NIMP| Alina Marinela BADEA (IONESCU), Author at NIMP

Education

2015-2018: PhD student at University of Bucharest, Faculty of Physics, Field of study: Condensed matter physics

2013-2015: Master Degree in Physics at University of Bucharest, Faculty of Physics, Field of study: Advanced materials physics and nanostructures

2010 - 2013: Bachelor Degree in Physics at University of Craiova, Faculty of Exact Science, Field of study: Physics Informatics

Positions

2018-present: Scientific researcher at National Institute of Materials Physics, Magurele, Romania

2019 - 2020: Post doctoral fellowship at Max Planck Institute for Intelligent Systems, Stuttgart, Germany

2013 – 2018: Research assistant at National Institute of Materials Physics, Magurele, Romania

Research interests

Vortex dynamics and magnetic characterization of different types of superconductors such as REBCO thin films, MgB2 bulk samples obtained by Spark Plasma Sintering, iron-based single crystals and YBCO/ferromagnet heterostructures.

Expertize

Magnetic measurements at SQUID

Deposition of thin films by PLD

X-ray diffraction measurements

X-ray microscopy at low temperatures

Data processing, interpretation of magnetization curves, determination of the critical current density.

Data processing, interpretation of magnetic relaxation curves.

Books

Miu, L, Ivan, I, Ionescu AM, Crisan, A, Miu, D, Petrisor, T, Mele, P, “High Vortex Activation Energies in the AC Magnetic Response of Superconductors Close to the DC Irreversibility Line”, in Superconductivity: From Materials Science to Practical Applications, Springer (2020)

Badica, P, Aldica, G, Burdusel, M, Grigoroscuta, M, Ionescu, AM, Sandu, V, Popa, S, Enculescu, M, Pasuk, I, Kuncser, A, “Control of the Critical Current Density Through Microstructural Design by Ho2O3 and Te Co-addition into MgB2 Processed by Ex Situ Spark Plasma Sintering”, in Superconductivity: From Materials Science to Practical Applications, Springer (2020)

Miu, L, Ionescu, AM, Miu, D, Ivan, I, Crisan, A, “Behaviour of the second magnetization peak in self-nanostructured La2-xSrxCuO4 single crystals”, in Vortices and Nanostructured Superconductors, Springer (2017)

Badica, P, Aldica, G, Ionescu, AM, Burdusel, M, Batalu, D, „The Influence of Different Additives on MgB2 Superconductor Obtained by Ex Situ Spark Plasma Sintering: Pinning ForceAspects”, in Correlated Functional Oxides, Springer (2017)

Miu, L, Mele, P, Ivan, I, Ionescu, AM, Crisan, A, Badica, P, Miu, D, “Magnetization relaxation in superconducting YBa2Cu3O7 films with embedded nanorods and nanoparticles”, in Size Effects In Nanostructures, Springer (2014)

Patents

Acoperiri supraconductoare de YBa2Cu3O7-x cu centri de fixare artificiali cu structura mixta si procedeu de obtinere, Ion Ivan; Gheorghe Virgil Aldica; Enculescu Monica; Miu Lucica; Ionescu Alina- Marinela; Ioan Adrian Crisan, A000745/15.11.2019

Metoda de procesare in mediu activ de sinterizare si banda/fir supraconductoare in teaca metalica usoara cu miez pe baza de MgB2, Badica Petre; Ionescu Alina Marinela; Grigoroscuta Mihai Alexandru; Burdusel Mihail; Aldica Gheorghe Virgil A00351/11.06.2019

Publications

1. 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.

2. 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.

3. 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

4. Influence of on-off pulsed current pattern on processes during spark plasma sintering of MgB2 superconductor

Authors: Aldica, G; Sandu, V; Popa, S; Pasuk, I; Enculescu, M; Ionescu, AM; Badica, P

Published: DEC 2023, OPEN CERAMICS, 16, 100441, DOI: 10.1016/j.oceram.2023.100441

High density samples (92-94.5 %) of MgB2 were prepared by Spark Plasma Sintering (SPS). The on-off pulsed current patterns of SPS processing were 8-4, 12-2, 24-2, 99-1. Patterns with more on pulses favor formation of a higher amount of the secondary MgB4 phase through the decomposition of MgB2. They also promote enhancement of larger MgO crystallites without a strong increase in the amount of this phase. Densification rate and pressure in the SPS chamber show a similar behavior, but their amplitude varies and the temperatures defining different stages present some shifts. As-revealed differences induced by pulsed patterns impact superconducting properties in a complex manner. An attempt to assess correlations between the pattern and different superconducting parameters is presented.

