best parameters to put at 77 K in order to exploit the material for applications.

5. Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa₂Cu₃O_7-x Superconducting Film (vol 14, 4379, 2024)

Authors: Galluzzi, A; Crisan, A; Ionescu, AM; Ivan, I; Leo, A; Grimaldi, G; Polichetti, M

Published: SEP 2024, APPLIED SCIENCES-BASEL, 14, 7664, DOI: 10.3390/app14177664

6. Vortex Glass-Vortex Liquid Transition in BaFe2(As1-xPx)(2) and CaKFe4As4 Superconductors from Multi-Harmonic AC Magnetic Susceptibility Studies

Authors: Ivan, I; Ionescu, AM; Crisan, DN; Crisan, A

Published: APR 26 2023, INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES, 24, 7896, DOI: 10.3390/ijms24097896

For practical applications of superconductors, understanding the vortex matter and dynamics is of paramount importance. An important issue in this context is the transition of the vortex glass, which is a true superconducting phase, into a vortex liquid phase having a linear dissipation. By using multi-harmonic susceptibility studies, we have investigated the vortex glass-vortex liquid phase transitions in CaKFe4As4 and BaFe2(As0.68P0.32)(2) single crystals. The principle of our method relates the on-set of the third-harmonic susceptibility response with the appearance of a vortex-glass phase in which the dissipation is non-linear. Similar to the high-critical temperature cuprate superconductors, we have shown that even in these iron-based superconductors with significant lower critical temperatures, such phase transition can be treated as a melting in the sense of Lindemann's approach, considering an anisotropic Ginzburg-Landau model. The experimental data are consistent with a temperature-dependent London penetration depth given by a 3D XY fluctuations model. The fitting parameters allowed us to extrapolate the vortex melting lines down to the temperature of liquid hydrogen, and such extrapolation showed that CaKFe4As4 is a very promising superconducting material for high field applications in liquid hydrogen, with a melting field at 20 K of the order of 100 T.

7. Novel Rare Earth (RE)-Free Nanocomposite Magnets Derived from L1(0)-Phase Systems

Authors: Crisan, AD; Crisan, O

Published: MAR 2023, NANOMATERIALS, 13, 912, DOI: 10.3390/nano13050912

In the quest for novel rare earth (RE)-free magnetic materials, which also exhibit other additional properties such as good corrosion resistance and potential to operate at higher temperatures, an alloy deriving from the binary FePt system, with Mo and B addition, has been synthesized for the first time, using the out-of-equilibrium method of rapid solidification form the melt. The alloy with the composition Fe49Pt26Mo2B23 has been subjected to thermal analysis through differential scanning calorimetry in order to detect the structural disorder - order phase transformation as well as to study the crystallization processes. For the stabilization of the formed hard magnetic phase, the sample has been annealed at 600 degrees C and further structurally and magnetically characterized by means of X-ray diffraction, transmission electron microscopy, Fe-57 Mossbauer spectrometry as well as magnetometry experiments. It has been proven that after annealing at 600 degrees C the tetragonal hard magnetic L1(0) phase emerges via crystallization from a disordered cubic precursor and becomes the predominant phase in terms of relative abundance. Moreover, it has been revealed by quantitative analysis via Mossbauer spectroscopy that the annealed sample exhibits a complex phase structure, where the L1(0) hard magnetic phase is accompanied by few other soft magnetic phases, in minority abundance: the cubic A1, orthorhombic Fe2B and residual intergranular region. The magnetic parameters have been derived from 300 K hysteresis loops. It was shown that, contrary to the as-cast sample which behaves as a typical soft magnet, the annealed sample presents strong coercivity and high remanent magnetization, accompanied by a large saturation magnetization. These findings offers good insight into the potential developing of novel class of RE-free permanent magnets, based on Fe-Pt-Mo-B, where the magnetic performance emerges from the co-existence of hard and soft magnetic phases in controlled and tunable proportions, capable of finding good applicability in fields requiring good catalytic properties and strong corrosion resistance.

8. Measurements of Surface Impedance in MgB2 in DC Magnetic Fields: Insights in Flux-Flow Resistivity

Authors: Alimenti, A; Torokhtii, K; Garcia, PV; Silva, E; Grigoroscuta, MA; Badica, P; Crisan, A; Pompeo, N

Published: JAN 2023, MATERIALS, 16, 205, DOI: 10.3390/ma16010205

We present the multifrequency measurements of the surface resistance of spark-plasma-sintered MgB2 performed through a dielectric loaded resonator operating at 16.5 and 26.7 GHz. By normally applying magnetic fields <= 1.2 T to the sample surface, we drove it in the mixed state. By means of data-rooted analysis, we found that the sample vortex dynamics could be fully described within a single-component approach. Pinning phenomena were present and characterized by a depinning frequency smaller than the measurement ones. The multiband nature of the superconductor emerged in the flux-flow resistivity, whose field dependence could be interpreted well within theoretical models. By exploiting them, the upper critical field was extracted in the low-temperature range, which exhibited a consistent temperature trend with the values obtained at the onset of the resistive transition near Tc, and was well in line with literature data on other polycrystalline samples.

9. Vortex Glass-Vortex Liquid Transition in BaFe₂(As_1-xP_x)₂ and CaKFe₄As₄ Superconductors from Multi-Harmonic AC Magnetic Susceptibility Studies

Authors: Ivan, I; Ionescu, AM; Crisan, DN; Crisan, A

Published: APR 26 2023, INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES, 24, 7896, DOI: 10.3390/ijms24097896

10. Novel Rare Earth (RE)-Free Nanocomposite Magnets Derived from L1₀-Phase Systems

Authors: Crisan, AD; Crisan, O

Published: MAR 2023, NANOMATERIALS, 13, 912, DOI: 10.3390/nano13050912

11. Remarkable Magnetic Properties in a Mn_73.6Ga_26.4 Alloy Produced via Out-of-Equilibrium Method

Authors: Crisan, O; Crisan, AD

Published: DEC 2023, NANOMATERIALS, 13, 3014, DOI: 10.3390/nano13233014

Rare-earth-free permanent magnets with the L1(0) phase are actively researched for their potential as a future class of magnetic materials, capable of operating at higher temperatures and in challenging corrosion environments such as renewable energy applications. Among these classes, MnGa shows potential, being cost effective and having interesting magnetic properties. A MnGa magnetic alloy, with composition Mn73.6Ga26.4 in atomic percent, was produced via the out-of-equilibrium method, and its structural and magnetic properties were assessed using X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and extended magnetic characterization. We show that the MnGa alloy submitted to thermal annealing in optimal conditions exhibits a two-phase microstructure, where small nanocrystals of tetragonal L1(0)/D0(22) magnetic phase are embedded within a D0(19) MnGa matrix of a non-collinear antiferromagnetic nature. These co-existing, magnetically different phases produce an optimal set of promising magnetic properties, larger than the values reported in the literature for single-phase MnGa alloys and thin films. Such large values are explained by the exchange coupling between competing non-collinear magnetic sublattices of the D0(19) MnGa with the net moment of the small magnetic nanocrystals of tetragonal symmetry.

12. Highly Coercive L1₀-Phase Dots Obtained through Low Temperature Annealing for Nano-Logic Magnetic Structures

Authors: Crisan, O; Crisan, AD; Schinteie, G; Kuncser, V; Carotenuto, G

Published: DEC 2023, COATINGS, 13, 2068, DOI: 10.3390/coatings13122068

Nano-logic magnetic structures are of great interest for spintronic applications. While the methods used for developing arrays of magnetic L1(0)-phase dots are, in most cases, based on deposition followed by annealing at high temperatures, usually around 700 degrees C, we demonstrate here a technique where a much lower annealing temperature (i.e., 400 degrees C) is needed in order to promote fully the disorder-order phase transformation and achievement of highly coercive L1(0)-phase dots. In order to develop building blocks based on arrays of L1(0)-phase FePt dots for further spintronic applications, an engraving technique using electron beam lithography is employed. This paper describes the fabrication, as well as the morphological and magnetic characterization, of regularly placed FePt dots of various shapes, as pre-requisites for integration into nano-logic devices. As a proof of concept, regular arrays of FePt circular dots were devised and their structural characterization, using X-ray diffraction (XRD) and transmission electron microscopy (TEM), was performed. It has been shown that annealing at only 400 degrees C for 30 min proved the occurrence of the tetragonal L1(0) phase. Moreover, structural characterization showed that the disorder-order phase transformation was complete with only the L1(0) phase detected in high resolution TEM. The magnetic characterization provided more insight into the potential of such arrays of magnetic devices with convenient values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of various applications.

13. Vortex Dynamics and Pinning in CaKFe₄As₄ Single Crystals from DC Magnetization Relaxation and AC Susceptibility

Authors: Ionescu, AM; Ivan, I; Miclea, CF; Crisan, DN; Galluzzi, A; Polichetti, M; Crisan, A

Published: DEC 2023, CONDENSED MATTER, 8, 93, DOI: 10.3390/condmat8040093

Among various "families" of iron-based superconductors, the quite recently discovered AeAFe(4)As(4) (where Ae is an alkali-earth metal and A is an alkali metal) has high critical current density, a very high upper critical field, and a low anisotropy, and has recently received much interest for the possibility of high magnetic field applications at the liquid hydrogen temperature. We have performed DC magnetization relaxation and frequency-dependent AC susceptibility measurements on high-quality single crystals of CaKFe4As4 with the aim of determining the pinning potential U*. The temperature dependence of U* displays a clear crossover between elastic creep and plastic creep. At temperatures around 27-28 K, U* has a very high value, up to 1200 K, resulting in an infinitesimally small probability of thermally activated flux jumps. From the dependence of the normalized pinning potential on irreversible magnetization, we have determined the creep exponents in the two creep regimes, which are in complete agreement with theoretical models. The estimation of the pinning potential from multifrequency AC susceptibility measurements was possible only near the critical temperature due to equipment limitations, and the resulting value is very close to the one that resulted from the magnetization relaxation data. Magnetic hysteresis loops revealed a second magnetization peak and very high values of the critical current density.

14. Measurements of Surface Impedance in MgB₂ in DC Magnetic Fields: Insights in Flux-Flow Resistivity

Authors: Alimenti, A; Torokhtii, K; García, PV; Silva, E; Grigoroscuta, MA; Badica, P; Crisan, A; Pompeo, N

Published: JAN 2023, MATERIALS, 16, 205, DOI: 10.3390/ma16010205

15. The Depairing Current Density of a Fe(Se,Te) Crystal Evaluated in Presence of Demagnetizing Factors

Authors: Galluzzi, A; Buchkov, K; Tomov, V; Nazarova, E; Leo, A; Grimaldi, G; Crisan, A; Polichetti, M; Gencer, A; Bussmann-Holder, A; Ruiz, JJC; Vinokur, V; de la Fuente, GF

Published: DEC 2023, CONDENSED MATTER, 8, 91, DOI: 10.3390/condmat8040091

The effect of the demagnetizing factor, regarding the determination of the de-pairing current density Jdep, has been studied in the case of a Fe(Se,Te) crystal, using DC magnetic measurements as a function of a magnetic field (H) at different temperatures (T). First, the lower critical field Hc1(T) values were obtained, and the demagnetization effects acting on them were investigated after calculating the demagnetizing factor. The temperature behaviors of both the original Hc1 values and the ones obtained after considering the demagnetization effects (Hc1demag) were analyzed, and the temperature dependence of the London penetration depth lambda L(T) was obtained in both cases. In particular, the lambda L(T) curves were fitted with a power law dependence, indicating the presence of low-energy quasiparticle excitations. Furthermore, by plotting lambda L-2 as a function of T, we found that our sample behaves as a multigap superconductor, which is similar to other Fe-11 family iron-based compounds. After that, the coherence length xi values were extracted, starting with the Hc2(T) curve. The knowledge of lambda L and xi allowed us to determine the Jdep values and to observe how they are influenced by the demagnetizing factor.

16. Ferromagnetism and Superconductivity in CaRuO3/YBa2Cu3O7-delta Heterostructures

Authors: Ionescu, AM; Ivan, I; Locovei, C; Onea, M; Crisan, A; Soltan, S; Schutz, G; Albrecht, J

Published: APR 2022, MATERIALS, 15, 2345, DOI: 10.3390/ma15072345

The deposition of a ferromagnetic layer can affect the properties of high-temperature superconductors underneath. We investigated the influence of ferromagnetic CaRuO3 on the properties of YBa2Cu3O7-x (YBCO) superconducting thin films when the layers are either in direct contact or separated by a barrier layer of 5 nm SrTiO3. Detailed measurements of the magnetic moment of the superconductor and ferromagnet as a function of temperature and magnetic field have been performed using SQUID magnetometry. Magnetometry and relaxation measurements show that the modification of the superconducting properties of YBCO strongly depends on the interaction with the ferromagnetic layer on top. The barrier layer has a significant impact on both the supercon-ducting properties of the YBCO film and the ferromagnetic ordering of CaRuO3. The physical properties mentioned above were discussed in correlation with the materials' structure determined by XRD analysis.

17. Morpho-Structural Investigations and Carbon Nanoclustering Effects in Cr-Al-C Intermetallic Alloys

Authors: Crisan, AD; Crisan, O

Published: SEP 2022, NANOMATERIALS, 12, 3225, DOI: 10.3390/nano12183225

Intermetallic Cr-Al-C thin films from the 211 class of MAX phases were fabricated via ion beam deposition and structural investigations were undertaken to obtain information about morpho-structural effects propelled by carbon excess in the stoichiometry of the films. In order to promote the occurrence of the Cr2AlC MAX phase, the stoichiometric thin films were subsequently annealed at two temperature values: 650 degrees C and 700 degrees C in UHV conditions for 30 min. The morpho-structural effects in both as-deposited and annealed films were monitored using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. XRD analysis showed that the as-deposited sample was almost completely crystallized in the hexagonal Cr2AlC structure, with a remaining amorphous fraction of about 17%, most probably rich in carbon. Raman analysis allowed the identification of three spectral regions, two of them encompassing the Raman optical modes belonging to the Cr2AlC 211 MAX phase, while the third one gave strong evidence of highly intense and large D- and G-bands of carbon. Structural parameters such as the crystal lattice parameters as well as the volume of the crystal unit cell were found to decrease upon annealing; this decrease is attributed to the grain growth. The average crystallite dimension was proven to increase after annealing, while the lattice micro-strain lowered to approximately 63% in the annealed thin film compared to the as-deposited one. Well-formed and intense Raman peaks attributed to D- and G-bands of carbon were also observed and, corroborated with the structural data, seemed to indicate an overall increased level of crystal ordering as well as potential carbon nanoclustering after thermal treatments with thin Cr2AlC films. This observed phenomenon concords with previously documented reports on ab initio modelling of possible Cr2AlC structures with carbon excess.

18. Pinning Potential of the Self-Assembled Artificial Pinning Centers in Nanostructured YBa2Cu3O7-x Superconducting Films

Authors: Ivan, I; Ionescu, AM; Crisan, DN; Andrei, A; Galluzzi, A; Polichetti, M; Mosqueira, J; Crisan, A

Published: MAY 2022, NANOMATERIALS, 12, 1713, DOI: 10.3390/nano12101713

For high-field power applications of high-temperature superconductors, it became obvious in recent years that nano-engineered artificial pinning centers are needed for increasing the critical current and pinning potential. As opposed to the artificial pinning centers obtained by irradiation with various particles, which is a quite expensive approach, we have studied superconducting samples having self-assembled defects, created during the sample fabrication, that act as effective pinning centers. We introduced a simple, straight-forward method of estimating the frequency-dependent critical current density by using frequency-dependent AC susceptibility measurements, in fixed temperatures and DC magnetic fields, from the positions of the maxima in the dependence of the out-of-phase susceptibility on the amplitude of AC excitation magnetic field. The results are compatible with a model that stipulates a logarithmic dependence of the pinning potential on the probing current. A mathematical derivation allowed us to estimate from the experimental data the pinning potentials in various samples, and in various DC magnetic fields. The resulted values indicate large pinning potentials, leading to very small probability of magnetic flux escaping the pinning wells, hence, leading to very high critical currents in high magnetic fields.

19. Thermal Stability, Blocking Regime and Superparamagnetic Behavior in Mn-Al-C Melt Spun Ribbons

Authors: Crisan, AD; Leca, A; Dan, I; Crisan, O

Published: NOV 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11112898

Alloys possessing nominal compositions Mn53Al45C2 and Mn52Al46C2 were prepared by the melt spinning method and were subjected to complex structural, morphological and magnetic investigations. As these alloys can exhibit tetragonal L1(0)-type and tau phase, they have good potential as rare earth (RE)-free magnets. It is, therefore, important to monitor the epsilon-tau phase transformation and the stability and the magnetic features of the tetragonal phase in an entire temperature interval. By using synchrotron X-ray diffraction, it has been proven that the epsilon-tau phase transformation occurs gradually, with the tau phase becoming predominant only after 450 & DEG;C. Moreover, this phase has been proven to be quite stable without any grain growth even at the highest temperature investigated at 800 & DEG;C. Low temperature behavior was thoroughly investigated by using a complex combination of major and minor hysteresis loops combined with the zero field cooled-field cooled magnetization protocols (ZFC-FC). Two different regimes, blocking and superparamagnetic, were documented. A spin reorientation transition was proven to occur at 55 K while a maximum magnetization observed in ZFC-FC curves proved that at about 75 K, a transition from ferro to superparamagnetic state occurs. The existence of a blocking regime below 55 K that is characteristic to nanogranular systems with superparamagnetic behavior has shown further development towards obtaining RE-free magnets.

20. Role of Disordered Precursor in L1(0) Phase Formation in FePt-Based Nanocomposite Magnet

Authors: Crisan, AD; Dan, I; Crisan, O

Published: NOV 2021, MAGNETOCHEMISTRY, 7, DOI: 10.3390/magnetochemistry7110149

In order to prove the usefulness of having a structurally disordered precursor to the formation of FePt L1(0) phase and to facilitate the co-existence of exchange coupled hard and soft magnetic phases with optimized magnetic properties in various conditions of annealing, a Fe-Pt-Zr-B melt spun alloy has been synthesized and detailed structural and magnetic investigations have been undertaken to probe its phase evolution during annealing. The dynamics of formation of the hard magnetic L1(0) phase during the gradual disorder-order phase transformation has been monitored by using a complex combination of X-ray diffraction methods and Fe-57 Mossbauer spectroscopy methods, over a wide range of annealing temperatures. Multiple phases co-existing in the annealed sample microstructures, observed in XRD, have been reconfirmed by the Mossbauer spectra analysis and, moreover, accurate quantitative data have been acquired in what concerns the relative abundance of each of the observed crystalline phases in every stage of annealing. It is shown that the formation of the hard magnetic phase, emerging from the chemically disordered precursor, is gradual and occurs via complex mechanisms, involving the presence of a disordered Fe-Zr-B-rich intergranular region which contributes to an increase in the abundance of the L1(0) phase for higher annealing temperatures. Magnetic measurements have confirmed the good performances of these alloys in terms of coercivity and remanence. These results contribute to the development of these alloys as the next generation of rare earth, free permanent magnets.

21. New superconductor/ferromagnet heterostructure formed by YBa2Cu3O7-x and CaRuO3

Authors: Ivan, I; Pasuk, I; Crisan, A; Sandu, V; Onea, M; Leca, A; Cosar, C; Burdusel, M

Published: NOV 2021, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 34, DOI: 10.1088/1361-6668/ac2622

Almost all proposed configurations and practical achievements based on superconductor/ferromagnet (S/F) heterostrucutres focus on s-wave superconductors. However, several attempts targeted also high temperature superconductors, most of them using manganite ferromagnets LaXMnO3 (X: Ca or Sr) and Y1Ba2Cu3O7-x (YBCO). Here we propose a new ferromagnetic material that can be used with YBCO for the fabrication of S/F hybrid structures. We show that a ferromagnetic order can be induced in a thin layer (similar to 130 nm thickness) of CaRuO3 grown by pulsed laser deposition on epitaxial YBCO film. Detailed magnetic and structural investigations show that the observations of the weak ferromagnetism are consistent with the magnetic order induced by in-plane tensile strain of about 1.7% and the easy-magnetization axis forms an angle of similar to 180o with the layer plane. The value of the Curie temperature T (Curie) estimated using the Curie-Weiss law was 340 K. An unusual temperature dependence of the magnetic moment around the superconducting transition was observed in both field-cooled and zero-field-cooled configurations which is attributed to the paramagnetic Meissner effect.

22. Magnetism and epsilon-tau Phase Transformation in MnAl-Based Nanocomposite Magnets

Authors: Crisan, AD; Leca, A; Bartha, C; Dan, I; Crisan, O

Published: APR 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11040896

Melt spun ribbons of Mn53Al45C2 and Mn52Al46C2 have been synthesized by rapid quenching of the melt with the purpose of monitoring the epsilon-tau phase transformation to show technologically feasible ways to increase magnetic parameters and to illustrate the viability of these alloys as the next generation of rare earth (RE)-free magnets. By differential scanning calorimetry (DSC), activation energies and temperatures of onset of the epsilon-tau phase transformation were obtained. Structural analysis was performed using X-ray diffraction (XRD) and the resulting XRD patterns were quantitatively assessed using full profile Rietveld-type analysis. Appropriate annealing was performed in order to enable the epsilon-tau phase transformation. While hcp epsilon-phase was found to be predominant in the as-cast samples, after appropriate annealing, the tetragonal tau-phase, the one that furnishes the relevant magnetic response, was found to be predominant with an abundance of about 90%. The data suggested a mechanism of hcp epsilon-phase decomposition controlled by the segregation towards the interfacial regions, having the rate of transformation governed by antiphase boundary diffusion processes. Magnetic measurements of annealed sample Mn53Al45C2, consisting of predominant tetragonal tau-phase, showed high values of magnetization and increased coercivity, consistent with an energy product of about 10 MGOe, similar with previously reported magnetization measurements, providing further insight into the realization of future class of RE-free low-cost permanent magnets.

23. EFFECT OF Mn SUBSTITUTION ON THE STRUCTURAL, MAGNETIC AND MAGNETOSTRICTIVE PROPERTIES OF Fe-Pd FERROMAGNETIC SHAPE MEMORY RIBBONS

Authors: Sofronie, M; Enculescu, M; Crisan, AD; Tolea, F

Published: 2020, ROMANIAN REPORTS IN PHYSICS, 72, DOI:

This work reports the effect of the Mn substitution, rapid solidification technique and heat treatments on the martensitic transformation, magnetic and magnetostrictive properties on the Fe70-xPd30Mnx (x = 1, 3) ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The thermal treatments induce significant changes in the microstructure and magnetocrystalline anisotropy of the martensitic phase, for Fe67Pd30Mn3 compared to Fe69Pd30Mn1. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.

24. Transport, AC susceptibility, DC magnetization, and magnetic relaxation studies of YBa2Cu3O7 films with synergetic pinning centers grown by PLD

Authors: Crisan, A; Ivan, I; Miu, L; Becherscu, N; Dang, VS

Published: JUL 20 2020, MODERN PHYSICS LETTERS B, 34, DOI: 10.1142/S0217984920400606

We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa2Cu3O7 films with artificial pinning centers, with various impurities, various architectures and thickness, and various techniques of nano-scale pinning engineering: substrate decoration, BaZrO3 (BZO) nano-inclusions, and (quasi) multilayer architecture. Here we will present the results regarding vortex matter, dynamics, and pinning, in some of our best samples. Magnetic relaxation studies with magnetic field perpendicular to the film show that in both cases the splayed defects and/or nanoscale inclusions help reduce the dissipation in certain field-temperature range, by inhibiting the detrimental double vortex-kink formation. The response of the vortex system to AC excitation is rather complex but extremely useful for the characterization of the vortex dynamics. We have measured AC susceptibility in various DC fields, with various AC fields and frequencies and found that the effective vortex activation energy U-eff has a logarithmic dependence on the AC-field-induced current density J. Synergetic pinning centers assured a low anisotropy of the critical current for various field orientations, a property desired in coil/solenoid applications.

25. Hybrid nanoelectronic-magnetic device with magnetoresistive core-shell Fe/FeC nanoparticles

Authors: Crisan, O; Crisan, AD; Dumitrache, F; Luculescu, C

Published: FEB 17 2020, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 126, DOI: 10.1007/s00339-020-3378-y

We propose a concept of hybrid nanoelectronic-magnetic device made of magnetic Fe-C core-shell nanoparticles deposited onto prepatterned Si (111) substrate with basic circuitry made of metallic conductive lines. The synthesis of magnetic material and the creation of nanoelectronic prepatterned interdigitated die are reported and to prove the effectiveness in devices, their magnetotransport properties are investigated. Magnetic Fe/FeC nanoparticles, 11 nm diameter, with a core-shell structure have been prepared by laser pyrolysis. Two different layouts of prepatterned interdigitated die, have been conceived using e-beam lithography, with various geometries. A range of microscopy techniques, transmission electron, scanning and optical, were employed for morphological characterization of the as-obtained structures. Magnetic and magnetotransport characterization using SQUID magnetometry has been performed onto both the core-shell nanoparticles and onto the hybrid device obtained by depositing centrifugated and dispersed core-shell nanoparticles from liquid carrier solutions. From magnetotransport measurements, it has been revealed that the hybrid device made of Fe/FeC nanosized materials on prepatterned interdigitated die exhibit a large giant magnetoresistive (GMR) effect of about 8% at 300 K. This result is promising in view of the use of such devices as arrays of nanosensors and in spintronic applications.

26. Magnetic Phase Coexistence and Hard-Soft Exchange Coupling in FePt Nanocomposite Magnets

Authors: Crisan, O; Dan, I; Palade, P; Crisan, AD; Leca, A; Pantelica, A

Published: AUG 2020, NANOMATERIALS, 10, DOI: 10.3390/nano10081618

With the aim of demonstrating phase coexistence of two magnetic phases in an intermediate annealing regime and obtaining highly coercive FePt nanocomposite magnets, two alloys of slightly off-equiatomic composition of a binary Fe-Pt system were prepared by dynamic rotation switching and ball milling. The alloys, with a composition Fe(53)Pt(47)and Fe55Pt45, were subsequently annealed at 400 degrees C and 550 degrees C and structurally and magnetically characterized by means of X-ray diffraction,Fe-57 Mossbauer spectrometry and Superconducting Quantum Interference Device (SQUID) magnetometry measurements. Gradual disorder-order phase transformation and temperature-dependent evolution of the phase structure were monitored using X-ray diffraction of synchrotron radiation. It was shown that for annealing temperatures as low as 400 degrees C, a predominant, highly ordered L1(0)phase is formed in both alloys, coexisting with a cubic L1(2)soft magnetic FePt phase. The coexistence of the two phases is evidenced through all the investigating techniques that we employed. SQUID magnetometry hysteresis loops of samples annealed at 400 degrees C exhibit inflection points that witness the coexistence of the soft and hard magnetic phases and high values of coercivity and remanence are obtained. For the samples annealed at 500 degrees C, the hysteresis loops are continuous, without inflection points, witnessing complete exchange coupling of the hard and soft magnetic phases and further enhancement of the coercive field. Maximum energy products comparable with values of current permanent magnets are found for both samples for annealing temperatures as low as 500 degrees C. These findings demonstrate an interesting method to obtain rare earth-free permanent nanocomposite magnets with hard-soft exchange-coupled magnetic phases.

