NIMP| Ovidiu CRISAN, Author at NIMP

Projects

1. Novel RE-free permanent magnet systems for automotive applications

Project Type: PNIII Mobilities, Start Date: 2019-01-01 End Date: 2020-01-01

2. Nanomagnet-logic integrated platform with arrays of magnetic tunnel junctions, with laser reversed magnetization for spintronic memories and nanosensors

Project Type: PCE, Start Date: 2017-07-01 End Date: 2019-12-01

Publications

1. 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.

2. Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers

Authors: Kanistras, N; Scheuer, L; Anyfantis, DI; Barnasas, A; Torosyan, G; Beigang, R; Crisan, O; Poulopoulos, P; Papaioannou, ET

Published: JAN 2024, NANOMATERIALS, 14, 215, DOI: 10.3390/nano14020215

THz radiation emitted by ferromagnetic/non-magnetic bilayers is a new emergent field in ultra-fast spin physics phenomena with a lot of potential for technological applications in the terahertz (THz) region of the electromagnetic spectrum. The role of antiferromagnetic layers in the THz emission process is being heavily investigated at the moment. In this work, we fabricate trilayers in the form of Co/CoO/Pt and Ni/NiO/Pt with the aim of studying the magnetic properties and probing the role of very thin antiferromagnetic interlayers like NiO and CoO in transporting ultrafast spin current. First, we reveal the static magnetic properties of the samples by using temperature-dependent Squid magnetometry and then we quantify the dynamic properties with the help of ferromagnetic resonance spectroscopy. We show magnetization reversal that has large exchange bias values and we extract enhanced damping values for the trilayers. THz time-domain spectroscopy examines the influence of the antiferromagnetic interlayer in the THz emission, showing that the NiO interlayer in particular is able to transport spin current.

3. 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.

4. 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.

5. 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

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. Unexpected magneto-functionalities of amorphous Fe-Gd thin films crossing the magnetization compensation point

Authors: Stanciu, AE; Schinteie, G; Kuncser, A; Iacob, N; Trupina, L; Ionita, I; Crisan, O; Kuncser, V

Published: MAR 15 2020, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 498, DOI: 10.1016/j.jmmm.2019.166173

A complex investigation of the morpho-structural, magnetic, magneto-optical and magneto-transport properties of amorphous Fe-Gd thin films crossing the magnetization compensation point is reported and the unexpected observed magneto-functionalities are discussed. A tendency of magnetic domain formation with increasing the Fe content over the compensation concentration is observed. The switch from a reversed Magneto-optical Kerr Effect loop to a direct loop when increasing the Fe content over the compensation point is explained via the specific contribution to the rotation of the polarization vector from each magnetic sublattice, belonging to Fe and Gd, respectively. Local atomic configurations and magnetic interactions ascertained the amorphous character and revealed an out-ofplane orientation of the magnetic moment of Fe above the compensation point. The thermomagnetic curves prove a concentration dependent behavior, explained by weakly coupled magnetization relaxation processes of the two magnetic sub-lattices. On the other hand, the magnetic hysteresis loops gave evidence of two exchange coupled magnetic phases with different coercive fields. According to structural and Fe-57 Mossbauer Spectroscopy results, the two phases correspond to definite nanosized volumes of two different average concentrations (one of them closer to the compensation point) which are randomly distributed in the film. The unexpected single step-like behavior of the magneto-resistivity curves was explained by dissimilar switching of the spins in these two magnetic phases distributed in nano-sized volumes.

13. 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.

14. 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.

15. 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.

16. 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.

17. 3D Superparamagnetic Scaffolds for Bone Mineralization under Static Magnetic Field Stimulation

Authors: Paun, IA; Calin, BS; Mustaciosu, CC; Mihailescu, M; Moldovan, A; Crisan, O; Leca, A; Luculescu, CR

Published: SEP 2019, MATERIALS, 12, DOI: 10.3390/ma12172834

We reported on three-dimensional (3D) superparamagnetic scaffolds that enhanced the mineralization of magnetic nanoparticle-free osteoblast cells. The scaffolds were fabricated with submicronic resolution by laser direct writing via two photons polymerization of Ormocore/magnetic nanoparticles (MNPs) composites and possessed complex and reproducible architectures. MNPs with a diameter of 4.9 +/- 1.5 nm and saturation magnetization of 30 emu/g were added to Ormocore, in concentrations of 0, 2 and 4 mg/mL. The homogenous distribution and the concentration of the MNPs from the unpolymerized Ormocore/MNPs composite were preserved after the photopolymerization process. The MNPs in the scaffolds retained their superparamagnetic behavior. The specific magnetizations of the scaffolds with 2 and 4 mg/mL MNPs concentrations were of 14 emu/g and 17 emu/g, respectively. The MNPs reduced the shrinkage of the structures from 80.2 +/- 5.3% for scaffolds without MNPs to 20.7 +/- 4.7% for scaffolds with 4 mg/mL MNPs. Osteoblast cells seeded on scaffolds exposed to static magnetic field of 1.3 T deformed the regular architecture of the scaffolds and evoked faster mineralization in comparison to unstimulated samples. Scaffolds deformation and extracellular matrix mineralization under static magnetic field (SMF) exposure increased with increasing MNPs concentration. The results are discussed in the frame of gradient magnetic fields of similar to 3 x 10(-4) T/m generated by MNPs over the cells bodies.

18. 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 (?).

