National Institute Of Materials Physics - Romania

Publications

903 articles found

661. Criteria for the occurrence of negative resonant magnetoresistance in magnetic tunnel junctions

Authors: Tolea, M; Aldea, A; Tolea, F

Published: SEP 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, R86, DOI: 10.1002/pssb.200642347

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.

662. Mossbauer spectroscopy and magnetic measurements on Fe-Ni-Co-Ti shape memory alloys

Authors: Tolea, F; Schinteie, G; Popescu, B

Published: AUG 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1506, DOI:

Fe-Ni-Co-Ti shape memory alloys were prepared by various techniques. A comparison between samples prepared by classical metallurgy and by melt spinning technique is performed in respect to the microscopic mechanisms responding for the shape memory effects. X-ray diffraction, thermomagnetic measurements and Mossbauer spectroscopy were applied for a complete structural and magnetic characterization. Only samples supporting an aging treatment gave evidence for shape memory effects and correlated structural transformations.

663. Synthesis, structure and properties of heterotrinuclear carboxylate complexes [Fe2M(Ca, Sr, Ba)O(CCl3COO)(6)(THF)n]

Authors: Prodius, D; Turta, C; Mereacre, V; Shova, S; Gdaniec, M; Simonov, Y; Lipkowski, J; Kuncser, V; Filoti, G; Caneschi, A

Published: JUL 10 2006, POLYHEDRON, 25, 2182, DOI: 10.1016/j.poly.2006.01.019

Three novel mu(3)-oxo trinuclear carboxylates [Fe2CaO(CCl3COO),(THF)(4)] center dot THF (1), [Fe2SrO(CCl3COO)(6)(THF)(6)] center dot 0.5H(2)O center dot 0.5THF (2) and [Fe2BaO(CCl3COO)(6)(THF)(6)] center dot 0.5H(2)O center dot 0.5THF (3) have been synthesised and subsequently characterised by X-ray structure analysis, thermal gravimetry (TG), infra red (IR) spectroscopy and magnetic measurements. Compounds I and 3 are molecular complexes with s- and d-metals displayed in the apexes of isosceles triangles with a triply bridging oxygen atom at its centre. The Ca center dot center dot center dot Fe and Ba center dot center dot center dot Fe separations have the values 3.750(1) and 4.177(1) angstrom, respectively. In comparison with these. Fe center dot center dot center dot Fe separations, 3.229(2) angstrom in 1 and 3.201(1) angstrom in 3, appear much shorter. The molecular complexes obey C-2 symmetry with the twofold axis passing through s metal and mu(3)-oxo atom. The coordination numbers are 6, 7 and 9 for iron. calcium and barium correspondingly. Magnetic studies reveal the presence of an antiferromagnetic exchange in the isosceles triangular skeletons of the trinuclear species. Using the spin Hamiltonian H = -2J(12)[S1S2], the fitting parameters g(Fe) = 2.00. J(Fe-Fe) = -58.90 cm(-1) for 1 and -60.40 cm(-1) for 3; g(Fe) = 2.14, J(Fe-Fe) = -75.40 cm(-1) for 2 have been estimated. (c) 2006 Elsevier Ltd. All rights reserved.

664. S-wave spin-triplet order in superconductors without inversion symmetry: Li2Pd3B and Li2Pt3B

Authors: Yuan, HQ; Agterberg, DF; Hayashi, N; Badica, P; Vandervelde, D; Togano, K; Sigrist, M; Salamon, MB

Published: JUL 7 2006, PHYSICAL REVIEW LETTERS, 97, DOI: 10.1103/PhysRevLett.97.017006

We investigate the order parameter of noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B via the behavior of the penetration depth lambda(T). The low-temperature penetration depth shows BCS-like behavior in Li2Pd3B, while in Li2Pt3B it follows a linear temperature dependence. We propose that broken inversion symmetry and the accompanying antisymmetric spin-orbit coupling, which admix spin-singlet and spin-triplet pairing, are responsible for this behavior. The triplet contribution is weak in Li2Pd3B, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in Li2Pt3B allows the spin-triplet component to be larger in Li2Pt3B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of lambda(T). The experimental data are in quantitative agreement with theory.

665. A new growth method of high-quality precipitate-free HTS thin films applied for electronics

Authors: Endo, K; Badica, P; Sato, H; Akoh, H

Published: JUL 2006, JOURNAL OF ELECTROCERAMICS, 16, 603, DOI: 10.1007/s10832-006-9926-9

We report successful growth of high-quality thin films with clean and completely precipitate-free surface, suitable for device applications, by applying a new concept and method to the substrates. The concept consists in generation of artificial steps of controlled height and width, and desired shape on the surface of the substrate. The width of the step is chosen so that it is equal to the double of the migration (surface diffusion) length of the atomic species in the growth process of the film. If precipitates occur, they will be selectively gathered to the step edge where the free energy is lowest. Using this new method, we have successfully obtained by MOCVD high-quality precipitate-free Bi-2223 and Bi-2223/Bi-2212-superlattice thin films on (100) SrTiO3 substrates with artificial steps of controlled width and height. These as-grown films have been further used to fabricate patterned intrinsic Josephson junctions. Completely precipitate-free films offer a strong advantage for integration, and generate new possibilities for the device fabrication.