27. Second magnetization peak, rhombic-to-square Bragg vortex glass transition, and intersecting magnetic hysteresis curves in overdoped BaFe2(As1-xPx)(2) single crystals

Authors: Miu, L; Ionescu, AM; Miu, D; Burdusel, M; Badica, P; Batalu, D; Crisan, A

Published: OCT 14 2020, SCIENTIFIC REPORTS, 10, DOI: 10.1038/s41598-020-74156-z

The second magnetization peak (SMP) in the fourfold symmetric superconducting single crystals (such as iron pnictides and tetragonal cuprates) has been attributed to the rhombic-to-square transition (RST) of the quasi-ordered vortex solid (the Bragg vortex glass, BVG). This represents an alternative to the pinning-induced BVG disordering as the actual SMP mechanism. The analysis of the magnetic response of BaFe2(As1-xPx)(2) specimens presented here shows that the SMP is not generated by the RST. However, the latter can affect the pinning-dependent SMP onset field if this is close to the (intrinsic) RST line, through the occurrence of a "shoulder" on the magnetic hysteresis curves m(H), and a maximum in the temperature variation of the DC critical current density. These features disappear in AC conditions, where the vortex system is dynamically ordered in the RST domain, emphasizing the essential role of vortex dislocations for an efficient accommodation of the vortex system to the pinning landscape and the SMP development. The m(H) shoulder is associated with a precipitous pinning-induced proliferation of dislocations at the RST, where the BVG elastic "squash" modulus softens. The DC magnetization relaxation indicates that the pinning-induced vortex system disordering continues above the RST domain, as the basic SMP mechanism.

28. Mn-Induced Thermal Stability of L1(0)Phase in Fept Magnetic Nanoscale Ribbons

Authors: Crisan, AD; Leca, A; Pantelica, D; Dan, I; Crisan, O

Published: JUL 2020, NANOMATERIALS, 10, DOI: 10.3390/nano10071278

Magnetic nanoscale materials exhibiting the L1(0)tetragonal phase such as FePt or ternary alloys derived from FePt show most promising magnetic properties as a novel class of rare earth free permanent magnets with high operating temperature. A granular alloy derived from binary FePt with low Pt content and the addition of Mn with the nominal composition Fe(57)Mn(8)Pt(35)has been synthesized in the shape of melt-spun ribbons and subsequently annealed at 600 degrees C and 700 degrees C for promoting the formation of single phase, L1(0)tetragonal, hard magnetic phase. Proton-induced X-ray emission spectroscopy PIXE has been utilized for checking the compositional effect of Mn addition. Structural properties were analyzed using X-ray diffraction and diffractograms were analyzed using full profile Rietveld-type analysis with MAUD (Materials Analysis Using Diffraction) software. By using temperature-dependent synchrotron X-ray diffraction, the disorder-order phase transformation and the stability of the hard magnetic L1(0)phase were monitored over a large temperature range (50-800 degrees C). A large interval of structural stability of the L1(0)phase was observed and this stability was interpreted in terms of higher ordering of the L1(0)phase promoted by the Mn addition. It was moreover found that both crystal growth and unit cell expansion are inhibited, up to the highest temperature investigated (800 degrees C), proving thus that the Mn addition stabilizes the formed L1(0)structure further. Magnetic hysteresis loops confirmed structural data, revealing a strong coercive field for a sample wherein single phase, hard, magnetic tetragonal L1(0)exists. These findings open good perspectives for use as nanocomposite, rare earth free magnets, working in extreme operation conditions.

29. Microwave investigation of pinning in Te- and cubic-BN- added MgB2

Authors: Alimenti, A; Torokhtii, K; Grigoroscuta, M; Badica, P; Crisan, A; Silva, E; Pompeo, N

Published: 2020, 14TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS2019), 1559, DOI: 10.1088/1742-6596/1559/1/012039

MgB2 has great potential for many applications, thanks to its relatively high critical temperature and low fabrication cost. Large efforts are done to improve the current carrying capabilities of bulks and tapes in view of different application fields, e.g. with the addition of Te and cubic-BN to MgB2. To elucidate the vortex pinning physics exploiting a different dynamic regime, we present here a microwave study of the pinning properties of spark plasma sintered bulk MgB2 with and without the addition of 0.01 % at. Te or cubic-BN. We show the surface resistance R-s of the MgB2 samples measured with a dielectric-loaded resonator at similar to 16.5 GHz and similar to 26.7 GHz in the 10 K-Tc temperature range at fields up to 1.0 T. Then, the MgB2 R., is studied with high frequency vortex motion models in order to obtain the pinning constant (Labusch parameter) and the depinning frequency. Finally, the microwave behavior of MgB2 in the mixed state is compared with the recent results obtained on Nb3Sn.

30. Structure and magnetic properties of highly coercive L1(0) nanocomposite FeMnPt thin films

Authors: Crisan, O; Vasiliu, F; Crisan, AD; Mercioniu, I; Schinteie, G; Leca, A

Published: JUN 2019, MATERIALS CHARACTERIZATION, 152, 252, DOI: 10.1016/j.matchar.2019.04.028

Among the rare-earth-free systems that are currently investigated in search for novel permanent magnet solutions for various applications, with special emphasis on the magnets required to operate in extreme conditions, the FePt binary system, where the tetragonal hard magnetic L1(0) phase can be formed by suitable microstructure processing via annealing, has been extensively studied. A variation of this system, the ternary FeMnPt system, has been also recently shown to exhibit good permanent magnet behavior due to the suitable formation of the L1(0) phase. In addition to be likely to form the L1(0) phase as its parent binary system, the ternary FeMnPt benefits from the reduced costs due to the reduced amount of Pt and may exhibit particular magnetic structure due to the influence of the antiferromagnetic Mn. In the present work, we have employed a mixed sputtering technique, based on the use of both elemental and compound target for developing L1(0) FeMnPt thin films with specific structural features that triggers better magnetic performances in terms of coercivity and maximum energy products. The as-obtained films have been thermally annealed and characterized by means of transmission electron microscopy, X-ray diffraction, Mossbauer spectroscopy, magneto-optic Kerr effect (MORE) and SQUID magnetometry. The aim is to correlate the Mn induced microstructural and lattice changes with the magnetic properties and to optimize the microstructure for an early formation of the ordered L1(0) phase and increased coercivity compared to the as-prepared, structurally disordered, face centred cubic initial state of the films.

31. Gd3+ co-doping influence on the morphological, up-conversion luminescence and magnetic properties of LiYF4:Yb3+/Er3+ nanocrystals

Authors: Secu, CE; Bartha, C; Matei, E; Negrila, C; Crisan, A; Secu, M

Published: JUL 2019, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 130, 241, DOI: 10.1016/j.jpcs.2019.03.003

The influence of the Gd3+ co-dopant on the structure, morphology and up-conversion luminescence/magnetic properties of the LiYF4:Gd3+/Yb3+/Er3+ nanocrystals was investigated and compared to those of Gd-free samples. Electron microscopy has indicated an inhibiting effect of the agglomeration and collapsing process of the nanocrystals compared to the Gd-free powder samples. The surface analysis of nanocrystals have not shown oxygen-metal bonds related to the metal oxidation within the surface nanometric layer. The paramagnetic properties are related to the magnetic moment of the Gd3+ ions. The up-conversion luminescence of the LiYF4:Gd3+/Yb3+/Er3+ nanocrystals is about six times stronger than in LiYF4:Yb3+/Er3+ nanocrystals; the enhancement effect is most probably due to the lattice distortion induced by the Gd co-doping.

32. Vortex dynamics driven by AC magnetic field in YBCO thin films with complex pinning structures

Authors: Ivan, I; Ionescu, AM; Sandu, V; Crisan, A; Miu, L

Published: OCT 2018, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 31, DOI: 10.1088/1361-6668/aadbfd

We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2O3 nanoparticles at a static magnetic field H-dc lower than the matching field H-Phi. We proposed a practical formula for the determination of the induced current J during AC susceptibility measurements, which allows us to directly obtain the pinning potential U-c from the characteristics of electric field E versus J. We show that the dynamic critical current J(d) induced at the depinning frequency f(d) can be experimentally obtained by measuring m'(T) and m ''(T) at different frequencies and amplitudes. It was found that the value of f(d) obtained by extrapolation of J to J(d) in the PHz domain is much higher than the frequency reported for ordinary YBa2Cu3O7 thin films as determined by microwave impedance measurements. AC susceptibility, vortex dynamics, pinning energy, depinning frequency, dynamic critical current, microwave impedance measurements

33. Pinning-Induced Vortex-System Disordering at the Origin of the Second Magnetization Peak in Superconducting Single Crystals

Authors: Ionescu, AM; Miu, D; Crisan, A; Miu, L

Published: AUG 2018, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 31, 2337, DOI: 10.1007/s10948-017-4487-5

The nature of the second magnetization peak (SMP) appearing on the dc magnetic hysteresis curves of superconducting single crystals with random pinning is still under debate. Many interesting SMP models and mechanisms were proposed and considered so far, and it is believed at present that this effect is system dependent. We analyzed the dc magnetization curves and the magnetic relaxation in the SMP domain for various single-crystal specimens (superconducting cuprates and iron-based superconductors), of different pinning strengths, with the external magnetic field H oriented along the crystallographic c axis or perpendicular to it. The sample independent aspects revealed by the relaxation results are the absence of single-vortex collective pinning around the SMP onset field and the sign changing of the vortex creep exponent between the onset field and the peak field. This general behavior supports strongly the pinning-induced disordering of the low-H quasi-ordered vortex solid as the actual scenario for the SMP.

34. LOW-TEMPERATURE FORMATION OF 312 PHASE IN Ti-Si-C TERNARY COMPOUND

Authors: Crisan, AD; Crisan, O

Published: JAN-MAR 2018, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 13, 162, DOI:

We report on the formation of the Ti3SiC2 nanolaminate phase in the Ti-Si-C thin film system, using a UHV magnetron sputtering technique with top mounted sample holder, from elemental and compound targets. The formation of the Ti3SiC2 (or 312 phase) has been evidenced by detailed X-ray diffraction analysis followed by full-profile quantitative analysis of the obtained diffractograms. It has been proven that for deposition temperatures as low as 500 degrees C, there is a significant amount of 312 phase obtained in the deposited films, in co-existence with the majority TiC phase. This amount increased to about 21% when the deposition temperature was raised to 650 degrees C. The ternary 312 phase becomes predominant at around 60% relative abundance for slight off-stoichiometric, Si increased, content of the alloy, for temperatures as low as 650 degrees C. The conditions for improving the relative abundance of the 312 phase within our experimental setup are pointed out and explained in terms of a nucleation and growth model of nanostructure formation from the amorphous precursor (?).

35. Annealing-Induced High Ordering and Coercivity in Novel L1(0) CoPt-Based Nanocomposite Magnets

Authors: Crisan, AD; Vasiliu, F; Mercioniu, I; Bartha, C; Enculescu, M; Crisan, O

Published: JUN 2018, METALS, 8, DOI: 10.3390/met8060466

A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L1(0) phase. A Co48Pt28Ag6B18 alloy has been prepared by rapid solidification from the melt and subjected to various isothermal annealing procedures. The structure and magnetism of both as-cast and annealed samples as well as the phase evolution with temperature are investigated by means of thermal analysis, X-ray, and selected area electron diffraction, scanning and high-resolution electron microscopy, and magnetic measurements. The X-ray diffraction (XRD) analysis shows that both the as-cast alloy and the sample annealed at 400 degrees C (673 K) have a nanocrystalline structure where fcc CoPt phase predominates. Annealing at 473 degrees C promotes the formation of L1(0) phase triggered by the disorder-order phase transformation as documented in the differential scanning calorimetry results. The sample annealed at 670 degrees C (943 K) shows full formation of L1(0) CoPt as revealed by XRD. Magnetic measurements showed coercivity values ten times increased compared to the as-cast state. This confirms the full formation of L1(0) CoPt in the annealed samples. Moreover, detailed atomic resolution HREM images and SAED patterns show the occurrence of the rarely seen (003) superlattice peaks, which translated into a high ordering of the L1(0) CoPt superlattice. Such results spur more interest in finding novel classes of nanocomposite magnets based on L1(0) phase.

36. AC magnetic response of superconducting single crystals exhibiting a second peak on the DC magnetization curves

Authors: Miu, L; Ionescu, AM; Miu, D; Petrisor, T; Park, A; Tamegai, T; Crisan, A

Published: DEC 15 2018, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 555, 6, DOI: 10.1016/j.physc.2018.10.002

The strong modulation of the AC magnetic response in the magnetic field-temperature region of the DC magnetization peak effect (PE) occurring just below the DC irreversibility line (IL) of weakly pinned superconducting single crystals is well known. However, it is not yet clear if the AC magnetic signal registered at usual frequencies and amplitudes is significantly affected by the presence of the second magnetization peak (SMP) in the DC magnetic hysteresis curves, since the SMP appears in specimens with stronger pinning and is located well below the IL. We investigated the AC magnetic response of several La2 - xSrxCuO4 and 122-type pnictide superconducting single crystals exhibiting a pronounced DC SMP. It was found that in the case of small demagnetization effects, the AC magnetic signal across the SMP remains in the linear regime, with no detectable distortions in the SMP range (within the high sensitivity of a Physical Property Measurement System). This indicates reduced values of the Campbell penetration depth compared with the London magnetic penetration depth. A clearly nonmonotonic, SMP-related variation has only been obtained for the DC field dependence of the out of phase component of the AC magnetic moment in the nonlinear regime. The nonlinear AC Bean regime far below the IL has been attained in the conditions of enhanced demagnetization effects (plate-like specimens in perpendicular magnetic fields).

37. Thermodynamic, structural and magnetic studies of phase transformations in MnAl nanocomposite alloys

Authors: Crisan, AD; Vasiliu, F; Nicula, R; Bartha, C; Mercioniu, I; Crisan, O

Published: JUN 2018, MATERIALS CHARACTERIZATION, 140, 8, DOI: 10.1016/j.matchar.2018.03.034

We have undertaken a temperature-dependent, investigation of the thermodynamics, structure, morphology and magnetism of two MnAl nanocomposite alloys (Mn60Al40 and Mn55Al45). Differential scanning calorimetry (DSC) studies allowed the determination of the exo-effects occurring in the structural phase transformation of MnAI, while by using temperature-dependent X-ray diffraction of synchrotron radiation we were able to monitor the phase transformation effects and the evolution with temperature of various structural phases occurring in the samples. We have shown that slight changes in stoichiometry (5 at.%) give rise to different phase structure in the as-cast state. While in Mn60Al40 only hcp epsilon phase can be found, in Mn55Al45 as-cast alloy, there is a quite complex phase structure with a mixture of gamma(2) (Al8Mn5) and epsilon as well as ferromagnetic MnAI T-phase. Activation energies of about 165 kJ/mol and 290 kJ/mol have been calculated from the Ozawa-Flynn-Wall analysis of DSC experimental data. These findings are in good agreement with the results obtained from XRD. For the Mn55Al45 as cast alloy, we have confirmed by temperature-dependent synchrotron XRD that the hcp e phase decomposes through the migration of interphase interfaces with the transformation rate controlled by boundary diffusion processes. High-resolution transmission electron microscopy have confirmed the phase structure obtained by XRD, while magnetic properties, obtained for the as-cast alloys are consistent with the multiphase character of the samples and in good agreement with previously reported results. The magnetization does not saturate for the maximum applied field of about 4 x 10(6) A/m and a marked coercivity of about 160 kA/m is obtained for the Mn55Al45 as-cast alloy.

38. Incipient low-temperature formation of MAX phase in Cr-Al-C films

Authors: Crisan, O; Crisan, AD

Published: JUN 2018, JOURNAL OF ADVANCED CERAMICS, 7, 151, DOI: 10.1007/s40145-018-0265-5

Ceramic-metallic MAX phase of chromium aluminium carbide ternary compounds was successfully obtained through deposition by DC sputtering onto Si substrates. A study of the influence of substrate temperature and in-air post-annealing on the film crystallinity and oxidation was undertaken. Scanning electron microscopy (SEM), wavelength-dispersive X-ray analysis (WDSX), and X-ray diffraction (XRD) were used for film characterization. It is shown that, at substrate temperature of about 450 degrees C, as-deposited films are amorphous with small nanocrystals. Subsequent annealing in air at 700 degrees C leads to film crystallization and partial oxidation. WDSX spectroscopy shows that the films oxidise to a depth of around 120 nm, or 5% of total film thickness which amounts at around 2.68 mu m. As a novelty, this demonstrates the possibility of in-air crystallization of Cr2AlC films without significant oxidation. Materials Analysis Using Diffraction (MAUD) software package for a full-profile analysis of the XRD patterns (Rietveld-type) was used to determine that, as a result of annealing, the average crystallite size changes from 7 to 34 nm, while microstrain decreases from 0.79% to 0.24%. A slight tendency of preferential growth along the (10 (1) over bar0) direction has been observed. Such texturing of the microstructure has the potential of inducing beneficial anisotropic fracture behaviour in the coatings, potentially interesting for several industrial applications in load-bearing devices.

39. Flux Channeling in YBa2Cu3O7 Superlattices

Authors: Mollatt, HJS; Dang, VS; Qureishy, T; Mikheenko, P; Crisan, A

Published: 2018, PROCEEDINGS OF THE 2018 IEEE 8TH INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATION & PROPERTIES (NAP-2018), DOI:

We report unusual effect of channeled magnetic flux motion in YBa2Cu3O7-delta/PrBa2Cu3O7-delta superlattices grown by pulsed laser deposition. Magneto-optical imaging reveals that flux moves along a set of parallel and perpendicular lines, while optical microscopy does not show any features on the surface that may be linked to this effect. In contrast, scanning electron microscopy registers sub-micron fractures in the superlattices corresponding to these flux lines, but the magnetic flux channels are much wider than the width of these fractures. To further clarify the origin of flux channels, electrical transport measurements on the superlattices have been performed. Their current-voltage characteristics reveal the presence of distinctive branches related to the flux motion along the selective channels, following which magnetic flux crosses sample in a shortest and least resistive way. The application of very large current overheated the superlattice along these channels, evaporating superconducting material and exposing, wider than in the superconductor, fractures in the substrate. It is concluded that motion of flux in the channels is controlled not only by the presence of nano-fractures in YBa2Cu3O7-delta/PrBa2Cu3O7_delta, but also by stress developed in the superconducting material due to the fracturing of the substrate.

40. Synergetic pinning centres in BaZrO3-doped YBa2Cu3O7-x films induced by SrTiO3 nanolayers

Authors: Crisan, A; Dang, VS; Mikheenko, P; Ionescu, AM; Ivan, I; Miu, L

Published: APR 2017, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 30, DOI: 10.1088/1361-6668/aa5edf

We report on the enhancement of critical current density (J(c)) and the unusual behaviour of its dependence on field orientation in YBa2Cu3O7-x (YBCO) nanostructured films by a combination of substrate decoration with Ag nano-dots, of the incorporation of BaZrO3 (BZO) nano-particles and nano-rods, and of multilayer architecture (a thin SrTiO3 layer separating two 1.5 mu m-thick YBCO layers). SrTiO3 insulating layers were 15, 30 or 45 nm thick. The highest improvement of J(c) in applied magnetic fields along the c-axis and smaller than 1 T occurs in the bi-layer with 30 nm-thick STO, but the influence of STO thickness is small. Our thick nanostructured films show significant improvement of J(c) in the magnetic field along the ab-plane direction. The presence of BZO nano-rods, ab-plane defects and nano particles of BZO and Y2O3 was observed in transmission electron microscopy (TEM) images of the film. The peculiarities of artificial pinning centres revealed in the TEM images of the nanostructured films are used to explain an unusual split of the peak in the J(c) dependence on the magnetic field along the ab-plane of YBCO. Effective pinning potentials in high magnetic fields have rather high values for such thick films.

41. Vortex-glass state in the isovalent optimally doped pnictide superconductor BaFe2(As0.68P0.32)(2)

Authors: Salem-Sugui, S; Mosqueira, J; Alvarenga, AD; Sonora, D; Crisan, A; Ionescu, AM; Sundar, S; Hu, D; Li, SL; Luo, HQ

Published: MAY 2017, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 30, DOI: 10.1088/1361-6668/aa62f1

We report on isochamp magneto-resistivity and ac susceptibility curves obtained in a high-quality single crystal of the isovalent optimally doped pnictide BaFe2(As0.68P0.32)(2) with superconducting temperature T-c = 27.8 K for H||c-axis. Plots of the logarithmic derivative of the resistivity curves allowed the identification of a vortex-glass (VG) phase and to obtain the values of the critical glass temperature T-g, the temperature T* marking the transition to the liquid phase and of the critical exponent s. The presence of the VG phase is confirmed by detailed measurements of the third harmonic signal of the ac magnetic susceptibility. The modified VG model was successfully applied to the data allowing the obtention of the temperature independent VG activation energy U-b. The activation energy U-0 obtained from the Arrhenius plots in the flux-flow region are compared with U-b and with U-0 obtained from flux-creep measurements on a M(H) isothermal in the same sample. A phase diagram of the studied sample is constructed showing the T-g glass line, the T* line representing a transition (melting) to the liquid phase, the mean field temperature T-c(H) line and the H-p line obtained from the peaks in isothermal critical current, J(c)(H) curves, which are explained in terms of a softening of the vortex lattice. The glass line was fitted by a theory presented in the literature which considers the effect of disorder.

42. Nano-engineered pinning centres in YBCO superconducting films

Authors: Crisan, A; Dang, VS; Mikheenko, P

Published: FEB 15 2017, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 533, 132, DOI: 10.1016/j.physc.2016.06.011

For practical applications of superconducting materials in applied magnetic fields, artificial pinning centres in addition to natural ones are required to oppose the Lorentz force. These pinning centres are actually various types of defects in the superconductor matrix. The pinning centres can be categorised on their dimension (volume, surface or point) and on their character (normal cores or AK cores). Different samples have been produced by Pulsed Laser Deposition, with various thicknesses, temperatures and nanostructured additions to the superconducting matrix. They have been characterized by SQUID Magnetic Properties Measurement System and Physical Properties Measurement System, as well as by Transmission Electron Microscopy (TEM). Correlations between pinning architecture, TEM images, and critical currents at various fields and field orientations will be shown for a large number of YBa2Cu3Ox films with various types and architectures of artificial pinning centres. (C) 2016 Elsevier B.V. All rights reserved.

43. Behavior of the Second Magnetization Peak in Self-nanostructured La2-xSrxCuO4 Single Crystals

Authors: Miu, L; Ionescu, AM; Miu, D; Ivan, I; Crisan, A

Published: 2017, VORTICES AND NANOSTRUCTURED SUPERCONDUCTORS, 261, 184, DOI: 10.1007/978-3-319-59355-5_6

44. Specific Changes in the Magnetoresistance of Ni-Fe-Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions

Authors: Tolea, F; Tolea, M; Sofronie, M; Popescu, B; Crisan, A; Leca, A; Valeanu, M

Published: APR 2017, IEEE TRANSACTIONS ON MAGNETICS, 53, DOI: 10.1109/TMAG.2016.2628386

In this paper, we investigate the influence of Cu, Co, and Al substitutions on the transport properties, and in particular, the magnetoresistive effect in Ni-Fe-Ga ferromagnetic shape memory alloys (FSMAs) prepared as ribbons by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetometry, and magnetoresistive characterizations were performed. In the range of the martensitic transformation (MT), different FSMA compositions show a rich spectrum of different behaviors. For one of the compositions (Ni52Fe20Co2Ga23Al3), the magnetoresistance (MR) showed a local minimum or, on the contrary, a local maxima of reduced amplitude on cooling, in the range of the MT, depending on the performed thermal treatments. In the same composition, by replacing one Al atom with a Co one, no local extremes are seen, the alloy having a concomitant magneto-structural transition. When the magnetic field was varied, the MR showed a nonmonotonic variation in the martensite phase for some compounds, possibly due to the dynamics of the martensite variants realignment. From the studied compositions, the highest MR found on the MT of -9% for 5 T is for Ni50Fe20Ga27Cu3.

45. ELASTIC PROPERTIES AND CRYSTALLIZATION OF THERMAL BARRIER COATINGS OF Cr2AlC

Authors: Crisan, O; Crisan, AD

Published: OCT-DEC 2017, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 12, 1174, DOI:

The elastic and structural properties of thermal barrier coatings of Cr2AlC DC sputtered from elemental targets onto Si(100) substrate at 200 degrees C are studied. The material was subsequently submitted to re-crystallization procedure by annealing the film at 650 degrees C and 700 degrees C in high vacuum and in air for 30 min. As-deposited and annealed films are shown to be homogeneous and dense over large areas. The re-crystallization process was monitored using X-ray diffraction and Raman spectroscopy. As-deposited films have an amorphous-like Cr-C-Al solid solution structure and annealed samples crystallize into single-phase hexagonal Cr2AlC. While XRD analysis and Raman spectroscopy of samples annealed in-air show the presence of 5% Cr2O3, the high vacuum samples annealed at 650 degrees C and 700 degrees C are fully crystallized and comprise only single-phase Cr2AlC hexagonal structure. Oxygen presence in the in-air annealed sample is shown to cause an elongation of the hexagonal unit cell along the c axis which coincides with the direction of stacking of Cr6C octahedral building blocks with alternate Al layers. Whereas the average grain size is shown to increase upon annealing, lattice microstrain, calculated using the integral breadth method, is seen to decrease by almost 70% in the annealed films. Raman spectra of as-deposited films show characteristic MAX-phase bands with broad, overlapping peaks in the region of 120-400 cm(-1) but also peaks in the range of 550-900 cm(-1), attributed to other Raman-active modes of the Cr2AlC structure. After annealing, the Raman peaks corresponding to Cr2AlC single-phase are narrower, more intense and better defined than in the as-deposited case. The occurrence of the disorder-induced D carbon band is observed in the Raman spectrum of the as-deposited film while, after high vacuum annealing, a sharp and relatively intense peak attributed to the carbon G band is observed that suggests that there may be carbon nanoclustering in the coatings upon annealing. This observation is consistent with and comes as a confirmation of previously reported ab initio modelling of possible Cr2AlC off-stoichiometric structures.