19. 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.

20. 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.

21. Local configurations and atomic intermixing in as-quenched and annealed Fe1-xCrx and Fe1-xMox ribbons

Authors: Stanciu, AE; Greculeasa, SG; Bartha, C; Schinteie, G; Palade, P; Kuncser, A; Leca, A; Filoti, G; Birsan, A; Crisan, O; Kuncser, V

Published: 2018, PHILOSOPHICAL MAGAZINE, 98, 1067, DOI: 10.1080/14786435.2018.1425556

Local atomic configuration, phase composition and atomic intermixing in Fe-rich Fe1-xCrx and Fe1-xMox ribbons (x = 0.05, 0.10, 0.15), of potential interest for high-temperature applications and nuclear devices, are investigated in this study in relation to specific processing and annealing routes. The Fe-based thin ribbons have been prepared by induction melting, followed by melt spinning and further annealed in He at temperatures up to 1250 degrees C. The complex structural, compositional and atomic configuration characterisation has been performed by means of X-ray diffraction (XRD), transmission Mossbauer spectroscopy and differential scanning calorimetry (TG-DSC). The XRD analysis indicates the formation of the desired solid solutions with body-centred cubic (bcc) structure in the as-quenched state. The Mossbauer spectroscopy results have been analysed in terms of the two-shell model. The distribution of Cr/Mo atoms in the first two coordination spheres is not homogeneous, especially after annealing, as supported by the short-range order parameters. In addition, high-temperature annealing treatments give rise to oxidation of Fe (to haematite, maghemite and magnetite) at the surface of the ribbons. Fe1-xCrx alloys are structurally more stable than the Mo counterpart under annealing at 700 degrees C. Annealing at 1250 degrees C in He enhances drastically the Cr clustering around Fe nuclei.

22. 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.

23. 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.

24. New concept for information storage in magnetic films exchange-coupled through non-magnetic layer

Authors: Klein, T; Rohlsberger, R; Burkel, E; Crisan, O

Published: 2017, 2017 IEEE 12TH INTERNATIONAL CONFERENCE ON NANO/MICRO ENGINEERED AND MOLECULAR SYSTEMS (NEMS), 686, DOI:

We describe a new concept to encode information in magnetic films that goes beyond the conventional way of digital magnetic recording. In our approach the information is stored via a continuous variable, namely the remanent coupling angle between two magnetic films that are separated by a nonmagnetic spacer layer. Using the technique of nuclear resonant scattering (NRS) [1, 2] we show with good precision, how this coupling angle can be conveniently adjusted with high degree of remanence by shortly applied external magnetic fields. Moreover this effect is explained using a micromagnetic model [3, 4]. Extremely important for future applications of this concept, we demonstrate, that the remanent coupling angles can be read out via magneto-optical or magneto-resistance effects. In principle, this approach allows to design novel memory cells for advance data storage devices, where multiple states per unit cell can be generated and recorded.

25. 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.

26. Nitriding-induced texture, ordering and coercivity enhancement in FePtAgB nanocomposite magnets

Authors: Crisan, O; Vasiliu, F; Palade, P; Mercioniu, I

Published: MAR 1 2016, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 401, 968, DOI: 10.1016/j.jmmm.2015.11.017

FePt system attracts currently a great deal of interest for applications as future RE free permanent magnets. Among the key issues to be solved one may count the decreasing of the ordering temperature and improvement of magnetic behavior. For that purpose we have studied the effect of a nitriding post-synthesis procedure on the FePtAgB melt spun ribbons, aimed at refining the microstructure and enhancing the magnetic performances. Deep structural characterization by transmission electron microscopy, electron diffraction, energy dispersive X-ray spectroscopy and X-ray diffraction allowed us to observe the morphology and to correctly assign and identify the nature of the main granular phases observed. Nitriding procedure is shown to strongly enhance the (001) texturing and the degree of ordering of the L1(0) FePt phase, as well as largely increase of coercivity, compared to the as-cast state. These changes are interpreted in terms of Ag segregation towards intergranular region associated to N diffusion and creation of vacancies that favor consistently the process of ordering the FePt grains into the L1(0) tetragonal phase. (C) 2015 Elsevier B.V. All rights reserved.

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. Tuning exchange spring effects in FePt/Fe(Co) magnetic bilayers

Authors: Reddy, VR; Crisan, O; Gupta, A; Banerjee, A; Kuncser, V

Published: JAN 1 2012, THIN SOLID FILMS, 520, 2189, DOI: 10.1016/j.tsf.2011.10.143

Structural and magnetic properties of exchange spring magnets consisting of hard magnetic (FePt) and soft magnetic (Fe and Co) bilayers. prepared by ion beam sputtering method are studied via X-ray diffraction (XRD), magneto-optic Kerr effect (MOKE) and vibrating sample magnetometry (VSM). Thin tracer layers of Fe-57 were introduced in the soft layer in order to observe the Fe spin structure and interfacial diffusion by Conversion Electron Mossbauer Spectroscopy (CEMS). The observed in-plane exchange spring behavior extends also to the magnetic hard layer, whose switching field can be tuned in an unexpected manner via the top soft magnetic layer. To explain the observed phenomenon it is suggested that the increased switching field, found in the system with a Co/Fe bilayer acting as a single soft magnetic layer, is compatible with a peculiar behavior of the stiffness coefficient of the heterogeneous soft magnetic layer. According to this observation, possibilities to maximize the exchange spring effects via suitably chosen non-homogeneous soft magnetic layers are open. (C) 2011 Elsevier B.V. All rights reserved.

