National Institute Of Materials Physics - Romania

Publications

892 articles found

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

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

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

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

512. Non-centro-symmetric superconductors Li2Pd3B and Li-2(Pd0.8Pt0.2)(3)B: amplitude and phase fluctuation analysis of the experimental magnetization data

Authors: Badica, P; Salem-Sugui, S; Alvarenga, AD; Jakob, G

Published: OCT 2010, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 23, DOI: 10.1088/0953-2048/23/10/105018

We report on magnetization data obtained as a function of temperature and magnetic field in Li-2(Pd0.8Pt0.2)(3)B and Li2Pd3B non-centro-symmetric superconductors. Reversible magnetization curves were plotted as M-1/2 versus T. This allows study of the asymptotic behavior of the averaged order parameter amplitude (gap) near the superconducting transition. Results of the analysis show, as expected, a mean field superconducting transition for Li2Pd3B. By contrast, a large deviation from the mean field behavior is revealed for Li-2(Pd0.8Pt0.2)(3)B. This is interpreted as due to the strength of the non-s-wave spin-triplet pairing in this Pt-containing compound which produces nodes in the order parameter and, consequently, phase fluctuations. The diamagnetic signal above T-c(H) in Li2Pd3B is well explained by superconducting Gaussian fluctuations and agrees with the observed mean field transition. For Li-2(Pd0.8Pt0.2)(3)B the diamagnetic signal above T-c(H) is much higher than the expected Gaussian values and appears to be well explained by three-dimensional critical fluctuations of the lowest-Landau-level type, which somehow agrees with the scenario of a phase mediated transition.

513. Flux jumps anomalous behaviour in FAST-processed MgB2 composites

Authors: Aldica, G; Badica, P; Plapcianu, C; Groza, JR; Popa, S

Published: SEP 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 2003, DOI:

Superconductor MgB2 has a significant potential for practical applications, but undesirable flux jumps in magnetization curves at low temperature and at moderate or intermediate magnetic fields prevent its widespread use. In the present work, we analyzed the flux-jumps tendency in MgB2 bulk doped with SiC, B4C and metallic Mg. Samples were prepared by field-assisted-sintering (FAST) as composites with up to three intercalated Fe-foils. Samples produced by FAST showed quite different phase structures clearly influencing pinning and thermo-magnetic instability features due to inhomogeneous materials structure or due to dopants. The largest flux jumps are observed in samples with high critical current density, as usually observed in conventional MgB2 bulks. However, jumps patterns are different and do not perfectly follow the jumps critical current dependence, especially for the samples with Mg-addition.

514. Improvement of the critical current density of spark plasma sintered MgB2 by C-60 addition

Authors: Miu, L; Aldica, G; Badica, P; Ivan, I; Miu, D; Jakob, G

Published: SEP 2010, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 23, DOI: 10.1088/0953-2048/23/9/095002

We investigated the influence of fullerene (C-60) addition on the superconducting parameters of MgB2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5 wt% C-60 addition leads to the reduction of the critical temperature by a few K, whereas the magnetically determined critical current density increases by approximately one order of magnitude in an applied magnetic field H = 70 kOe at temperature T = 10 K. This indicates substitution of B by C in C-60 added MgB2 processed by SPS, contrary to some data reported in the literature obtained using conventional methods. For our relatively large specimens, the occurrence of macroscale flux jumps was observed over a wide (H, T) domain. The relaxation of the irreversible magnetization suggests that the macroscale flux jumps transform into microscale flux jumps with increasing H and T

515. Interfacial atomic diffusion in AF/Fe/Cu/Fe (AF=Fe50Mn50 and Ir50Mn50) multilayer systems

Authors: Kuncser, V; Keune, W; von Hoersten, U; Schinteie, G; Stefan, N; Palade, P; Filoti, G

Published: AUG 31 2010, THIN SOLID FILMS, 518, 5985, DOI: 10.1016/j.tsf.2010.05.100

Spin valve like AF/Fe/Cu/Fe (AF = Fe50Mn50 and Ir50Mn50) multilayer systems have been prepared by molecular beam epitaxy. Thin tracer layers enriched in the Fe-57 isotope were artificially grown at the AF/Fe and Fe/Cu interfaces and the interfacial atomic diffusion was observed via Fe-57 conversion electron Mossbauer spectroscopy. The results show that the atomic interdiffusion at all involved interfaces is lower in the IrMn based structures as compared to the FeMn based ones. (C) 2010 Elsevier B.V. All rights reserved

516. Thermodynamic destabilization of Li-N-H system by Si addition

Authors: Palade, P; Lungu, GA; Husanu, AM

Published: AUG 27 2010, JOURNAL OF ALLOYS AND COMPOUNDS, 505, DOI: 10.1016/j.jallcom.2010.03.249