46. Pinning-Engineered YBa2Cu3Ox Thin Films

Authors: Mele, P; Crisan, A; Adam, MI

Published: 2017, VORTICES AND NANOSTRUCTURED SUPERCONDUCTORS, 261, 63, DOI: 10.1007/978-3-319-59355-5_2

47. Vortices and Nanostructured Superconductors Preface

Authors: Crisan, A

Published: 2017, VORTICES AND NANOSTRUCTURED SUPERCONDUCTORS, 261, IX, DOI:

48. Interfacial mechanisms of novel laser-irradiated L1(0)-based nanocomposite magnets

Authors: Crisan, O; Crisan, AD; Enculescu, M

Published: APR 2016, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 122, DOI: 10.1007/s00339-016-9947-4

In melt-spun FePtB-based ribbons, the addition of Ag has been proven to decrease the temperature of phase transformation from the A1 fcc FePt phase to the hard magnetic tetragonal L1(0) phase. Alloys with 6 and 9 at.% Ag added to the initial FePtB have been synthesized by rapid solidification from the melt. The samples have been laser irradiated and submitted to nitriding procedure. This procedure has been proven beneficial for inducing complete transformation of A1 to L1(0) phase and a strong (001) texturing. Ag segregation combined to mechanisms of creation of vacancies and diffusion of N give rise to the formation of an intergranular disordered region and due to an improved interfacial coupling between FePt grains, enhanced coercivity and two-phase magnetic behavior is obtained.

49. STRUCTURAL AND MAGNETIC PROPERTIES OF Sr2FeMoO6 OBTAINED AT LOW TEMPERATURES

Authors: Bartha, C; Plapcianu, C; Crisan, A; Enculescu, M; Leca, A

Published: JUL-SEP 2016, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 11, 780, DOI:

The double-perovskite Sr2FeMoO6 has been obtained by solid state method at low temperature (1060 degrees C) and a very short time of synthesis (up to 4h). Both, X-ray diffraction and scanning electron microscopy (SEM) confirmed the formation of Sr2FeMoO6 oxide with grain sizes around 160 mm, and a small amount of SrMoO4 as an impurity. Mossbauer spectroscopy revealed a mixed site population with Fe and Mo ions generating a structure type with population inversion. This structure has a critical influence on the magnetic properties, as confirmed by the magnetization and TC values, i.e 3.56 mu(B)/f.u and 415 K, respectively. The Sr2FeMoO6 behavior was interpreted in terms of ferrimagnetic couplings generated by the various distributions of local interactions between Fe and Mo neighbors while comparing the ideal structure should show antiferromagnetic coupling between the two sublattices.

50. Columnar Growth of Transparent Semiconducting Films: Learning from Nature

Authors: Wang, Y; Crisan, A; Mikheenko, P

Published: 2016, INTERNATIONAL CONFERENCE ON NANOMATERIALS: APPLICATION & PROPERTIES (NAP), DOI:

A two-dimensional array of a large number of identical densely-packed indium-oxide nano-columns was prepared by pulsed laser deposition. The structure closely mimics tubular structure of the under-cap layer of tubular mushrooms. The growth was initiated by a dense array of gold or silver nanodots. The resulting structure forms a transparent semiconducting layer whose resistance is highly sensitive to composition of gases in atmosphere. A simple device based on the array of nano-columns is able to accurately monitor gas balance in an open or closed-space environment. The simplicity of the device, its compactness and a well-established process of deposition would allow incorporating it in various equipment of general or personal use. The application of the sensor could range from health monitoring during the exhale to the check of the car exhaust or gas content in homes, offices, workplaces or in the dangerous and threatening environment.

51. Development and structural characterization of exchange-spring-like nanomagnets in (Fe,Co)-Pt bulk nanocrystalline alloys

Authors: Crisan, O; Crisan, AD; Mercioniu, I; Nicula, R; Vasiliu, F

Published: MAR 1 2016, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 401, 715, DOI: 10.1016/j.jmmm.2015.10.114

FePt-based alloys are currently under scrutiny for their possible use as materials for perpendicular magnetic recording. Another possible application is in the field of permanent magnets without rare-earths, magnets that may operate at higher temperatures than the classic Nd-Fe-B magnets. Within this study, FeCoPt alloys prepared by rapid solidification from the melt are structurally and magnetically characterized. In the as-cast FeCoPt ribbons, a three-phase structure comprising well-ordered CoFePt and CoPt L1(0) phases embedded in a disordered fcc FePt matrix was evidenced by XRD, HREM and SAED. Extended transmission electron microscopy analysis demonstrates the incipient formation of ordered L1(0) phases. X-ray diffraction was used to characterize the phase structure and to obtain the structural parameters of interest for L1(0) ordering. In the as-cast state, the co-existence of hard magnetic CoFePt and CoPt L1(0) tetragonal phases with the soft fcc FePt phase is obtained within a refined microstructure made of alternatively disposed grains (grain sizes from 1 to 7 nm). Following a thermal treatment of 1 h at 670 degrees C, the soft magnetic fcc matrix phase transforms to tetragonal L1(0) phases (disorder-order transition). The resulting CoPt and CoFePt L1(0) phases have grains of around 5-20 nm in size. In the as-cast state, magnetic measurements show a quite large remanence (0.75 T), close to the value of the parent L1(0) FePt phase. Coercive fields of about 200 kA/m at 5 K were obtained, comparable with those reported for some FePt-based bulk alloys. Upon annealing both remanence and coercivity are increased and values of up to 254 kA/m at 300 K are obtained. The polycrystalline structure of the annealed FeCoPt samples, as well as the formation of multiple c-axis domains in different CoPt and CoFePt regions (which leads to a reduction of the magneto-crystalline anisotropy) may account for the observed coercive fields that are lower than in the case of very thin FeCoPt films. A Curie temperature of about 820 K (close to 550 degrees C) is reported for the Fe35Co15Pt50 alloy which opens wide possibilities for the use of such magnets in high operating temperature industrial applications. The present results indicate that ternary FeCoPt alloys hold a great potential as a novel class of rare earth free exchange-spring coupled nanocomposite magnets. (C) 2015 Elsevier B.V. All rights reserved.

52. Magnetoelastic properties in polycrystalline ferromagnetic shape memory Heusler alloys

Authors: Sofronie, M; Tolea, F; Crisan, AD; Popescu, B; Valeanu, M

Published: 2016, 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), 2, 1537, DOI: 10.1016/j.prostr.2016.06.194

The influence of the heat treatments on the martensitic transformation, magnetic properties and thermo- and magnetic induced strain on Ni50Fe20Ga27Cu3 ferromagnetic shape memory alloy prepared as ribbons by melt spinning technique are investigated. The degree of atomic order as effect of different thermal treatments produces important changes in the magneto-crystalline anisotropy of the martensite phase. The anomalies evidenced in the thermo-and magnetic-strain curves are discussed and correlated with the thermo-magnetic data. The transformation-induced strains with and without magnetic field have been measured, the results setting out the influence of the pre-martensitic transformation. Copyright (C) 2016 The Authors. Published by Elsevier B.V.

53. Effect of thermal treatments in Ni-Fe-Ga with Co substitutions and Ni-Mn-Ga melt spun ribbons

Authors: Tolea, F; Sofronie, M; Crisan, AD; Popescu, B; Tolea, M; Valeanu, M

Published: 2016, 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), 2, 1480, DOI: 10.1016/j.prostr.2016.06.187

The effect of "in situ" thermal treatments (by DSC measurements) on the martensitic transformation in two representative Ni-Fe-Ga and Ni-Mn-Ga alloys has been studied and discussed by correlating the structural and magnetic properties. The alloys were prepared from high purity elements, by arc melting under argon protective atmosphere as bulk and also as melt-spun ribbons - an alternative preparation route that also allows to assess the influences of grains size and strain induced by this processing method. All samples presented reversible thermo-elastic transformations. The thermal treatments promote a reduction of the martensitic transformation temperatures in the Ni-Fe-Ga investigated samples, as opposed to the stoichiometric Ni2MnGa where the temperatures increase with increasing the annealing temperatures. Interestingly however, the off-stoichiometric Ni-Mn-Ga with increased Ni content recovers the behaviour with reduction of transformation temperatures by thermal treatments. The precipitation of the secondary FCC (gamma) phase is inherently found in Ni-Fe-Ga alloys with Ga <= 27% at, and also-although in lower amounts- in the off-stoichiometric Ni-Mn-Ga. The gamma phase is considered to contribute to the decrease of the MT temperatures (via valence electrons concentration depletion of the main matrix) and of the transformation heat as well as to the final structural degradation if the temperature of the thermal treatments is further increased. In addition, this phase, located mainly at the grain boundaries, is responsible for the improved ductility of Ni-Fe-Ga based alloys. Changes in the transformation heat due to thermal treatments are observed and discussed in both types of alloys, the maxima of the transformation heat being associated with the highest atomic order. Thermo-magnetic measurements show that Ni-Fe-Ga alloys have close magnetic and structural transitions temperatures, with promising applications for magnetic refrigeration. Copyright (C) 2016 The Authors. Published by Elsevier B.V.

54. Magnetic and Martensitic Transformations in the bulk and melt spun ribbons of Ni57-xNdxFe18Ga25 Ferromagnetic Shape Memory Alloys

Authors: Tolea, F; Crisan, AD; Sofronie, M; Tolea, M; Valeanu, M

Published: 2015, MATERIALS TODAY-PROCEEDINGS, 2, 878, DOI: 10.1016/j.matpr.2015.07.421

Influence of Nd substitution for Ni on the magnetic properties and the martensitic transformation (MT) characteristics are investigated on Ni57-xNdxFe18Ga25 (x=0 divided by 4) ferromagnetic shape memory alloys (FSMAs) in bulk and also in ribbons prepared by melt spinning method and subjected to different thermal treatments. Increasing the Nd content induces a decrease of both the Curie and the MT temperatures. (C) 2015 The Authors. Published by Elsevier Ltd.

55. High-Field Pinning Potential in YBCO Films with Nanoengineered Pinning Centres

Authors: Collazos, JC; Guner, SB; Celik, ME; Oprea, A; Gencer, A; Crisan, A

Published: FEB 2015, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 28, 360, DOI: 10.1007/s10948-014-2671-4

Frequency-dependent AC susceptibility measurements have been used to estimate the critical current densities and pinning potential of two 1.1-mu m-thick YBCO films with nanoengineered pinning centres. The films were grown by pulsed laser deposition using a target with 4 % BaZrO3 nanoinclusions. Data were analysed in the framework of the Anderson-Kim model, collective pinning model, and a model due to Zeldov that implies a logarithmic dependence of pinning potential on the current density. It was found that the latter describes better our experimental data, and that the resulting pinning potential has reasonable high values in high magnetic field up to 9 T, indicating a high current-carrying capability in high magnetic fields.

56. Effect of Mn addition on the thermal stability and magnetic properties of rapidly-quenched L1(0) FePt alloys

Authors: Crisan, O; Crisan, AD; Mercioniu, I; Pantelica, D; Pantelica, A; Vaucher, S; Nicula, R; Stir, M; Vasiliu, F

Published: OCT 2015, INTERMETALLICS, 65, 87, DOI: 10.1016/j.intermet.2015.06.008

Nano-composite magnets with L1(0) structure derived from binary FePt alloys and prepared as melt-spun ribbons are of current interest due to their higher operating temperature and the ability to be cast as a two-phase magnet with exchange spring magnetic properties, as both soft and hard magnetic phase may emerge from the same metastable precursor, i.e. the disordered cubic A1 phase. The present paper studies the effect of Mn addition on the thermal stability and phase structure, on the abundance of the hard magnetic phase and relative proportion of the soft ones, on the microstructure of the alloy as a function of temperature and on the overall magnetic properties. The interplay of the various magnetic sublattices in the ordering of the L1(0) phases as a consequence of introducing antiferromagnetically coupled Mn atoms in the alloy composition is discussed and interpreted in terms of microstructural changes induced by this addition as revealed by high resolution transmission electron microscopy and Xray diffraction. The temperature evolution of the phase composition and structural parameters is monitored using synchrotron radiation powder diffraction, while the compositional aspects are investigated using proton-induced X-ray emission and energy dispersive X-ray spectroscopy. Magnetic measurements reveal the magnetic parameters of interest (coercivity, remanence, Curie temperature, saturation magnetization), as well as the exchange-coupled two-phase nature of these magnets and provide information that hints at possible spin reorientation transitions in the Mn-containing planes of the L1(0) superlattice. (C) 2015 Elsevier Ltd. All rights reserved.

57. Thermal stability, thermal expansion and grain-growth in exchange-coupled Fe-Pt-Ag-B bulk nanocomposite magnets

Authors: Nicula, R; Crisan, O; Crisan, AD; Mercioniu, I; Stir, M; Vasiliu, F

Published: FEB 15 2015, JOURNAL OF ALLOYS AND COMPOUNDS, 622, 870, DOI: 10.1016/j.jallcom.2014.10.181

Rare-earth free (RE-free) exchange coupling nanocomposite magnets are intensively studied nowadays due to their potential use in applications demanding stable high-temperature operation and corrosion resistance. In this respect, the FePt alloy system is one of the most actively addressed potential permanent magnet solutions. In FePt alloys, promising magnetic features arise from the co-existence of hard magnetic L1(0) FePt and soft magnetic L1(2) Fe3Pt phases emerged from the same metastable precursor. The present work deals with an in-situ temperature-resolved synchrotron radiation study of the thermal stability, thermal expansion and microstructure evolution in exchange-coupled FePtAgB alloys. The as-cast microstructural state as well as the optimized magnetic behavior are given as reference and correlated to the observed microstructural evolution with temperature. The melt-spun Fe48Pt28Ag6B18 alloy ribbons were examined in situ by synchrotron X-ray powder diffraction from ambient temperature up to 600 degrees C. The FePt-Fe3Pt exchange-coupled microstructure achieved by rapid solidification is not significantly altered during the high temperature exposure. The thermal expansion of the FePt L1(0) unit cell has been found to be strongly anisotropic, being essentially an in-plane expansion which may be seen as an anisotropic invar effect. For the FePt L1(0) phase, a significant deviation from linear thermal expansion is observed at the Curie temperature T-C = 477 degrees C. This non-linear behavior above T-C is tentatively linked to a diffusion/segregation mechanism of Ag. The promising hard magnetic properties as well as the direct formation of the L1(0) phase from the as-cast state are directly related to the presence of Ag in the intergranular regions. (C) 2014 Elsevier B.V. All rights reserved.

58. Effect of thermal treatments on the structural and magnetic transitions in melt-spun Ni-Fe-Ga-(Co) ribbons

Authors: Tolea, F; Sofronie, M; Crisan, AD; Enculescu, M; Kuncser, V; Valeanu, M

Published: NOV 25 2015, JOURNAL OF ALLOYS AND COMPOUNDS, 650, 670, DOI: 10.1016/j.jallcom.2015.07.296

The effect of thermal treatments on the martensitic transformation in three representative Ni-Fe-Ga alloys with or without Co substitutions has been studied by calorimetry, X-ray diffractometry, scanning electron microscopy and magnetometry. The alloys were prepared as ribbons, by the melt spinning technique. The thermal treatments promote a reduction of the martensitic transformation temperature in all investigated samples, with the most pronounced decrease for the alloys with lower Ga content. Three different mechanisms induced by specific thermal treatments and responsible for the characteristic behaviour of the martensitic transformation, with respect to temperature and heat of transition, were observed and discussed in details. (C) 2015 Elsevier B.V. All rights reserved.

59. Magnetocaloric effect in Ni-Fe-Ga Heusler alloys with Co and Al substitutions

Authors: Tolea, F; Sofronie, M; Crisan, AD; Tolea, M; Valeanu, M

Published: 2015, ESOMAT 2015 - 10TH EUROPEAN SYMPOSIUM ON MARTENSITIC TRANSFORMATIONS, 33, DOI: 10.1051/matecconf/20153305008

The functionality of the ferromagnetic shape memory alloys is related to the martensitic and magnetic order-disorder transformations, both of which may be tailored by doping with other elements or by suitable thermal treatments, so that alloys with concomitant (or sequential but close) structural and magnetic phase transitions may be obtained. Concerning the magnetocaloric applications, it is assumed that the thin melt-spun ribbons assure a more efficient heat transfer. In the present work we investigate the influence of Co and Al substitutions on magnetocaloric effect characteristics of NiFeGa in bulk and also in ribbons prepared by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetocaloric and magnetoresistive characterizations have been performed. The results highlight the differences between the bulk and the ribbons (both as prepared and annealed) and the role of substitutions.

60. Role of Ag addition in L1(0) ordering of FePt-based nanocomposite magnets

Authors: Crisan, AD; Vasiliu, F; Mercioniu, I; Crisan, O

Published: JAN 12 2014, PHILOSOPHICAL MAGAZINE, 94, 189, DOI: 10.1080/14786435.2013.850549

The FePt system has important perspectives as high-temperature corrosion-resistant magnets. In the form of rapidly solidified melt-spun ribbons, FePt-based magnets may exhibit in certain cases a two-phase hard-soft magnetic behaviour. The present paper deals with a microstructural and magnetic study of FePtAgB alloys with increasing Ag content. The aim is to identify and confirm the effect of Ag addition in decreasing the temperature of the FePt disorder-order structural phase transformation. A detailed high-resolution transmission electron microscopy study is employed, and the alternative disposal of hard and soft regions within the two-phase microstructure is observed and interpreted with respect to the X-ray diffraction results. In the as-cast Ag-containing samples, it is shown that there is an optimum of the Ag content for which best magnetic properties are obtained. Ag addition creates a nonlinear behaviour of the coercive field and the ordering parameter, similar to the RKKY interaction-induced interlayer exchange coupling (IEC) observed in magnetic layers separated by non-magnetic spacer layers. Direct formation of the L1(0) phase from the as-cast state in the FePtAgB alloys is reported with magnetic parameters compatible to other exchange spring permanent nanomagnets. These findings open novel perspectives into utilization of such alloys in applications requiring magnets operating in high-temperature industrial environments.

61. Investigation of the bulk pinning force in YBCO superconducting films with nano-engineered pinning centres

Authors: Crisan, A; Dang, VS; Yearwood, G; Mikheenko, P; Huhtinen, H; Paturi, P

Published: AUG 15 2014, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 503, 93, DOI: 10.1016/j.physc.2014.03.028

For practical applications of superconducting materials in applied magnetic fields, artificial pinning centres in addition to natural ones are required to oppose the Lorentz force. These pinning centres are actually various types of defects in the superconductor matrix. The pinning centres can be categorised on their dimension (volume, surface, or point) and on their character (normal cores or Delta kappa cores). We have used the Dew Hughes approach to determine the types of pinning centres present in various samples, with various thicknesses, temperatures and nanostructured additions to the superconducting matrix. Results show that normal surface pinning centres are present throughout almost all the samples, as dominant pinning mechanism. Such 2D extended pinning centres are mainly due to dislocations, grain boundaries, nanorods. Strong normal point pinning centres were found to be common in BZO doped YBCO samples. Other types of pinning centres, in various (minor) concentrations were also found in some of the samples. (C) 2014 Elsevier B.V. All rights reserved.

62. Magnetization Relaxation in Superconducting YBa2Cu3O7 Films with Embedded Nanorods and Nanoparticles

Authors: Miu, L; Mele, P; Ivan, I; Ionescu, AM; Crisan, A; Badica, P; Miu, D

Published: 2014, SIZE EFFECTS IN NANOSTRUCTURES: BASICS AND APPLICATIONS, 205, 317, DOI: 10.1007/978-3-662-44479-5_9

Vortex pinning on natural and artificial defects is essential for large scale applications of superconducting materials. One of the most promising solutions for the creation of efficient pinning structures is to combine the strong pinning supplied by columnar defects (with the radius of the order of the superconducting coherence length) and the presence of random quenched disorder, which inhibits the detrimental vortex kink formation. A strong pinning is revealed by high values of the vortex activation energy in the magnetic relaxation process. We present a critical analysis of the interpretation of the relaxation data at long- and short time scales, by extracting the so called normalized vortex-creep activation energy. This allowed us to find the actual temperature interval for the characteristic vortex excitations in YBa2Cu3O7 films with embedded BaZrO3 nanorods 9preferentially oriented along the c axis), and to unambiguously determine the characteristic vortex pinning energy. The observed drastic change of magnetic relaxation at short time scales (attained in standard AC measurements) is attributed to a large contribution of the pinning enhanced viscosity to the vortex hopping activation energy.

63. Evolution of vortex dynamics in YBa2Cu3O7 films with nanorods by adding nanoparticles

Authors: Miu, L; Mele, P; Crisan, A; Ionescu, A; Miu, D

Published: MAY 15 2014, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 500, 43, DOI: 10.1016/j.physc.2014.03.002

Vortex excitations and creep regimes in YBa2Cu3O7 (YBCO) films with nonsuperconducting nanorods (columnar defects) and/or nanoparticles in a low external magnetic field H oriented perpendicular to the film surface have been identified using standard DC magnetization relaxation measurements. It was found that by increasing the pinning energy dispersion through nanoparticle addition the relevant changes in the temperature T variation of the normalized magnetization relaxation rate S appear at relatively high temperatures (T similar to 65K for H = 2 kOe), owing to the inhibition of double vortex- kink formation. This indicates that the often observed S(T) maximum at low T (around 30 K) is not related to the excitation of double vortex kinks and super-kinks in specimens with columnar defects, as previously believed. The low-T S(T) maximum is present in the case of strongly pinned specimens without nanorods, as well, and depends on the film thickness, which definitely shows that it is caused by thermo-magnetic instabilities, as recently suggested. (C) 2014 Elsevier B.V.All rights reserved.

64. Cobalt chromite obtained by thermal decomposition of oxalate coordination compounds

Authors: Mindru, I; Gingasu, D; Marinescu, G; Patron, L; Calderon-Moreno, JM; Bartha, C; Andronescu, C; Crisan, A

Published: NOV 2014, CERAMICS INTERNATIONAL, 40, 15258, DOI: 10.1016/j.ceramint.2014.07.009

Cobalt chromites (CoCr2O4) powders have been synthesized via thermal decomposition of oxalate precursor compounds (NH4)(4)[CoCr2(C2O4)(4)(OH)(4)]center dot 7H(2)O (I) and (Na-4)(12)[CoCr2(C2O4)(8)(OH)(4)]center dot(NH4)(2)C2O4 center dot 12H(2)O (II). The oxalate precursors have been characterized by infrared (IR) and ultraviolet visible spectroscopy (UV-vis), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis. The structure, morphology and magnetic properties of CoCr2O4 powders have been investigated by XRD, SEM, IR and Raman spectroscopy (RS), UV-vis and magnetic measurements. XRD patterns confirmed the formation of spinel-type cobalt chromite, with average crystallite sizes calculated from XRD patterns of 38 nm and 58 nm, for spinel chromites from precursors I and II, respectively. SEM micrographs revealed particle sizes between 30 and 130 nm, with a low degree of aggregation of primary nanocrystallites. Both CoCr2O4 powders presented ferrimagnetic ordering below the Currie temperature (T-c) and a phase transition at T-s similar to 20 K attributed to the onset of long-range spiral magnetic order. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

65. Interpretation of Abnormal AC Loss Peak Based on Vortex-Molecule Model for a Multicomponent Cuprate Superconductor (vol 46, pg 134, 2007)

Authors: Tanaka, Y; Crisan, A; Shivagan, DD; Iyo, A; Tokiwa, K; Watanabe, T

Published: SEP 2014, JAPANESE JOURNAL OF APPLIED PHYSICS, 53, DOI: 10.7567/JJAP.53.099202

66. Anomalous AC Susceptibility Response of (Cu,C)Ba2Ca2Cu3Oy: Experimental Indication of Two-Component Vortex Matter in Multi-Layered Cuprate Superconductors (vol 46, pg L451, 2007)

Authors: Crisan, A; Tanaka, Y; Shivagan, DD; Iyo, A; Cosereanu, L; Tokiwa, K; Watanabe, T

Published: SEP 2014, JAPANESE JOURNAL OF APPLIED PHYSICS, 53, DOI: 10.7567/JJAP.53.099201

67. Inhibition of the detrimental double vortex-kink formation in thick YBa2Cu3O7 films with BaZrO3 nanorods

Authors: Miu, D; Ivan, I; Crisan, A; Mele, P; Jakob, G; Miu, L

Published: APR 2013, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 26, DOI: 10.1088/0953-2048/26/4/045008

We investigated the temperature (T) variation of the normalized magnetization relaxation rate S and of the corresponding normalized vortex-creep activation energy U* = T/S for YBa2Cu3O7 films containing BaZrO3 nanorods, with the external magnetic field H oriented perpendicular to the film surface. It was found that by increasing the film thickness and using nanodot decorated substrates the high-T S(T) maximum appearing at low H is substituted by a minimum in S(T). As revealed by the analysis of the current density dependence of U*, this behaviour is due to the inhibition of vortex excitations involving double vortex-kinks and superkinks formation in the investigated thick films, owing to the large nanorod splay and pinning energy dispersion.

68. High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

Authors: Miu, L; Ivan, I; Miu, D; Mele, P; Matsumoto, K; Mikheenko, P; Dang, VS; Crisan, A

Published: APR 2013, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 26, 1173, DOI: 10.1007/s10948-012-2019-x

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB2Cu3O7 (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T (dp) very close to the critical temperature T (c) (T (dp)similar to 0.95T (c)), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T (dp) (similar to 0.5T (c)). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO3 (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T (c) does not support a low T (dp). The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T (dp)) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T (dp) close to T (c). By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.

69. AFM studies of metallic and organic nanosized structures ion beam sputtered onto functionalised surfaces

Authors: Crisan, AD; Crisan, O

Published: APR 2012, MATERIALS SCIENCE AND TECHNOLOGY, 28, 466, DOI: 10.1179/1743284711Y.0000000083

The growth of submonolayer metallic or molecular nanostructures via ion beam sputtering onto reconstructed semiconductor surfaces followed by in situ scanning probe imaging of the formed nanostructures provides an interesting basis for future development of new molecular multifunctional nanoarchitectured materials for various applications. The observed growth modes, structure and topology of pentacene, which is one of the most important candidates in the field of organic thin film electronic molecules, and Au metallic nanostructures deposited in the submonolayer regime onto reconstructed InP (0 0 1) surface, are discussed. During the initial stages of growth, a uniaxial diffusion channel dominates, and long pentacene molecular chains self-organise parallel to the [110] crystallographic direction on the InP surface. The study is performed by in situ non-contact atomic force microscopy (AFM) investigations with atomic resolution. It is shown that self-assembling of molecular structures onto flat terraces is dependent on the flatness and orientation of the terraces reconstructed onto the semiconductor surface. Moreover, it is possible to create functional molecular nanoarchitectures by nanomanipulation of single molecules with the AFM tip. This procedure may have large impact for technological applications such as organic thin film transistors and molecular nanowires.