33. Magnetic Interfaces at the Nanoscale: From Fundamentals to Technological Applications

Authors: Angelakeris, M; Crisan, O; Martinez-Boubeta, C

Published: 2012, JOURNAL OF NANOMATERIALS, 2012, DOI: 10.1155/2012/619485

34. 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.

35. 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.

36. 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

37. 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.

38. 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]

39. 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.

40. Structure and magnetic properties of Fe/Fe oxide clusters

Authors: Crisan, O; von Haeften, K; Ellis, AM; Binns, C

Published: 2008, JOURNAL OF NANOPARTICLE RESEARCH, 10, 199, DOI: 10.1007/s11051-008-9463-3

Fe clusters have been synthesised in ultrahigh-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.

41. Novel gas-stabilized iron clusters: synthesis, structure and magnetic behaviour

Authors: Crisan, O; von Haeften, K; Ellis, AM; Binns, C

Published: DEC 17 2008, NANOTECHNOLOGY, 19, DOI: 10.1088/0957-4484/19/50/505602

A novel method for the synthesis of nanostructured films produced by depositing gas-phase magnetic nanoparticles is presented and the properties of the films are reported. The technique mixes metal vapour and small argon clusters produced in a supersonic expansion. The condensed clusters are subsequently deposited in situ onto copper grids. The cluster size is controlled by the vapour pressure of the metal inside the pick-up chamber. Detailed analysis of the transmission electron micrographs of the Fe clusters shows that there is a simple linear relationship between the average metal cluster diameter and the metal vapour pressure during deposition. Furthermore, the nanoparticles show a relatively narrow size distribution for a given set of experimental conditions. Structural and magnetic investigations have been performed on Fe cluster samples, and the influence of the metal vapour pressure has been studied. Detailed analysis of the magnetic and structural data has been performed and valuable information such as cluster size distributions, strength of the interparticle dipolar interactions and average magnetic moment per cluster are derived. It is shown that, at room temperature, the magnetic behaviour of the films is consistent with nanoparticle supermoments interacting via dipolar interactions.

42. 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.

43. Iron oxide in a silica matrix prepared by the sol-gel method

Authors: Predoi, D; Crisan, O; Jitianu, A; Valsangiacom, MC; Raileanu, M; Crisan, M; Zaharescu, M

Published: JUN 4 2007, THIN SOLID FILMS, 515, 6323, DOI: 10.1016/j.tsf.2006.11.148

FexOy-SiO2 nanocomposites were prepared by sol-gel method by using two SiO2 sources and Fe(SO4)(2)center dot 7H(2)O as raw materials. The amorphous gels were thermally treated up to 1000 degrees C. The initial gel and the thermally treated samples were characterized by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) and infrared spectroscopy. The presence of hematite was confirmed by the obtained Mossbauer spectra which showed the characteristic sextet. The total amount and the size distributions of the hematite nanoparticles can be controlled via the initial precursors and subsequent by annealing conditions. (C) 2006 Elsevier B.V. All rights reserved.

44. 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.

45. 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.

46. Hybrid approach to the synthesis of FePt/Fe3B nanocomposite magnets

Authors: Crisan, O; Angelakeris, M; Kehagias, T; Filoti, G

Published: SEP 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2737, DOI:

Exchange coupling between alternatively dispersed hard and soft magnetic phases, gives rise to exchange spring magnets with enhanced energy product (BH)(max) that are believed to represent a novel class of permanent magnets with improved properties. The synthesis of a hybrid nanocomposite system made of soft magnetic Fe3B melt spun ribbon and hard magnetic Fe-Pt thin film, obtained by e-beam co-deposition in ultrahigh vacuum onto the ribbons, is reported. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction are used for structural characterization of these hybrid samples. Magnetic measurements provide observation of a drastic decrease of the saturation magnetization, compatible with the exchange coupling between hard and soft magnetic phases in the melt spun/thin film samples.

47. 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.

48. 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.

49. Magnetic structure and interlayer exchange coupling in spring magnets - studied via nuclear resonant scattering

Authors: Klein, T; Rohlsberger, R; Crisan, O; Schlage, K; Burkel, E

Published: DEC 5 2006, THIN SOLID FILMS, 515, 2534, DOI: 10.1016/j.tsf.2006.03.035

Magnetic properties of FePt/Ag/Fe and FePt/Pd/Fe layer systems, prepared by magnetron sputtering, were investigated using the nuclear resonant forward scattering of synchrotron radiation. This technique allows the accurate determination of magnetic hyperfine field orientations by using an extremely thin 57 Fe probe layer suitably embedded within the soft magnetic layer. From an evaluation of these measurements within a one-dimensional micromagnetic model, the interlayer exchange coupling constants between the magnetically hard (FePt) and soft (Fe) layers were determined as function of the Ag and I'd interlayer thickness. The interlayer thickness dependence of the bilinear coupling constants provides evidence for the superposition of Ruderman-Kittel-Kasuya-Yoshida coupling and a magnetostatic interaction between the magnetic layers. (c) 2006 Elsevier B.V. All rights reserved.