Li-N-H and Li-Si-N-H composites have been synthesized by ball milling starting from lithium amide, lithium hydride and silicon powders. Hydrogen sorption behaviour was investigated for the Si-containing composites and a Li-N-H reference sample. The as-milled Si-containing sample showed the presence of the constituent powders but after few absorption-desorption cycles an important amount of Li(2)SiN(2) appeared in both de-hydrogenated and re-hydrogenated composites. This phase was present in higher amount in the de-hydrogenated sample than in the re-hydrogenated one. The presence of this phase was also confirmed by XPS measurements. Li-N-H and Li-Si-N-H composites in de-hydrogenated state also contained lithium imide. X-ray diffraction data indicated a reversible hydrogen generating reaction between lithium amide, lithium hydride and silicon to produce Li(2)SiN(2). lithium imide and hydrogen. The plateau pressure of Li-N-Si-H system is twice the one for Li-N-H at the same temperature (about 265 degrees C), as shown by pressure-composition isotherms in desorption mode. For the first time it has been observed a thermodynamic destabilization of Li-N-H by mixing it with Si. (C) 2010 Elsevier B.V. All rights reserved.

517. Comparison of the critical current density of YBCO films obtained by dc sputtering and pulsed laser deposition

Authors: Ivan, I; Miu, L; Popa, S; Miu, D; Mele, P; Matsumoto, K

Published: AUG 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1245, DOI:

We measured the critical current density J(c) of optimally doped YBa(2)Cu(3)O(7) (- delta) thin films obtained by dc sputtering on (100) oriented SrTiO(3) substrates and by pulsed laser deposition (PLD) on (100) oriented MgO substrates with SrTiO(3) buffers. J(c) at various temperatures T was determined from the magnetization curves registered with a vibrating sample magnetometer for an external magnetic field H (oriented along the c axis) up to 90 kOe. It was found that J(c) of the films obtained by PLD usually overcomes the J(c) values of the sputtered films in the whole investigated (H, T) domain.

518. Magnetic properties of Fe-Co ferromagnetic layers and Fe-Mn/Fe-Co bilayers obtained by thermo-ionic vacuum arc

Authors: Kuncser, V; Schinteie, G; Palade, P; Jepu, I; Mustata, I; Lungu, CP; Miculescu, F; Filoti, G

Published: JUN 4 2010, JOURNAL OF ALLOYS AND COMPOUNDS, 499, 29, DOI: 10.1016/j.jallcom.2010.03.129

Simple Fe-Co ferromagnetic films and Fe-Mn antiferromagnetic films of different compositions were grown on epitaxial Si substrates with Ta buffer layers via thermo-ionic vacuum arc method. Fe-Co/Fe-Mn bilayer structures were obtained by following similar growing conditions as for the simple films. The magnetic behavior concerning easy axis distribution, anisotropy energy and coercive field of the Fe-Co films exchange coupled to the antiferromagnetic layers were discussed with respect to the magnetic behavior of the simple, uncoupled, ferromagnetic films. The composition of the antiferromagnetic films has a sensible influence on the magnetic parameters of the exchange coupled ferromagnetic films. The largest coupling and the narrowest easy axis distribution are induced in the bilayer system with equatomic composition of the Fe-Mn antiferromagnetic layer. (C) 2010 Elsevier B.V. All rights reserved.

519. Martensitic transformation and accompanying magnetic changes in Ni-Fe-Ga-Co alloys

Authors: Sofronie, M; Tolea, F; Kuncser, V; Valeanu, M

Published: JUN 1 2010, JOURNAL OF APPLIED PHYSICS, 107, DOI: 10.1063/1.3429231

Two series of cobalt substituted Ni-Fe-Ga ferromagnetic shape memory alloys with lower gallium content (< 27 at. %) were studied by differential scanning calorimetry, x-ray diffraction, and thermomagnetic measurements. Co substitution for Fe or Ni promotes an increase in the Curie temperatures and a variation in the martensitic transformation (MT) temperatures, in accord with the alloy valence electron concentration change. For alloys with MTs below room temperature, a field dependent thermomagnetic hysteresis was evidenced and discussed in connection with the effect of cobalt substitution, on the magnetic hardness of the martensite phase. A direct interpretation of the evolution of the thermomagnetic hysteresis versus the applied field was provided. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3429231]

520. Inter layer magnetic coupling in exchange bias and spin valve structures with Fe-Mn and Ir-Mn antiferromagnetic layers

Authors: Kuncser, V; Keune, W; von Horsten, U; Schinteie, G

Published: JUN 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 1393, DOI:

Exchange bias AF/Fe and spin valve AF/Fe/Cu/Fe (AF=Fe50Mn50 and Ir50Mn50) multilayer systems have been prepared by molecular beam epitaxy. Thin tracer layers enriched in the Fe-57 isotope were artificially grown at the AF/Fe interface and the phase composition of the ferromagnetic layer, as well as the interfacial atomic diffusion were observed via Fe-57 conversion electron Mossbauer spectroscopy. The dependence of the magnetization reversal process on training and temperature associated effects was studied by low temperature vibrating sample magnetometry, whereas the interlayer magnetic coupling was analyzed via longitudinal magneto-optic Kerr effect.