70. Improved Critical Current Densities in YBa2Cu3O7-delta Multilayer Films Interspaced with Palladium Nano-dots

Authors: Sarkar, A; Mikheenko, P; Dang, VS; Kechik, MMA; Abell, JS; Crisan, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 177, DOI: 10.1007/s10948-010-0914-6

Power applications of superconducting coated conductors in high magnetic fields require thick films with high critical current density J (c) and strong artificial pinning centers. Here, we report on the artificial pinning centers induced in YBCO quasi-multilayer films interspaced with palladium (Pd) nano-dots. Quasi-multilayered (QM) YBa2Cu3O7-delta (YBCO) films composed of YBCO layers interspaced with quasi-layers of palladium nano-dots were grown by pulsed laser deposition on SrTiO3(100) substrates. DC magnetization and frequency-dependent measurements showed high J (c) comparable with best YBCO films in thin quasi-multilayers and significant improvement of J (c) in thick quasi-multilayers. TEM study shows regions of planar defects, stacking faults, and pore formations suitable for immobilizing vortices. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.

71. Intermittent trapping of a liquid-like vortex state visualized by scanning Hall probe microscopy

Authors: Crisan, A; Bending, SJ; Li, ZZ; Raffy, H

Published: NOV 2011, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 24, DOI: 10.1088/0953-2048/24/11/115001

We have used scanning Hall probe microscopy to investigate vortex structures and vortex dynamics in Bi2Sr2CaCu2O8+delta thin films in very low perpendicular magnetic fields. After nominally zero field cooling in the Earth's field we find that the vortices appear to be in a stable glassy state in our highly disordered samples. After applying a cancellation field of a few Oersted at low temperature, however, the system enters a new regime at very low magnetic induction when the only image contrast is due to vortices that are intermittently trapped on strong pinning centres. This state shares many of the signatures of the re-entrant vortex liquid phase that has been theoretically predicted in these highly anisotropic materials at very low vortex densities. Analysing the trapping times for vortices in the fluctuating state we estimate that the pinning potential of typical strong pinning centres is about 900 K under our experimental conditions. To our knowledge, this is the first direct experimental evidence for the existence of a dynamic liquid-like vortex state in this highly anisotropic material at very low magnetic induction.

72. Domains in multiband superconductors

Authors: Tanaka, Y; Yanagisawa, T; Crisan, A; Shirage, PM; Iyo, A; Tokiwa, K; Nishio, T; Sundaresan, A; Terada, N

Published: NOV 2011, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 471, 750, DOI: 10.1016/j.physc.2011.05.043

Multiband superconductors can have several types of domains that are inhibited in conventional single-band superconductors. These domains are phase domains and chiral domains and their domain wall are an interband phase difference soliton. In a superconductor with an odd number of electronic bands (five or more) and with positive interband Josephson interactions, we find other types of domains with different interband phase differences. We call these domains configuration domains because pseudo-order parameters for each band are dispersed in the complex plain and several configurations, which have several local minima. Fractional vortices serve as hubs for phase difference solitons (configuration domain walls). The divergence of the number of configurations with local minima would pose a serious problem for the stability of superconductivity. (C) 2011 Elsevier B.V. All rights reserved.

73. Combination of Ag Substrate Decoration with Introduction of BaZrO3 Nano-Inclusions for Enhancing Critical Current Density of YBa2Cu3O7 Films

Authors: Dang, VS; Mikheenko, P; Sarkar, A; Kechik, MMA; Abell, JS; Paturi, P; Huhtinen, H; Crisan, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 509, DOI: 10.1007/s10948-010-0981-8

The combination of two methods: Ag substrate decoration and introduction of BZO nano-inclusions has been used in a pulsed laser deposition (PLD) method to increase the critical density (J (c) ) of YBCO films. The films were deposited on single crystal SrTiO3 (STO) substrates decorated with various architecture of Ag nano-dots. We have studied the diameter and density of Ag nano-dots and their influence on J (c) of BZO-added YBCO films. We found that 15 laser pulses on the Ag target gives an optimum result in increasing J (c) in comparison with BZO-doped YBCO films of the same thickness in self-field and low applied magnetic fields. A higher number of laser pulses on the Ag target led to increasing critical current density in high applied magnetic fields only (above 2 T). We have studied films of the thickness from 0.4 mu m to 3.8 mu m and found that the highest J (c) at all applied fields investigated is achieved for a 1.2 mu m thick film. The transmission electron microscopy clearly shows BZO nano-rods that provide strong c-axis pinning centres in the films.

74. Martensitic transformation of Ti50Ni30Cu20 alloy prepared by powder metallurgy

Authors: Valeanu, M; Lucaci, M; Crisan, AD; Sofronie, M; Leonat, L; Kuncser, V

Published: MAR 31 2011, JOURNAL OF ALLOYS AND COMPOUNDS, 509, 4498, DOI: 10.1016/j.jallcom.2011.01.154

Phase transformation behavior of Ti50Ni30Cu20 shape memory alloys prepared by powder metallurgy is analyzed with respect to the duration of mechanical alloying. The processed blends were studied by differential scanning calorimetry and room temperature X-ray diffraction. The martensitic transformations evidenced by thermal scans are discussed in correlation with the relative phase content obtained from the refinement of the X-ray diffraction patterns. (c) 2011 Elsevier B.V. All rights reserved.

75. Direct formation of L1(0) FePt in as-cast FePt-based magnetic nanocomposite ribbons without post-synthesis annealing

Authors: Crisan, AD; Crisan, O

Published: SEP 14 2011, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 44, DOI: 10.1088/0022-3727/44/36/365002

A novel nanocomposite FePt-based exchange-coupled magnet has been synthesized and structurally and magnetically characterized. We report for the first time the direct formation of the L1(0) FePt phase without the need for post-synthesis annealing procedures in Fe-Pt-based melt-spun ribbons, obtained by a conventional melt spinning method. The structure and magnetic properties are investigated and the occurrence of the L1(0) ordered phase in the as-cast state of Fe-Pt-Ag-B melt-spun ribbons is confirmed by XRD and magnetic measurements. A microstructure consisting of fine, uniformly dispersed, 22-24 nm FePt grains dispersed within a soft magnetic matrix is observed by scanning transmission electron microscopy imaging. Coercive fields as high as 727 kAm(-1), saturation magnetization of about 1.2 T and energy product around 87 kJm(-3) are determined from 270 K hysteresis loops of the as-cast ribbons, making one of the best FePt-based nanocomposite magnet ribbons even without further annealing treatments.

76. Synergetic Pinning Centers in YBa2Cu3OX Films Through a Combination of Ag Nano-Dot Substrate Decoration, Ag/YBCO Quasi-Multilayers, and the Use of BaZrO3-Doped Target

Authors: Mikheenko, P; Dang, VS; Kechik, MMA; Sarkar, A; Paturi, P; Huhtinen, H; Abell, JS; Crisan, A

Published: JUN 2011, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 21, 3188, DOI: 10.1109/TASC.2010.2086041

We report on an original technique for nanoengineered pinning centers that combine three previously reported approaches: substrate decoration, quasi-multilayers and targets with secondary phase nanoinclusions. We have used a 4% BZO-doped YBCO target, and Ag nanodots, all grown by PLD. Such an approach gave interesting results in terms of pinning landscape, as proved by TEM studies. Angle-dependent measurements of critical current I-c showed that, for smaller fields, the absolute maximum in I-c occur for fields perpendicular to the a-b planes, while at larger fields the absolute maximum in I-c occurs for fields parallel to the a-b planes, in both cases with a clear second, local maxima. Measurements also showed a smooth change in the character of pinning with magnetic field, from the out-of-plane to in-plane-dominant pinning. For the out-of-plane magnetic field, the highest Ic-w (critical current per cm width) obtained so far, at 77.3 K, occurred in a 5.8 mu m Ag nanodots / BZO-doped YBCO trilayer: 782 A/cm-w in self-field, 167 A/cm-w in 1 T and 18 A/cm-w in 3 T.

77. Exotic Vortex Matter: Pancake Vortex Molecules and Fractional-Flux Molecules in Some Exotic and/or Two-Component Superconductors

Authors: Crisan, A; Tanaka, Y; Iyo, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 6, DOI: 10.1007/s10948-010-0892-8

The discovery of superconductivity in MgB2 and, more recently, in iron pnictides, revived the interest in two-component and other exotic superconductors, including in the field of vortex matter, as was shown recently by the discovery of "type 1.5" superconductivity. However, even some "older" exotic cuprates grown by high-pressure techniques showed interesting anomalies related to the interplay between Josephson and magnetic coupling and/or two-gap superconductivity. Here, we will present two such examples of exotic vortex matter: magnetically-coupled pancake-vortex molecules in super-multilayered cuprates, and vortex molecules composed of fractional flux quanta glued by an interband phase difference soliton, and will discuss the major differences between MgB2 and such multilayer cuprates in terms of the relative strength of the interband interaction and, respectively, in terms of the coherence and penetration lengths.

78. 'Beautiful' unconventional synthesis and processing technologies of superconductors and some other materials

Authors: Badica, P; Crisan, A; Aldica, G; Endo, K; Borodianska, H; Togano, K; Awaji, S; Watanabe, K; Sakka, Y; Vasylkiv, O

Published: FEB 2011, SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS, 12, DOI: 10.1088/1468-6996/12/1/013001

Superconducting materials have contributed significantly to the development of modern materials science and engineering. Specific technological solutions for their synthesis and processing helped in understanding the principles and approaches to the design, fabrication and application of many other materials. In this review, we explore the bidirectional relationship between the general and particular synthesis concepts. The analysis is mostly based on our studies where some unconventional technologies were applied to different superconductors and some other materials. These technologies include spray-frozen freeze-drying, fast pyrolysis, field-assisted sintering (or spark plasma sintering), nanoblasting, processing in high magnetic fields, methods of control of supersaturation and migration during film growth, and mechanical treatments of composite wires. The analysis provides future research directions and some key elements to define the concept of 'beautiful' technology in materials science. It also reconfirms the key position and importance of superconductors in the development of new materials and unconventional synthesis approaches.

79. Phase transformation and exchange bias effects in mechanically alloyed Fe/magnetite powders

Authors: Crisan, O; Crisan, AD

Published: JUN 9 2011, JOURNAL OF ALLOYS AND COMPOUNDS, 509, 6527, DOI: 10.1016/j.jallcom.2011.03.147

Nanostructured powders processed by ball milling of a mixture of Fe and Fe3O4 at room temperature are shown to undergo an incomplete redox reaction with formation of FeO during the milling process. This reaction is favored by the high energy introduced during the mechano-alloying process. Concurrent effects of milling such as grain refinement down to the nanometre scale lead at the end of the milling processes to a mixed multiphase powder of nanograins, with Fe and Fe oxide grains inter-dispersed. We show that in the as-milled Fe/Fe3O4 powder, during milling process, wustite (FeO) is formed as a consequence of the redox reaction. Moreover, with increasing temperature, the system undergoes an inverse phase transformation towards the initial Fe and Fe3O4 phases until about 450 degrees C. Above this temperature the reduction reaction Fe + Fe3O4 = 4FeO is reinitiated, resulting in sharp decrease of Fe and Fe3O4 content from about 550 degrees C and almost complete disappearance of these phases at about 900 degrees C. This transformation was investigated via an energy-dispersive in situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray patterns after few minutes exposure, at selected temperatures, ranging between 20 degrees C and 1000 degrees C. The structural and magnetic characterizations of the nanograin powders, as-milled and annealed at several temperatures, are studied using XRD, SEM and magnetic measurements. Such ferromagnetic-antiferromagnetic composites are extensively studied as they exhibit exchange bias effect, with a large impact in technological applications. The magnetic behaviour and intrinsic mechanisms leading to the occurrence of exchange bias effects are discussed and related to the samples microstructural features. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanograin powder. (C) 2011 Elsevier B.V. All rights reserved.

80. Nano Techniques for Enhancing Critical Current in Superconducting YBCO Films

Authors: Mikheenko, P; Dang, VS; Kechik, MMA; Wang, Y; Sarkar, A; Tanner, JL; Abell, JS; Crisan, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 1064, DOI: 10.1007/s10948-010-0861-2

A range of nano techniques is explored in order to increase the critical current in pulsed laser deposited (PLD) superconducting YBa2Cu3O (x) (YBCO) thin films. The structural measurements are linked with magnetic and transport measurements of the films. The effectiveness of PLD techniques is analyzed from the point of view of the dimensionality of nanostructures formed prior to and during the film growth. It is shown that a combination of two-dimensional substrate decoration with nanoparticles before the film deposition and one-dimensional growth of external phase nanorods during the deposition offers a high critical current in magnetic field both along the c-axis and in the ab-plane of YBCO.

81. Field-assisted organization, substrate effects and magnetic behavior of Ag30Co70 core-shell nanoparticles

Authors: Crisan, AD; Angelakeris, M; Simeonidis, K; Tsiaoussis, I; Crisan, O

Published: NOV 2010, SOLID STATE SCIENCES, 12, 1911, DOI: 10.1016/j.solidstatesciences.2010.06.006

In core-shell systems with non-magnetic core and magnetic shell the electron transport and magnetic properties are expected to show enhanced behavior due to the particular morpho structural features of the conductive and magnetic regions This may lead to novel advanced GMR materials and spin valves This is the case of core-shell Ag-Co colloidal nanoscale particles that organize into regular arrays An insight on the structure and morphology of the newly synthesized Ag-Co nanoparticles deposited on different substrates will be presented The influence of the substrate on different morphologies and organization dynamics is discussed It is shown that the magnetic behavior of the Ag-Co nanoparticles is highly influenced by the corona-like morphology of Co shell chemical environment of the magnetic atoms and by the fact that they exhibit strongly reduced coordination due to the surface states (C) 2010 Elsevier Masson SAS All rights reserved

82. Self-assembled artificial pinning centres in thick YBCO superconducting films

Authors: Mikheenko, P; Abell, JS; Sarkar, A; Dang, VS; Kechik, MMA; Tanner, JL; Paturi, P; Huhtinen, H; Babu, NH; Cardwell, DA; Crisan, A

Published: 2010, 9TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS 09), 234, DOI: 10.1088/1742-6596/234/2/022022

Strong, artificial pinning centres are required in superconducting films of large thickness for power applications in high magnetic fields. One of the methods for the introduction of pinning centres in such films is substrate decoration, i.e., growing nanoscale islands of certain materials on the substrate prior to the deposition of the superconducting film. Two other methods are building up a layered distribution of a second phase and homogeneous incorporation of second phase inclusions from a compositional target. In this paper, we compare the effectiveness of these methods in terms of the type of the self-assembly of nanoparticles. The comparison is made over a large set of YBa2Cu3O7 films of thickness up to 6.6 mu m deposited with Au, Ag, Pd, LaNiO3, PrBa2Cu3O7, YBCO, BaZrO3 and Gd2Ba4CuWOy nanoparticles. It is found that substrate-decoration self-assembly is able to provide higher critical current in low magnetic field than the incorporation of homogeneous second phase in the sample microstructure. By specific modification of substrate decoration we achieved the self-field critical current per centimetre of width of 896 A/cm at 77.3 K and 1620 A/cm at 65 K in a film of thickness of 4.8 mu m.

83. Topological structure of the inter-band phase difference soliton in two-band superconductivity

Authors: Tanaka, Y; Iyo, A; Tokiwa, K; Watanabe, T; Crisan, A; Sundaresan, A; Terada, N

Published: NOV 1 2010, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 470, 1012, DOI: 10.1016/j.physc.2010.05.022

Two-component superconductivity based on the two-band superconductor has a functional topology such as an inter-band phase difference soliton (i-soliton) to realize topological electronics (topolonics). Many gauge field theories are applied to investigate the topology of two-band superconductivity. To ease experimental and electronics applications, these theories should be refined. Weinberg-Salam theory and SU(2) (two-dimensional special unitary symmetry) gauge field theory are proper starting points. An effective extra force field because of the crystal structure and inter-band Josephson interaction, rather than spontaneous symmetry breaking, simplifies the conventional gauge field theory. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.

84. Topology of two-band superconductors

Authors: Tanaka, Y; Iyo, A; Tokiwa, K; Watanabe, T; Crisan, A; Sundaresan, A; Terada, N

Published: DEC 2010, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 470, S967, DOI: 10.1016/j.physc.2009.10.036

Two-band superconductivity has a topology different from that in single-band superconductivity. The topology is not always stabilized in an infinitely homogeneous sample. The morphology, grain shape, and pattern of the device (topology of the superconducting materials) is effective in stabilizing the topology. In this report, we discuss a vortex having a small magnetic flux but a large winding number as one plausible topology in a two-band superconductor. (C) 2009 Elsevier B.V. All rights reserved.

85. Pinning potential in thick PrBa2Cu3Ox/YBa2Cu3O7-delta quasi-multilayers

Authors: Crisan, A; Dang, VS; Mikheenko, P; Tse, YY; Sarkar, A; Bowen, J; Abell, JS

Published: JAN 1 2010, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 470, 60, DOI: 10.1016/j.physc.2009.10.006

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa2Cu3Ox (PrBCO) nano-dots and layers of YBa2Cu3O7-delta (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 rim, and total thickness between 1.13 and 5.31 mu m. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J(c) at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J(c) only in high fields. We have also investigated the frequency dependence of J(c) from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nanodots, both isotropic and c-axis correlated. (C) 2009 Elsevier B.V. All rights reserved.

86. Surface-functionalized core-shell nanoparticles by novel gas/cluster aggregation method

Authors: Crisan, O; Crisan, AD

Published: FEB 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 192, DOI:

A simple and novel method for production of metal clusters as building blocks for nanoscale devices is presented. With this procedure, called the gas / cluster aggregation method, a wide range of clusters may be synthesised and, more important, these clusters may be subsequently modified and functionalized in-situ by adding atoms/molecules of different nature, on the surface of readily formed clusters. The cluster size is extremely well controlled by the vapour pressure of the picked-up species. Moreover, the method is versatile, since it allows multiple pick-up processes within the same rare gas cluster for producing, for example, core-shell nanoparticles with metal core and non-metallic shell or vice-versa, nano-onions, with different species successively attached to the surface of the initial picked-up cluster, and so on. Initial formation of Fe gas-stabilised clusters and core-shell nanoparticles with Fe core and Fe oxide shell, as well as their structure and morphology, are presented and discussed. The core-shell nanoparticles show incipient self-organization into hexagonal cluster superlattice. Structural, magnetic and Mossbauer spectroscopy investigations have been performed on the Fe cluster samples. The magnetic properties of supported Fe clusters show marked differences compared to the bulk. A small hysteresis is observed in the parallel applied field while in the perpendicular case, lack of saturation at the highest applied field is noticed. Such behaviour has been also observed in FeRh [1] and AgCo [2] bimetallic nanoparticles. This behaviour marks the occurrence of a strong planar magnetic anisotropy in the sample and may also be a consequence of increased surface spin disorder and finite size effects, which are typical for nanoparticles in the reported size range.

87. Improved critical current densities in thick YBa2Cu3O7-delta multilayer films interspaced with non-superconducting YBa2Cu3Ox nanodots

Authors: Sarkar, A; Dang, VS; Mikheenko, P; Kechik, MMA; Abell, JS; Crisan, A

Published: NOV 1 2010, THIN SOLID FILMS, 519, 879, DOI: 10.1016/j.tsf.2010.08.102

We have fabricated and studied quasi-multilayered thick YBa2Cu3O7-delta (YBCO) films composed of several YBCO layers Interspaced with quasi-layers of non-superconducting YBa2Cu3Ox nanodots grown by Pulsed Laser Deposition on SrTiO3 (100) substrates Magnetization J(c)(B) at 773 K for these thick films showed significant improvement as compared to pure YBa2Cu3O7-delta films of same or even smaller thickness A high J(c)(B) in our quite thick films (1 mu m to 6 mu m) provides a very high total critical current per centimetre of the film width Ic-w Critical current as high as 830 A per cm width in self field and 77 3 K was achieved in 5 mu m thick quasi-multilayer film with non-superconducting YBa2Cu3Ox nanodots Frequency-dependent susceptibility measurements showed also an increase in the pinning potential The angular dependence of Ic-w at 86 5 K in 3 T shows a clear indication of anisotropic pinning centres aligned along the c-direction (C) 2010 Elsevier B V All rights reserved

88. Integrated nanotechnology of pinning centers in YBa2Cu3Ox films

Authors: Mikheenko, P; Dang, VS; Tse, YY; Kechik, MMA; Paturi, P; Huhtinen, H; Wang, Y; Sarkar, A; Abell, JS; Crisan, A

Published: DEC 2010, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 23, DOI: 10.1088/0953-2048/23/12/125007

A controlled pinning change from the ab-plane dominant to the c-axis dominant has been achieved in a novel method of nanostructured YBa2Cu3Ox (YBCO) growth. The method is a synchronous self-assembly of BaZrO3 (BZO) and Ag-assisted YBCO nanothreads. The formation of entangled nanothreads increases the critical current density while keeping the critical temperature close to that in pure YBCO films. The nanothreads extend through the whole thickness of thick films, making the method suitable for increasing total critical current density per centimeter of width (Ic-w). Two growth mechanisms, the formation of BZO nanorods and YBCO nanocolumns, complement each other, form a coherent structure and produce samples with strong correlated pinning. In addition to the increase in Ic-w, correlated pinning leads to an increase in vortex melting temperature in a wide range of magnetic fields. The films grown by this method have high Ic-w both in low magnetic fields along the c axis and high magnetic fields in the ab plane of YBCO. Such a superconductor would be suitable for both cable and magnet applications.

89. Nanodots induced columnar growth of YBa2Cu3Ox films

Authors: Mikheenko, P; Tanner, JL; Bowen, J; Sarkar, A; Dang, VS; Abell, JS; Crisan, A

Published: DEC 2010, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 470, S236, DOI: 10.1016/j.physc.2009.11.064

We report columnar growth of YBa2Cu3Ox (YBCO) films by pulsed laser deposition using nanodots of Au, Ag and Pd deposited on SrTiO3 substrate prior to the growth of the film. The nanodots initiate formation of densely packed YBCO columns with c-axis along the length of the columns. The obtained columnar structure offers enhanced pinning of magnetic flux and provides high critical current density in a magnetic field directed along the c-axis of YBCO while keeping high critical current density for other directions of magnetic field. The evidence of columnar structure is given by the planar and cross-sectional atomic force microscopy, transmissional election microscopy and rotational transport measurements of critical current. By creating the columnar structure, an improvement of critical current up to about 200% is achieved comparable to the reference samples without nanodots in a wide range of magnetic field and thickness of the samples. The largest increase in critical current is observed in magnetic fields close to the matching field calculated from the density of nanoparticles or density of YBCO columns. (C) 2009 Elsevier BM. All rights reserved.

90. Increased critical current density and pinning in thick Ag/YBa2Cu3O7-x multilayers

Authors: Dang, VS; Mikheenko, P; Sarkar, A; Kechik, MMA; Abell, JS; Crisan, A

Published: NOV 1 2010, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 470, 1241, DOI: 10.1016/j.physc.2010.05.083

We have investigated the superconducting properties of Ag/YBa2Cu3O7-x thick multilayers grown by Pulsed Laser Deposition, and found that the artificial pinning centres induced by Ag nanodots lead to a significant increase in critical current, especially in high applied magnetic fields. Transmission Electron Microscopy showed a columnar growth of YBa2Cu3O7-x, induced by Ag nanodots, while angle-dependent transport measurements revealed the existence of strong, both isotropic and c-axis correlated, artificial pinning centres. (C) 2010 Elsevier B.V. All rights reserved.

91. Compositional studies and thermal analysis in amorphous and nanocrystalline FePtNbB melt spun ribbons

Authors: Crisan, AD

Published: FEB 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 256, DOI:

The FePtNbB alloys are of interest as a new class of exchange spring magnets. In strictly defined compositional range and after proper conditions of annealing, they develop a microstructure made of small nanograins of hard and soft magnetic phases alternatively disposed and coupled through exchange interactions. Taking advantage of the high uniaxial aniosotropy of the tetragonal hard magnetic FePt phase and high saturation magnetization of the cubic soft magnetic FePt phases, an exchange coupled nanocomposite magnet can be derived. A compositional study of FePtNbB alloys is provided in order to understand the role of each additional element. The possibility of casting alloys where two or more magnetic phases can be obtained is discussed in terms of the eutectic point of the FePt phase diagram. Also, the formation and evolution of the magnetic phases is studied by differential scanning calorimetry (DSC) and detailed analysis of these DSC scans is provided. Activation energy determined from DSC scans provides essential informations about crystallization and disorder-order phase transformation in these alloys.

92. Artificial pinning in thick YBCO films: Pinning potential and c-axis correlation

Authors: Crisan, A; Mikheenko, P; Sarkar, A; Dang, VS; Kechik, MMA; Abell, JS; Paturi, P; Huhtinen, H

Published: OCT 1 2010, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 470, 843, DOI: 10.1016/j.physc.2010.02.033

We have introduced artificial pinning centres in thick (>1 mu m) YBCO films grown by Pulsed Laser Deposition using substrate decoration, quasi-multilayers, and target doping approaches. We have found that the frequency dependence of critical current density is consistent with a logarithmic dependence of pinning potential on current density. For most of materials used as nano-dots, artificially-induced pinning centres have a larger potential than natural ones. From angle-dependent in-field transport measurements and from Transmission Electron Microscopy we have found evidence of c-axis correlated pinning centres. (C) 2010 Elsevier B.V. All rights reserved.