50. Structure effects on the magnetism of AgCo nanoparticles

Authors: Crisan, O; Angelakeris, M; Simeonidis, K; Kehagias, T; Komninou, P; Giersig, M; Flevaris, NK

Published: NOV 2006, ACTA MATERIALIA, 54, 5260, DOI: 10.1016/j.actamat.2006.06.053

Fabrication by colloidal chemistry and an extended structural and magnetic investigation are reported for AgCo bimetallic colloidal nanoparticles using X-ray diffraction, transmission electron microscopy and superconducting quantum interference device magnetometry. The AgCo nanoparticles exhibit a core-shell structure, with a face-centred cubic Ag core and a hexagonal close-packed Co (either complete or partial) shell. A bimodal size distribution of superparamagnetic (SPNI) nanoparticles together with a small fraction of nanoparticles ferrornagnetic at room temperature has been determined and their influence on magnetic properties is discussed. The arrays of self-assembled nanoparticles exhibit a lack of saturation of magnetisation and typical SPM behaviour. The annealing of the arrays greatly enhances the ferromagnetic character of the samples. Finally, the observed magnetic behaviour of the AgCo nanoparticles is correlated with surface spin disorder induced by the particular structural features of the sample, high fraction of magnetic atoms in surface states and finite-size effects. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

51. 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.

52. Influence of the SMCo5 hard magnetic phase on the magnetoresistive properties of Cu-Fe ribbons

Authors: Larde, R; Le Breton, JM; Richomme, F; Teillet, J; Hauet, A; Maignan, A; Crisan, O

Published: OCT 20 2004, JOURNAL OF PHYSICS-CONDENSED MATTER, 16, 7492, DOI: 10.1088/0953-8984/16/41/026

The SmCo5 hard magnetic phase was added to magnetoresistive granular Cu-Fe alloys in order to investigate the influence of the presence of a hard magnetic phase on the magnetoresistive properties of a granular alloy that contains a soft magnetic phase. Cu-80(Sm0.17Co0.83)(x)Fe20-x ribbons, with x = 20, 15, 10, 5, obtained by melt spinning, were investigated. The ribbons are composed of magnetic Fe, SmCo5, and Co precipitates embedded in a Cu matrix. In the as-quenched state, the magnetic interactions between magnetic precipitates lead to the formation of magnetic coherent regions and the magnetoresistance effect is only observed at high field (>1T). After annealing, the strength of interactions decreases and a magnetoresistance effect is observed at low field (<1T). The largest magnetoresistance effect (16%) is observed at 5K for the Cu-80(Sm0.17Co0.83)(10)Fe-10 alloy annealed at 450degreesC.

53. Magnetic interactions and spin structure in composite Fe/Nd2Fe14B systems

Authors: Kuncser, V; Valeanu, M; Lifei, F; Predoi, D; Palade, P; Schinteie, G; Crisan, O; Filoti, G

Published: MAY 2004, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 272, 799, DOI: 10.1016/j.jmmm.2003.12.1279

Local magnetic interactions and spin configurations in alpha-Fe/Nd2Fe14B composite exchange-spring magnets have been investigated by Mossbauer spectroscopy and the magnetic behaviour was analysed by magnetometry. A new method for the evaluation of the coupling strength between soft and hard magnetic phases is proposed. (C) 2003 Elsevier B.V. All rights reserved.

54. Correlation of structure and magnetism of AgCo nanoparticle arrays

Authors: Crisan, O; Angelakeris, M; Nogues, M; Kehagias, T; Komninou, P; Sobal, N; Giersig, M; Flevaris, NK

Published: MAY 2004, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 272, E1254, DOI: 10.1016/j.jmmm.2003.12.304

The bimetallic AgCo nanoparticles synthesized by colloidal chemistry and deposited onto Si(100) substrate are investigated. Size-selective mechanisms of colloidal crystallization during the self-organization are evidenced. The magnetic behavior is studied and correlated with the nanoparticles structure and morphology. (C) 2003 Elsevier B.V. All rights reserved.

55. Magnetic interactions and spin configuration in FeRh and Fe/FeRh systems

Authors: Kuncser, V; Keune, W; Sahoo, B; Duman, E; Acet, M; Radu, F; Valeanu, M; Crisan, O; Filoti, G

Published: MAY 2004, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 272, 350, DOI: 10.1016/j.jmmm.2003.11.199

The magnetic interactions and the Fe spin structure have been studied in Fe(6 nm)/FeRh systems by magnetometry, magneto-optic Kerr effect and conversion electron Mossbauer spectroscopy. A spin-flop coupling mechanism, with the interfacial spins of the ferromagnetic phase perpendicular to the spins of the antiferromagnetic phase was experimentally proved. (C) 2004 Elsevier B.V. All rights reserved.

56. 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.

57. 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.

