National Institute Of Materials Physics - Romania

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.

We have determined the valence band discontinuity Delta E(upsilon) of strained Si1-xGex on unstrained (100) Si using temperature-dependent current-voltage characteristics (I-V-T) of Si/Si0.83Ge0.17/Si heterostructures. In a first step, the measurements were performed on a Schottky diode, and in a second step, on a sample with ohmic contacts. The values of Delta E(upsilon) obtained by these two different procedures are comparable. Moreover, they are in good agreement with the theoretical value of Delta E(upsilon)=0.84x=143 meV predicted by Van de Walle measurements. (C) 1996 American Institute of Physics.

Several iron complexes containing a tri-iron-oxo-cation linked to short chain fatty acids have been used in the preparation of films by self-assembling from solution on glass slides via two procedures: direct evaporation and dragging out. The ordered lamellar structure thus obtained has been investigated by X-ray diffraction. A relation between the lamellar spacing and the disorder in the film has been obtained from the X-ray diffraction data.

The Hall effect and magnetoresistance of a mesoscopic system having a four-probe ring geometry are calculated in a tight-binding model, either ordered or disordered, as function of magnetic flux and Fermi energy. The changes induced in the spectrum by the magnetic field affect differently the Aharonov-Bohm (AB) oscillating pattern of the two resistances: after the gap reaches the Fermi energy, the Hall effect remains bounded while the magnetoresistance increases exponentially with the sample size. The results are shown to depend significantly on the number and geometry of the probes.

Temperature induced surface transitions in liquid crystal tilted homogeneous cells are experimentally studied. The results are theoretically explained by an elastic theory of surface transitions and the resulting values for the effective splay-bend elastic constant (k) over bar(13) for each liquid crystal material are compared with the results from experience with other types of surface anchoring.