5. 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.

6. Bulk MgB2 superconductor for levitation applications fabricated with boron processed by different routes

Authors: Savaskan, B; Ozturk, UK; Guner, SB; Abdioglu, M; Bahadir, MV; Acar, S; Somer, M; Ionescu, AM; Locovei, C; Enculescu, M; Badica, P

Published: OCT 25 2023, JOURNAL OF ALLOYS AND COMPOUNDS, 961, 170893, DOI: 10.1016/j.jallcom.2023.170893

Bulk MgB2 discs were prepared by an in situ route from mixtures of magnesium and boron powders. The boron powders were produced by two methods. The first one consisted of a self-propagating high tem-perature magnesiothermic synthesis (SHS) process followed by acid and fluorine cleaning and a heat treatment in inert atmosphere. This approach produced boron with purities between 86 % and 97 %, where the main impurity was Mg. Depending on the final heat treatment, these boron powders were amorphous or crystalline. In the second route, high purity nano powders (99 %) of boron were obtained by a diborane pyrolysis process. Bulks of MgB2 were characterized by structural, microstructural, and magnetic mea-surements. Critical current density, pinning force aspects and levitation force (including guiding force) details were assessed. Amorphous lower purity boron (86-97 %) obtained by the first processing route was found to promote the largest levitation forces of the MgB2 bulks and, among these samples, the best le-vitation results were recorded when using boron with a purity of 95-97 %. Use of a lower purity boron that decreases the cost of MgB2 promotes large scale production at industrial level of bulk MgB2 super-conducting magnets for levitation applications and enhances the applicability potential of MgB2 super-conductor. The relationship between levitation force and specific features of the samples such as pinning force details are discussed.& COPY; 2023 Elsevier B.V. All rights reserved.

7. 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.

8. MgB2 with Addition of Cubic BN and Ge2C6H10O7 Obtained by Spark Plasma Sintering Technique

Authors: Ionescu, AM; Aldica, G; Popa, S; Enculescu, M; Sandu, V; Pasuk, I; Burdusel, M; Grigoroscuta, MA; Miu, L; Badica, P

Published: JUL 2022, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, DOI: 10.1007/s10948-022-06350-1

High density (94-98% of the theoretical density) MgB2 samples added with C6H10Ge2O7 and cubic BN with compositions (MgB2)(1-x)(Ge2C6H10O7)(0.0028)(cBN)(x) (x = 0.003, 0.005, 0.007, 0.01) and (MgB2)(1-y)(Ge2C6H10O7)(y)(cBN)(0.005) (y = 0.0014, 0.0028, 0.005, 0.0075) were obtained by spark plasma sintering technique. For optimum doped samples with x = 0.005-0.007 and y = 0.0028-0.005, a weak enhancement of zero-field critical current density J(c0), irreversibility field H-irr, and volume pinning force F-p,F-max was determined. This behavior is very different from similar samples added with a single additive for which H-irr has a large enhancement. Consequently, it suggests the presence of opposite structural and microstructural effects induced by the additives. These effects, on the one hand, are discussed to decrease the sensitivity of MgB2 superconducting properties in the co-added samples comparative to samples added with C6H10Ge2O7, and, on the other hand, they contribute to anomalies that were found when assessing the pinning force-related parameters by the universal scaling law.