93. The influence of microstructure on magnetic properties of nanocrystalline Fe-Pt-Nb-B permanent magnet ribbons

Authors: Randrianantoandro, N; Crisan, AD; Crisan, O; Marcin, J; Kovac, J; Hanko, J; Greneche, JM; Svec, P; Chrobak, A; Skorvanek, I

Published: NOV 1 2010, JOURNAL OF APPLIED PHYSICS, 108, DOI: 10.1063/1.3504245

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and Fe-57 Mossbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L1(0)-FePt hard-magnetic grains (15-45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 degrees C for 1 h exhibit promising hard-magnetic properties at room temperature: M-r/M-s=0.69; H-c=820 kA/m and (BH)(max)=70 kJ/m(3). Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive delta M-peak in the Henkel plot. The magnetic properties measured from 5 to 750 K reveals that the hard characteristics remains rather stable up to 550 K, indicating a good prospect for the use of these permanent magnets in a wide temperature range. (C) 2010 American Institute of Physics. [doi:10.1063/1.3504245]

94. Magnetism and Structural Phase Transformation in Fe/Fe oxide Nanopowders

Authors: Crisan, O; Crisan, AD; Skorvanek, I; Kovac, J

Published: 2009, 13TH INTERNATIONAL CONFERENCE ON RAPIDLY QUENCHED AND METASTABLE MATERIALS, 144, DOI: 10.1088/1742-6596/144/1/012027

Fe / Fe3O4 (magnetite) powders obtained by ball milling at room temperature, undergo an incomplete redox reaction with formation of FeO. This reaction is favoured due to the high energy developed during the milling and alloying. Concurrent effects of the milling, such as grain refinement down to the nanometric scale lead at the end of the milling processes to a mixed multiphased nanopowder, with a homogeneous dispersion of Fe and Fe oxide grains. Such ferromagnetic - antiferromagnetic systems are extensively studied due to their exchange bias properties, extremely useful in technological applications. We study the phase transformation that leads to a multiphased metal / oxide microstructure with an energy-dispersive in-situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray spectra after few minutes exposure, at selected temperatures, ranging between 20 degrees C and 1000 degrees C. Magnetic behavior has been studied for as-milled and annealed samples and the obtained magnetic parameters are correlated to the microstructure and phase composition at each stage of annealing. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanogranular powder.

95. c-Axis correlated extended defects and critical current in YBa2Cu3Ox films grown on Au and Ag-nano dot decorated substrates

Authors: Mikheenko, P; Sarkar, A; Dang, VS; Tanner, JL; Abell, JS; Crisan, A

Published: JUL 15 2009, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 469, 804, DOI: 10.1016/j.physc.2009.05.001

We report measurements of critical current in YBa2Cu3Ox films deposited on SrTiO3 substrates decorated with silver and gold nanodots. An increase in critical current in these films, in comparison with the films deposited on non-decorated substrates, has been achieved. We argue that this increase comes from the c-axis correlated extended defects formed in the films and originated from the nanodots. Additionally to creating extended defects, the nanodots pin them and prevent their exit from the sample during the film growth, thus keeping a high density of defects and providing a lower rate of decrease of the critical current with the thickness of the films. The best pinning is achieved in the samples with silver nanodots by optimising their deposition temperature. The nanodots grown at a temperature of a few hundred degrees C have a small diameter of a few nanometres and a high surface density of 10(11)-10(12) particles/cm(2). We give evidence of c-axis correlated extended defects in YBa2Cu3Ox films by planar and cross-sectional atomic force microscopy, transmission electron microscopy and angle-dependent transport measurements of critical current. (C) 2009 Elsevier B.V. All rights reserved.

96. Ambiguity in the statistics of single-component winding vortex in a two-band superconductor

Authors: Tanaka, Y; Crisan, A

Published: MAY 1 2009, PHYSICA B-CONDENSED MATTER, 404, 1039, DOI: 10.1016/j.physb.2008.11.011

There are four topologically different paths to interchange two single-component winding vortices in a two-band superconductor having a weak inter-band interaction, where each band has its own superconducting component and its own phase, theta(v), other than the ordinary quantum phase, Theta (v is the index of the band and Theta is defined at the region of theta(1) = theta(2).). The acquired phase for the interchange of two single-winding vortices depends on the path, and three values Theta(soliton),-Theta(soliton) and 0, are observed. Theta(soliton) is the phase-slip accompanying the inter-band phase difference soliton (i-soliton). The interchange requires the creation and annihilation of the i-soliton and gives rise to an ambiguity in the statistics. (C) 2008 Elsevier B.V. All rights reserved.

97. Critical current density and pinning potential in YBa2Cu3O7-delta thick films ablated from a BaZrO3-doped nanocrystalline target

Authors: Crisan, A; Kechik, MMA; Mikheenko, P; Dang, VS; Sarkar, A; Abell, JS; Paturi, P; Huhtinen, H

Published: APR 2009, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 22, DOI: 10.1088/0953-2048/22/4/045014

We report on the pulsed laser deposition and characterization of thick (1.27 mu m) films ablated from a nanocrystalline YBa2Cu3O7-delta (YBCO) target doped with 4 wt% BaZrO3 (BZO), nYBCO/BZO. DC magnetization measurements have shown a four-to five-fold increase in the critical current density J(c) and a three- to four-fold increase in the maximum pinning force in comparison with 1 mu m thick films grown from a commercial microcrystalline YBCO target, at both 77.3 and 65 K. We have found that the frequency-dependent critical current densities J(c)(f(0)/f) estimated from susceptibility measurements are linear in a double-logarithmic plot ( where f is the frequency of the AC field and f(0) is a macroscopic attempt frequency of about 10(6) Hz), consistent with a logarithmic dependence of the pinning potential on the current density, and are again several times higher in nYBCO/BZO thick films. From the slopes of the J(c)(f(0)/f) dependences we have also estimated the values of pinning potentials in fields between 3 and 5 T.

98. Improvement of Pinning Force and Critical Current Density in Thick YBa (2) Cu (3) O (7-delta) Films Grown on SrTiO (3) Substrates Decorated with LaNiO (3) Nanodots

Authors: Crisan, A; Sarkar, A; Mikheenko, P; Dang, VS; Kechik, MMA; Abell, JS

Published: OCT 2009, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 22, 636, DOI: 10.1007/s10948-009-0452-2

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J (c)), pinning force (F (p)) and pinning potential in thick (1.3-1.6 mu m) YBa2Cu3O7-delta (YBCO) films grown by Pulsed Laser Deposition on SrTiO3 substrates decorated with LaNiO3 nanodots deposited by a few (5-15) laser pulses, in comparison with those of a 1 mu m thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO3 target, which have, at 77.3 K, a J (c) of 40-125% higher than in pure YBCO in fields between 1 and 2 T, and F (p) increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.

99. Phase diagram of a lattice of pancake vortex molecules

Authors: Tanaka, Y; Crisan, A; Shivagan, DD; Iyo, A; Shirage, PM; Tokiwa, K; Watanabe, T; Terada, N

Published: AUG-OCT 2009, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 469, 1131, DOI: 10.1016/j.physc.2009.05.202

On a superconducting bi-layer with thickness much smaller than the penetration depth, lambda, a vortex molecule might form. A vortex molecule is composed of two fractional vortices and a soliton wall. The soliton wall can be regarded as a Josephson vortex missing magnetic flux (degenerate Josephson vortex) due to an incomplete shielding. The magnetic energy carried by fractional vortices is less than in the conventional vortex. This energy gain can pay a cost to form a degenerate Josephson vortex. The phase diagram of the vortex molecule is rich because of its rotational freedom. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

100. YBa2Cu3O7-x Thin Films by Citrate-Based Non-fluorine Precursor

Authors: Cui, W; Mikheenko, P; Yu, LM; Button, TW; Abell, JS; Crisan, A

Published: NOV 2009, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 22, 815, DOI: 10.1007/s10948-009-0504-7

A new fluorine-free chemical solution deposition route has been developed for YBa2Cu3O7-x (YBCO) thin films. The precursor was formed by dissolving metal nitrates and citric acid in ethylene glycol. Polyethyleneimine was used to modify the solution. The preparation of YBCO thin films consisted of repeated dip coating, drying, heat-treating in air and annealing in flowing argon with a small addition of oxygen and with final treatment in pure oxygen. The films were characterized by XRD and SEM and show dominating and well-textured YBCO phase. The AC susceptibility and magnetization measurements reveal a critical temperature about 90 K and a high critical current density, comparable with that of pulse laser deposited films. The simplicity and high performance of the method may be attractive for large-scale superconducting applications.

101. Enhancing critical current in YBCO thick films: Substrate decoration and quasi-superlattice approach

Authors: Sarkar, A; Mikheenko, P; Dang, VS; Abell, JS; Crisan, A

Published: AUG-OCT 2009, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 469, 1553, DOI: 10.1016/j.physc.2009.05.242

For power applications of superconducting films. the critical current density (J(c)) and the thickness of the film (d) should be as high as possible. Since J(c) decreases with both thickness and magnetic field, artificial pinning centres in addition to natural ones are required to keep J(c) high. The earliest cost-effective method used for introducing artificial pinning centres was the so-called substrate decoration, i.e., growing nanoscale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on another version of this approach proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices). Several materials have been used for the Creation of artificial pinning centres. Here we report on the artificial pinning centres induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Pd and non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films. We observed an important shift of the position of the maximum in the thickness dependence of J(c)(B) towards higher thicknesses compared with pure YBCO films by both approaches. A high J(c)(B) in our quite thick films provides a very high total critical current per cm of the film width. Critical current as high as 800 A/cm width was achieved in a 2.4 mu m thick quasi-superlattice film with non-superconducting YBCO nano-dots. (C) 2009 Elsevier B.V. All rights reserved.

102. Pinning Centers Induced in YBCO Films by Nano-Dots in Substrate Decoration and Quasi-Superlattice Approaches

Authors: Mikheenko, P; Sarkar, A; Dang, VS; Tanner, JL; Kechik, MMA; Abell, JS; Crisan, A

Published: JUN 2009, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 19, 3494, DOI: 10.1109/TASC.2009.2018307

For power applications of superconducting films, the critical current density (J(c)) and the thickness of the film (d) should be as high as possible. Since J(c) decreases with both thickness and magnetic field, artificial pinning centers in addition to natural ones are required to keep J(c) high. The earliest cost-effective method used for introducing artificial pinning centers was the so-called substrate decoration, i.e., growing nanoscale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on other two approaches proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices) and distributing a secondary phase in the film from a compositionally changed target. Several materials have been used for the creation of artificial pinning centers. Here we report on the artificial pinning centers induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Ag, Au, Pd or non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.

103. All-self-assembled MgO nanorods and nanowires grown on Au-decorated MgO substrates by pulsed laser deposition

Authors: Crisan, A; Tanner, JL; Mikheenko, P; Abell, JS

Published: MAR 2009, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 3, 235, DOI:

We report on the fabrication of MgO nanorods and nanowires by a novel, all-self-assembled method, using Pulsed Laser Deposition. The seeds for the formation of these one-dimensional nanostructures consisted of self-assembled gold nanodots. Gold was used due to its well-known catalytic effect. Dimensions and surface density of the Au nano-dots, investigated by Atomic Force Microscopy, depend on the deposition conditions and the number of laser pulses. Morphology of the MgO nanorods and nanowires was investigated by SEM, and was found to depend strongly on the deposition conditions and on the architecture of the Au nanodots on the decorated substrates.

104. Artificial pinning centres in YBa2Cu3O7-delta thin films by Gd2Ba4CuWOy nanophase inclusions

Authors: Kechik, MMA; Mikheenko, P; Sarkar, A; Dang, VS; Babu, NH; Cardwell, DA; Abell, JS; Crisan, A

Published: MAR 2009, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 22, DOI: 10.1088/0953-2048/22/3/034020

Artificial pinning centres produced by Gd2Ba4CuWOy (2411W) nanophase inclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO target containing 1 mol% 2411W have been investigated. For comparison, YBCO thin films have been grown under the same conditions using a pure (commercial) YBCO target. The resulting films were characterized by AC susceptibility (superconducting transition) and SQUID magnetometry (critical current density, J(c)), and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition). J(c) of the film grown from the composite target is practically the same as that measured for the reference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatures higher than 50 K, however, the presence of the nano-inclusions leads to increased J(c). The relative enhancement in J(c) increases with further increase in temperature. J(c) of the film containing nanophase inclusions is between two and three times higher than that of the reference sample at 77.3 K and 1.5 T. This can be explained by considering the dimensions of nanophase inclusions compared with the temperature-dependent coherence length. Susceptibility measurements reveal that, apart from a large and sharp transition at about 91 K, there is a secondary, smaller and broader transition between 88 and 84 K in the YBCO film containing nanophase inclusions. This can be explained by substitutions of compatible atoms between YBCO and the 2411W phase (Gd with Y), which might also create additional pinning centres of delta T-c-type.

105. Thermally-induced self-assembling nanotechnology of gold nano-dots on CeO2-buffered sapphire for superconducting films

Authors: Crisan, A; Wordenweber, R; Hollmann, E; Kutzner, R; Button, TW; Abell, JS

Published: JUN 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 1373, DOI:

A typical facility for fabrication of devices based on high temperature superconducting (HTS) films grown on the less-expensive CeO2-buffered sapphire substrates comprises a transfer chamber, a chamber for CeO2 growth, a chamber for YBa2Cu3Oy (YBCO) growth, and a chamber for Au deposition at room temperature. Here we present the adaptation of our previously-reported method to create artificial pinning centres in HTS films through self-assembled nano-scale islands for this type of facility without making any modifications. We succeeded to grow Au nano-dots from an ultra-thin layer of Au grown at room temperature subsequently annealed in the chamber for YBCO deposition, in the same conditions as for YBCO growth. AFM and RBS were used to investigate the formation of Au nano-dots, while XRD and susceptibility measurements were used to study the properties of nano-structured YBCO films.

106. AC-Susceptibility study on vortex-molecule lattice in supermultilayer cuprate HgBa2Can-1CunO2n+2+delta (n=14)

Authors: Shivagan, DD; Shirage, PM; Crisan, A; Tanaka, Y; Iyo, A; Tokiwa, K; Watanabe, T; Terada, N

Published: SEP 15 2008, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 468, 1286, DOI: 10.1016/j.physc.2008.05.213

We investigated that in HgBa2Can-1CunO2n+2+delta, [Hg-1 2(n-1)n], for n >= 6, T-c of about 105 K, vortex melting lines remains constant and independent of number of inner plane CuO2 layers. However the vortex dynamics of the supermultilayer system could be crucially different from that of single or double layer cuprates. Therefore we measured the fundamental and third harmonic AC susceptibility responses to see the vortex dynamics and vortex matter phase diagram of Hg-12(n-1)n (n= 14), the higher member of super-multilayered cuprate superconductors. The Lindemann type vortex melting line was estimated using onset of third harmonic susceptibility (chi 3), at very low h(AC) (50 mOe), and represented by 3D Ginzburg Landau rescaling. Frequency dependence of out of phase susceptibility chi"(T) measured at lower fields of 500 and 1000 Oe. for 10 Hz to 10 kHz, showed resonance behaviour in the temperature region 55-68 K, suggesting rotational and translational creep dynamics of vortex molecule, even below usual melting line. We present schematic vortex phase diagram, considering the response of vortex dynamics to applied AC frequencies, taking into account the crossover temperatures obtained from critical slowing down of spin glass model and extended thermal activated model. (C) 2008 Elsevier B.V. All rights reserved.

107. Phase diagram of a lattice of vortex molecules in multicomponent superconductors and multilayer cuprate superconductors

Authors: Tanaka, Y; Shivagan, DD; Crisan, A; Iyo, A; Shirage, PM; Tokiwa, K; Watanabe, T; Terada, N

Published: AUG 2008, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 21, DOI: 10.1088/0953-2048/21/8/085011

The Abrikosov lattice in the multilayer cuprate superconductor CuBa2Ca3Cu3Oy (Cu-1223) has been experimentally and theoretically demonstrated to be composed of vortex molecules. Cu-1223 is considered to be a typical multicomponent superconductor. We show that in such a system the rotational freedom around the axis of the vortex molecular tube generates orientational disorder and the orientational glass (or crystal) phase, which is never present in conventional vortex lattices consisting of axisymmetric vortices. The emergence of the orientational glass phase and orientational order phase with orthorhombic distortion is a general property of vortex molecule lattices of the multiband type of multicomponent superconductors.

108. Pancake vortex molecules in HgBa2Can-1CunOy (n > 6) superconductors

Authors: Crisan, A; Iyo, A; Tanaka, Y; Matsuhata, H; Shivagan, DD; Shirage, PM; Tokiwa, K; Watanabe, T; Button, TW; Abell, JS

Published: APR 1 2008, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 468, 717, DOI: 10.1016/j.physc.2007.11.085

HgBa2Can-1CunOy (Hg: 12(n -1)n) phases have the same T-c independent of n and the same vortex melting line for n >= 6. In these materials, there are two types of pancake pairs: those separated by the thick block of (n-2) inner CuO2 planes, which are weakly coupled by the magnetic interaction, and, respectively, those separated by the thin HgBa2Ox charge reservoir layer, which are much stronger coupled by the Josephson coupling. The vortex melting line common to all Hg:12(n-1)n phases (n >= 6), separates two new vortex phases, pancake-molecule-glass, and pancake-molecule-liquid. (c) 2008 Elsevier B. V. All rights reserved.

109. Vortex Dynamics in Hg-based Multi- and Super-Multi-Layered Cuprates

Authors: Crisan, A; Tanaka, Y; Iyo, A; Matsuhata, H; Shivagan, DD; Shirage, PM; Button, TW; Abell, JS; Tokiwa, K; Watanabe, T

Published: 2008, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07), 97, DOI: 10.1088/1742-6596/97/1/012013

Hg-based multi- and super-multi-layered cuprates HgBa2Can-1CunOy (Hg:12(n-1)(n)) (n >= 4) are composed of the charge reservoir layer (CRL) HgBa2Ox, and of CuO2 planes: outer planes (OP) with pyramidal oxygen coordination and inner planes (IP) with square oxygen coordination, with different charge distribution. This fact gives rise to very interesting phenomena, like coexistence of superconductivity (SC) in OPs with antiferromagnetism (AF) in IPs in Hg:1245. The dependence of T-c on the number n of CuO2 planes is also interesting: for n increasing from 4 to 5 and to 6, T-c decrease from 125 to 108 and then to about 103 K; further increase in n keeps T-c constant at about 103 K. In Hg:1245 we show that the AF ordering at T-N of 60 K has no influence on vortex dynamics, and there is a quite robust c-axis supercurrent. Vortex melting lines of various samples showed that Hg-based multi- layered cuprates (n = 4, 5) behaves in the usual way (increasing n resulted in decrease of Josephson coupling (JC) and a less sharp melting line) and are well described by the usual anisotropic GL-based theory. Samples with n >= 6 have all the same melting line, suggesting that, in this case, the short-range JC becomes much less important than the long-range magnetic coupling (MC). However, since the JC between two pancakes in OPs of adjacent unit cells (pancakes separated by the thin CRL) is large, we suggest that the vortex matter in Hg-based super-multilayered cuprates is composed of magnetically-coupled pancake-vortex-molecules.

110. Manipulation of pancake vortices by rotating a Josephson vortex lattice

Authors: Crisan, A; Bending, SJ; Tamegai, T

Published: JAN 2008, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 21, DOI: 10.1088/0953-2048/21/01/015017

Scanning Hall probe microscopy has been used to demonstrate the manipulation of pancake vortices by rotating the Josephson vortex lattice in Bi(2)Sr(2)CaCu(2)O(8+delta) single crystals in the interacting crossing lattices regime. Creation of one-dimensional pancake vortex chains trapped on Josephson vortices, and the subsequent rotation of the chains were realized by independently controlling magnetic fields in three orthogonal directions. The anisotropy parameter determined from the in-plane distances between vortex chains in various in-plane fields is consistent with commonly accepted values.

111. Vortex melting line and anisotropy of a Ba2Ca3Cu4O8(O1-yFy)(2) multilayered superconductor

Authors: Shivagan, DD; Shirage, PM; Crisan, A; Tanaka, Y; Iyo, A; Kodama, Y; Tokiwa, K; Watanabe, T; Terada, N; Hamada, N

Published: SEP 2008, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 21, DOI: 10.1088/0953-2048/21/9/095002

We have measured the vortex melting lines of high-pressure-synthesized Ba2Ca3Cu4O8(O1-yFy)(2) (nominal composition, 2y = 1.3, 1.6 and 2.0) F(2y)-0234 multilayered high-T-c superconductor using fundamental and third harmonic susceptibility responses carried out on preferentially oriented crystallites with very low ac field amplitude (5 mu T) and in applied dc fields up to 6 T. The vortex melting lines of all three F-substituted samples show interesting doping dependence and are very well described by the commonly accepted theory of melting lines. Interestingly, we discovered that the models describing the temperature dependence of the in-plane London penetration depth also depend on the doping level: the 3D XY model for the nearly-optimum-doped sample [F(1.3)], the mean-field theory for the under-doped sample [F(1.6)] and the two-fluid model for the heavily-under-doped sample [F(2.0)]. We found that the vortex melting lines in F-0234 are determined by the interplay in the coupling of pancake vortices through the charge reservoir layer (CRL), and through the block of inner CuO2 planes (IPs), respectively. The anisotropy values (47 for the near-optimally-doped sample) derived from experimental vortex melting lines are consistent with the values obtained by first-principles electronic-band-structure calculations.

112. Vortex Molecule and i-soliton Studies in Multilayer Cuprate Superconductors

Authors: Shivagan, DD; Crisan, A; Shirage, PM; Sundaresan, A; Tanaka, Y; Iyo, A; Tokiwa, K; Watanabe, T; Terada, N

Published: 2008, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07), 97, DOI: 10.1088/1742-6596/97/1/012212

We observed two dissipation peaks in the field cooled AC susceptibility measurements on aligned multilayered cuprate Superconductor, (Cu,C)Ba2Ca2Cu3Oy [(Cu,C)-1223], and demonstrated that it is the direct experimental indication of the two-component vortex matter in the multilayered cuprates. We propose that, the second loss peak at lower temperature is due to the additional degree of freedom of the rotation of 'vortex molecule' composed of two fractional vortices, originated at two components, mediated by an i-soliton bond. To probe the dynamics of this vortex molecule, we measured frequency dependence of AC susceptibility response on aligned crystallites of (Cu,C)-1223 at different temperatures and DC fields. In second peak region, it gives resonance peak. The observed frequency dependence patterns of chi ''(T), for 0.5 T H-DC, are rescaled with the resonating frequencies and show tail at low frequency region indicating that the dissipation is due to rotation and twisting of vortex molecule. The temperature dependence of the average relaxation time shows that the vortex molecule rotation/twisting glass state follow critical slowing down process.

113. Magnetically coupled pancake vortex molecules in HgBa2Can-1CunOy (n >= 6)

Authors: Crisan, A; Iyo, A; Tanaka, Y; Matsuhata, H; Shivagan, DD; Shirage, PM; Tokiwa, K; Watanabe, T; Button, TW; Abell, JS

Published: APR 2008, PHYSICAL REVIEW B, 77, DOI: 10.1103/PhysRevB.77.144518

As recently shown, HgBa2Can-1CunOy (n >= 6) cuprate superconductors have a critical temperature of about 100 K independent of n. This remarkable property can be explained by the very imbalanced distribution of carriers among the inequivalent CuO2 planes in the unit cell. We discovered that these materials also have a common vortex melting line that resembles the theoretical melting lines of magnetically coupled pancake vortices. We suggest that there are two types of pancake pairs situated in the superconducting CuO2 outer planes: those separated by the thin charge reservoir layer are strongly (Josephson) coupled, while those separated by the thick block of (n-2) CuO2 nonsuperconducting inner planes are weakly (magnetically) coupled, forming magnetically coupled pancake vortex molecules.

114. Critical current densities and irreversibility fields of new high-T-c Ba2CaCu2O4(O,F)(2) superconductor

Authors: Shirage, PM; Shivagan, DD; Crisan, A; Tanaka, Y; Kodama, Y; Kito, H; Iyo, A

Published: APR 1 2008, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 468, 776, DOI: 10.1016/j.physc.2007.11.044

Ba2CaCu2O4(O2-yFy) (F-0212), a new high-T-c superconductor (HTSC) including fluorine at an apical site, is an attractive material for application because it shows a highest T-c of 108 K and has thin spacing between conduction layers that is favorable for high critical current density (J(c)) and irreversibility field (B-irr). The polycrystalline samples of F-0212 were synthesized under high pressure. A doping state was controlled from under doping (T-c = 82 K) to slightly-over doping (T-c = 106 K) via optimally-doping (T-c = 108 K). From the DC magnetization hysteresis loops, we determined the intragrain J(c) and B-irr using Bean's critical state model. The J(c) and B-irr properties of F-0212 were found to improve rapidly as the doping state changes from under doping to slightly-over doping to via optimally doping state. The irreversibility lines for the optimally- and slightly-over doped F-0212 are higher than optimally-doped Bi-2212. However, they are much lower than that of Cu-1212 (Y-123) though the spacing of F-0212 is thinner than that of Cu-1212. This indicates that property of the spacing is important to enhance B-irr as well as its thickness. An anisotropy factor for under doped F-0212 was estimated to 118 from a 3D-2D crossover field of about 0.28 T. (C) 2008 Elsevier B.V. All rights reserved.

115. Synthesis and magnetic properties of Fe51Pt27Nb2B20 melt spun ribbons

Authors: Crisan, AD; Crisan, O; Skorvanek, I; Randrianantoandro, N

Published: APR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 789, DOI:

Fe-Pt alloys have gained much interest for the occurrence of permanent magnetic features resulting from the L1(0) ordered face-centred-tetragonal FePt phase with very high magnetic crystalline anisotropy (7x10(6) J/m(3)). An amorphous melt spun ribbons of the composition Fe51Pt27Nb2B20 has been synthesized by the rapid solidification technique and its microstructure and magnetic properties were studied. After appropriate annealing, an ordered face-centred-tetragonal (f.c.t.) L1(0) phase is formed. X-ray analysis revealed a structural phase transformation from the body-centered-cubic Al to f.c.t. L1(0) phase and this produce magnetic hardening of the alloy, upon appropriate annealing conditions. Extremely performant magnetic properties, typical for exchange spring magnets, are obtained.

