National Institute Of Materials Physics - Romania

The low-voltage-level current-voltage characteristics of Bi2Sr2Ca2Cu3O10+x tapes and bulk samples with preferential crystallite orientation were measured in applied magnetic fields both parallel and perpendicular to the samples' surfaces. The data were interpreted within the framework of the collective pinning model. The collective pinning exponent alpha changes from approximate to 1 to approximate to 0.5 (close to T-c), while in the case of unoriented samples it is constant (and equal to the theoretical value 1/2) as shown in an earlier paper. We suggest that this temperature dependence of alpha is due to the change of the dimensionality of the intragranular vortices.

Polycrystalline p-type InxGa1-xSb (x=0.20) for optoelectronic devices has been obtained by rapid direct synthesis from high-purity (99.9999%) In, Ga, and Sb. The uniformity of the composition has been checked by neutron activation analysis and also by x-ray diffractometry (Vegard-law approximation). From Hall-effect measurements an acceptor concentration of 3.2x10(17) cm(-3) and a mobility of 628 cm(2)A/ s for holes, at liquid nitrogen temperature, have been determined, and from absorption measurements, a quite diffuse edge for the optical absorption coefficient resulted. This paper describes the method of rapid direct synthesis of the ternary compound and the techniques used for the characterization of this material for special optoelectronic applications. (C) 1996 Society of Photo-Optical Instrumentation Engineers.

The crystallization kinetics of the amorphous alloy Fe77Gd3B20 were examined under isothermal flash annealing conditions both in air and in a vacuum. Magnetic measurements were used to determine the reaction order (n) and activation energy (E(a)), and Mossbauer spectroscopy (MS) was used to identify some resulting crystalline phases.

TIN, films are deposited on glass in a modified de planar three-magnetron geometry. The ellipsometric and reflectance measurements reveal the influence of the substrate bias and of the Ar:N-2 partial pressure ratio on the stoichiometry and optical properties of the films. The stoichiometric structure is obtained for only certain bias voltage and Ar:N, ratio values. The value of the plasma energy omega(p) confirmed the film stoichiometry, as determined from x-ray diffractometry. (C) 1996 Society of Photo-Optical instrumentation Engineers.

From a Mossbauer spectroscopy study of-the BiPbSrCaFe6+z ferrate, prepared under different conditions, strong evidence for a variety of Morin-like spin-reorientation processes similar to the ones occuring in hematite was found. The phase evolution, their energetical and dynamical Mossbauer parameters as well as the spin-rotational process are described in detail and finally correlated with a proposed mechanism. (C) 1996 American Institute of Physics.

The effect of deposition parameters such as substrate temperature and argon pressure on the structure of continuous CrNi(65:35), CrNi(50:50) and CrNi(20:50) thin films r.f. sputtered onto air-cleaved and electrically coloured alkali halide substrates are investigated by transmission electron diffraction. The observed phases are: nanocrystalline phases belonging to the equilibrium solid state solutions [gamma-Ni(fcc) and alpha-Cr(bcc)] and CrO (NaCl-type) oxide; single icosahedral amorphous phase; amorphous separated phases, the icosahedral amorphous phase corresponding to CrNi and the other to CrO (NaCl-type) oxide; an icosahedral amorphous phase with alpha-Cr (bcc) solid solution nanocrystals; nanocrystalline phases belonging to gamma-Ni(fcc) solid solution and the CrO (NaCl-type) oxide; nanocrystalline phases belonging to [gamma-NiC(fcc) and alpha-Cr(bcc)] solid solutions and the CrO (NaCl-type) oxide; microcrystalline phases belonging to [gamma-Ni(fcc) and alpha-Cr(bcc)] solid solutions and CrO (NaCl-type) oxide. Thermodynamic and kinetic arguments are put forward in order to explain the complex dependences in the growth of various phases and their relation to the equilibrium phase diagram of the bulk Cr-Ni system.

Au L(3) x-ray-absorption near-edge spectra were measured with synchrotron radiation on a series of Au-X compounds, with X=Ga, In, Ge, and Sn. The white-line feature of the spectra, as a rough measure of charge transfer upon compound formation, was related to the local composition around the Au atoms. This enabled us to estimate the local An environment, in cases where its knowledge is missing.

In this paper we describe a modified liquid phase separation (double crucible) method for growth of superconducting Bi-Sr-Ca-Cu-O single crystals. We used alumina crucibles. The morphology and composition of as-grown crystals were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The stacking thin-plate single crystals of maximum 5 x 6 x 0.1 mm(3) were obtained. (C) 1997 Elsevier Science Ltd. All rights reserved.

Bi-Pb-Sr-Ca-Cu-O samples were produced by the spray-frozen, vacuum-freeze drying method from nitrate solutions. Structural modifications of crystallites during sintering were investigated in agreement with bulk density, phase structure of the samples, critical temperature T-c and critical current density j(c). A model for morphology changes of (2223) crystallites during sintering was proposed. Maximum j(c) in a zero magnetic field is reached on samples sintered for 200-250 h.