National Institute Of Materials Physics - Romania

Melt-spun Cu80Co10Fe10 ribbons are shown to exhibit a nanogranular structure with magnetic alpha-Fe(Co) grains segregated from the Cu-rich matrix after appropriate annealing. The Cu matrix purification as well as the magnetic properties of the alpha-Fe(Co) grains existing in a range of sizes are studied using X-ray diffraction, Mossbauer spectrometry and magnetic measurements. There is an optimum annealing for obtaining a high magnetoresistive effect. (C) 1999 Elsevier Science B.V. All rights reserved.

The effects of Mo substitution in Pr2Fe17-xMox and PrFe12-xMox compounds were investigated by X-ray diffraction, magnetic measurements and Mossbauer spectroscopy. Different fitting procedures, using magnetic sextets at low temperature, magnetic sextets and quadrupole doublets at room temperature and paramagnetic doublets well above the Curie points, had to be considered in order to analyze the Mossbauer spectra. The Mossbauer data obtained on the Mo-containing compounds stand for distributions of the hyperfine fields due to the random distributions of Mo over the 6c sites in the 2:17 and 8i sites in the 1:12 compounds. The variations in the isomer shifts and hyperfine fields with composition are discussed in relationship to the local environment details at the different iron sites. Site-specific information on the electronic effects due to Fe(Sd)-Mo(4d) states hybridization are derived. (C) 1999 Elsevier Science S.A. All rights reserved.

One step synthesis of the superconducting Bi(Pb)-2223 phase during heat treatment for non-equilibrium decomposition of the freeze dried Bi:Pb:Sr:Ba:Ca:Cu = 1.8.0.4: 1.8:0.2:2.2:3.0 nitrate powder has been studied. The heating rate was similar to 100 degrees C min(-1) and samples were maintained in air at 848 degrees C for 0, 5, 10, 20, 30, 60, 90, 240 and 1080 min. Each batch was investigated by x-ray diffraction analysis and ac magnetic susceptibility chi'(T). Evolution of phases and kinetics of phase formarion-decomposition processes were discussed. similar to 80% of Bi(Pb)-2223 has formed indicating the possibility of lowering the processing time of the freeze dried precursor powders by one order of magnitude comparative to the multistep synthesis route (from hundreds to tens of hours). A 30 min heat treatment step at 840 degrees C performed on the way up to the synthesis temperature is a forthcoming operation in order to block the formation of secondary undesired phases (e.g. (Ca1-xSrx)(14)Cu24O41).

Ru-zeolites prepared via deposition of aqueous solution of ruthenium chloride on X, L, ZSM-5 and APO-54 were investigated. The impregnated solids were characterized by the following techniques: N-2 sorption, H-2 chemisorption, TPR, XRD, XPS, EXAFS and Mossbauer spectroscopy. It was found that deposition of Ru is influenced by the pore structure and the chemical composition of the supports, in particular the concentration and strength of the protonic sites, with the formation of the different species. The packing of Ru as well as the degree of the reduction and the metal particle size are different according to the penetration depth of the metal and the nature of the zeolite. The amount of chlorine possibly remaining as adatoms of Ru is very low or inexistent. In calcined and reduced catalysts no chloride peak could be evidenced by XPS. The experiments confirmed that the use of large amounts of Ru chloride precursor leads to rather large metal particles. (C) 1999 Elsevier Science B.V. All rights reserved.

In this paper, we present some transport measurements on artificially layered (BaCuO2)(2)/(CaCuO2)(2) superlattices, engineered by the pulsed laser deposition (PLD) technique, and having critical temperatures (zero resistance) of about 80 K. The experimental current-voltage characteristics are discussed in the frame of current-induced unbinding of thermally created vortex-antivortex pairs (in zero-field) and in the frame of vortex glass (VG) transition for fields up to about 10 kG. The transport critical current density is estimated and compared to values reported from magnetisation measurements. The field dependence of the activation energy of vortices in the vortex liquid (VL) phase shows a strong effective pinning, due to the high disorder intrinsic to these artificially layered superconductors, at low magnetic fields, and a transition to a thermally activated plastic motion of an entangled VL in higher fields. From the temperature and field orientation dependencies of the vortex melting field, an anisotropy factor of about 20 is estimated for these superlattices. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

The resistive transition R(T) and the current-voltage characteristics (I-V) in the range 10(-9)-10(-6) V and close to the critical temperature (T-c) were measured in a single-grain of melt-textured YBa2Cu3O7-delta (YBCO) and in a detwinned YBCO single-crystal, with the transport current parallel to the (a,b) planes and in zero external magnetic field using a SQUID picovoltmeter. In the case of the melt-textured sample, we measured the I-V curves also at higher levels of dissipation. For T < T-c, we interpreted the lower part of the I-V curves in the framework of the Jensen-Minnhagen model of current-induced unbinding of thermally-created vortex-antivortex pairs, which leads to V proportional to I(I-I-c1)(a-1). Our results confirm a recently proposed current-temperature (I-T) phase diagram by S.W. Pierson. In addition, we determined for the first time the dependence of the characteristic (unbinding) current I-c1 with t = 1 - T/T-c for temperatures very close to T-c and low transport currents. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

The UV exposure of Fe:PVA thin films induces modifications of both the refractive index and the absorption coefficient. M-line spectroscopy showed the variation of the refractive index while Mossbauer measurements present the Fe2+/Fe3+ ratio versus the exposure energy. The diffraction efficiency experimental data were satisfactorily explained in correlation with the Fe2+/Fe3+ ratio.

The magnetic properties of the amorphous and nanocrystallized Fe-Cu-Nb-Si-B and Fe-Gd-Cu-Nb-Si-B ribbons are studied using temperature dependence of the specific magnetization. A two-phase ferromagnetic behaviour is obtained for the magnetization curves. Gd addition tends to stabilize the interfacial (grain boundary) phase by forming a metastable Gd-Fe-B phase, leading to the coexistence of two phases up to thermal treatments as high as 993K.

Crystallized Fe-(Cu)-Sm-B ribbons were investigated by means of X-ray diffraction, differential scanning calorimetry and Mossbauer spectrometry. The bcc alpha-Fe, tetragonal Fe3B and tetragonal Fe2B phases were found to crystallize after appropriate annealing. The relative proportion of alpha-Fe decreases with increasing Sm content, in agreement with the increase of the crystallization point. The metastable Fe3B phase decomposition into alpha-Fe and Fe2B was observed within a wide range of annealing temperature. The Sm ions randomly accommodate the Fe sites in the Fe3B lattice. After cumulative annealing at higher temperatures, the tetragonal Sm1.1Fe4B4 paramagnetic phase is found.

Iron-containing pyrochlores, having ions with various valencies on the B-sites, were investigated by magnetic measurements and Mossbauer spectroscopy. The influence of the ionic radius and of the valence state of the different cations accommodated on A or B sites on the super-exchange interactions and local distortions was emphasized. Three local configurations of iron were given as evidence in agreement with a model based on a random distribution of ions over the B-sites. (C) 1998 Elsevier Science S.A.