NIMP| Simona Greculeasa, Author at NIMP

Projects

1. Influence of structure on the magnetoelectric coupling of SrFe12O19 – BNT-BT composites

Project Type: , Start Date: End Date:

Publications

1. Lead-Free BNT-BT0.08/CoFe2O4 Core-Shell Nanostructures with Potential Multifunctional Applications

Authors: Cernea, M; Radu, R; Amorin, H; Greculeasa, SG; Vasile, BS; Surdu, VA; Ganea, P; Trusca, R; Hattab, M; Galassi, C

Published: APR 2020, NANOMATERIALS, 10, 672, DOI: 10.3390/nano10040672

Herein we report on novel multiferroic core shell nanostructures of cobalt ferrite (CoFe2O4) bismuth, sodium titanate doped with barium titanate (BNT-BT0.08), prepared by a two step wet chemical procedure, using the sol gel technique. The fraction of CoFe2O4 was varied from 1:0.5 to 1:1.5 = BNT-BT0.08/CoFe2O4 (molar ratio). X-ray diffraction confirmed the presence of both the spinel CoFe2O4 and the perovskite Bi0.5Na0.5TiO3 phases. Scanning electron microscopy analysis indicated that the diameter of the core shell nanoparticles was between 15 and 40 nm. Transmission electron microscopy data showed two phase composite nanostructures consisting of a BNT-BT0.08 core surrounded by a CoFe2O4 shell with an average thickness of 4-7 nm. Cole-Cole plots reveal the presence of grains and grain boundary effects in the BNT-BT0.08/CoFe2O4 composite. Moreover, the values of the dc conductivity were found to increase with the amount of CoFe2O4 semiconductive phase. Both X-ray photoelectron spectroscopy (XPS) and Mossbauer measurements have shown no change in the valence of the Fe3+, Co2+, Bi3+ and Ti4+ cations. This study provides a detailed insight into the magnetoelectric coupling of the multiferroic BNT-BT0.08/CoFe2O4 core shell composite potentially suitable for magnetoelectric applications.

2. Magnetic properties of BaNi x Fe 12-x O 19 (x=0.0-1.0) hexaferrites, synthesized by citrate -gel auto -combustion and sintered by conventional and spark plasma methods

Authors: Cernea, M; Greculeasa, SG; Radu, R; Aldica, G; Ganea, P; Surdu, VA; Tanasa, ET; Cioangher, M; Iacob, N; Costescu, RM

Published: AUG 5 2020, JOURNAL OF ALLOYS AND COMPOUNDS, 831, DOI:

3. Tuning structural and magnetic properties of Fe oxide nanoparticles by specific hydrogenation treatments

Authors: Greculeasa, SG; Palade, P; Schinteie, G; Leca, A; Dumitrache, F; Lungu, I; Prodan, G; Kuncser, A; Kuncser, V

Published: OCT 14 2020, SCIENTIFIC REPORTS, 10, 17174, DOI: 10.1038/s41598-020-74188-5

Structural and magnetic properties of Fe oxide nanoparticles prepared by laser pyrolysis and annealed in high pressure hydrogen atmosphere were investigated. The annealing treatments were performed at 200 degrees C (sample A200C) and 300 degrees C (sample A300C). The as prepared sample, A, consists of nanoparticles with similar to 4 nm mean particle size and contains C (similar to 11 at.%), Fe and O. The Fe/O ratio is between gamma-Fe2O3 and Fe3O4 stoichiometric ratios. A change in the oxidation state, crystallinity and particle size is evidenced for the nanoparticles in sample A200C. The Fe oxide nanoparticles are completely reduced in sample A300C to alpha-Fe single phase. The blocking temperature increases from 106 K in A to 110 K in A200C and above room temperature in A300C, where strong inter-particle interactions are evidenced. Magnetic parameters, of interest for applications, have been considerably varied by the specific hydrogenation treatments, in direct connection to the induced specific changes of particle size, crystallinity and phase composition. For the A and A200C samples, a field cooling dependent unidirectional anisotropy was observed especially at low temperatures, supporting the presence of nanoparticles with core-shell-like structures. Surprisingly high M-S values, almost 50% higher than for bulk metallic Fe, were evidenced in sample A300C.

