National Institute Of Materials Physics - Romania

Publications

892 articles found

701. Magnetization measurements on Li2Pd3B superconductor

Authors: Badica, P; Kondo, T; Kudo, T; Nakamori, Y; Orimo, S; Togano, K

Published: NOV 8 2004, APPLIED PHYSICS LETTERS, 85, 4435, DOI: 10.1063/1.1814433

Magnetization in dc magnetic fields and at different temperatures has been measured on the antiperovskite Li2Pd3B with a cubic structure composed of distorted Pd6B octahedrons. This material was recently found to exhibit superconductivity at 7-8 K. The critical fields H-c1(0) and H-c2(0) are determined to be 135 Oe and 4 T, respectively. Critical current density, scaling of the pinning force within the Kramer model, and irreversibility field data are presented. Several superconductivity parameters were deduced: Coherence length xi=9.1 nm, penetration depth lambda=194 nm, and Ginzburg-Landau parameter kappa=21. The material resembles other boride superconductors from the investigated points of view. (C) 2004 American Institute of Physics.

702. Growth of high-quality precipitate free thin films suitable for electronic devices: A new concept for substrates

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

Published: NOV 4 2004, ADVANCED MATERIALS, 16, +, DOI: 10.1002/adma.200400750

Precipitate-free thin films of multicomponent materials (e.g. superconductors) have been successfully grown (see Figure) on substrates onto whose surfaces artificial steps have been generated. The steps had widths that were equal to twice the migration length of the atomic species being deposited on the films. The resulting clean, high-quality films were due to the gathering of precipitates at the step edges, where the free energy is lowest.

703. 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.

704. Optical and electronic properties of polyvinyl alcohol doped with pairs of mixed valence metal ions

Authors: Bulinski, M; Kuncser, V; Plapcianu, C; Krautwald, S; Franke, H; Rotaru, P; Filoti, G

Published: SEP 7 2004, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 37, 2441, DOI: 10.1088/0022-3727/37/17/015

The electronic mechanisms induced by the UV exposure of thin films of polyvinyl alcohol doped with pairs of mixed valence metal ions were studied in relation to their optical behaviour by Mossbauer spectroscopy and optical absorption. The results obtained definitely point to the role of each element from the pair in the electronic mechanism involved, with influence on the optical properties regarding applications in real-time holography and integrated optics.

705. Application of elevated magnetic fields during growth of BiSrCaCuO superconducting whiskers and studies of growth defects for better understanding of the growth mechanism

Authors: Badica, P; Togano, K; Awaji, S; Watanabe, K; Iyo, A; Kumakura, H

Published: SEP 1 2004, JOURNAL OF CRYSTAL GROWTH, 269, 534, DOI: 10.1016/j.jcrysgro.2004.05.115

Superconducting whiskers of Bi-2212 have been grown by an airtight crucible method in elevated magnetic fields, up to 10 T. Growth behavior with and without application of a magnetic field, as well as the SEM detailed study of the growth defects of the whiskers allow to emphasize some aspects of the growth mechanism. The conclusion is that whiskers are growing mainly through continuous crystallization at the base-end and from the so-called 'micro-crucible' formed by the glassy or powdered substrates. The supply of cations by a vapor transfer is very probable, but is taking place to the micro-crucible reaction site rather than to the droplet, as in the classic vapor-liquid-solid (VLS) growth mechanism. Therefore, whiskers grow by a modified VLS-microcrucible mechanism (MVLS-MC). Application of magnetic fields during growth has proven to be a powerful tool to investigate and influence some processes. Observation of stoichiometric Bi(Pb)-2212, CuO and Ca2CuO3 whiskers is also reported. (C) 2004 Elsevier B.V. All rights reserved.

706. FexCu1-x alloys for permanent magnets

Authors: Kappel, W; Codescu, MM; Pasuk, I; Patroi, E; Kuncser, V; Valeanu, M; Predoi, D; Filoti, G

Published: SEP 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 978, DOI:

New magnetic composite materials, ribbon samples, with nominal compositions FexCu1-x (x=0.2; 0.5; 0.8) were obtained by the melt spinning technique. Various information was provided via X-ray diffraction, Mossbauer and magnetic measurements. The structural aspects: phase composition, size and texture effects, have been corroborated with magnetic properties. Finally, the possibility to choose the adequate composition and the suitable processing in order to obtain desired anisotropic magnetic materials were presented.