58. Monte Carlo simulation study of magnetic behaviour of core-shell bimetallic nanoparticles

Authors: Crisan, O; Tornau, EE; Petrauskas, V; Angelakeris, M

Published: 2004, SECOND SEEHEIM CONFERENCE ON MAGNETISM, PROCEEDINGS, 3763, DOI: 10.1002/pssc.200405549

The magnetic properties of core-shell-type nanoparticles with non-magnetic core and ferromagnetic shell are modeled using Monte Carlo simulation. The influence of the surface spin disorder on the magnetization and Curie temperature of magnetic states is studied. Due to competition between exchange coupling and surface anisotropy, rotation of the magnetization axis is observed, as the surface anisotropy of the ferromagnetic shell is increased. It is shown that the exchange coupling at the core-shell interface can influence the shape of the temperature dependence of total magnetization. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

59. Monte Carlo simulation of magnetic properties in nanocrystalline-like systems

Authors: Crisan, O; Labaye, Y; Berger, L; Greneche, JM

Published: SEP 24 2003, JOURNAL OF PHYSICS-CONDENSED MATTER, 15, 6344, DOI: 10.1088/0953-8984/15/37/003

Those nanocrystalline materials which consist of ferromagnetic nanograins embedded in a ferromagnetic matrix were modelled as a cubic lattice composed of a central sphere with strongly coupled spins surrounded by weakly coupled spins. The magnetic behaviour was studied by Monte Carlo simulation, especially the low temperature spin ordering and features exhibited in the temperature range between the Curie temperatures of the two phases. The magnetization and magnetic susceptibility are calculated as a function of temperature for different values of the interfacial exchange interactions. It is shown that the exchange coupling between matrix and nanograin influences the magnetic properties of the nanograin, the matrix and the interfacial regions differently. The magnetic behaviour of different regions has been explained in terms of a polarization mechanism acting on the surface and in the core, leading to magnetic correlation between the spins. Such features are quite consistent with the experimental results obtained on nanocrystalline alloys.

60. Nanocrystallization of soft magnetic Finemet-type amorphous ribbons

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

Published: SEP 15 2003, SENSORS AND ACTUATORS A-PHYSICAL, 106, 250, DOI: 10.1016/S0924-4247(03)00177-8

The nanocrystallization of two soft magnetic amorphous Finemet-type ribbons of the nominal composition Fe73.5Cu1Nb3Si13.5B9 and Fe73.5Cu1Gd1Nb2Si13.5B9 synthesized by a melt spinning technique is reported. The crystallization process was monitored by differential scanning calorimetry (DSC) that allows identification of the primary crystallization temperature as well as the other exothermic events occurring during crystallization process. The resulting phase structure of the properly annealed samples is investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) and is shown to consist of size-distributed body-centred cubic (bcc) alpha-Fe(Si) (rather than Fe3Si, DO3-type), nanograins, embedded into the amorphous Nb-rich residual matrix. The degree of dispersion of the nanograins in the amorphous matrix is estimated from the nanocrystalline volume fraction and the mean size of the nanograins obtained both from XRD data and TEM images. (C) 2003 Elsevier B.V. All rights reserved.

61. A Mossbauer investigation of amorphous Sm-Fe-B ribbons under applied field

Authors: Le Breton, JM; Crisan, O

Published: MAR 10 2003, JOURNAL OF ALLOYS AND COMPOUNDS, 351, 64, DOI: 10.1016/S0925-8388(02)01088-5

Applied field Mossbauer spectrometry is used to investigate the magnetic structure in two amorphous Sm-Fe-B ribbons with measured compositions Sm7.2Fe72.8B20 and Sm12Fe68B20. The applied field dependence of the hyperfine field agrees with the ferromagnetic coupling between the Fe and Sm magnetic moments. For the Sm7.2Fe72.8B20 ribbon, the magnetic moments are aligned in the applied field direction for fields higher than 4 T. For the Sm12Fe68B20 ribbon, the magnetic moments are not completely aligned in the direction of the applied field for fields up to 6 T. This spin canting is attributed to the influence of the samarium anisotropy, which probably dominates in the Sm12Fe68B20 ribbon, as the related composition lies in the concentration range where the local order around iron atoms is strongly influenced by the samarium atoms. (C) 2002 Elsevier Science B.V. All rights reserved.

62. 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.

63. 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.

64. Anisotropies in ferromagnetic nanoparticles: simulation and experimental approach

Authors: Crisan, O; Angelakeris, M; Flevaris, NK; Sobal, N; Giersig, M

Published: SEP 15 2003, SENSORS AND ACTUATORS A-PHYSICAL, 106, 133, DOI: 10.1016/S0924-4247(03)00150-X

Magnetic behavior of the isolated ferromagnetic bimetallic nanoparticles is investigated in relation with the competition between exchange interaction and surface and bulk anisotropies. Experimental results on AgCo bimetallic nanoparticles are presented and correlated with Monte Carlo simulation of low temperature spin ordering. The nature of spin disorder at the surface, responsible for the reduction of the overall saturation magnetization is briefly discussed. (C) 2003 Elsevier B.V. All rights reserved.

65. Magnetism and anisotropy in core-shell nanoparticles

Authors: Crisan, O; Angelakeris, M; Flevaris, NK; Filoti, G

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 962, DOI:

The subject of magnetic anisotropy in nanostructured materials is of considerable interest from both experimental and theoretical points of view. Systems that consist of either isolated or interacting nanoparticles are shown to exhibit different magnetic properties of the nanoparticle surface than those of the bulk. This altering of magnetic behavior is mainly related to surface spins structure cot-related with finite size effects, providing that the nanoparticles are sufficiently small so that the surface-to-volume atomic ratio would be high enough. This work reports on both the simulation and experimental approach on the magnetic behavior of ferromagnetic nanoparticles. We investigate the competition between surface and bulk magnetocrystalline anisotropy in small magnetic particles. The experimentally obtained magnetic data for ferromagnetic core-shell-type AgCo nanoparticles are interpreted consistently with the results of Monte Carlo simulation of the magnetic behavior of single domain nanoparticles.

66. 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.