9. Full Length Article Towards high degree of c-axis orientation in MgB2 bulks

Authors: Grigoroscuta, MA; Aldica, GV; Burdusel, M; Sandu, V; Kuncser, A; Pasuk, I; Ionescu, AM; Suzuki, TS; Vasylkiv, O; Badica, P

Published: AUG 2022, JOURNAL OF MAGNESIUM AND ALLOYS, 10, DOI: 10.1016/j.jma.2021.10.013

The paper presents fabrication and characterization of spark plasma sintered textured (001) MgB2 with a record degree of orientation of about 40% and 16% by high-energy ultra-sonication and slip casting in high magnetic field (12 T) and 0 T magnetic field, respectively. Structural characterization was performed by X-ray diffraction, and electron microscopy. The analysis revealed unexpected preferred orientation also in the MgO secondary phase due to the epitaxial growth of (111) MgO on (001) MgB2. The influence of oriented microstructure on the superconducting characteristics expressed by critical current density (Jc), irreversibility field ( H irr), and on the pinning properties were assessed. High anisotropy versus sample orientation in applied magnetic field, H , was observed for Jc, Hirr, pinning activation energy ( U *) extracted from relaxation measurements. The zero-field critical current, Jc0 and Fp,maxare weakly or not dependent on the direction of H , while the other indicated parameters are significantly influenced. Results enable control of superconducting parameters by further optimization of microstructure through MgB2 texturing as a novel and viable strategy for development of bulk MgB2 with enhanced properties when taking advantage of its anisotropy.(c) 2021 Chongqing University. Publishing services provided by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) Peer review under responsibility of Chongqing University

10. 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.

11. Transient increase of T-c and J(c) in superconducting/metallic heterostructures

Authors: Ionescu, AM; Bihler, M; Simmendinger, J; Miksch, C; Fischer, P; Cristiani, G; Rabinovich, KS; Schutz, G; Albrecht, J

Published: APR 15 2021, MATERIALS CHEMISTRY AND PHYSICS, 263, DOI: 10.1016/j.matchemphys.2021.124390

The presence of a metallic layer can influence the properties of high-temperature superconductors underneath. We investigate the influence of metallic structures deposited in form of nanoparticles or continuous layers by electron beam evaporation or ion beam sputtering on the properties of Y1Ba2Cu3O7-x (YBCO) thin films. To generally avoid diffusion of metal atoms an additional barrier layer is introduced. Detailed measurements of the magnetic moment of the superconductor as a function of temperature and magnetic field have been performed using SQUID magnetometry. It is found that the modification of the superconducting properties of coated YBCO strongly depends on the deposition method of the metal on top rather than the type of metal (Ni or Ag), its magnetic properties (ferromagnetic or paramagnetic) or its morphology (nanoparticles or thin film). The main result is a transient increase of the critical temperature T-c and critical current density J(c) that was observed for samples prepared by electron beam evaporation.

12. On the pinning force in high density MgB2 samples

Authors: Sandu, V; Ionescu, AM; Aldica, G; Grigoroscuta, MA; Burdusel, M; Badica, P

Published: MAR 15 2021, SCIENTIFIC REPORTS, 11, DOI: 10.1038/s41598-021-85209-2

An analysis of the field dependence of the pinning force in different, high density sintered samples of MgB2 is presented. The samples were chosen to be representative for pure MgB2, MgB2 with additives, and partially oriented massive samples. In some cases, the curves of pinning force versus magnetic field of the selected samples present peculiar profiles and application of the typical scaling procedures fails. Based on the percolation model, we show that most features of the field dependence of the critical force that generate dissipation comply with the Dew-Hughes scaling law predictions within the grain boundary pinning mechanism if a connecting factor related to the superconducting connection of the grains is used. The field dependence of the connecting function, which is dependent on the superconducting anisotropy, is the main factor that controls the boundary between dissipative and non-dissipative current transport in high magnetic field. Experimental data indicate that the connecting function is also dependent on the particular properties (e.g., the presence of slightly non-stoichiometric phases, defects, homogeneity, and others) of each sample and it has the form of a single or double peaked function in all investigated samples.

13. Soft-magnetic coatings as possible sensors for magnetic imaging of superconductors

Authors: Ionescu, AM; Simmendinger, J; Bihler, M; Miksch, C; Fischer, P; Soltan, S; Schutz, G; Albrecht, J

Published: JAN 2020, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 33, DOI: 10.1088/1361-6668/ab5984

Magnetic imaging of superconductors typically requires a soft-magnetic material placed on top of the superconductor to probe local magnetic fields. For reasonable results the influence of the magnet onto the superconductor has to be small. Thin YBCO films with soft-magnetic coatings are investigated using SQUID magnetometry. Detailed measurements of the magnetic moment as a function of temperature, magnetic field and time have been performed for different heterostructures. It is found that the modification of the superconducting transport in these heterostructures strongly depends on the magnetic and structural properties of the soft-magnetic material. This effect is especially pronounced for an inhomogeneous coating consisting of ferromagnetic nanoparticles.