116. Critical current densities and irreversibility fields of a HgBa2Can-1CunO2n+2+delta sample containing n=6-15 phases

Authors: Shirage, PM; Iyo, A; Shivagan, DD; Crisan, A; Tanaka, Y; Kodama, Y; Kito, H

Published: SEP 15 2008, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 468, 1290, DOI: 10.1016/j.physc.2008.05.049

A recent report on multilayered superconductors establishes unique constant T-c although the number of inner CuO2 plane (IP) increases. Many hypotheses proposed that the irreversibility field (B-irr) and critical current density (J(c)) are depends on the crystal structures. We have measured the J(c) and B-irr of the HgBa2Can-1CunO2n+2+delta sample containing n = 6-15 phases to investigate the effect of the number of IP's on the B-irr and J(c) and its pinning properties. The rate of fall of J(c) increases and irreversible lines (IL's) shift to lower temperatures with increasing the number of IP's, which suggests that anisotropy increases with n. The IL settled in between those for Bi2Sr2CaCu2Oy with high anisotropy value and YBa2Cu3O7 with less anisotropy value. The double logarithmic plot of irreversibility field versus [1 - (T/T-c)] analysis suggests that the flux line melting model is adopted. The flux pinning force density F-p (approximate to J(c)B) exhibits scaling behavior when the magnetic field B is normalized by the irreversibility field B;- Analysis of the normalized pinning force reveals that a surface pinning mechanism is dominant and reduced magnetic field b(max) = 0.2 agree with surface pinning mechanism with closely spaced pins. (C) 2008 Elsevier B.V. All rights reserved.

117. Interpretation of abnormal AC loss peak based on vortex-molecule model for a multicomponent cuprate superconductor

Authors: Tanaka, Y; Crisan, A; Shivagan, DD; Iyo, A; Tokiwa, K; Watanabe, T

Published: JAN 2007, JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, 46, 145, DOI: 10.1143/JJAP.46.134

An AC magnetic field can rotate a vortex-inolecule composed of two fractional vortices in a multiband type of multicomponent superconductor, where two components couple with each other through the intercomponent Josephson interaction. It may directly relate to an abnormal AC loss peak, which we have found recently in the multilayer cuprate supercondutor CuxBa2Ca2Cu3Oy (Cu-1223).

118. Crystallization processes and phase evolution in amorphous Fe-Pt-Nb-B alloys

Authors: Crisan, O; Crisan, AD; Randrianantoandro, N; Nicula, R; Burkel, E

Published: AUG 16 2007, JOURNAL OF ALLOYS AND COMPOUNDS, 440, L7, DOI: 10.1016/j.jallcom.2006.08.331

Fe-Pt system is nowadays widely studied due to its potential applications as magnetic recording media. The hard magnetic FePt L1(0) phase has extremely promising potential as permanent magnet with high magnetocrystalline anisotropy. Of recent interest is also the developing of the hard magnetic phase from an amorphous precursor by appropriate crystallization processes. The melt-spun amorphous Fe68Pt13Nb2B17 alloy has been submitted to dynamical annealing and its phase transformation during the process has been monitored by differential scanning calorimetry and in situ energy-dispersive X-ray diffraction of the synchrotron radiation. In the first stage of crystallization, alpha-Fe and cubic FePt phases are formed from the amorphous precursor. At around 600 degrees C superlattice Bragg reflections corresponding to tetragonal FePt are indexed in the XRD spectra and a-Fe phase diminishes drastically. Finally, between 900 degrees C and 975 degrees C the tetragonal superlattice peaks disappear and cubic FePt phase is formed again. This reversible order-disorder transformation is accompanied by a strong uniaxial lattice expansion of the cubic FePt unit cell. The system show promising features for the co-existence of hard and soft exchange coupled magnetic phases crystallized from FePt-based amorphous precursors. (c) 2006 Elsevier B.V. All rights reserved.

119. Coexistence of superconductivity and antiferromagnetism in HgBa2Ca4Cu5Oy: Multiharmonic susceptibility and vortex dynamics study

Authors: Crisan, A; Tanaka, Y; Iyo, A; Shivagan, DD; Shirage, PM; Tokiwa, K; Watanabe, T; Cosereanu, L; Button, TW; Abell, JS

Published: DEC 2007, PHYSICAL REVIEW B, 76, DOI: 10.1103/PhysRevB.76.212508

We have studied the multiharmonic ac susceptibility response of HgBa2Ca4Cu5Oy (Hg:1245), a multilayered high-temperature superconductor (HTS) having two crystallographically inequivalent CuO2 planes in a unit cell with very imbalanced carrier concentrations. Vortex melting lines are well described by the commonly accepted model of melting with moderate anisotropy factors of 40-50, depending on the doping level. The diamagnetic response with applied fields parallel to superconducting (a,b) planes also shows a quite robust supercurrent along the c axis. Our results are also discussed in connection with contradictory models and experiments regarding the coexistence of superconductivity and antiferromagnetism in HTSs.

120. Thermally and pressure activated phase evolution in Fe-Pt-Nb-B melt spun ribbons

Authors: Crisan, AD; Nicula, R; Crisan, O; Burkel, E

Published: SEP 2007, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 27, 1282, DOI: 10.1016/j.msec.2006.06.033

The Fe-Pt-rich alloys has recently attracted a lot of interest for their potential development of the hard magnetic L1(0) (also denoted gamma(1)) FePt phase with high magnetocrystalline anisotropy. An alloy with the nominal composition Fe65Pt25Nb2B8 has been synthesised by rapid quenching of the melt. The phase evolution in the as-cast ribbons was monitored by energy dispersive X-ray diffraction of the synchrotron radiation. It is shown that the Fe65Pt25Nb2B8 alloy is completely crystallized in the as-cast state. With increasing temperature a partial disorder-order phase transformation with occurrence of hard magnetic L1(0) phase is observed. This transformation is directly related to thermal and pressure effects in the sample. With applying pressure, the phase transformation between cubic and tetragonal FePt symmetry is more pronounced and is accompanied by a strong uniaxial lattice expansion. The results are extremely promising for the direct formation from the melt of an exchange coupled hard-soft magnetic alloy with initial formation of the tetragonal hard magnetic L1(0) FePt phase. (C) 2006 Elsevier B.V. All rights reserved.

121. Crystallization processes in Fe-Pt-Nb-B melt spun ribbons

Authors: Crisan, AD; Crisan, O; Randrianantoandro, N; Valeanu, M; Morariu, M; Burkel, E

Published: SEP 2007, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 27, 1285, DOI: 10.1016/j.msec.2006.08.031

Intermetallic Fe-Pt-Nb-B alloys with 3 different compositions have been synthesized by rapid solidification technique and their phase structure was characterized by means of X-ray diffraction and Mossbauer spectrometry. It is shown that Fe68Pt21Nb2B9 and Fe65Pt25Nb2B8 as-east samples consist mainly of A1 soft magnetic f.c.c. Fe-Pt phase, while the Fe68Pt13Nb2B17 as-cast sample exhibits topological short-range order, typical for amorphous ribbons. Crystallization processes in the amorphous sample and phase evolution with the temperature have been studied using differential scanning calorimetry (DSC). The occurrence of exothermic peaks is related to structural transformations in the alloys and the crystallization process is shown to be highly dependent upon the heating rate in the DSC process. (C) 2006 Elsevier B.V. All rights reserved.

122. Anomalous AC susceptibility response of (Cu,C)Ba2Ca2Cu3Oy: Experimental indication of two-component vortex matter in multi-layered cuprate superconductors

Authors: Crisan, A; Tanaka, Y; Shivagan, DD; Iyo, A; Cosereanu, L; Tokiwa, K; Watanabe, T

Published: MAY 2007, JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS, 46, L453, DOI: 10.1143/JJAP.46.L451

We have measured the AC susceptibility response in field-cooling conditions of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites. In a certain but large range of the applied DC magnetic field, the out-of-phase susceptibility has two dissipation peaks. Due to the characteristics of the sample, the shape of the second peak and its position's field-dependence, we ruled out the possibility that the anomalous second dissipation peak could be related to intergrain dissipation. Because this multilayered cuprate is known to be a two-gap superconductor, we believe that our results are an experimental indication of a two-component vortex matter in high-critical temperature superconductors.

123. Study of crystallization processes in Gd-substituted Finemet alloys

Authors: Crisan, O; Le Breton, JM; Crisan, AD; Filoti, G

Published: SEP 28 2006, JOURNAL OF ALLOYS AND COMPOUNDS, 422, 202, DOI: 10.1016/j.jallcom.2005.12.002

The crystallization processes that occur in amorphous melt-spun ribbons of nominal composition Fe73.5Cu1Nb3Si13.5B9 with Gd addition in different concentrations are studied by differential scanning calorimetry (DSC). The aim of Gd addition is to study the changes induced in the calorimetric behavior of Finemet alloys, the crystallization sequences at intermediate stages of annealing as well as the nature of obtained crystallization products. The nature and structural properties of the phases formed at the end of the calorimetric process are investigated using X-ray diffraction (XRD) and Mossbauer spectrometry (MS). The crystallization sequence and also the phase composition at different stages of annealing were investigated for the Fe68.5Gd5Cu1Nb3Si13.5B9 alloy. Structure and magnetic properties of resulting crystallized samples have been thoroughly investigated. It is shown that in the above-mentioned composition, the decomposition at around 690 degrees C of the metastable Gd3Fe62B14 phase formed at incipient stages of crystallization leads to the co-existence of Gd2Fe14B, Fe2B and alpha-Fe(Si) phases. Magnetic measurements of as-cast and annealed samples are in good agreement with the XRD and MS results and with the proposed crystallization sequences and decomposition of intermediate metastable phases. (c) 2005 Elsevier B.V. All rights reserved.

124. Anomalous vortex melting line in the two-component superconductor (Cu,C)Ba2Ca3Cu4O10+delta

Authors: Crisan, A; Tanaka, Y; Iyo, A; Cosereanu, L; Tokiwa, K; Watanabe, T

Published: NOV 2006, PHYSICAL REVIEW B, 74, DOI: 10.1103/PhysRevB.74.184517

Using the on-set of third-harmonic susceptibility we have measured vortex melting lines of high-pressure synthesized (Cu0.6C0.4)Ba2Ca3Cu4Oy [(Cu,C):1234] and (Cu0.5C0.5)Ba2Ca2Cu3Oy [(Cu,C):1223] with preferentially oriented crystallites and having various carrier concentrations. Vortex melting lines of all (Cu,C):1223 samples and of optimum-doped (Cu,C):1234 were very well described by the commonly accepted theory of melting lines within the two-fluid model. Overdoped (Cu,C):1234 proved to have an anomalous melting line, that was phenomenologically explained by the rather special gaps evolution, that is the significant opening of a second superconducting gap due to CuO2 outer planes at a temperature lower than the critical one, where the first superconducting gap due to inner planes opens. The experimental melting lines of overdoped (Cu,C):1234 were modelled theoretically by assuming an empirical formula for the temperature dependence of anisotropy factor.

125. Anisotropic vortex channeling in YBa2Cu3O7-delta thin films with ordered antidot arrays

Authors: Crisan, A; Pross, A; Cole, D; Bending, SJ; Wordenweber, R; Lahl, P; Brandt, EH

Published: APR 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.144504

Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-mu m-period square array of 2.5-mu m-sized holes (antidots). Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling (ZFC) to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.

126. Magnetic properties of nanostructured materials - Monte Carlo simulation and experimental approach for nanocrystalline alloys and core-shell nanoparticles

Authors: Crisan, O; Greneche, JM; Labaye, Y; Berger, L; Crisan, AD; Angelakeris, M; LeBreton, JM; Flevaris, NK

Published: 2005, PROPERTIES AND APPLICATIONS OF NANOCRYSTALLINE ALLOYS FROM AMORPHOUS PRECURSORS, 184, 266, DOI:

The magnetic properties of FINEMET-type nanocrystalline alloys and isolated ferromagnetic AgCo nanoparticles are investigated both experimentally and numerically. Theoretical models of spins that simulate ideal nanocrystalline alloys and isolated nanoparticles are considered while their magnetic properties are derived from Monte Carlo simulation of low-temperature spin ordering. Interesting features such as magnetic polarization of the matrix due to penetrating fields arising from nanograins and the role played by the crystalline fraction in the overall L magnetic behaviour, in the case of nanocrystalline alloys are investigated. For isolated nanoparticles it is shown that the competition between surface and bulk anisotropy gives rise to surface spin disorder that, together with finite-size effects, is responsible for the experimentally observed lack of saturation of the magnetization in high applied fields. These simulation results are confirmed by experimental data obtained on FINEMET nanocrystalline alloys and isolated ferromagnetic AgCo colloidal nanoparticles.

127. Flux structures in mesoscopic YBa2CU3O7-delta discs

Authors: Crisan, A; Bending, SJ; Pross, A; Aziz, A; Grigorenko, AN; Humphreys, RG

Published: MAR 2005, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 18, 213, DOI: 10.1088/0953-2048/18/3/001

Scanning Hall probe microscopy has been used to study flux Structures and dynamics in 5 mu m x 5 mu m YBCO thin film squares, which are mesoscopic with respect to the magnetic penetration depth, lambda(T), at temperatures close to T-C. A number of unusual vortex phenomena are observed in these microstructures which differ qualitatively from the expected behaviour of more macroscopic pieces of film. In field-cooled (FC) experiments to similar to 65 K a full Meissner state is generated for cooling fields less than similar to 6 Oe, reflecting the relatively small demagnetization factors in our samples. Cooling in higher fields, however, results in only a very weak diamagnetic response at low temperatures whose magnitude is almost independent of the cooling field. In contrast we observe considerable trapped flux upon field-removal whose magnitude grows monotonically with cooling field. Remarkably, all FC flux distributions exhibit almost perfect rotational symmetry, and can be nearly completely cancelled in a reversible fashion by tuning the field applied to the initially FC state. Our field-cooled and zero-field-cooled results have been analysed in terms of a Bean-like critical state model containing constant edge and bulk current densities, and most of the observed phenomena can be explained by considering the relative weight of these two components. Not all flux profiles call be described by our simple model, however, and under certain circumstances symmetry-breaking 'dipole'-like flux structures can form in several adjacent YBCO squares. We speculate that these are related to the unidirectional At-ion milling process which was used to pattern the squares and could have broken the expected four-fold symmetry. We note that our results could have important implications for the miniaturization of thin film HTS devices.

128. Observation of interacting crossing vortex lattices in Bi2Sr2CaCu2O8+delta thin films

Authors: Crisan, A; Bending, SJ; Popa, S; Li, ZZ; Raffy, H

Published: DEC 2005, PHYSICAL REVIEW B, 72, DOI: 10.1103/PhysRevB.72.214509

Scanning Hall probe microscopy and magnetometry have been used to study pancake vortices (PV's) in highly anisotropic Bi2Sr2CaCu2O8+delta (BSCCO) thin films. For small perpendicular applied fields we observe well-defined PV stacks which are randomly pinned due to a high density of relatively strong pinning centers in the films. When a strong in-plane magnetic field is added images reveal stripelike flux structures which are quite closely aligned to the in-plane field direction. We conclude that this is a manifestation of Josephson vortex (JV) "decoration" by PV's, which was recently demonstrated in highly ordered BSCCO single crystals in the crossing lattices regime, allowing independent estimates of the anisotropy parameter to be made. This interpretation is supported by the observation that the presence of an in-plane field suppresses the irreversible magnetization of thicker films. Estimates of the JV-PV crossing force and the PV pinning force are found to be comparable in magnitude and fully consistent with this scenario. We believe that this is the first confirmation of the existence of interacting crossing vortex lattices in strongly pinning BSCCO thin films and could find potential applications in superconducting devices.

129. Anomalies of AC susceptibility losses in the doped [Bi(Pb)](2)Sr2Ca2CU3Ox superconductor

Authors: Mihalache, V; Aldica, G; Badica, P; Crisan, A

Published: APR 2004, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 17, 730, DOI: 10.1088/0953-2048/17/4/030

AC susceptibility measurements have been performed on Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox (Bi-2223) samples doped with different Li-based compounds and prepared by the solid-state method. As-prepared samples and/or samples annealed in oxygen or argon, ground or unground, have been investigated in detail in order to understand the occurrence, nature and evolution of the anomalous peaks observed in chi " (T) curves versus the measuring parameters; we have detected up to four peaks instead of the usual two peaks observed in the non-doped samples. It was found that intrinsic physical-chemical properties, such as the melting temperature of the doping compound, are no less important for the final properties of the superconductor than their insertion properties into the crystal lattice of Bi-2223. Doping compounds with melting temperatures below or close to the phase formation temperature of the Bi-2223 phase can act as flux, changing the growth conditions. Intensification of some processes against others (e.g. decomposition-recovery of the BI-2223 phase, solubilization-precipitation of the secondary phases, changes in the properties of the liquid phase, etc) can lead to the formation of the Bi-2223 phase with different properties than for the non-doped superconductor. Generally, when using flux-type doping compounds, anomalous AC losses peaks are detected and the intra-granular critical current density is enhanced.

130. Vortex imaging and local magnetization studies in HTS by scanning Hall probe microscopy

Authors: Crisan, A; Pross, A; Cole, D; Bending, S

Published: AUG 2004, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 408, 557, DOI: 10.1016/j.physc.2004.03.199

We have used scanning Hall probe microscopy to correlate vortex images and local magnetisation measurements in two different high temperature superconducting samples. Near the edge of a continuous YBCO thin film we have observed local hysteresis inversion and negative remanent fields, which can be semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinitely long superconducting strip. In a YBCO film containing a regular square array of antidots we have further find that vortices trapped at antidots exhibit an unusual behaviour upon field sweep reversal. (C) 2004 Elsevier B.V. All rights reserved.

131. Flux lenses in the crossing lattices regime of layered superconductors

Authors: Cole, D; Crisan, A; Bending, SJ; Tamegai, T; van der Beek, K; Konczykowski, M

Published: MAY 1 2004, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 404, 102, DOI: 10.1016/j.physc.2003.09.108

We report here measurements of flux lensing behaviour which have been performed on Bi2Sr2CaCu2O8+delta single crystals placed on top of linear Hall probe arrays. In the crossing lattices regime we observe significant flux focussing as the in-plane field is swept up with a small constant density of pancake vortices. We also observe a sharp discontinous lock-in feature in local magnetic induction when the net field is close to the a-b plane. (C) 2004 Elsevier B.V. All rights reserved.

132. (Cu, C)Ba2Ca3Cu4Ox-(LiF)(y): addition of LiF - an effective way to synthesize overdoped superconductor

Authors: Badica, P; Iyo, A; Aldica, G; Kito, H; Crisan, A; Tanaka, Y

Published: MAR 2004, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 17, 437, DOI: 10.1088/0953-2048/17/3/022

(Cu, C)Ba2Ca3Cu4Ox superconductor with addition of y mol LiF has been synthesized by a high-pressure method. For the same synthesis conditions it was found that (almost) single-phase Cu, C-1234 samples can be synthesized for y(LiF) = 0-0.1 if the amount of z mol AgO oxidizer is increased linearly from Z(AgO) = 0.45 to 0.73 and for y(LiF) = 0 - 1-0.2 if ZAgO = 0.73 = constant. Transport measurements (rho(T) and room-temperature Seebeck coefficient) have shown that these samples are overdoped: LiF is an effective addition for synthesis of overdoped Cu, C-1234 with a controlled level of carriers. LiF addition continuously decreases T-c. The critical point at y(LiF) = 0.1 is discussed as the solubility limit of LiF and/or the point where the doping mechanism changes. It is proposed that the reason is the reaction of extra Li with C and O to form Li2CO3, inducing a lower concentration of C in Cu, C-1234/LiF crystals, and at the same time a possible substitution of Li not only for the Cu site but also for the Ca site, resulting in formation of a higher amount of residual Ca0.828CuO2 (for y(LiF) > 0.1). LiF induces the formation of a liquid phase and acts as a flux promoting the formation of Cu, C-12(n - 1)n with n greater than or equal to 4. LiF modifies to some degree the grain growth from a 3D to a 2D type (thinner platelike grains have been observed in the LiF added samples).

133. Magnetic nanostructures obtained by colloidal crystallization onto patterned substrates

Authors: Crisan, O; Angelakeris, M; Vouroutzis, N; Crisan, AD; Pavlidou, E; Kostic, I; Sobal, N; Giersig, M; Flevaris, NK

Published: MAY 2004, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 272, E1287, DOI: 10.1016/j.jmmm.2003.12.184

Colloidal solutions of magnetic nanoparticles are regularly dispersed onto patterned substrates in order to form novel magnetic nanostructures. The morphology of these nanostructures is investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM) and their structure is correlated with magnetic properties. It is shown that, depending on the nature of the substrate, different nanoparticle growth modes are identified during the colloidal crystallization. (C) 2003 Elsevier B.V. All rights reserved.

134. Influence of Gd addition on the magnetism and structure of Finemet-type nanocrystalline alloys

Authors: Crisan, O; Le Breton, JM; Crisan, AD; Machizaud, F

Published: MAY 2004, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 272, 1397, DOI: 10.1016/j.jmmm.2003.12.1284

The effect of rare earth addition in the structure and magnetism of melt spun nanocrystalline Finemet-type alloys devitrified from amorphous precursor ribbons is discussed. Starting with the initial composition Fe73.5Cu1Nb3Si13.5B9 an amount of 5 at% Gd is introduced into the primary alloy. The purpose is to enable after appropriate recrystallization the occurrence of hard and soft magnetic, suitably dispersed, exchange-coupled nanograins and to determine the transformation sequences of the crystallization process and the obtained crystallization products. (C) 2003 Elsevier B.V. All rights reserved.

135. Enhancement of critical current density in YBa2Cu3O7-delta thin films grown using PLD on YSZ (001) surface modified with Ag nano-dots

Authors: Ionescu, M; Li, AH; Zhao, Y; Liu, HK; Crisan, A

Published: JUL 7 2004, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 37, 1828, DOI: 10.1088/0022-3727/37/13/014

Y123 thin films were grown by pulsed laser deposition (PLD) on YSZ (001) substrate. Prior to the film deposition, a discontinuous layer of Ag was deposited on the substrate, also using PLD, in the form of separate islands. Atomic force microscopy (AFM) investigation of the Ag layer showed that its morphology consisted of self-assembled islands of nanometre size, randomly distributed on the surface of the substrate, called nano-dots. The Y123 superconducting films grown on such a surface were characterized using AFM, x-ray diffraction, secondary electron microscopy, ac susceptibility and dc magnetization. The results show that there is no significant difference in surface morphology, crystallographic orientation, phase composition or superconducting transition temperature between the Y123 films grown on YSZ (001) with an Ag nano-dots layer and a control Y123 film grown on a virgin YSZ (001) surface. On the other hand, at 77 K, the magnetic critical current density (J(c)(m)) was three times higher for the Y123 film grown on YSZ with the modified (001) surface than for the film grown on YSZ with a virgin (001) surface. At 5 K the enhancement of J(c)(m) was approximately seven times, at both low and high fields. This suggests an increase in pinning, caused presumably by point defects formed in the Y123 film above the Ag islands.

136. Electron-doped superconductivity induced by oxygen vacancies in as-grown Sr0.6Ca0.4CuO2-delta infinite-layer films

Authors: Nie, JC; Badica, P; Hirai, M; Sundaresan, A; Crisan, A; Kito, H; Terada, N; Kodama, Y; Iyo, A; Tanaka, Y; Ihara, H

Published: JAN 2003, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 16, L3, DOI: 10.1088/0953-2048/16/1/101

Thin films of the infinite-layer compound Sr0.6Ca0.4CuO2-delta have been prepared by off-axis rf magnetron sputtering on SrTiO3 (001). For an opt mum level of oxygen vacancy superconductivity was observed (T-c onset = 42 K and T-c(rho=0) = 11 K). Structural and transport data suggesting n-type superconductivity are presented and discussed. Infinite-layer thin films of Sr0.5Ca0.4CuO2-delta, without trivalent cation doping and showing n-type superconductivity, are reported for the first time.

137. Intra- and inter-grain critical current density in (Cu,C): 1234 superconductors

Authors: Crisan, A; Badica, P; Hirai, M; Kito, H; Iyo, A; Tanaka, Y

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 422, DOI: 10.1016/S0921-4534(02)02550-9

From DC magnetization studies in fields up to 14 T and temperatures between 20 and 100 K, we estimated the intra-and inter-grain critical current density J(c) of (Cu,C): 1234 high-T-c superconductors, in the frame of critical state models. The inter-grain J(c) was determined by comparing the magnetization loops of as-grown sample and of the ground sample. Finally, short comments on J(c) resulted from AC susceptibility measurements and of the impressive increase of intra-grain J(c) due to heavy-ion and neutron irradiation are presented. (C) 2003 Elsevier Science B.V. All rights reserved.

138. TlBa(2)Ca(2)Cu(3)O(y) superconducting films on MgO with different morphologies

Authors: Badica, P; Sundaresan, A; Crisan, A; Nie, JC; Hirai, M; Fujiwara, S; Kito, H; Ihara, H

Published: JAN 1 2003, , DOI: 10.1016/S0921-4534(02)01836-1

TlBa(2)Ca(2)Cu(3)O(y) (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 degreesC. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities J(c). Critical current density J(c) determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of J(c) is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 degreesC. In these samples, the substrate's flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 degreesC and showing rough surface and Ca-segregates, J(c) was lower. However, J(c) was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended. (C) 2002 Elsevier Science B.V. All rights reserved.

139. Electron-doped superconductivity in Sr1-xCaxCuO2-delta infinite-layer thin films

Authors: Nie, JC; Badica, P; Hirai, M; Kodama, Y; Crisan, A; Sundaresan, A; Tanaka, Y; Ihara, H

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 442, DOI: 10.1016/S0921-4534(02)02568-6

Thin films of infinite-layer compound Sr1-xCaxCuO2-delta have been prepared by rf magnetron sputtering. For an optimum level of oxygen vacancy superconductivities were observed (T-c (onset) = 42 and 50 K, T-c(rho=0) = 11 and 20 K). Structural and transport data suggest that the doping mechanism is electron-type. The superconducting transitions were also confirmed by ac susceptibility. Further increase of doping showed to destroy superconductivity. (C) 2003 Elsevier Science B.V. All rights reserved.

140. TlBa2Ca2Cu3Oy superconducting films on MgO with different morphologies

Authors: Badica, P; Sundaresan, A; Crisan, A; Nie, JC; Hirai, M; Fujiwara, S; Kito, H; Ihara, H

Published: JAN 1 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 383, 490, DOI: 10.1016/S0921-4534(02)01836-1

TlBa2Ca2Cu3Oy (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 degreesC. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities J(c). Critical current density J(c) determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of J(c) is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 degreesC. In these samples, the substrate's flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 degreesC and showing rough surface and Ca-segregates, J(c) was lower. However, J(c) was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended. (C) 2002 Elsevier Science B.V. All rights reserved.