4. Complex exchange coupling mechanisms in SRO/BFO/Fe heterostructures

Authors: Greculeasa, SG; Schinteie, G; Hrib, LM; Stancu, V; Pasuk, I; Kuncser, A; Kuncser, V

Published: JAN 30 2019, JOURNAL OF ALLOYS AND COMPOUNDS, 773, DOI: 10.1016/j.jallcom.2018.09.208

Temperature dependent interfacial coupling mechanisms in SRO/BFO/Fe layered structures were investigated. The BFO/Fe heterostructures were prepared by PLD and sputtering, respectively, on the STO(0 0 1) substrate with a 20 nm SRO buffer layer. An annealing treatment in external magnetic field was further applied. Complex characterizations with X-ray diffraction, atomic force microscopy, Transmission Electron Microscopy, Mossbauer spectroscopy, magneto-optic Kerr effect and SQUID magnetometry were performed. Before annealing, the films show good crystallization and epitaxy of the SRO and BFO layers with smooth interfaces. Two coupling mechanisms of the ferromagnetic layers (top Fe and bottom SRO, respectively) to the epitaxial BFO film with mainly antiferromagnetic structure were evidenced in the as deposited samples at low temperatures. Negative exchange bias fields of up to 67(10) Oe and 37(5) Oe at low temperatures were observed for the two ferromagnetic components, respectively, depending on the thickness of the Fe layer. The field annealing treatments induce a specific morphology and magnetic spin structure at both interfaces of the BFO spacer layer, giving rise to a long range magnetostatic coupling between the two ferromagnetic films, in addition to the interfacial couplings. Moreover, the experimentally evidenced Fe clusters penetrating the BFO/Fe interface toward the BFO layer give support for this interaction. As an additional consequence, a considerable enhancement of both uniaxial and unidirectional anisotropies as well as an increased blocking temperature of exchange bias were obtained. The involved exchange coupling mechanisms were discussed in detail. (C) 2018 Elsevier B.V. All rights reserved.

5. Synthesis and Mechanical Properties of Polyurea-Based Hybrid Composites for Ballistic Individual Protection

Authors: Petre, R; Zecheru, T; Petrea, N; Ginghina, R; Sandu, S; Muresan, M; Matache, LC; Sava, AC; Neatu, F

Published: SEP 2018, MATERIALE PLASTICE, 55, DOI:

During conflict situations, the combat staff is exposed to a wide variety of aggressions, such as temperature and pressure variations and dynamic impacts (from ammunition or fragments). Textiles used in the manufacture of the military uniforms and devices have always played an important role in defending the military against these hazards, and an adequate level of individual protection equipment is required. In this respect, novel fibre-reinforced polymer composite materials for military application, such as reducing blunt trauma for ballistic protection equipment, have been studied in terms of thermal and mechanical properties and ballistic protection, obtaining very good results.

6. Strain induced magnetism and superexchange interaction in Cr substituted nanocrystalline cobalt ferrite

Authors: Kumar, L; Kumar, P; Kuncser, V; Greculeasa, S; Sahoo, B; Kar, M

Published: JUN 1 2018, MATERIALS CHEMISTRY AND PHYSICS, 211, DOI: 10.1016/j.matchemphys.2018.02.008

Distribution of magnetic relaxation time using Mossbauer spectroscopy has been observed for CoFe2-xCrxO4 (0.1 <= x <= 0.4) annealed at lower temperature (<= 400 degrees C). The estimated cation distribution over A and B sites observed by Mossbauer spectroscopy is consistent with that obtained by the Rietveld analysis of the X-ray diffraction patterns. Saturation magnetization and magnetocrystalline anisotropy constant increase up to 20% of Cr (x = 0.2) substitution and, decrease with further increase of x. However, the coercive field as well as the Neel temperature decreases with increase in Cr3+ concentration, i.e., a crossover from the hard magnetic to soft magnetic nature by Cr substitution at the Fe site of CoFe2O4 has been observed. This behavior is attributed to the competition between strain induced magnetism and superexchange interaction between cations. (C) 2018 Elsevier B.V. All rights reserved.

7. 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, 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.