67. Exchange coupling effects in nanocrystalline alloys studied by Monte Carlo simulation

Authors: Crisan, O; Labaye, Y; Berger, L; Coey, JMD; Greneche, JM

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 8729, DOI: 10.1063/1.1456420

The magnetic behavior of nanocrystalline alloys has been modeled using atomic Monte Carlo simulation. The model consists of a cubic lattice composed of a ferromagnetic nanograin in a ferromagnetic matrix. The magnetizations of nanograin core, surface and interface regions and matrix were studied as a function of the exchange coupling between the nanograin and the matrix, as well as of the nanograin/matrix volume ratio, equivalent to the crystalline fraction in the nanocrystalline alloys. The mechanism of polarization of the matrix by fields penetrating from the nanograin is discussed and correlated with the matrix-nanograin exchange coupling. Competition between interface anisotropy and magnetocrystalline anisotropy produces spin-glass-like magnetic order of the interfacial regions. (C) 2002 American Institute of Physics.

68. Magnetism and phase structure of crystallized Sm-Fe-B melt spun ribbons

Authors: Crisan, O; Le Breton, JM; Nogues, M; Machizaud, F; Filoti, G

Published: DEC 2 2002, JOURNAL OF PHYSICS-CONDENSED MATTER, 14, 12609, DOI: 10.1088/0953-8984/14/47/331

The crystallization of amorphous SmxFe80-xB20 melt spun ribbons is studied over an extended range of composition (0 < x < 8). Differential scanning calorimetry scans reveal different endo- and exothermic effects depending upon the composition. X-ray diffraction and Mossbauer studies for samples annealed at temperatures close to the onset of the first exothermic effect prove that, during the primary crystallization of the amorphous ribbons, both alpha-Fe and Fe3B phases appear-with Sm ions randomly accommodating the Fe sites in the tetragonal Fe3B lattice-while the subsequent exothermic reactions correspond to the decomposition of the metastable Fe3B phase into alpha-Fe and Fe2B and to the formation of the ternary Sm1.1Fe4B4 phase. Fully crystallized ribbons with Sm contents lower than about 7 at.% show the co-existence between alpha-Fe and Fe2B soft magnetic phases and the Sm2Fe14B magnetic one. The enhancement of magnetic properties with increasing relative proportion of magnetic phase is discussed and correlated with exchange coupling and spin wave stiffness data obtained from the magnetic measurements.

69. High temperature magnetic behaviour of iron-based nanocrystalline alloys

Authors: Randrianantoandro, N; Labaye, I; Berger, L; Crisan, O; Grafoute, M; Calvayrac, F; Greneche, JM

Published: JUN 2002, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 4, 198, DOI:

After a presentation of structural aspects of iron-based nanocrystalline alloys, the magnetic behaviours of those two-phase magnetic structures are conceptually discussed at high temperatures as a function of the crystalline volumetric fraction, on the basis of the magnetic correlation length compared to the distances between crystalline grains. In addition, some results predicted by an approach based on Monte-Carlo simulation are then briefly presented: they show a qualitative agreement with experimental results.

70. Monte Carlo simulation of the magnetization of a ferromagnet with antiphase boundaries

Authors: Berger, L; Labaye, Y; Crisan, O; Greneche, JM; Coey, JMD

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 7636, DOI: 10.1063/1.1456404

The magnetic properties of a ferromagnetic cylinder containing an antiphase boundary with antiferromagnetic interactions are studied using atomic Monte Carlo simulation. The approach to saturation shows a reduced magnetization compared with the completely ferromagnetic sample, even in huge applied fields. The magnetization profiles are studied as a function of the number of ferromagnetic planes in the cylinder with 80 atoms per plane and also as function of the ratio of the bulk-to-antiphase exchange coupling. Hysteresis appears after removing the applied field for relatively low antiphase boundary densities. It is shown that the antiferromagnetic coupling across the antiphase boundary leads to a progressive rotation of spins as the two half "domain walls" at the boundary with opposite chirality narrow with increasing applied field. (C) 2002 American Institute of Physics.

71. Surface anisotropy in ferromagnetic nanoparticles

Authors: Labaye, Y; Crisan, O; Berger, L; Greneche, JM; Coey, JMD

Published: MAY 15 2002, JOURNAL OF APPLIED PHYSICS, 91, 8717, DOI: 10.1063/1.1456419

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a "throttled" spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a "hedgehog" configuration with no net magnetization. Implications for FePt nanoparticles are discussed. (C) 2002 American Institute of Physics.

72. Magnetic properties of nanocrystalline materials: A Monte Carlo simulation

Authors: Crisan, O; Labaye, Y; Berger, L; Greneche, JM

Published: FEB 23 2002, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 189, 848, DOI: 10.1002/1521-396X(200202)189:3<845::AID-PSSA845>3.0.CO;2-#

The magnetic properties of interfacial regions between ferromagnetic nanograins and the residual matrix in nanocrystalline materials, are modelled using Monte Carlo simulation. The exchange coupling between matrix and nanograin influences differently the nanograin surface and the interface between nanograin and matrix. This behaviour has been explained in terms of a polarisation mechanism of the interfacial zones leading to magnetic correlation between spins of nanograin surface and core.