707. Effect of Li2CO3 addition on the Bi1.7Pb0.4Sr1.5Ca2.5CU3.6Ox superconducting system

Authors: Mihalache, V; Aldica, G; Popa, S; Lifei, F; Miu, D

Published: SEP 2004, MATERIALS LETTERS, 58, 3044, DOI: 10.1016/j.matlet.2004.05.037

We have investigated the effect of Li2CO3 addition on the transport and magnetic properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox, samples in connection with the structural changes observed by scanning electronic microscopy (SEM) and X-ray diffraction. In addition to the results reported in literature, we have extracted new information from the magnetic hysteresis loops and AC susceptibility measurements. One such information is the splitting of the AC loss peak corresponding to the dissipation inside the Bi-2223 grains. The results reported here have been compared with those obtained for Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) and Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) specimens, and the effect of halogen is pointed out. (C) 2004 Elsevier B.V. All rights reserved.

708. Vortex imaging and local magnetization studies in HTS by scanning Hall probe microscopy

Authors: Crisan, A; Pross, A; Cole, D; Bending, S

Published: AUG 2004, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 408, 557, DOI: 10.1016/j.physc.2004.03.199

We have used scanning Hall probe microscopy to correlate vortex images and local magnetisation measurements in two different high temperature superconducting samples. Near the edge of a continuous YBCO thin film we have observed local hysteresis inversion and negative remanent fields, which can be semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinitely long superconducting strip. In a YBCO film containing a regular square array of antidots we have further find that vortices trapped at antidots exhibit an unusual behaviour upon field sweep reversal. (C) 2004 Elsevier B.V. All rights reserved.

709. Enhancement of critical current density in YBa2Cu3O7-delta thin films grown using PLD on YSZ (001) surface modified with Ag nano-dots

Authors: Ionescu, M; Li, AH; Zhao, Y; Liu, HK; Crisan, A

Published: JUL 7 2004, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 37, 1828, DOI: 10.1088/0022-3727/37/13/014

Y123 thin films were grown by pulsed laser deposition (PLD) on YSZ (001) substrate. Prior to the film deposition, a discontinuous layer of Ag was deposited on the substrate, also using PLD, in the form of separate islands. Atomic force microscopy (AFM) investigation of the Ag layer showed that its morphology consisted of self-assembled islands of nanometre size, randomly distributed on the surface of the substrate, called nano-dots. The Y123 superconducting films grown on such a surface were characterized using AFM, x-ray diffraction, secondary electron microscopy, ac susceptibility and dc magnetization. The results show that there is no significant difference in surface morphology, crystallographic orientation, phase composition or superconducting transition temperature between the Y123 films grown on YSZ (001) with an Ag nano-dots layer and a control Y123 film grown on a virgin YSZ (001) surface. On the other hand, at 77 K, the magnetic critical current density (J(c)(m)) was three times higher for the Y123 film grown on YSZ with the modified (001) surface than for the film grown on YSZ with a virgin (001) surface. At 5 K the enhancement of J(c)(m) was approximately seven times, at both low and high fields. This suggests an increase in pinning, caused presumably by point defects formed in the Y123 film above the Ag islands.

710. A modified airtight two-crucible method for growth of Bi-2212 whiskers from glassy pellets

Authors: Badica, P; Togano, K; Kumakura, H

Published: JUL 2004, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 17, 898, DOI: 10.1088/0953-2048/17/7/011

Bi2Sr2CaCu2Ox, whiskers have been grown by a modified airtight two-crucible method, employing radial and vertical thermal gradients resulting from the relative position of the crucibles. A heating profile with a constant heating rate applied from 842 to 846degreesC allows formation of whiskers up to 2 cm in length for a certain level of Al impurity in the glass. A cation-rich atmosphere is found to be a key intrinsic parameter. Our data confirm the importance of the thermal gradients, but this parameter is a somewhat extrinsic one and from a practical point of view should be correlated to other intrinsic parameters that influence growth atmosphere (i.e. to total and partial pressures). As-grown whiskers have composition Bi:Sr:Ca:Cu = (2.9-3.5):2:(1.7-2.05):(2.4-3.0) (normalized to SrA zero-resistance critical temperature T-c(R=0) = 76-98 K and maximum transport critical current density above J(c) = 10(4) A cm(-2) at 77 K in self-field.