8. Significant change of local atomic configurations at surface of reduced activation Eurofer steels induced by hydrogenation treatments

Authors: Greculeasa, SG; Palade, P; Schinteie, G; Kuncser, A; Stanciu, A; Lungu, GA; Porosnicu, C; Lungu, CP; Kuncser, V

Published: APR 30 2017, APPLIED SURFACE SCIENCE, 402, DOI: 10.1016/j.apsusc.2017.01.056

Reduced-activation steels such as Eurofer alloys are candidates for supporting plasma facing components into kamak-like nuclear fusion reactors. In order to investigate the impact of hydrogen/deuterium insertion in their crystalline lattice, annealing treatments in hydrogen atmosphere have been applied on Eurofer slabs. The resulting samples have been analyzed with respect to local structure and atomic configuration both before and after successive annealing treatments, by X-ray diffractometry (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and conversion electron Mossbauer spectroscopy (CEMS). The corroborated data point out for a bcc type structure of the non-hydrogenated alloy, with an average alloy composition approaching Fe0.9Cr0.1 along a depth of about 100 nm. EDS elemental maps do not indicate surface inhomogeneities in concentration whereas the Mossbauer spectra prove significant deviations from a homogeneous alloying. The hydrogenation increases the expulsion of the Cr atoms toward the surface layer and decreases their oxidation, with considerable influence on the surface properties of the steel. The hydrogenation treatment is therefore proposed as a potential alternative for a convenient engineering of the surface of different Fe-Cr based alloys.(C) 2017 Elsevier B.V. All rights reserved.

9. Magnetite-based glass-ceramics prepared by controlled crystallization of borosilicate glasses: Effect of nucleating agents on magnetic properties and relaxation

Authors: Sandu, V; Cimpoiasu, E; Greculeasa, S; Kuncser, A; Nicolescu, MS; Kuncser, V

Published: FEB 15 2017, CERAMICS INTERNATIONAL, 43, DOI: 10.1016/j.ceramint.2016.11.188

The specific magnetic structure and magnetic relaxation phenomena in magnetite nanoentities grown in a glassy matrix by controlled crystallization of Fe-containing borosilicate and boroaluminosilicate glasses in the presence of two types of nucleating agents, Cr2O3 and P2O5, were investigated. The structure, morphology and magnetic properties are strongly influenced by the nucleating agents. Cr2O3 generates magnetite-based glass ceramics with magnetite configurations showing an upward relaxation of magnetization at low and high temperatures but downward at intermediate temperatures. The magnetite grown with P2O5 displays only downward relaxation but with different signs of the temperature derivative of the relaxation rate S in different temperature ranges. The observed effects are discussed with respect to the following factors: i) the existence of a multimodal size distribution of the magnetite (nano)particles as revealed by high resolution electron microscopy; ii) the degree of occupation of different sublattices of the magnetite structure with Fe3+ and Fe2+ ions; and the interplay between the relaxation mechanisms in different temperature ranges.

10. Effect of Cr2O3 on the magnetic properties of magnetite-based glass-ceramics obtained by controlled crystallization of Fe-containing aluminoborosilicate glass

Authors: Sandu, V; Greculeasa, S; Kuncser, A; Nicolescu, MS; Kuncser, V

Published: AUG 2017, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 37, DOI: 10.1016/j.jeurceramsoc.2017.03.055

The phase structure and magnetic properties of magnetite-based glass-ceramics obtained by crystallization of Fe-containing boroaluminosilicate glass melts are presented. The use of Cr2O3 as nucleating agent generated magnetite configurations showing a complex temperature dependence of the relaxation of the remanent magnetization. Specifically, the expected decrease in time of the remanent magnetization occurs only in a limited temperature range, whereas it increases at low and high temperatures (upward relaxation). We tentatively attribute these effects to the complex spin structure of the tiny magnetite nanoparticles, their complex size distribution and the interplay between the relaxation mechanisms in different temperature ranges. (C) 2017 Elsevier Ltd. All rights reserved.