666. Mossbauer spectral study of RFe11.3W0.7 compounds (R = Dy, Ho, Er, and Lu)

Authors: Lazar, DP; Plugaru, N; Kuncser, V; Valeanu, M; Filoti, G; Bartolome, J; Rubin, J

Published: JUL 2006, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 302, 67, DOI: 10.1016/j.jmmm.2005.08.030

The series of compounds RFe11.3W0.7, where R = Dy, Ho, Er, and Lu, has been measured by Mossbauer spectroscopy between 15 and 295 K. A model which takes into account the Wigner-Seitz cell local environment of each iron site, the distribution of tungsten atoms on the 8i site, and the temperature-dependent magnetic structures of the compounds has been used to analyse the spectra. The assignment and the temperature dependencies of the hyperfine fields and the isomer shifts are in complete agreement with the Wigner-Seitz; cell analysis of the three iron sites in RFe11.3W0.7 compounds. The plausible orientations of the iron magnetic moments in the canted magnetic phase of ErFe11.3W0.7 compound have been established by a thorough Mossbauer spectral analysis. The Mossbauer spectra of DyFe11.3W0.7 clearly show the influence of the two spin re-orientations occurring in this compound and indicate the directions of the iron magnetic moments in both canted and planar magnetic arrangements. (c) 2005 Elsevier B.V. All rights reserved.

667. Modification of the sperconducting parameters of Bi-Sr-Ca-Cu-O by iodine intercalation

Authors: Mihalache, V; Aldica, G; Miu, D

Published: JUN 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1291, DOI:

We investigated the superconducting parameters of iodine-intercalated BiSrCaCuO high-temperature superconducting materials. While for samples containing a significant amount of Bi2Sr2CaCu2O8+x, (Bi-2212) iodine intercalation results in the dramatic decrease of the superconducting parameters, it was found that for samples with a nearly single Bi2Sr2Ca2Cu3O10-y (Bi-2223) phase these parameters can increase. We argue that this different behavior of the superconducting parameters is the result of a different doping regime, and can be explained based on the parabolic dependence between T, and the number of holes per CuO2 layer. Iodine intercalation appears to be an efficient way to bring the usually under-doped superconductor to its optimal T-c and flux pinning properties.

668. Optical and electronic properties of (Fe+Sb): PVA for real time holography

Authors: Kuncser, V; Bulinski, M; Krautwald, S; Franke, H; Wagner, FE; Cristea, D; Palade, P; Plapcianu, C; Filoti, G

Published: JUN 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1229, DOI:

The electron transfer processes induced by the ultraviolet exposure of the Fe and Sb doped polyvinyl alcohol (PVA) thin films were studied in comparison with those appeared in the correspondent mono (Fe or Sb) doped ones. Samples with different thickness and subjected to different exposures were analysed by optical absorption as well as by Fe-57 and Sb-121 Mossbauer spectroscopy. The influence of each element from the pair on the electronic mechanism activated by the ultraviolet exposure was investigated. Accordingly, the (Fe+Sb) doped PVA presents improved electronic characteristics, in respect to the specific behaviour of single metal doped polymer recording media for real time holography.

669. Hydrogen storage in Mg-Ni-Fe compounds prepared by melt spinning and ball milling

Authors: Palade, P; Sartori, S; Maddalena, A; Principi, G; Lo Russo, S; Lazarescu, M; Schinteie, G; Kuncser, V; Filoti, G

Published: MAY 18 2006, JOURNAL OF ALLOYS AND COMPOUNDS, 415, 176, DOI: 10.1016/j.jallcom.2005.08.017

Magnesium-rich Mg-Ni-Fe intermetallic compounds have been prepared by two different routes: (a) short time ball milling of ribbons obtained by melt spinning; (b) long time ball milling of a mixture of MgH2, Ni and Fe powders. The first type of samples displays an hydrogen desorption kinetics better than the second one. Pressure composition isotherm measurements exhibit for both type of samples two plateaux, the lower and wider corresponding to the MgH2 phase and the upper and shorter corresponding to the Mg2NiH4 phase. The presence of the two types of hydrides is confirmed by X-ray diffraction analysis. Mossbauer spectroscopy shows that in melt spun and subsequently milled samples iron is mainly in a disordered structure and segregates after hydrogenation, while in directly milled powders remains mainly unalloyed. After multiple hydrogen absorption/desorption cycles the main part of iron is in metallic state in samples of both types, those of first type preserving better hydrogen desorption kinetics. (c) 2005 Elsevier B.V. All rights reserved.

670. Magnetic behaviour of UB4 at high temperatures

Authors: Galatanu, A; Yamamoto, E; Haga, Y; Onuki, Y

Published: MAY 1 2006, PHYSICA B-CONDENSED MATTER, 378-80, 1000, DOI: 10.1016/j.physb.2006.01.383

UB4 is a moderate heavy fermion compound which does not exhibit my magnetic order down to 0.3 K. In this work, the magnetic properties of UB4 are investigated in an extended temperature range, 2-800 K. on high-quality single crystals. A cross-over from a low temperature 5f itinerant behaviour to a high temperature 5f localized behaviour is observed, marked by broad peaks in the magnetic susceptibility measured along the main crystallographic axes. (c) 2006 Elsevier B.V. All rights reserved.