14. Enhanced critical current density at high magnetic fields in MgB2 with Ga/In acetylacetonate processed by spark plasma sintering

Authors: Batalu, D; Aldica, G; Burdusel, M; Grigoroscuta, M; Pasuk, I; Kuncser, A; Ionescu, AM; Badica, P

Published: MAY-JUN 2020, JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T, 9, 3733, DOI: 10.1016/j.jmrt.2020.01.109

Dense samples (94-96%) with starting composition (MgB2)(0.99)(X-acac)(0.01) (X-acac denotes Ga or In acetylacetonate) were obtained by spark plasma sintering. The resulting material is a superconducting composite, where carbon substitutes for boron in the crystal structure of MgB2. Added samples show enhanced critical current density at high magnetic fields and this is reflected in high values of irreversibility field (H-irr) at temperatures below 25 K when compared to a pristine sample. More efficient is In-acac addition and it promotes a H-irr of similar to 12.4T (100 A/cm(2) criterion) at 5 K. Carbon substitution for boron in the crystal structure of MgB2 has a strong influence on the pinning force and its related parameters and promotes in the added samples a grain boundary pinning mechanism as the dominant one, whereas the pristine sample with a low amount of carbon shows a major mechanism of point pinning type. However, our analysis indicates on the synergetic effects of the carbon substituting for boron and of the microstructural details on pinning and critical current density. The result strongly emphasizes the significantly different behavior of the additive during processing of the MgB2 samples, although thermal analysis experiments on both additives show very similar decomposition patterns. (C) 2020 The Authors. Published by Elsevier B.V.

15. 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.

16. Effect of proton fluence on the superconducting properties of MgB2. irradiated with protons of high energy

Authors: Sandu, V; Ionescu, AM; Ivan, I; Craciun, L; Aldica, G

Published: NOV 15 2020, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 578, DOI: 10.1016/j.physc.2020.1353734

Effects of proton irradiation on the superconducting properties of sintered MgB2 samples were investigated as a function of proton fluence at fixed energy. All experimental data suggest that critical current densities, irreversibility field and creep activation energy are enhanced because of the complex disorder, which mainly affects the grain boundaries, brought on the crystalline structure by the energetic protons.

17. Bound and stable vortex-antivortex pairs in high-T-c superconductors

Authors: Simmendinger, J; Bihler, M; Ionescu, AM; Weigand, M; Schutz, G; Albrecht, J

Published: DEC 2020, NEW JOURNAL OF PHYSICS, 22, DOI: 10.1088/1367-2630/abd123

18. 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

19. 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.

20. Ex Situ Spark Plasma Sintering of Short Powder-in-Tube MgB2 Tapes with Open and Closed Ends

Authors: Aldica, G; Burdusel, M; Popa, S; Pasuk, I; Ionescu, AM; Kuncser, A; Badica, P

Published: NOV 2018, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 31, 3432, DOI: 10.1007/s10948-018-4616-9

Short powder-in-tube tapes of MgB2 in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB2 core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB2 crystallite size, a weak contact between Fe and MgB2 core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB2 cores, the detected impurity phase is MgO, while at the interface with Fe, MgB4 also occurs. Impurity phases found at interface, MgO and MgB4, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB2 powder has an important contribution to observed differences, and its removal or control is recommended.