11. Tuning magneto-transport properties of Fe-Au granular thin films by cluster organization

Authors: Stanciu, AE; Kuncser, A; Schinteie, G; Palade, P; Leca, A; Greculeasa, SG; Catrina, A; Kuncser, V

Published: JUL 12 2017, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 50, 275004, DOI: 10.1088/1361-6463/aa7405

A comparative study of morpho-structural, magnetic and magneto-transport properties of two Fe-Au granular films with different concentrations of Fe nanoclusters of almost similar size is reported. Different organizations of the Fe clusters, i.e. in lamellar-like or random-like configuration, were obtained by varying the amount of Fe in the Fe-Au films. The specific magnetic behaviour was investigated with respect to local structure and morpho-structural aspects by combining magneto-optic Kerr effect and superconducting quantum interference device magnetometry, Fe-57 conversion electron Mossbauer spectroscopy and a wide range of electron microscopy techniques. A strong in-plane magnetic texture with uniaxial anisotropy was observed in the case of the lamellar-like organization of the clusters (specific to the Fe-Au film with higher Fe concentration) whereas a superparamagnetic behaviour was evidenced in the case of random distribution of the clusters (specific to the Fe-Au film with lower Fe concentration), despite the similar average size of the clusters in the two samples. Specific magnetoresistance effects were investigated with respect to both the involved magnetic configurations and magnetic interactions of the Fe clusters.

12. MAGNETIC AND MOSSBAUER SPECTROSCOPY STUDY OF Fe-Cr-Al THIN FILMS SPUTTERED ON Si SUBSTRATES

Authors: Greculeasa, SG; Schiniteie, G; Palade, P; Filoti, G; Ghita, IS; Kuncser, V

Published: 2016, ROMANIAN REPORTS IN PHYSICS, 68, DOI:

Fe and Fe-Cr-Al thin films, both as sputtered on Si substrates as well as after subsequent annealing treatments in hydrogen atmosphere, have been investigated by means of X-ray reflectometry, magneto-optic Kerr effect and Mossbauer spectroscopy with respect to structural and related magnetic characteristics. The as deposited films are showing chemically disordered bcc structures, assigned to Fe-Si and Fe-Cr-Al-Si phases, respectively, with soft magnetism and magnetic texture, both influenced by the presence of Si, Cr and Al atoms. The softness of the films as well as the magnetic texture are strongly modified via hydrogenation treatments. In addition, such treatments induce the expulsion of Si, Cr and Al atoms from the as deposited phases, providing almost pure Fe films of much well crystallized bcc structure.

13. Effect of P2O5 on the Structural and Magnetic Properties of Magnetite-Based Glass-Ceramics

Authors: Sandu, V; Greculeasa, S; Kuncser, A; Kuncser, V

Published: DEC 2016, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 99, DOI: 10.1111/jace.14457

The specific morphology and magnetic properties of magnetite-based glass-ceramics obtained by crystallization of Fe-containing borosilicate glassmelts in the presence of P2O5 as nucleating agent are investigated. We found that the distribution of the tiny nanoparticles of magnetite determines the low temperature response to magnetic field. The observed effects are discussed with respect to the following factors: (1) the existence of a multimodal size distribution of the tiny grains as revealed by Mossbauer spectroscopy, magnetometry, and high-resolution electron microscopy; (2) the existence of a disordered layer at the grain surface which is driven by field in a magnetically ordered state; and (3) the interplay between the relaxation mechanisms in different temperature ranges.

14. Be/W and W/Be bilayers deposited on Si substrates with hydrogenated Fe-Cr and Fe-Cr-Al interlayers for plasma facing components

Authors: Greculeasa, SG; Palade, P; Schinteie, G; Lungu, GA; Porosnicu, C; Jepu, I; Lungu, CP; Kuncser, V

Published: DEC 2016, JOURNAL OF NUCLEAR MATERIALS, 481, DOI: 10.1016/j.jnucmat.2016.09.010

Be/W and W/Be bilayers, of interest in regard to the specific behavior of plasma facing components (PFCs) were deposited on Si substrates by thermionic vacuum arc, with Fe, Fe-Cr and Fe-Cr-Al interlayers. The interlayers, with compositions approaching the one of the reduced activation steels used in supporting PFCs, were subsequently annealed in hydrogen atmosphere. The multilayers were characterized with respect to morphologic, structural, diffusional and atomic intermixing aspects via XRD, XRR, X-ray photoemission spectroscopy and Mossbauer spectroscopy. All as-prepared samples present partially amorphous structures. A main alpha-Fe phase is observed, as well as (superparamagnetic) secondary Fe oxides, metallic Fe with Si, Cr, W and Be neighbors, Be-rich Fe-Be and Fe-Si phases. High amounts of tungsten and tungsten oxides were also evidenced in the Fe layer. The strong atomic intermixing of W and Be layers was indirectly supported by the unusual densities of Wand Be layers and Fe-57 Mossbauer spectroscopy results. (C) 2016 Elsevier B.V. All rights reserved.