73. Crystallization processes and resulting phase structure of Sm-Fe-B melt-spun ribbons

Authors: Crisan, O; Le Breton, JM; Machizaud, F; Labaye, Y; Filoti, G

Published: APR 2002, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 242, 1299, DOI: 10.1016/S0304-8853(01)01268-9

The crystallization behaviour of amorphous melt-spun SmxFe80-xB20 ribbons is investigated by differential scanning calorimetry, X-ray diffraction and Mossbauer spectrometry. On increasing the Sm content. the crystallization temperature reaches a maximum for a critical Sm content (x(c) approximate to 8) and the main crystallization products are different below and above x,. This is attributed to a change in the short-range local order in the amorphous alloy. (C) 2002 Elsevier Science B.V. All rights reserved.

74. A magnetic study of magnetoresistive Cu-(SmCo5)-Fe heterogranular alloys

Authors: Crisan, O; Le Breton, JM; Jianu, A; Maignan, A; Nogues, M; Teillet, J; Filoti, G

Published: AUG 2001, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 234, 99, DOI: 10.1016/S0304-8853(01)00279-7

The magnetic interactions between the nanograins in heterogeneous granular Cu-(SmCo5)-Fe ribbons were evaluated with a classical mean field model to obtain the exchange coupling as well as the magnetic moment per nanoparticle. The samples were subjected to various thermal treatments and a significant magnetoresistive effect was obtained when the optimal annealing conditions had been achieved. (C) 2001 Elsevier Science B.V. All rights reserved.

75. Structural refinement effect on the magnetoresistive properties of annealed melt spun Cu-Co-(Fe-Si) ribbons

Authors: Crisan, O; Le Breton, JM; Maignan, A; Machizaud, F; Jianu, A; Teillet, J; Filoti, G

Published: MAY 1999, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 195, 436, DOI: 10.1016/S0304-8853(99)00160-2

Structural and magnetic properties of melt-spun Cu-Co-(Fe-Si) ribbons, as-quenched and after appropriate annealing, are investigated and correlated with their magnetoresistive properties. The effect of structural refinement of the magnetic phases and of the coexistence of multiple magnetic phases on the magnetoresistance is discussed. The existence of an optimum in the annealing conditions for obtaining a significant magnetoresistive effect, is inferred. (C) 1999 Elsevier Science B.V. All rights reserved.

76. Structural and magnetic properties of magnetoresistive Cu-Co-Fe ribbons

Authors: Crisan, O; Le Breton, JM; Jianu, A; Maignan, A; Nogues, M; Teillet, J; Filoti, G

Published: MAY 1999, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 196, 469, DOI: 10.1016/S0304-8853(98)00822-1

Melt-spun Cu80Co10Fe10 ribbons are shown to exhibit a nanogranular structure with magnetic alpha-Fe(Co) grains segregated from the Cu-rich matrix after appropriate annealing. The Cu matrix purification as well as the magnetic properties of the alpha-Fe(Co) grains existing in a range of sizes are studied using X-ray diffraction, Mossbauer spectrometry and magnetic measurements. There is an optimum annealing for obtaining a high magnetoresistive effect. (C) 1999 Elsevier Science B.V. All rights reserved.

77. Influence of Gd addition on the magnetic properties of Fe-Cu-Nb-Si-B ribbons

Authors: Crisan, O; Le Breton, JM; Nogues, M; Machizaud, F; Jianu, A; Teillet, J; Filoti, G

Published: JUN 1998, JOURNAL DE PHYSIQUE IV, 8, 118, DOI: 10.1051/jp4:1998227

The magnetic properties of the amorphous and nanocrystallized Fe-Cu-Nb-Si-B and Fe-Gd-Cu-Nb-Si-B ribbons are studied using temperature dependence of the specific magnetization. A two-phase ferromagnetic behaviour is obtained for the magnetization curves. Gd addition tends to stabilize the interfacial (grain boundary) phase by forming a metastable Gd-Fe-B phase, leading to the coexistence of two phases up to thermal treatments as high as 993K.

78. On the passivation of the metallic surfaces from the nuclear plant water circuits

Authors: Filoti, G; Kuncser, V; Palade, P; Micu, IF; Crisan, O

Published: 1998, HYPERFINE INTERACTIONS, 112, 38, DOI: 10.1023/A:1011063905776

Samples obtained from the hot conditioning and hot performance procedures have been investigated by GEMS. Various types of substrates were considered. The surface phases were assigned to magnetite, alpha-Fe, wustite and to spinel solid solutions. The data have been analyzed taking into account the compound stoichiometry, the number of iron unequivalent positions appeared in each identified phase and the relative ratio of the formed products.

79. A Mossbauer spectroscopy and magnetic study of FeRh

Authors: Rosenberg, M; Kuncser, V; Crisan, O; Hernando, A; Navarro, E; Filoti, G

Published: JAN 1998, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 177, 136, DOI: 10.1016/S0304-8853(97)00662-8

A Mossbauer and magnetic study of FeRh has been undertaken in order to understand better the remarkable properties of both melt-spun BCC and ball-milled FCC-phases of the anti-invar FeRh alloy. (C) 1998 Elsevier Science B.V. All rights reserved.