21. A Comparative Study of Ge-Based Organometallic Additions to MgB2

Authors: Batalu, D; Nakamura, T; Enculescu, M; Popa, S; Pasuk, I; Aldica, G; Ionescu, AM; Badica, P

Published: JUN 2018, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, 28, DOI: 10.1109/TASC.2018.2808366

Four Ge-based organometallic (OM = C6H10 Ge2O7) polymers with similar chemical composition were used as additions for MgB2. MgB2 (OM) 0.0014 dense samples with a relative density higher than 97% were obtained by ex situ spark plasma sintering. The critical current density and the irreversibility magnetic field of the added samples are slightly improved at high magnetic fields by repagermanium (Alfa Aesar) addition when compared to the pristine reference sample. Addition of Ge-straight-chain polymer shows some improvement of the maximum volume pinning force, F-p (max). There is no significant Ge substitution in the crystal lattice of MgB2, while C substitutes for B. Pinning mechanism is mainly of a point pinning type, with the grain boundary pinning mechanism strengthening at lower temperatures and when additives are not used. A relatively high amount of carbon in the samples washes out the Ge effects on pinning-force-related parameters. The structure and morphology of the polymer additive play an important role in dispersion when mixing, impacting the quality of the superconducting properties. The nanosize dimension of the pristine MgB2 powder shows also some drawbacks in mixing and sintering.

22. 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).

23. Dwell Time Influence on Spark Plasma-Sintered MgB2

Authors: Aldica, G; Popa, S; Enculescu, M; Pasuk, I; Ionescu, AM; Badica, P

Published: FEB 2018, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 31, 325, DOI: 10.1007/s10948-017-4236-9

Samples of MgB2 with relative density above 95% were obtained by spark plasma sintering (SPS) at 1150 A degrees C for a heating time of 100 A degrees C/min and a maximum pressure of 95 MPa. Dwell time was of 1, 4, 7, 10, and 20 min. Samples show that dwell time has a low influence on superconducting characteristics. However, small differences were observed and they are discussed from the viewpoint of density, structural, microstructural, critical current density, irreversibility field, and pinning type. The dominant contribution for all investigated samples is given by point and delta T (c) pinning.

24. 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.

25. 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.

26. 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

27. POWDER-IN-TUBE TAPES OF MgB2 IN Fe-SHEATH PROCESSED BY EX-SITU SPARK PLASMA SINTERING

Authors: Burdusel, M; Ionescu, AM; Grigoroscuta, M; Batalu, D; Enculescu, M; Popa, S; Mihalache, V; Aldica, G; Badica, P

Published: 2017, UNIVERSITY POLITEHNICA OF BUCHAREST SCIENTIFIC BULLETIN SERIES B-CHEMISTRY AND MATERIALS SCIENCE, 79, 172, DOI:

Commercial MgB2 powder was loaded into a Fe-tube, by plastic deformation a tape of similar to 0.5 mm in thickness and 6.9 mm in width was obtained. Short pieces were processed by Spark Plasma Sintering (SPS) at 950, 1050 and 1150 degrees C for 3 min. The optimum sintering temperature is 1050 degrees C. From magnetic/electrical measurements, the onset critical temperature and the irreversibility field at 5 K were 38.7 / 38.9 K and 6.2 / 13.5 T, respectively. The pinning-force-related parameters indicate that the dominant flux pinning mechanism is of point pinning type. Contribution of grain boundary pinning is stronger at lower temperatures.

28. From an Anomalous Peak Effect to a Second Magnetization Peak in Nb-rich Nb-Ti Alloys

Authors: Ionescu, AM; Ivan, I; Enculescu, M; Grigoroscuta, M; Miu, D; Valeanu, M; Badica, P; Miu, L

Published: APR 2017, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 30, 1108, DOI: 10.1007/s10948-016-3670-4

We investigated the evolution of the DC magnetic hysteresis curves of Nb0.89Ti0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 degrees C in vacuum and cold rolling (similar to 50% thickness reduction). Starting with a rectangular piece (similar to 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak ( where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

29. Physical Properties of Polycrystalline CuGeO3 Prepared by Field-assisted Sintering Technique

Authors: Ion, I; Sandu, V; Enculescu, M; Aldica, G; Ionescu, AM

Published: MAR 2016, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 29, 780, DOI: 10.1007/s10948-015-3343-8

The Peierls transition of polycrystalline CuGeO3 samples was investigated by thermal and magnetic investigations. We find that spin disorder induced by chain breaking and grain boundaries has similar effects to doping but without suppressing the spin-Peierls transition temperature or the spin-Peierls gap.