15. Effects of annealing in Be/W and Be/C bilayers deposited on Si(001) substrates with Fe buffer layers

Authors: Schinteie, G; Greculeasa, SG; Palade, P; Lungu, GA; Porosnicu, C; Jepu, I; Lungu, CP; Filoti, G; Kuncser, V

Published: FEB 2015, JOURNAL OF NUCLEAR MATERIALS, 457, DOI: 10.1016/j.jnucmat.2014.11.029

Atomic intermixing processes in relation to structural aspects and phase formation in Be based thin films subjected to different annealing treatments simulating the case of re-deposited layered structures on plasma facing components in nuclear fusion devices are reported. Accordingly, bilayers of Be/W and Be/C have been deposited on Si(001) substrates with Fe buffer layers. The Fe films have been prepared by radiofrequency sputtering and further processed by annealing in hydrogen atmosphere at 300 degrees C, for 90 min, at a pressure of 10 bars of H-2. After the Be/W and Be/C bilayer deposition by means of thermionic vacuum arc method, annealing in vacuum at 600 degrees C, for 10 min has been applied to the complex structures. The influence of annealing on the phase composition and atomic intermixing processes in the complex structures has been studied by means of X-ray photoelectron spectroscopy (XPS) and conversion electron Mossbauer spectroscopy (CEMS). The layered structures present an oxidation gradient with oxide phases in the uppermost layers and non-oxidized phases in the lower layers, as observed from the XPS data. The CEMS results revealed that the as-deposited structures contain a main metallic Fe phase and secondary superparamagnetic Fe oxide phases at the Fe/Be interface, while annealed samples present a large contribution of Fe-Be and Fe-C mixtures. The annealing treatment induces considerable atomic interdiffusion, strongly dependent on the nature of the upper layer. In the case of Be/W system, the annealing provides a much rougher Be/W interface, while in case of the Be/C structure, the annealing treatment only homogenize the structure over the whole depth. (C) 2014 Elsevier B.V. All rights reserved.

16. Electron Reconfiguration and Enhanced Phonon Activation in the Superconducting State of a FeSe0.3Te0.7 Single Crystal, as Evidenced by Mossbauer Spectroscopy

Authors: Greculeasa, S; Miu, L; Badica, P; Nie, J; Tolea, M; Kuncser, V

Published: JAN 15 2015, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 84, 014701, DOI: 10.7566/JPSJ.84.014701

The Mossbauer spectra of a FeSe0.3Te0.7 single crystal grown by the Bridgman method were analysed across the superconducting transition by considering the interplay between the structure and electron configuration of the transition metal. The magnetically determined superconducting critical temperature is T-C similar to 14 K. The Fe-57 Mossbauer spectra collected in the temperature range from 5 to 200K mainly have an asymmetric doublet pattern, which was conveniently fitted by the full Hamiltonian method. No effective magnetic moment ascribed to the superconducting phase was observed down to 5K. The unusual behaviour observed below similar to 17K for the chemical isomer shift and quadrupole splitting may be associated with an electron reconfiguration process intimately related to an unusual lattice distortion accompanying the superconducting transition. The decreasing trend of the total absorption spectral area and second-order Doppler shift during cooling the sample below the critical temperature, point to enhanced phonon activation in the superconducting state.

17. Tuning structural and magnetic properties of Fe films on Si substrates by hydrogenation processing

Authors: Sandu, SG; Palade, P; Schinteie, G; Birsan, A; Trupina, L; Kuncser, V

Published: FEB 2014, MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS, 181, DOI: 10.1016/j.mseb.2013.11.004

In order to study specific phenomena at ferromagnetic/semiconducting interfaces, of potentially high interest in spintronics and information technology, structural aspects and magnetic properties of Fe thin films grown on Si(001) substrates by RF sputtering have been investigated using Fe-57 conversion electron Mossbauer spectroscopy (CEMS) and magneto-optic Keer effect (MOKE). Films of different thicknesses have been deposited either directly on crystalline Si substrates or on Cu buffer layers. An inherent Fe oxide layer is observed in all as prepared films, with a relative thickness decreasing drastically with the deposition time. The Cu buffer layer does not diminish either the interfacial diffusion or the oxidation process. An efficient method to prepare sharper oxygen- and silicon-free interfaces for an improved spin injection, via thermal treatment in hydrogen atmosphere, is proposed. Accordingly, the hydrogenation treatments are very efficient in the modification of the ferromagnetic film structure, phase composition, magnetic properties and interfacial mixing. (C) 2013 Elsevier B.V. All rights reserved.