80. Investigation of the structural properties of crystallized Fe-(Cu)-Sm-B ribbons

Authors: Crisan, O; Le Breton, JM; Machizaud, F; Jianu, A; Teillet, J; Filoti, G

Published: JUN 1998, JOURNAL DE PHYSIQUE IV, 8, 50, DOI: 10.1051/jp4:1998210

Crystallized Fe-(Cu)-Sm-B ribbons were investigated by means of X-ray diffraction, differential scanning calorimetry and Mossbauer spectrometry. The bcc alpha-Fe, tetragonal Fe3B and tetragonal Fe2B phases were found to crystallize after appropriate annealing. The relative proportion of alpha-Fe decreases with increasing Sm content, in agreement with the increase of the crystallization point. The metastable Fe3B phase decomposition into alpha-Fe and Fe2B was observed within a wide range of annealing temperature. The Sm ions randomly accommodate the Fe sites in the Fe3B lattice. After cumulative annealing at higher temperatures, the tetragonal Sm1.1Fe4B4 paramagnetic phase is found.

81. Structural properties of amorphous and nanocrystallized Fe-Cu-Nb-Si-B and Fe-Gd-Cu-Nb-Si-B ribbons

Authors: Crisan, O; Le Breton, JM; Jianu, A; Teillet, J; Filoti, G

Published: NOV 14 1997, JOURNAL OF ALLOYS AND COMPOUNDS, 262, 389, DOI: 10.1016/S0925-8388(97)00417-9

The influence of Gd addition on the structural properties of Fe-Cu-Nb-Si-B nanocrystallized and amorphous alloys is studied. The crystallization temperature increases and the microstructure of the annealed samples changes. Gd addition induces the formation of Gd-Fe-B phases. In fully crystallized Fe-Gd-Cu-Nb-Si-B alloys the alpha-Fe(Si), Fe-Nb-B, Gd3Fe62B14 and Gd1.1Fe4B4 phases are observed. The evolution of the microstructure is followed as a function of the cumulative effects of annealing time and temperature. The results suggest the transformation of the metastable Gd3Fe62B14 phase into Gd1.1Fe4B4 and alpha-Fe. The hyperfine parameters of the Gd3Fe62B14 Mossbauer contribution are reported. (C) 1997 Elsevier Science S.A.

Dr. Ovidiu CRISAN

Scientific Researcher I

Projects

1. Novel RE-free permanent magnet systems for automotive applications

2. Nanomagnet-logic integrated platform with arrays of magnetic tunnel junctions, with laser reversed magnetization for spintronic memories and nanosensors

Publications

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

2. Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers

3. High Magnetic Performance in MnGa Nanocomposite Magnets

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

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

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

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

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

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

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

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

12. Unexpected magneto-functionalities of amorphous Fe-Gd thin films crossing the magnetization compensation point

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

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

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

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

17. 3D Superparamagnetic Scaffolds for Bone Mineralization under Static Magnetic Field Stimulation

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

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

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

21. Local configurations and atomic intermixing in as-quenched and annealed Fe1-xCrx and Fe1-xMox ribbons

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

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

24. New concept for information storage in magnetic films exchange-coupled through non-magnetic layer

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

26. Nitriding-induced texture, ordering and coercivity enhancement in FePtAgB nanocomposite magnets

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

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

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

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

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

32. Tuning exchange spring effects in FePt/Fe(Co) magnetic bilayers

33. Magnetic Interfaces at the Nanoscale: From Fundamentals to Technological Applications

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

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

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

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

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

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

40. Structure and magnetic properties of Fe/Fe oxide clusters

41. Novel gas-stabilized iron clusters: synthesis, structure and magnetic behaviour

42. Synthesis and magnetic properties of Fe51Pt27Nb2B20 melt spun ribbons

43. Iron oxide in a silica matrix prepared by the sol-gel method

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

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

46. Hybrid approach to the synthesis of FePt/Fe3B nanocomposite magnets

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

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

49. Magnetic structure and interlayer exchange coupling in spring magnets - studied via nuclear resonant scattering

50. Structure effects on the magnetism of AgCo nanoparticles

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

52. Influence of the SMCo5 hard magnetic phase on the magnetoresistive properties of Cu-Fe ribbons

53. Magnetic interactions and spin structure in composite Fe/Nd2Fe14B systems

54. Correlation of structure and magnetism of AgCo nanoparticle arrays

55. Magnetic interactions and spin configuration in FeRh and Fe/FeRh systems

56. Magnetic nanostructures obtained by colloidal crystallization onto patterned substrates

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

58. Monte Carlo simulation study of magnetic behaviour of core-shell bimetallic nanoparticles

59. Monte Carlo simulation of magnetic properties in nanocrystalline-like systems

60. Nanocrystallization of soft magnetic Finemet-type amorphous ribbons

61. A Mossbauer investigation of amorphous Sm-Fe-B ribbons under applied field

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

63. Magnetism of nanocrystalline Finemet alloy: experiment and simulation

64. Anisotropies in ferromagnetic nanoparticles: simulation and experimental approach

65. Magnetism and anisotropy in core-shell nanoparticles

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

67. Exchange coupling effects in nanocrystalline alloys studied by Monte Carlo simulation

68. Magnetism and phase structure of crystallized Sm-Fe-B melt spun ribbons

69. High temperature magnetic behaviour of iron-based nanocrystalline alloys

70. Monte Carlo simulation of the magnetization of a ferromagnet with antiphase boundaries

71. Surface anisotropy in ferromagnetic nanoparticles

72. Magnetic properties of nanocrystalline materials: A Monte Carlo simulation

73. Crystallization processes and resulting phase structure of Sm-Fe-B melt-spun ribbons

74. A magnetic study of magnetoresistive Cu-(SmCo5)-Fe heterogranular alloys

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

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

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

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