30. Vortex activation energy in the AC magnetic response of superconducting YBa2Cu3O7 thin films with complex pinning structures

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

Published: SEP 2016, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 29, DOI: 10.1088/0953-2048/29/9/095013

The vortex activation energy U-AC in the AC magnetic response of superconductors exhibits a logarithmic variation with the screening current density J (regardless of the pinning structure details), and takes surprisingly high values in the vicinity of the DC irreversibility line, especially at low external DC magnetic fields, as often reported. This is essentially different from the behaviour of the vortex-creep activation energy at long relaxation time scales in DC magnetic measurements, and is not completely understood. We investigated the DC relaxation and the AC response for YBa2Cu3O7 films containing nanorods and nanoparticles, with the DC and AC fields oriented perpendicular to the film surface. It is shown that the large U-AC values in the vicinity of the DC irreversibility line, where the critical-state-related AC signal occurs, are generated by a non-diffusive vortex motion during the AC cycle, with the mean vortex hopping length longer than the average distance between the pinning centres. In these conditions, the smearing of the vortex pinning potential by thermally induced vortex fluctuations is weak, and U-AC mainly results from the strong influence of the pinning-enhanced viscous drag on the vortex hopping process. The logarithmic U-AC(J) dependence is consistent with a high U-AC.

31. AC magnetic response of superconducting YBa2Cu3O7/PrBa2Cu3O7 superlattices

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

Published: JUN 2016, AIP ADVANCES, 6, DOI: 10.1063/1.4955290

Vortex activation energy U-AC in the critical-state related AC magnetic response of superconductors (appearing in the vicinity of the DC irreversibility line) takes large values, as often reported, which is not yet understood. This behavior is essentially different from that of the vortex-creep activation energy at long relaxation time scales, and may become important for AC applications of superconductors. To elucidate this aspect, we investigated the AC signal of almost decoupled [YBa2Cu3O7](n)/[PrBa2Cu3O7](4) superlattices (with n = 11 or 4 units cells) in perpendicular DC and AC magnetic fields. In these model samples, the length of the hopping vortex segment is fixed by the thickness of superconducting layers and vortices are disentangled, at least at low DC fields. It is shown that the high U-AC values result from the large contribution of the pinning enhanced viscous drag in the conditions of thermally activated, non-diffusive vortex motion at short time scales, where the influence of thermally induced vortex fluctuations on pinning is weak. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

32. Superconductivity in MgB2 irradiated with energetic protons

Authors: Sandu, V; Craciun, L; Ionescu, AM; Aldica, G; Miu, L; Kuncser, A

Published: SEP 15 2016, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 528, 34, DOI: 10.1016/j.physc.2016.07.006

A series of MgB2 samples were irradiated with protons of 11.3 and 13.2 MeV. Magnetization data shows an insignificant reduction of the critical temperatures but a continuous decrease of the Meissner fraction with increasing fluence or energy. All samples show a consistent improvement of the critical current density compared to the virgin sample and an increase of the pinning energy at high fields as resulted from relaxation data. (C) 2016 Elsevier B.V. All rights reserved.

33. Possible Enhancement of Spin Fluctuations by Ag addition to SmFeAsO1-x F (x)

Authors: Sandu, V; Suo, HL; Aldica, G; Ionescu, AM; Suo, C; Liu, ZY

Published: FEB 2016, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 29, 308, DOI: 10.1007/s10948-015-3279-z

We investigate the effect of silver addition to polycrystalline SmFeAsO1-x F (x) (Sm-1111) superconductor. Transport and magnetic measurement data reveal the enhancement of the magnetic fluctuation with increasing silver content.

34. Reactive spark plasma sintering of MgB2 in nitrogen atmosphere for the enhancement of the high-field critical current density

Authors: Badica, P; Burdusel, M; Popa, S; Pasuk, I; Ivan, I; Borodianska, H; Vasylkiv, O; Kuncser, A; Ionescu, AM; Miu, L; Aldica, G

Published: OCT 2016, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 29, DOI: 10.1088/0953-2048/29/10/105020

High density bulks (97%-99%) of MgB2 were prepared by spark plasma sintering (SPS) in nitrogen (N-2) atmosphere for different heating rates (10, 20 and 100 degrees C min(-1)) and compared with reference samples processed in vacuum and Ar. N-2 reacts with MgB2 and forms MgB9N along the MgB2 grain boundaries. The high-field critical current density is enhanced for the sample processed in N-2 with a heating rate of 100 degrees C min(-1). At 2-35 K, this sample shows the strongest contribution of the grain boundary pinning (GBP). All samples are in the point pinning (PP) limit and by increasing temperature the GBP contribution decreases.