18. Engineering Magnetic Properties of Nanostructures via Size Effects and Interphase Interactions

Authors: Kuncser, V; Palade, P; Kuncser, A; Greculeasa, S; Schinteie, G

Published: 2014, SIZE EFFECTS IN NANOSTRUCTURES: BASICS AND APPLICATIONS, 205, DOI: 10.1007/978-3-662-44479-5_7

Finite size effects and interfacial interactions as well as their influence on the magnetic properties of nanosized systems are discussed by starting from very basic principles of magnetism. Some preparation and subsequent processing tools for a proper engineering of the properties of such magnetic nanosized systems are introduced together with specific characterization tools. A summary of the most important technological applications related to size effects and interfacial interactions, with exemplifications starting from bio-medical applications of magnetic fluids to magnetoresitive multilayers for sensor applications are also provided.

19. Spectroscopic Investigation of Iron Substitution in EuCoO3: Related Impact on the Catalytic Properties in the High-Temperature N2O Decomposition

Authors: Wu, Y; Dujardin, C; Granger, P; Tiseanu, C; Sandu, S; Kuncser, V; Parvulescu, VI

Published: JUL 11 2013, JOURNAL OF PHYSICAL CHEMISTRY C, 117, DOI: 10.1021/jp402211c

20. Magnetic properties of BaxSr1-xFe12O19 (x=0.05-0.35) ferrites prepared by different methods

Authors: Cernea, M; Sandu, SG; Galassi, C; Radu, R; Kuncser, V

Published: JUN 5 2013, JOURNAL OF ALLOYS AND COMPOUNDS, 561, DOI: 10.1016/j.jallcom.2013.01.081

Hexagonal ferrites BaxSr1-xFe12O19 (x = 0.05, 0.15, 0.25, 0.35) were prepared by sol-gel and conventional solid state reaction techniques. Their magnetic properties, investigated by magnetometry and Mossbauer spectroscopy, were discussed in correlation to structural aspects. Whereas the hexaferrites obtained by sol-gel are almost single phase, the ones prepared by the conventional solid state reaction technique present a low amount of BaFe2O4 as secondary phase. All samples show a relative high coercive field and remanent magnetization, as specific features for permanent magnet behavior. The five Fe3+ sites of the specific elemental cell have been revealed and their relative occupancy has been derived from the Mossbauer spectra. The analyzed samples show a sensitivity of the magnetic properties (magnetic moment and anisotropy) with respect to the occupancy of the different Fe sites. (C) 2013 Elsevier B. V. All rights reserved.

21. Interface characterization and atomic intermixing processes in Be/W bilayers deposited on Si(001) substrates with Fe buffer layers

Authors: Kuncser, V; Palade, P; Schinteie, G; Sandu, SG; Trupina, L; Lungu, GA; Gheorghe, NG; Teodorescu, CM; Porosnicu, C; Jepu, I; Lungu, CP; Filoti, G

Published: JAN 25 2012, JOURNAL OF ALLOYS AND COMPOUNDS, 512, DOI: 10.1016/j.jallcom.2011.09.063

Structural aspects and atomic intermixing processes in Be/W bilayers deposited on Si(0 0 1) substrates with Fe buffer layers enriched in the (57)Fe Mossbauer isotope have been studied via atomic force microscopy, grazing incidence X-ray diffractometry, X-ray reflectometry, X-ray photoelectron spectroscopy and conversion electron Mossbauer spectroscopy. The mentioned investigations allowed a full sequential characterization of the involved interfaces. Various ionic configurations appeared for Fe or W, while an amorphous state was observed in the case of Be. It has been proven that the Be layer has a negative influence on the roughness of the whole structure, which however presents an oxidation gradient from more oxidized elements at the surface towards more reduced elements in deeper layers. A strong diffusion of the W atoms inside the Be layer, induced by the deposition method, as well as of the Fe atoms inside the Be layer, induced by thermal annealing, has been evidenced. (C) 2011 Elsevier B.V. All rights reserved.

Dr. Simona Greculeasa

Scientific Researcher