141. Metastable phases and crystallization products in nanocrystalline FINEMET-type alloys with RE addition

Authors: Crisan, AD; LeBreton, JM; Crisan, O; Filoti, G

Published: SEP 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 711, DOI:

The melt spun nanocrystalline alloys such as Fe73.5Cu1Nb3Si13.5B9 (FINEMET) have attracted great deal of interest in recent years due to their excellent soft magnetic properties mostly related to the exchange coupling between nanograins, through the amorphous matrix. The microstructural evolution of both nanocrystalline and amorphous residual phases during annealing of the ribbons give rise to crystallization products that determine the expected magnetic properties for specific applications. Upon addition of the rare earth (RE), the evolution of the crystalline phases which emerge from metastable precursors during annealing is investigated. It is expected that the RE presence should strongly modify both the phase structure via new ternary metastable precursors and the magnetic properties of the ribbons by inducing enhanced exchange correlation due to novel ternary crystalline phases.

142. Magnetic properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) superconducting system

Authors: Mihalache, V; Aldica, G; Popa, S; Crisan, A

Published: FEB 15 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 384, 457, DOI: 10.1016/S0921-4534(02)02018-X

AC complex susceptibility (with and without a superimposed DC field) and DC magnetization measurements on (Bi,Pb)-2223 high-temperature superconductors with various amounts of LiF are presented. The results are discussed in the frame of the critical state model. The values of the intra- and inter-granular critical current density as well as of fields for full penetration are estimated. It can be seen that, in Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y), various amounts of LiF change the superconducting properties of both the grains and the inter-grain matrix. The intra-granular critical current density has similar values from both AC susceptibility and DC magnetization measurements, its highest value being attained for y = 0.15. For all LiF-doped samples, a split of the peak corresponding to the dissipation in the grains was observed. (C) 2002 Elsevier Science B.V. All rights reserved.

143. Magnetism of nanocrystalline Finemet alloy: experiment and simulation

Authors: Crisan, O; Greneche, JM; Le Breton, JM; Crisan, AD; Labaye, Y; Berger, L; Filoti, G

Published: JUL 2003, EUROPEAN PHYSICAL JOURNAL B, 34, 162, DOI: 10.1140/epjb/e2003-00207-3

Mossbauer spectrometry and magnetic measurements are employed to experimentally investigate the magnetic behavior of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons obtained by appropriate annealing of the amorphous precursor. A detailed analysis of the correlation between the microstructure of annealed samples and their magnetic properties is provided. Thermomagnetic data allow the Curie temperatures of both amorphous residual matrix and nanocrystalline phase to be estimated. The differences between Curie temperatures of amorphous residual matrix and amorphous precursor are investigated and explained in terms of magnetic polarization of the matrix by exchange fields arising from the nanocrystalline grains. Theoretical systems of spins consisting of a single ferromagnetic nanocrystalline grain immersed in weakly ferromagnetic environment, quite similar to our real samples, are considered and their magnetic behavior is investigated by Monte Carlo simulation of low temperature spin ordering, with emphasize on the matrix-nanocrystalline grain interface which is shown to exhibit peculiar magnetic behavior. The magnetic features of the matrix-nanocrystalline grain interface are studied, as depending on matrix-nanocrystalline grain exchange coupling as well as crystalline fraction of the nanocrystalline systems.

144. Nanodots-induced pinning Centers in thin films: Effects on critical current density, activation energy and flux jump rate

Authors: Crisan, A; Badica, P; Fujiwara, S; Nie, JC; Sundaresan, A; Iyo, A; Tanaka, Y

Published: JUN 2003, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 13, 3729, DOI: 10.1109/TASC.2003.812527

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of TI-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the, actiation energy for the flux jumps, which resulted to. be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T-c dc SQUID's.

145. Preparation of Tl-2212 and -1223 superconductor thin films and their microwave properties

Authors: Sundaresan, A; Asada, H; Crisan, A; Nie, JC; Kito, H; Iyo, A; Tanaka, Y; Kusunoki, M; Ohshima, S

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 474, DOI: 10.1016/S0921-4534(02)02585-6

Tl2Ba2CaCu2Oy (Tl-2212) and Tl(Ba,Sr)(2)Ca2Cu3Oy (Tl-1223) superconductor thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process namely, amorphous phase epitaxy (APE) method. In the case of Tl-1223, an amorphous layer of TlSr2CaCu2Oy (TlSr-1212) was deposited on top Of CeO2 layer to avoid chemical reaction between Tl-1223, containing Ba, and CeO2 layers. All films are epitaxial with smooth surface and excellent crystallinity. Superconducting transition temperatures (T-c) of Tl-2212 and 1223 are around 95 and 104 K, respectively. Critical current density (J(c)) at 77 K measured on 5 x 5 mm. films by inductive technique is as high as 2 MA/cm(2). Surface resistance R-s of Tl-2212 films measured by a dielectric resonator technique at 38 GHz is comparable to the best YBCO films. (C) 2003 Elsevier Science B.V. All rights reserved.

146. Hall probe imaging of local hysteresis inversion and negative remanent fields near the edge of a YBCO thin film

Authors: Crisan, A; Pross, A; Humphreys, RG; Bending, S

Published: JUN 2003, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 16, 698, DOI: 10.1088/0953-2048/16/6/307

We report vortex imaging and local magnetometry studies near the edge of a YBCO thin film using scanning Hall probe microscopy (SHPM). Our data show some unusual features, namely local hysteresis inversion and negative remanent fields, which are semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinite-long thin superconducting strip.

147. Vortex melting line and anisotropy of high-pressure-synthesized TlBa2Ca2Cu3O10-y high-temperature superconductor from third-harmonic susceptibility studies

Authors: Crisan, A; Iyo, A; Tanaka, Y

Published: JUL 21 2003, APPLIED PHYSICS LETTERS, 83, 508, DOI: 10.1063/1.1593824

The fundamental and third-harmonic susceptibility response of YBa2Cu3O7-delta (YBCO) and TlBa2Ca2Cu3O10-y (Tl:1223) high-critical temperature (T-c) superconductors with preferentially oriented crystallites has been measured in various magnetic fields up to 14 T, and with very low ac field amplitude, to probe the melting transition of the vortex matter. This method does not require large single crystals, being therefore suitable for high-T-c superconductors with small crystallites, prepared with high-pressure synthesis techniques. We determine experimentally the melting line of vortices and the anisotropy factor in high-pressure-grown Tl:1223 with a high T-c of 133.5 K. Results from similar measurements on YBCO with preferentially oriented grains are consistent with both commonly accepted models of the melting line and values of the anisotropy factor, supporting the validity of our approach. (C) 2003 American Institute of Physics.

148. LiF addition to (Cu,C) Ba2Ca3Cu4Oy superconductor

Authors: Badica, P; Crisan, A; Hirai, M; Iyo, A; Kito, H; Aldica, G; Tanaka, Y

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 396, DOI: 10.1016/S0921-4534(02)02535-2

Samples of (Cu0.6C0.4)Ba2Ca3Cu4Oy ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes' the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba2Can-1CunOy series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. T-c decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and J(c) has shown a fish-tail effect. (C) 2003 Elsevier Science B.V. All rights reserved.

149. Preparation of Tl-2212 and Tl-1223 superconductor thin films and their microwave surface resistance

Authors: Sundaresan, A; Asada, H; Crisan, A; Nie, JC; Kito, H; Iyo, A; Tanaka, Y; Kusunoki, M; Ohshima, S

Published: JUN 2003, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 13, 2916, DOI: 10.1109/TASC.2003.812045

Epitaxial Tl2Ba2CaCu2Oy and Tl(Ba,Sr)(2)Ca-2 Cu3Oy thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the TI-2212 films showed well connected and smaller sized grains of 1 mum or less. In the case of TI-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm(2) at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm x 10 mm. The temperature dependence of surface resistance of TI-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the TI-1223 exhibits slightly higher values resulting from an inferior surface morphology.

150. Annealing effect of the irreversibility fields in (Cu,C)Ba2Can-1CunOy (n=3 and 4)

Authors: Hirai, M; Iyo, A; Kito, H; Crisan, A; Tokiwa, K; Watanabe, T; Arai, J; Tanaka, Y

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 428, DOI: 10.1016/S0921-4534(02)02552-2

Annealing effect of the irreversibility fields (H-irr) in (Cu,C)Ba2Ca2Cu3Oy ((Cu,C)-1223) and (Cu,C)Ba2Ca3Cu4Oy ((Cu,C)-1234) have been investigated. The samples prepared under high pressure were post-annealed in nitrogen gas in the temperature range of 400-500degreesC. H-irr of (Cu,C)-1234 was improved after annealing at 450degreesC while T-c remained almost constant. This suggests that carrier density of five-coordinated CuO2 planes or conductivity of charge reservoir layers is changed by annealing while keeping that of four-coordinated CuO2 planes almost constant. Both samples show H-irr similar or equal to 4 T at 90 K by the post-annealing, which is quite attractive for practical application. (C) 2003 Elsevier Science B.V. All rights reserved.

151. Heavy-ion irradiation dependence of the superconducting properties of (Cu,C)Ba2Ca3Cu4O10.5-delta

Authors: Kito, H; Iyo, A; Crisan, AI; Hirai, M; Tokumoto, M; Okayasu, S; Sasase, M; Ihara, H; Tanaka, Y

Published: MAY 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 388, 712, DOI: 10.1016/S0921-4534(02)02488-7

To enhance or improve critical currents density (J(c)) and irreversibility field (H-irr) of (Cu,C)Ba2Ca3Cu4O10.5-delta ((Cu,C)-1234), pinning centers were introduced by heavy-ion irradiation. The polycrystalline samples were irradiated with An(15+) ions (240 MeV energy) at various fluence of 1 x 10(11), 2.5 x 10(11) and 5 x 10(11) ions/cm(2). J(c) and H-irr were determined for the irradiated samples as well as unirradiated samples. J(c) (77 K, 1 T) increase from 3.9 x 10(4) A/cm(2) to 4.1 x 10(6) A/cm(2) for at fluence of 1 x 10(11) ions/cm(2) of heavy-ion irradiated sample and decreases with further increase of fluence. These results indicate the possibility of further enhancement of Jc and of achieving a very high H-irr of (Cu,C)-1234 below a fluence of 1 x 10(11) ions/cm(2). (C) 2003 Elsevier Science B.V. All rights reserved.

152. Fabrication of novel magnetic nanostructures by colloidal bimetallic nanocrystals and multilayers

Authors: Angelakeris, M; Crisan, O; Papaioannou, E; Vouroutzis, N; Tsiaoussis, I; Pavlidou, E; Crisan, AD; Kostic, I; Sobal, N; Giersig, M; Flevaris, NK

Published: DEC 15 2003, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 23, 878, DOI: 10.1016/j.msec.2003.09.147

Recent developments of lithographic techniques as well as improved chemical synthesis methods allow researchers to engineer novel nanostructured materials consisting of arrays of self-organized nanocrystals and multilayers grown as patterns on different substrates. In our case, the magnetic nanostructures consist either of multilayers directly deposited on pre-patterned substrates to form regular arrays of stripes and grooves or colloidal solutions of self-organized bimetallic Ag/Co nanoparticles on patterned and nonpatterned substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed in order to study the surface morphology of the 2D patterning arrays and the 3D nanostructures. The development of periodic arrays of magnetic patterns of micrometer size is strongly dependent on technological parameters such as: film, thickness, distances, size and shape of the patterns. Moreover, it is shown that the substrate morphology significantly affects the colloidal crystallization of magnetic nanoparticles and leads to different growth modes. This will ultimately affect the overall magnetic behavior of the nanostructures. Consequently, the combination of self-assembly and patterning allows for the controlled fabrication of the novel magnetic nanostructures at a macroscopic level and the study of fundamental aspects in magnetism such as quantum tunneling magnetization and magneto-transport properties along well-defined nanosized patterns. (C) 2003 Elsevier B.V. All rights reserved.

153. Magnetization and susceptibility studies on BaZrO3-doped YBa2Cu3O7-(x) bulk superconductors

Authors: Bradea, I; Popa, S; Aldica, G; Mihalache, V; Crisan, A

Published: AUG 2002, JOURNAL OF SUPERCONDUCTIVITY, 15, 242, DOI: 10.1023/A:1019911311340

We studied the YBa2Cu3O7-x bulk superconductor doped with BaZrO3 up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO3 doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO3 does not lead to degradation of the superconducting properties of bulk YBa2Cu3O7-x.

154. Growth of TlBa2Ca2Cu3Oy superconducting thin film on CeO2 buffered sapphire substrate

Authors: Sundaresan, A; Nie, JC; Hirai, M; Crisan, A; Fujiwara, S; Asada, H; Badica, P; Ishiura, Y; Kito, H; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 1286, DOI: 10.1016/S0921-4534(02)01692-1

We report the growth of TlBa2Ca2Cu3Oy superconducting thin film on CeO2 buffered r-cut sapphire substrate by amorphous phase epitaxy method, wherein an amorphous phase of a composition, TlBa2Ca3Cu4Oy was deposited by rf magnetron sputtering and subsequently crystallized by annealing at high temperatures in a closed system. There exists an optimum thickness of about 200 Angstrom of CeO2 buffer layer that gives a smooth buffer layer surface. At the temperature around 840 degreesC we get only Tl-2212 phase and for a longer annealing time it results in Tl-1212 phase. At higher temperatures Ba reacts with Ce to form BaCeO3. To avoid this reaction process we introduced another amorphous buffer layer, TlSr2CaCu2Oy onto CeO2 layer. By this method we could prepare a good quality TI-1223 thin film which showed a T-c of 104 K and a J(c) = 0.3 MA/cm(2) at 77 K and 0.1 T field. (C) 2002 Elsevier Science B.V. All rights reserved.

155. AC susceptibility and higher harmonics studies of heavy-ion irradiated CuBa2Ca3Cu4Oy bulk superconductor with highest irreversibility field above liquid-nitrogen temperature

Authors: Crisan, A; Iyo, A; Kito, H; Tanaka, Y; Hirai, M; Sasase, M; Okayasu, S; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 117, DOI: 10.1016/S0921-4534(02)01393-X

A 50 mum thick slice of (Cu,C)Ba2Ca3Cu4Oy (CU:1234) bulk superconductor grown by high-pressure synthesis technique was irradiated with high-energy (240 MeV) Au15+ ions (10(11) cm(-2)). Complimentary DC magnetization measurements up to 5 T showed an increase in critical current density by two orders of magnitude. AC susceptibility studies were performed in DC magnetic fields up to 14 T and at various temperatures, using various AC field amplitudes (up to 15 Oe) and frequencies (up to 10 kHz). Critical current densities at various temperatures and fields were estimated from critical state model. Using the third harmonic response of the sample we were able to separate different contributions to the dissipation: inter-grain regions, grains without columnar defects (heavy-ions penetration depth is about one quarter of sample's thickness), and grains with columnar defects. In irradiated grains, we could observe a peak-effect in the field dependence of critical current density at high DC fields and temperatures between 85 and 105 K, suggesting the possibility of the highest irreversibility field at liquid nitrogen temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

156. (Cu,Tl)Ba2Ca3Cu4Ox compositions: II. Heating rate applied to synthesis of superconducting ceramics

Authors: Badica, P; Iyo, A; Crisan, A; Ihara, H

Published: JUN 2002, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 15, 982, DOI: 10.1088/0953-2048/15/6/324

Superconducting samples with a starting composition Cu0.5TlxBa2Ca3Cu4O12 (x = 1-0.35) were prepared in Au tubes from oxides by using different heating rates. The optimum heating rate was 0.11 degreesC min(-1) applied between 860 and 880 degreesC. These samples show the highest content of 1234 phase and the lowest of Ba-Cu-O phases as well as the highest critical temperatures. The highest values were T-c = 119.2 K and T-c0 = 115.1 K. All samples show J(c) values around 6 x 10(5) A cm(-2), at 60 K and 0.5 T. In the rest of the samples, heated by using lower or higher heating rates, a high concentration of 1223 and/or 1245 phases is detected. The content of the 1234 phase and of Ba(Ca.Tl)-Cu-O residual liquid phase, in the samples synthesized for the optimum heating rate, depends on the Tl content in the starting mixture. The influence of the oxygen content in the starting mixture on phase composition is not as strong as of Tl, but it has a major role in establishing the superconducting behaviour of the 1234 grains. Some arguments that suggest the dependence of the transport properties of the non-superconducting matrix on oxygen content will be discussed. In our technological arrangement the determined optimum heating rate can be applied to the synthesis of other single-layered superconducting phases, e.g. 1212 phase from a Cu0.25Tl0.75Ba2CaCu2O8.115 starting composition.

157. Decomposition of Ca : Cu=1 : 1 nitrate powder: thermal analysis and structural studies

Authors: Badica, P; Aldica, G; Crisan, A

Published: FEB 1 2002, JOURNAL OF MATERIALS SCIENCE, 37, 594, DOI: 10.1023/A:1013777826319

Thermal decomposition processes in mixtures of Ca : Cu = 1 : 1 nitrate powders produced by a conventional technique and by freeze drying method have been investigated by thermal analysis and X-ray diffraction. Results have been compared with literature values and data obtained by our investigations of individual calcium and copper nitrates. Temperatures at which decomposition processes occur change with by +/-30degreesC and depend also on the level of mixing of the nitrates. An exemption is the temperature for CuO phase formation, which was determined for the Cu-nitrate and for both 1 : 1-nitrate powders to be at 266degreesC. The domains of stability for different phases are shown to be variable. Also, for 1 : 1-powders, alpha-type and beta-type Ca(NO3)(2) . 2H(2)O phases coexist, while in Ca-nitrate only the alpha-type phase was observed. (C) 2002 Kluwer Academic Publishers.

158. The influence of Tl and O content from the starting mixture on phase formation in (Cu,Tl)-1234 system

Authors: Badica, P; Crisan, A; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 687, DOI: 10.1016/S0921-4534(02)01521-6

Superconducting samples have been synthesized from starting compositions CU0.5TlxBa2Ca3Cu4O12+y, x(Tl) = 0.35-1 in the sealed Au-tubes. Starting oxygen content was y = 0 and/or y = +/-0.225. Samples contain non-superconducting phases and only one main superconducting phase, 1234. The energy dispersive spectroscopy grain composition was Tl0.87-1.3Ba1.9-2.05Ca2.55-3.1Cu4Ox (normalized to Cu = 4). The starting Tl and oxygen content do not influence the occurrence of the phases, grain size, morphology and composition. On the other hand the phase content strongly depends on Tl content and almost does not depend on oxygen content. Both Tl and oxygen content are important for the transport properties of the superconducting grains and it seems that also of the non-superconducting matrix. T-c of the samples was in the range 119.1-111.3 K. (C) 2002 Elsevier Science B.V. All rights reserved.

159. (Cu,Tl)Ba2Ca3Cu4Ox compositions: I. The influence of synthesis time and temperature on the phase formation and evaporation-condensation mechanism

Authors: Badica, P; Iyo, A; Crisan, A; Ishiura, Y; Sundaresan, A; Ihara, H

Published: JUN 2002, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 15, 974, DOI: 10.1088/0953-2048/15/6/323

Temperature and time dependence of the phase formation in the (Cu-0.5,Tl-0.5)-1234 composition have been studied. The liquid-vapour deposition mechanism for the formation of 12(n - 1)n superconducting phases is presented. XRD, SEM, EDS and weight loss data are discussed. Samples were superconducting with maximum T-c and J(c) (at 60 K and 0.5 T), determined from M(H) loops of 118 K and 4 x 10(5) A cm(-2), respectively.

160. Strong reduction of thermally activated flux jump rate in superconducting thin films by nanodot-induced pinning centers

Authors: Crisan, A; Badica, P; Fujiwara, S; Nie, JC; Sundaresan, A; Tanaka, Y; Ihara, H

Published: MAY 13 2002, APPLIED PHYSICS LETTERS, 80, 3568, DOI: 10.1063/1.1478782

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices. (C) 2002 American Institute of Physics.

161. Cum-1Bam(Sr,Ca)(n)Cun+1O2m+2n+1 superlattice thin film by layer-by-layer growth techniques

Authors: Nie, JC; Sundaresan, A; Hirai, M; Ishiura, Y; Badica, P; Crisan, A; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 1282, DOI: 10.1016/S0921-4534(02)01690-8

Thin films of Cum-1Bam(Sr,Ca)(n)Cun+1O2m+2n+1, i.e., [BaCuO2](m)/[(Sr,Ca)CuO2](n) superlattices have been grown by layer-by-layer growth method with rf magnetron sputtering on NdGaO3(1 1 0) substrate. Two targets with the compositions BaCuO2 and (Sr,Ca)CuO2, which form the charge reservoir and superconducting layers, respectively, were used. [BaCuO2](m)/[(Sr,Ca)CuO2](n) superlattice was grown at 460-520 degreesC in a gas mixture of Ar and N2O by alternatively depositing the charge reservoir layer and superconducting layer. The superlattice modulation is close to that of high pressure synthesized bulk materials. By means of the layer-by-layer growth method, a system of Cu-based artificial lattice structures is formed. Electrical transport properties of these superlattices were also investigated. (C) 2002 Elsevier Science B.V. All rights reserved.

162. The effect of pinning centers in Zn-doped CuBa2Ca3Cu4O12-y high-temperature superconductors

Authors: Crisan, A; Agarwal, SK; Koganezawa, T; Kuroda, R; Tokiwa, K; Watanabe, T; Iyo, A; Tanaka, Y; Ihara, H

Published: JUN-AUG 2002, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 63, 1076, DOI: 10.1016/S0022-3697(02)00030-6

CuBa2Ca3Cu4O12-y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J(c) of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J(c) of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J(c), we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu: 1234 there is a second critical temperature T-c2 of about 70-80 K (in zero field, and 50-60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO2 plane). (C) 2002 Elsevier Science Ltd. All rights reserved.

163. Third-harmonic susceptibility for studying dissipation in heavy ion irradiated (Cu,C)Ba2Ca3Cu4O12-y high-temperature superconductors

Authors: Crisan, A; Badica, P; Hirai, M; Kito, H; Iyo, A; Tanaka, Y

Published: AUG 2002, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 15, 1243, DOI: 10.1088/0953-2048/15/8/313

A slice of the (Cu,C)Ba2Ca3Cu4O12-y bulk superconductor grown by high-pressure synthesis was polished down to a thickness of 50 mum, and then irradiated with high-energy (240 MeV) Au15+, ions (10 11 cm(-2)). Since the length of the ion tracks was estimated to be 12.2 mum, our sample consists of two different types of grain, with and without columnar defects, having very different critical current densities. Using the third-harmonic response of the sample, we were able to separate the small dissipation in the irradiated grains from the much larger dissipation in the non-irradiated grains, in a certain experimental range of temperature, field and current density. The critical current density was estimated from critical state models. For temperatures in the range of 85-105 K, we observed a peak effect at high dc magnetic fields, suggesting the possibility of a record irreversibility field at 77.3 K.

164. Superconducting properties of the heavy-ions and neutron irradiated (Cu,C)Ba2Can-1CunO2n+4-delta (n-3, 4 and 5)

Authors: Kito, H; Iyo, A; Hirai, M; Crisan, A; Tokumoto, M; Okayasu, S; Sasase, M; Ihara, H

Published: OCT 1 2002, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 378, 332, DOI: 10.1016/S0921-4534(02)01437-5

For further enhancement of critical currents density J(c) and irreversibility field H-irr of (Cu1-xCx) Ba2Can-1CunO2n+4-delta ((Cu,C)-(n - 1)n, n = 3, 4 and 5) family, pinning centers were introduced by ion and neutron irradiation in these compounds. The polycrystalline samples were irradiated with 3.6 x 10(17) neutrons/cm(2) or with Au15+ ions (240 MeV energy) at a fluence of 1 x 10(11) ions/cm(2). J(c) and H-irr were determined for the irradiated samples as well as unirradiated samples. The intragrain J(c) was evaluated from M-H curves using Bean's critical state model. In (Cu,C-12(n - 1) n, J(c) (77 K, 1 T) shows a remarkable increase from 3.9 x 10(4) to 9.1 x 10(5) A/cm(2) for neutron irradiated (Cu,C- 1234 and from 3.9 x 10(4) to 4.1 x 10(6) A/cm(2) for heavy-ion irradiated (Cu,C)-1234. These results indicate the possibility of further enhancement of J(c) and of achieving a very high H-irr of (Cu,C)-1234 in (Cu,C)-12(n - 1)n family. (C) 2002 Elsevier Science B.V. All rights reserved.

165. TlSr2CaCu2Oy template for the growth of superconducting Tl(Ba, Sr)(2)Ca2Cu3Oy thin films on CeO2 buffered sapphire

Authors: Sundaresan, A; Crisan, A; Nie, JC; Hirai, M; Asada, H; Kito, H; Tanaka, Y; Ihara, H

Published: JUN 2002, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 15, 963, DOI: 10.1088/0953-2048/15/6/322

Thin films of Tl(Ba, Sr)(2)Ca2Cu3Oy superconductor have been grown on CeO2 buffered r-cut sapphire substrate with the use of a template TlSr2CaCu2Oy by the amorphous phase epitaxy method. By using this template containing Sr, we have overcome the problem of chemical reactions between Ba and Cc present in the superconducting and buffer layers, respectively, at the normal processing temperatures of 840-890 degreesC. Films grown by this method have an excellent c-axis orientation as well as in-plane alignment with the TlSr2CaCu2Oy template, which in turn is aligned with the CeO2 layers. These films have superconducting critical temperature values up to 104 K and critical current density values as high as 1 MA cm(-2) at 77.3 K and 0 T.

166. Superconducting properties from AC susceptibility and harmonic generation in CuBa2Ca3Cu4Oy bulk superconductors

Authors: Crisan, A; Iyo, A; Tanaka, Y; Hirai, M; Tokumoto, M; Ihara, H

Published: MAY 15 2001, PHYSICA C, 353, 240, DOI: 10.1016/S0921-4534(00)01744-5

We measured for the first time the fundamental and higher harmonic components of AC susceptibility of CuBa2Ca3Cu4Oy superconductor produced by the high-pressure technique, in various experimental conditions (temperature, DC magnetic field, AC held amplitude and frequency). The results are discussed qualitatively in the frame of the critical state and other models in the literature. By using a very simple frequency-dependent critical state model, we estimated the parameters of practical interest, critical current density and pinning potential at various DC fields and temperatures and the irreversibility field. We also extrapolated our estimations to both practical temperature of 77 K and longer time scale as of DC magnetization and transport measurements. The results appeared to be quite self-consistent. (C) 2001 Published by Elsevier Science B.V.