35. Spark plasma sintered MgB2 co-added with c-BN and C-60

Authors: Ionescu, AM; Aldica, G; Popa, S; Enculescu, M; Sandu, V; Pasuk, I; Kuncser, A; Miu, L; Badica, P

Published: FEB 15 2016, MATERIALS CHEMISTRY AND PHYSICS, 170, 209, DOI: 10.1016/j.matchemphys.2015.12.040

High density (above 92%) superconducting bulks of MgB2 co-added with cubic BN (c-BN) and fullerenes (C-60) were obtained by the ex-situ spark plasma sintering (SPS). Compositions were (MgB2)((1-x))(C)(x)(c-BN)(0.01), x = 0.0125, 0.025, 0.05, 0.075. The co-added sample (MgB2)(0.975)(C)(0.025)(c-BN)(0.01) shows a marginally higher critical current density J(c) at intermediate magnetic fields and below 15 K than for optimum samples added with c-BN or C-60. For this sample, pinning is in the point pinning limit and the delta T-c mechanism is dominant. At high magnetic fields co-added samples are inferior to samples added with one additive, but are superior to pristine sample. Co-addition of c-BN and C-60 is not effective for vortex pinning when compared with individual addition. The result is discussed based on phase formation aspects, microstructural details and residual strain. It was found that in the presence of C-60, c-BN consumption with formation of MgNB9 is intensified with implications on different elements that influence pinning. (C) 2015 Elsevier B.V. All rights reserved.

36. Behaviour of the second magnetization peak in La2-xSrxCuO4 single crystals upon entering the doping domain of static stripe order

Authors: Miu, L; Ionescu, AM; Ivan, I; Miu, D; Adachi, T; Omori, K; Koike, Y

Published: DEC 15 2015, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 519, 84, DOI: 10.1016/j.physc.2015.08.013

The occurrence of a second magnetization peak (SMP) on the dc magnetization curves of superconducting single crystals with random quenched disorder in increasing external magnetic field H is quite common. Many models for this intriguing phenomenon have been proposed so far, but its origin is still under strong debate. Optimally doped and overdoped La2-xSrxCuO4 single crystals (x >= 0.15) exhibit an SMP over a large temperature interval, up to very close to the critical temperature. By decreasing doping, it was found that with H oriented along the c axis the SMP completely disappears in the doping domain of well developed static charge and spin stripes (x similar to 1/8), reappearing for x <= 0.10. This behaviour follows the instability of the Bragg vortex glass generated by the static stripe order (as revealed by small-angle neutron scattering experiments), which is confirmed by our magnetization relaxation measurements. If H is along the (a, b) planes, the SMP is observed for all investigated specimens. The presented analysis supports the scenario in which the SMP is generated by the pinning-induced disordering of the low-field Bragg vortex glass in the dynamic conditions of dc magnetic measurements. (C) 2015 Elsevier B.V. All rights reserved.

37. DC Magnetization Relaxation and the AC Susceptibility of YBCO Films with Strong Pinning

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

Published: FEB 2015, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 28, 365, DOI: 10.1007/s10948-014-2652-7

AC susceptibility and DC magnetization relaxation measurements have been performed for YBa2Cu3O7 films containing BaZrO3 nanorods preferentially oriented along the c axis, with the external magnetic field perpendicular to the film surface. It was found that the effective vortex activation energy U (e) supplied by the standard analysis of the frequency dependent AC magnetic response remains very high even in the vicinity of the irreversibility line indicated by the DC relaxation. We show that the large U (e) values appearing in the AC signal are related to the pinning enhanced viscous drag and significant vortex velocities.

38. 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.

Dr. Alina Marinela BADEA (IONESCU)

Scientific Researcher III