167. The influence of BaZrO3 on the transport properties of the (Bi,Pb)2223 superconducting system

Authors: Mihalache, V; Aldica, G; Crisan, A

Published: MAY 2001, JOURNAL OF MATERIALS SCIENCE LETTERS, 20, 891, DOI: 10.1023/A:1010912429361

168. Some superconducting properties of the inter-domain border of melt-textured YBa2Cu3O7-delta

Authors: Crisan, A; Gordeev, SN; de Groot, PAJ; Beduz, C

Published: JUN 15 2001, PHYSICA C, 355, 237, DOI: 10.1016/S0921-4534(01)00023-5

The resistive transitions R(T) and the current-voltage (I-V) in fields up to 5 T were measured in a melt-textured YBa2Cu7-delta (YBCO) sample consisting of two single-grain domains separated by an inter-domain border, and in a single-grain sample cut from the same pellet and oxygenated in such conditions that ensure the optimum oxygenation of the grains. From comparing the two sets of results, we were able to suggest that the inter-domain border consists of oxygen-deficient YBCO with thickness smaller than 6 mum, having a slightly lower critical temperature, a higher anisotropy factor and without strong pinning centers. From applications point of view, our work shows that, for improving the superconducting properties of large multiple-domain melt-textured pellets. an oxygenation process much longer than that required for optimum grain properties is needed. (C) 2001 Published by Elsevier Science B.V.

169. The influence of BaZrO3 on the magnetic response of (Bi,Pb): 2223 high-temperature superconductors

Authors: Mihalache, V; Aldica, G; Popa, S; Nita, P; Crisan, A

Published: JUN 2001, JOURNAL OF SUPERCONDUCTIVITY, 14, 386, DOI: 10.1023/A:1011174321955

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO3 are presented. The results are discussed in the frame of the critical state model. and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system. BaZrO3 impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.

170. Sputtered nanodots: A costless method for inducing effective pinning centers in superconducting thin films

Authors: Crisan, A; Fujiwara, S; Nie, JC; Sundaresan, A; Ihara, H

Published: DEC 31 2001, APPLIED PHYSICS LETTERS, 79, 4549, DOI: 10.1063/1.1428632

A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3-5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude. (C) 2001 American Institute of Physics.

171. Fluctuation magnetoconductivity of BSCCO-2212 films in parallel magnetic field

Authors: Balestrino, G; Crisan, A; Livanov, DV; Manokhin, SI; Milani, E

Published: JUN 1 2001, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 355, 139, DOI: 10.1016/S0921-4534(00)01771-8

The in-plane (delta sigma (ab)(T,H)) and out-of-plane (delta sigma (c)(T,H)) components of the magnetoconductivity tenser of Bi2Sr2CaCu2O8+x films have been simultaneously measured in magnetic fields parallel to the film conducting planes (B less than or equal to 1 T). A negative c-axis magnetoresistance is measured for the first time in this geometry. Both delta sigma (ab) and delta sigma (c) are well described by the fluctuation theory in the temperature range T-c < T < T-c + 20 K, using a single set of microscopical parameters for both directions. The negative c-axis magnetoresistance in the field parallel to conducting layers is shown to be induced by the density of states Zeeman fluctuation contribution. (C) 2001 Elsevier Science B.V. All rights reserved.

172. Improvement of the superconducting transport properties of YBa2Cu3O7-delta by BaZrO3 doping

Authors: Aldica, G; Bradea, I; Jaklovszky, J; Crisan, A

Published: JUN 2001, JOURNAL OF SUPERCONDUCTIVITY, 14, 409, DOI: 10.1023/A:1011186724681

We have doped the YBa2Cu3O7-delta,S superconducting ceramics with BaZrO3 up to 75 wt.% and studied the changes of some physical properties. The most important finding is the enhancement of the critical current density, which has a maximum at around 5 wt.% doping level. Compared with the undoped samples, the critical current density is four times higher if the doping compound is introduced prior to the calcination treatment, or two times higher if BaZrO3 is introduced prior to the sintering treatment, The dependence of the critical current density on the doping level is consistent with the dependence of the Seebeck coefficient. We also observed a very small decrease of the critical temperature. The real density of the samples decreases with increasing doping level as expected from the ratio between the theoretical densities of YBa2Cu3O7-delta and BaZrO3, the effect being important for doping levels higher than 25 wt.%.

173. [(BaCuO2)(n)/(CaCuO2)(m)](N) artificial superconducting superlattices

Authors: Crisan, A; Medaglia, PG; Balestrino, G

Published: SEP 2000, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 2, 234, DOI:

In this review are presented the main results of a complex research project devoted to preparation and investigations of physical properties of [(BaCuO2)(n)/(CaCuO2)(m)](N) artificial superconducting superlattices. The paper starts with the layer-by-layer pulsed laser deposition technique developed for this project and discuss the structure of this new material. The superconducting properties and the effects of structural disorder on them are also presented.

174. Synthesis of the Ca0.45Cu0.55O peritectic phase using a mixture of nitrate powders

Authors: Badica, P; Aldica, G; Alexe, G; Crisan, A

Published: APR 2000, MATERIALS LETTERS, 43, 184, DOI: 10.1016/S0167-577X(99)00256-6

The Ca0.45CU0.55O phase was successfully produced from a mixture of Ca and Cu nitrate powders with cation ratio Ca:Cu = 1:1. The heat treatment for decomposition and the synthesis of the indicated phase were pet-formed at 710 degrees C in air and flowing oxygen, respectively. The Ca0.45Cu0.55O phase concentration increases with synthesis time. After 209 h of heat treatment, the concentration of the Ca0.45Cu0.55O phase determined by X-ray diffraction (XRD) analysis has reached 87% and the kinetic data have indicated that the material enters the saturation stage of the reaction, The secondary phases are CaO and CuO. (C) 2000 Elsevier Science B.V. All rights reserved.

175. Transport measurements, vortex dynamics and anisotropy in artificially layered (BaCuO2)(2/)(CaCuO2)(2) superconducting films

Authors: Crisan, A; Balestrino, G; Lavanga, S; Medaglia, PG; Milani, E

Published: FEB 10 1999, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 313, 78, DOI: 10.1016/S0921-4534(99)00006-4

In this paper, we present some transport measurements on artificially layered (BaCuO2)(2)/(CaCuO2)(2) superlattices, engineered by the pulsed laser deposition (PLD) technique, and having critical temperatures (zero resistance) of about 80 K. The experimental current-voltage characteristics are discussed in the frame of current-induced unbinding of thermally created vortex-antivortex pairs (in zero-field) and in the frame of vortex glass (VG) transition for fields up to about 10 kG. The transport critical current density is estimated and compared to values reported from magnetisation measurements. The field dependence of the activation energy of vortices in the vortex liquid (VL) phase shows a strong effective pinning, due to the high disorder intrinsic to these artificially layered superconductors, at low magnetic fields, and a transition to a thermally activated plastic motion of an entangled VL in higher fields. From the temperature and field orientation dependencies of the vortex melting field, an anisotropy factor of about 20 is estimated for these superlattices. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

176. Investigations of the zero-field (a,b)-plane conductivity of YBa2Cu3O7-delta near the critical temperature

Authors: Crisan, A; Gordeev, SN; Manton, S; Rassau, AP; Popa, S; Beduz, C; de Groot, PAJ; Gagnon, R; Taillefer, L

Published: DEC 1 1998, PHYSICA C, 309, 7, DOI: 10.1016/S0921-4534(98)00578-4

The resistive transition R(T) and the current-voltage characteristics (I-V) in the range 10(-9)-10(-6) V and close to the critical temperature (T-c) were measured in a single-grain of melt-textured YBa2Cu3O7-delta (YBCO) and in a detwinned YBCO single-crystal, with the transport current parallel to the (a,b) planes and in zero external magnetic field using a SQUID picovoltmeter. In the case of the melt-textured sample, we measured the I-V curves also at higher levels of dissipation. For T < T-c, we interpreted the lower part of the I-V curves in the framework of the Jensen-Minnhagen model of current-induced unbinding of thermally-created vortex-antivortex pairs, which leads to V proportional to I(I-I-c1)(a-1). Our results confirm a recently proposed current-temperature (I-T) phase diagram by S.W. Pierson. In addition, we determined for the first time the dependence of the characteristic (unbinding) current I-c1 with t = 1 - T/T-c for temperatures very close to T-c and low transport currents. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

177. Current-voltage characteristics of long Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes at different bending strains

Authors: AlMosawi, MK; Crisan, A; Beduz, C; Phillips, D; Haldar, P

Published: SEP 15 1997, PHYSICA C, 289, 69, DOI: 10.1016/S0921-4534(97)01587-6

The current-voltage (I-V) characteristics of long Bi2Sr2Ca2Cu3O10/Ag (Bi:2223) multifilamentary tapes with different bending strains were measured in a large range of dissipation (10(-12)-10(-3) V/cm). The experimental data were successfully interpreted in terms of current-induced unbinding of the thermally-created vortex-antivortex pairs by taking into account the size limitation of vortex fluctuations. It was found that the shape of the I-V curves can be explained by the contributions to the dissipation of superconducting intergrain contacts having different qualities (i.e. different critical currents) and by the appearance of micro-cracks during the bending process. (C) 1997 Elsevier Science B.V.

178. Field and temperature dependence of the current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes

Authors: Crisan, A; Miu, L; Popa, S; Beduz, C

Published: JUN 1997, JOURNAL OF SUPERCONDUCTIVITY, 10, 219, DOI: 10.1007/BF02770554

The current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.

179. Current-voltage characteristics of Bi2Sr2Ca2Cu3O10+x/Ag multifilamentary tapes in zero applied magnetic field

Authors: Crisan, A; Miu, L; Popa, S; Yang, Y; Beduz, C

Published: MAY 1997, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 10, 303, DOI: 10.1088/0953-2048/10/5/006

Current-voltage characteristics of multifilamentary Bi2Sr2Ca2Cu3O10/Ag tapes (short samples) produced by the 'powder in tube' technique were measured at different temperatures close to the mean-field critical temperature, and in zero applied magnetic field. After performing the required corrections due to the current flowing in the silver matrix, the I-V curves were interpreted in terms of current-induced unbinding of the thermally created vortex-antivortex pairs. Two possible mechanisms for appearance of a finite critical current in zero applied magnetic field are discussed: the Jensen-Minnhagen quasi-two-dimensional (2D) approach, that takes into account the interlayer Josephson coupling, and a model of size limitation of vortex fluctuations. From our analysis, it seems that the latter model is more suitable for this kind of superconducting material, due probably to an accentuated intrinsic anisotropy.

180. The influence of the structure on the electrical and magnetic properties of Bi2Sr2CaCu2O8+delta single crystals

Authors: Aldica, G; Crisan, A; VelterStefanescu, M; Mandache, S; Bunescu, MC

Published: MAR 1 1997, JOURNAL OF MATERIALS SCIENCE, 32, 1199, DOI: 10.1023/A:1018579817471

The correlation between the structure, chemical composition, electrical and magnetic properties of Bi2Sr2CaCu2O8+delta single crystals is presented. As revealed by SEM measurements, the small samples (approximately 1 mm x 1 mm x 0.2 mm) are stacks of thin plates (1-2 mu m). Microcompositional measurements (energy dispersive spectroscopy) show the Bi2Sr2CaCu2O8+delta (2212)-phase and a small quantity of residual phases enriched in bismuth and strontium and calcium-deficient. Magnetically modulated microwave absorption measurements (MAMMA) lead to a value for the anisotropy of the upper critical field of about 7, which is more than twice the reported value for YBa2Cu3O7-delta single crystals (also determined from MAMMA responses). The plot of the resistance versus temperature suggests that only the first 10-20 plates from the surface of the sample have a contribution to the sample conductivity. The imaginary part, chi'', of the magnetic susceptibility of our samples shows three peaks.

181. Magnetic ac susceptibility in Bi2Sr2CaCu2O8 single crystals

Authors: Mandache, S; Crisan, A; Aldica, G; Popa, S

Published: JUN 1997, JOURNAL OF SUPERCONDUCTIVITY, 10, 214, DOI: 10.1007/BF02770553

We measured the temperature dependence of the ac susceptibility in Bi2Sr2CaCu2O8 single crystals. The penetration field H* was determined, at different temperatures, from the field dependence of chi '' in the framework of the critical state model. Using a very simple formula, we estimated the values of the critical current density near T-c, which are consistent with the usually reported data for this material.

182. A new approach for the growth of high-T-c superconductors

Authors: Jaklovszky, J; Aldica, G; Rusu, C; Crisan, A

Published: APR 1996, APPLIED SUPERCONDUCTIVITY, 4, 194, DOI: 10.1016/S0964-1807(96)00023-3

In this paper we describe a modified liquid phase separation (double crucible) method for growth of superconducting Bi-Sr-Ca-Cu-O single crystals. We used alumina crucibles. The morphology and composition of as-grown crystals were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The stacking thin-plate single crystals of maximum 5 x 6 x 0.1 mm(3) were obtained. (C) 1997 Elsevier Science Ltd. All rights reserved.

183. Temperature-dependent collective pinning exponent in Bi2Sr2Ca2Cu3O10+x tapes and bulk samples with preferential crystallite orientation

Authors: Crisan, A; Popa, S; Aldica, G; Jaklovszky, J

Published: JUN 1996, JOURNAL OF SUPERCONDUCTIVITY, 9, 291, DOI: 10.1007/BF00727550

The low-voltage-level current-voltage characteristics of Bi2Sr2Ca2Cu3O10+x tapes and bulk samples with preferential crystallite orientation were measured in applied magnetic fields both parallel and perpendicular to the samples' surfaces. The data were interpreted within the framework of the collective pinning model. The collective pinning exponent alpha changes from approximate to 1 to approximate to 0.5 (close to T-c), while in the case of unoriented samples it is constant (and equal to the theoretical value 1/2) as shown in an earlier paper. We suggest that this temperature dependence of alpha is due to the change of the dimensionality of the intragranular vortices.

184. CURRENT-VOLTAGE CHARACTERISTICS OF HIGH-TEMPERATURE SUPERCONDUCTING CERAMICS WITH THE PEAK EFFECT IN THE MAGNETIC-FIELD DEPENDENCE OF THE TRANSPORT CRITICAL-CURRENT DENSITY

Authors: CRISAN, A

Published: APR 1995, JOURNAL OF SUPERCONDUCTIVITY, 8, 320, DOI: 10.1007/BF00732386

The experimentally determined current-voltage characteristics of high-T-c superconducting ceramics exhibiting the ''peak-effect'' in the magnetic field dependence of the transport critical-current density were described in terms of thermally activated flux creep at grain boundaries; taking into account the collective pinning of intergranular vortices. The peak effect was explained by considering the increase of the intergrain pinning potential at intermediate fields through the interaction of intergranular (Josephson type) vortices and intragranular (Abrikosov) vortices. The magnetic field dependences of the effective pinning potential and of the collective pinning exponent were experimentally determined, and the features of the I-V curves were explained through these dependences.

185. ON A MODIFIED KIM ANDERSON MODEL OF THE CURRENT-VOLTAGE CHARACTERISTICS OF HIGH-TEMPERATURE SUPERCONDUCTING CERAMICS

Authors: CRISAN, A

Published: AUG 1994, JOURNAL OF SUPERCONDUCTIVITY, 7, 691, DOI: 10.1007/BF00721648

In comparison with the ''classical'' Kim-Anderson model for the current-voltage characteristics of type-II superconductors, another model with a less sharp pinning potential is presented. In addition to the approximate solution, for currents close to the critical one, a complete solution of this model is derived, which better fits the experimentally obtained current-voltage characteristics.

186. DISSIPATION IN CURRENT-CARRYING HIGH-T(C) SUPERCONDUCTING CERAMICS IN APPLIED MAGNETIC-FIELD

Authors: MIU, L; POPA, S; CRISAN, A; ALDICA, G; JAKLOVSZKY, J

Published: AUG 1993, JOURNAL OF SUPERCONDUCTIVITY, 6, 284, DOI: 10.1007/BF00626270

The dissipation mechanism in high-T(c) superconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.

187. ON THE LIMITING FACTORS OF THE CRITICAL CURRENT-DENSITY IN HIGH-TC SUPERCONDUCTING CERAMICS

Authors: MIU, L; CRISAN, A; POPA, S; SANDU, V; NISTOR, L

Published: DEC 1990, JOURNAL OF SUPERCONDUCTIVITY, 3, 394, DOI: 10.1007/BF00617464

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa2Cu3O7-y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.

188. TEA-CO2 LASER-DEPOSITED YBA2CU3O7 SUPERCONDUCTING THIN-FILMS

Authors: SANDU, V; CRISAN, A; GRIGORESCU, D; MIU, L; CHIS, I; ALDICA, G; CRUCEANU, E; GRIGORIU, C; DRAGULINESCU, D

Published: MAY 1989, JOURNAL OF MATERIALS SCIENCE LETTERS, 8, 510, DOI: 10.1007/BF00720279

Dr. Alina CRISAN

Scientific Researcher II

Publications

1. High Magnetic Performance in MnGa Nanocomposite Magnets

2. Lithographically Ordered FePt L10 Dots with High Coercivity for Logic-Conditioned Magnetic Nanostructures

3. Magnetic Memory Effects in BaFe2(As0.68P0.32)2 Superconducting Single Crystal

4. Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa2Cu3O7-x Superconducting Film

5. Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa2Cu3O7-x Superconducting Film (vol 14, 4379, 2024)

6. Vortex Glass-Vortex Liquid Transition in BaFe2(As1-xPx)(2) and CaKFe4As4 Superconductors from Multi-Harmonic AC Magnetic Susceptibility Studies

7. Novel Rare Earth (RE)-Free Nanocomposite Magnets Derived from L1(0)-Phase Systems

8. Measurements of Surface Impedance in MgB2 in DC Magnetic Fields: Insights in Flux-Flow Resistivity

9. Vortex Glass-Vortex Liquid Transition in BaFe2(As1-xPx)2 and CaKFe4As4 Superconductors from Multi-Harmonic AC Magnetic Susceptibility Studies

10. Novel Rare Earth (RE)-Free Nanocomposite Magnets Derived from L10-Phase Systems

11. Remarkable Magnetic Properties in a Mn73.6Ga26.4 Alloy Produced via Out-of-Equilibrium Method

12. Highly Coercive L10-Phase Dots Obtained through Low Temperature Annealing for Nano-Logic Magnetic Structures

13. Vortex Dynamics and Pinning in CaKFe4As4 Single Crystals from DC Magnetization Relaxation and AC Susceptibility

14. Measurements of Surface Impedance in MgB2 in DC Magnetic Fields: Insights in Flux-Flow Resistivity

15. The Depairing Current Density of a Fe(Se,Te) Crystal Evaluated in Presence of Demagnetizing Factors

16. Ferromagnetism and Superconductivity in CaRuO3/YBa2Cu3O7-delta Heterostructures

17. Morpho-Structural Investigations and Carbon Nanoclustering Effects in Cr-Al-C Intermetallic Alloys

18. Pinning Potential of the Self-Assembled Artificial Pinning Centers in Nanostructured YBa2Cu3O7-x Superconducting Films

19. Thermal Stability, Blocking Regime and Superparamagnetic Behavior in Mn-Al-C Melt Spun Ribbons

20. Role of Disordered Precursor in L1(0) Phase Formation in FePt-Based Nanocomposite Magnet

21. New superconductor/ferromagnet heterostructure formed by YBa2Cu3O7-x and CaRuO3

22. Magnetism and epsilon-tau Phase Transformation in MnAl-Based Nanocomposite Magnets

23. EFFECT OF Mn SUBSTITUTION ON THE STRUCTURAL, MAGNETIC AND MAGNETOSTRICTIVE PROPERTIES OF Fe-Pd FERROMAGNETIC SHAPE MEMORY RIBBONS

24. Transport, AC susceptibility, DC magnetization, and magnetic relaxation studies of YBa2Cu3O7 films with synergetic pinning centers grown by PLD

25. Hybrid nanoelectronic-magnetic device with magnetoresistive core-shell Fe/FeC nanoparticles

26. Magnetic Phase Coexistence and Hard-Soft Exchange Coupling in FePt Nanocomposite Magnets

27. Second magnetization peak, rhombic-to-square Bragg vortex glass transition, and intersecting magnetic hysteresis curves in overdoped BaFe2(As1-xPx)(2) single crystals

28. Mn-Induced Thermal Stability of L1(0)Phase in Fept Magnetic Nanoscale Ribbons

29. Microwave investigation of pinning in Te- and cubic-BN- added MgB2

30. Structure and magnetic properties of highly coercive L1(0) nanocomposite FeMnPt thin films

31. Gd3+ co-doping influence on the morphological, up-conversion luminescence and magnetic properties of LiYF4:Yb3+/Er3+ nanocrystals

32. Vortex dynamics driven by AC magnetic field in YBCO thin films with complex pinning structures

33. Pinning-Induced Vortex-System Disordering at the Origin of the Second Magnetization Peak in Superconducting Single Crystals

34. LOW-TEMPERATURE FORMATION OF 312 PHASE IN Ti-Si-C TERNARY COMPOUND

35. Annealing-Induced High Ordering and Coercivity in Novel L1(0) CoPt-Based Nanocomposite Magnets

36. AC magnetic response of superconducting single crystals exhibiting a second peak on the DC magnetization curves

37. Thermodynamic, structural and magnetic studies of phase transformations in MnAl nanocomposite alloys

38. Incipient low-temperature formation of MAX phase in Cr-Al-C films

39. Flux Channeling in YBa2Cu3O7 Superlattices

40. Synergetic pinning centres in BaZrO3-doped YBa2Cu3O7-x films induced by SrTiO3 nanolayers

41. Vortex-glass state in the isovalent optimally doped pnictide superconductor BaFe2(As0.68P0.32)(2)

42. Nano-engineered pinning centres in YBCO superconducting films

43. Behavior of the Second Magnetization Peak in Self-nanostructured La2-xSrxCuO4 Single Crystals

44. Specific Changes in the Magnetoresistance of Ni-Fe-Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions

45. ELASTIC PROPERTIES AND CRYSTALLIZATION OF THERMAL BARRIER COATINGS OF Cr2AlC

46. Pinning-Engineered YBa2Cu3Ox Thin Films

47. Vortices and Nanostructured Superconductors Preface

48. Interfacial mechanisms of novel laser-irradiated L1(0)-based nanocomposite magnets

49. STRUCTURAL AND MAGNETIC PROPERTIES OF Sr2FeMoO6 OBTAINED AT LOW TEMPERATURES

50. Columnar Growth of Transparent Semiconducting Films: Learning from Nature

51. Development and structural characterization of exchange-spring-like nanomagnets in (Fe,Co)-Pt bulk nanocrystalline alloys

52. Magnetoelastic properties in polycrystalline ferromagnetic shape memory Heusler alloys

53. Effect of thermal treatments in Ni-Fe-Ga with Co substitutions and Ni-Mn-Ga melt spun ribbons

54. Magnetic and Martensitic Transformations in the bulk and melt spun ribbons of Ni57-xNdxFe18Ga25 Ferromagnetic Shape Memory Alloys

55. High-Field Pinning Potential in YBCO Films with Nanoengineered Pinning Centres

56. Effect of Mn addition on the thermal stability and magnetic properties of rapidly-quenched L1(0) FePt alloys

57. Thermal stability, thermal expansion and grain-growth in exchange-coupled Fe-Pt-Ag-B bulk nanocomposite magnets

58. Effect of thermal treatments on the structural and magnetic transitions in melt-spun Ni-Fe-Ga-(Co) ribbons

59. Magnetocaloric effect in Ni-Fe-Ga Heusler alloys with Co and Al substitutions

60. Role of Ag addition in L1(0) ordering of FePt-based nanocomposite magnets

61. Investigation of the bulk pinning force in YBCO superconducting films with nano-engineered pinning centres

62. Magnetization Relaxation in Superconducting YBa2Cu3O7 Films with Embedded Nanorods and Nanoparticles

63. Evolution of vortex dynamics in YBa2Cu3O7 films with nanorods by adding nanoparticles

64. Cobalt chromite obtained by thermal decomposition of oxalate coordination compounds

65. Interpretation of Abnormal AC Loss Peak Based on Vortex-Molecule Model for a Multicomponent Cuprate Superconductor (vol 46, pg 134, 2007)

66. Anomalous AC Susceptibility Response of (Cu,C)Ba2Ca2Cu3Oy: Experimental Indication of Two-Component Vortex Matter in Multi-Layered Cuprate Superconductors (vol 46, pg L451, 2007)

67. Inhibition of the detrimental double vortex-kink formation in thick YBa2Cu3O7 films with BaZrO3 nanorods

68. High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

69. AFM studies of metallic and organic nanosized structures ion beam sputtered onto functionalised surfaces

70. Improved Critical Current Densities in YBa2Cu3O7-delta Multilayer Films Interspaced with Palladium Nano-dots

71. Intermittent trapping of a liquid-like vortex state visualized by scanning Hall probe microscopy

72. Domains in multiband superconductors

73. Combination of Ag Substrate Decoration with Introduction of BaZrO3 Nano-Inclusions for Enhancing Critical Current Density of YBa2Cu3O7 Films

74. Martensitic transformation of Ti50Ni30Cu20 alloy prepared by powder metallurgy

75. Direct formation of L1(0) FePt in as-cast FePt-based magnetic nanocomposite ribbons without post-synthesis annealing

76. Synergetic Pinning Centers in YBa2Cu3OX Films Through a Combination of Ag Nano-Dot Substrate Decoration, Ag/YBCO Quasi-Multilayers, and the Use of BaZrO3-Doped Target

77. Exotic Vortex Matter: Pancake Vortex Molecules and Fractional-Flux Molecules in Some Exotic and/or Two-Component Superconductors

2. Lithographically Ordered FePt L1₀ Dots with High Coercivity for Logic-Conditioned Magnetic Nanostructures

3. Magnetic Memory Effects in BaFe₂(As_0.68P_0.32)₂ Superconducting Single Crystal

4. Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa₂Cu₃O_7-x Superconducting Film

5. Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa₂Cu₃O_7-x Superconducting Film (vol 14, 4379, 2024)

9. Vortex Glass-Vortex Liquid Transition in BaFe₂(As_1-xP_x)₂ and CaKFe₄As₄ Superconductors from Multi-Harmonic AC Magnetic Susceptibility Studies

10. Novel Rare Earth (RE)-Free Nanocomposite Magnets Derived from L1₀-Phase Systems

11. Remarkable Magnetic Properties in a Mn_73.6Ga_26.4 Alloy Produced via Out-of-Equilibrium Method

12. Highly Coercive L1₀-Phase Dots Obtained through Low Temperature Annealing for Nano-Logic Magnetic Structures

13. Vortex Dynamics and Pinning in CaKFe₄As₄ Single Crystals from DC Magnetization Relaxation and AC Susceptibility

14. Measurements of Surface Impedance in MgB₂ in DC Magnetic Fields: Insights in Flux-Flow Resistivity