National Institute Of Materials Physics - Romania

Scanning Hall probe microscopy with an effective spatial resolution of similar to 1 mu m has been used to investigate the vortex structures in superconducting Bi2Sr2CaCu2O8+delta single crystals in the temperature range 77.3-81.3 K and zero applied field (in the presence of the earth field). The vortex images were obtained in real time mode as the temperature increased slowly for 3.36 h. At 77.3 K, the vortices were arranged in a chain structure. With the increase of the temperature, two jumps in the vortex array occur at 77.3 K, immediately when the temperature starts to rise, and at 79.2 K with a good stability between jumps. The second jump is accompanied by the jump in the average magnetic induction when bundles of 4-5 additional vortices enter the scanning area and the vortex array get disordered. These directly visualized transitions in the vortex lattice are consistent with a vortex creep over the surface barriers at high temperatures. A short movie is presented. (c) 2008 Elsevier B.V. All rights reserved.

Structural, thermodynamic and,transport, properties of CeAg1-xNixSb2 compounds are investigated. The Ni substitution of Ag decreases the magnetic ordering temperature from approximate to 9.5 to approximate to 6 K, while the characteristic temperatures, considered as the temperature of the resistivity maximum, increase from 26 to 65 K. (c) 2007 Elsevier B.V. All rights reserved.

Ba2CaCu2O4(O2-yFy) (F-0212), a new high-T-c superconductor (HTSC) including fluorine at an apical site, is an attractive material for application because it shows a highest T-c of 108 K and has thin spacing between conduction layers that is favorable for high critical current density (J(c)) and irreversibility field (B-irr). The polycrystalline samples of F-0212 were synthesized under high pressure. A doping state was controlled from under doping (T-c = 82 K) to slightly-over doping (T-c = 106 K) via optimally-doping (T-c = 108 K). From the DC magnetization hysteresis loops, we determined the intragrain J(c) and B-irr using Bean's critical state model. The J(c) and B-irr properties of F-0212 were found to improve rapidly as the doping state changes from under doping to slightly-over doping to via optimally doping state. The irreversibility lines for the optimally- and slightly-over doped F-0212 are higher than optimally-doped Bi-2212. However, they are much lower than that of Cu-1212 (Y-123) though the spacing of F-0212 is thinner than that of Cu-1212. This indicates that property of the spacing is important to enhance B-irr as well as its thickness. An anisotropy factor for under doped F-0212 was estimated to 118 from a 3D-2D crossover field of about 0.28 T. (C) 2008 Elsevier B.V. All rights reserved.

We study the superconducting order parameter in the non-centrosymmetric compounds Li-2(Pd1-xPtx)(3)B (x = 0, 0.3, 0.7 and 1) by measuring magnetic penetration depth lambda(T). The low temperature lambda(T) shows a linear temperature dependence for x >= 0.3, but follows exponential-like behavior for lower Pt contents. These findings suggest that a spin-triplet state might gradually develop with increasing x due to the broken inversion symmetry. Published by Elsevier B.V.

Recently, MgB2 was discovered to be a superconductor with a surprisingly high critical temperature of about 39 K and with significant potential for applications. This material is usually prepared from a mixture of Mg and B in sealed tubes with excess Mg. As-prepared samples usually are very porous and mechanically weak. High density superconducting samples of MgB2 undoped and doped with SiC and B4C were produced by Field Assisted Sintering Technique (FAST) (T-c = 38.5 K). Samples are relatively large of 1.9 cm in diameter, apparently uniform, and can be easily extracted from the die. Processing times are short for easily attainable and relatively low temperatures. The effects of FAST consolidation on crystalline morphology revealed by microtomography and Scanning Electron Microscopy (SEM) and bulk density were investigated. Most samples have bulk density above 90% of the theoretical one. Despite such high bulk density, microtomography in combination with SEM revealed that in the samples there are some regions of low and high density. High density regions are not the consequence of FAST processing and they were observed in the raw material.

As recently shown, HgBa2Can-1CunOy (n >= 6) cuprate superconductors have a critical temperature of about 100 K independent of n. This remarkable property can be explained by the very imbalanced distribution of carriers among the inequivalent CuO2 planes in the unit cell. We discovered that these materials also have a common vortex melting line that resembles the theoretical melting lines of magnetically coupled pancake vortices. We suggest that there are two types of pancake pairs situated in the superconducting CuO2 outer planes: those separated by the thin charge reservoir layer are strongly (Josephson) coupled, while those separated by the thick block of (n-2) CuO2 nonsuperconducting inner planes are weakly (magnetically) coupled, forming magnetically coupled pancake vortex molecules.

Double perovskite Sr2FeMoO6 type compounds show special properties, such as magnetoresistance response at relatively small-applied fields and at rather high temperatures (ca. 400 K). Perovskite precursors were obtained unconventionally, in liquid phase, using oxalic acid as complexing agent, in the presence of ethylene glycol as polymerization agent. Ordering control concerning the magnetic properties is predictable to be strongly influenced by the synthesis route. The thermal stability of the precursors and the phase transitions of the oxide samples were analyzed by thermal analysis method, IR spectroscopy, Mossbauer spectroscopy, X-ray diffraction method and magnetic measurements. The experimental data for the perovskite compounds prepared by using oxalic acid as complexing agent revealed a double perovskite structure of analyzed samples, convenient magnetorezistive response (3.51 mu(B)/f.u), in good agreement with the Mossbauer spectra. Moreover, Mossbauer spectreoscopy allows the distinction between the ordered and the disordered phases of the perovskite structure and put in evidence the strong dependence of the structural and magnetic characteristics of the double perovskites on the synthesis route.

In the last years the ternary stabilized zirconia ceramics have attracted a considerable attention as a source of materials for structural ceramics. In present paper, the simultaneous influence of Eu3+ and Y3+ cations on zirconia powders (obtained by co-precipitation of aqueous salts solutions) on stabilization process in conjunction with the structural characteristics was investigated. A conventional sintering method (in electric furnace) at 1350 degrees C temperature range was applied. SEM+EDAX and XPS (X-ray Photoelectron Spectroscopic) analysis were performed. The Eu3+ Mossbauer parameters: isomer shift and quadrupole splitting have been obtained for 21.6 keV radiation of Eu-151 are in good agreement with the XPS and XRD-obtained crystallographic data of the studied compositions.

Fe-Pt alloys have gained much interest for the occurrence of permanent magnetic features resulting from the L1(0) ordered face-centred-tetragonal FePt phase with very high magnetic crystalline anisotropy (7x10(6) J/m(3)). An amorphous melt spun ribbons of the composition Fe51Pt27Nb2B20 has been synthesized by the rapid solidification technique and its microstructure and magnetic properties were studied. After appropriate annealing, an ordered face-centred-tetragonal (f.c.t.) L1(0) phase is formed. X-ray analysis revealed a structural phase transformation from the body-centered-cubic Al to f.c.t. L1(0) phase and this produce magnetic hardening of the alloy, upon appropriate annealing conditions. Extremely performant magnetic properties, typical for exchange spring magnets, are obtained.

The catalytic effect of the addition of lanthanum oxide (La2O3), in the range 0.5-2.0 mol%, on the hydrogen storage properties of MgH2 prepared by ball milling has been studied. The addition of La2O3 reduces the formation during milling of the metastable orthorhombic gamma-MgH2 phase. The desorption rate of samples with 1 and 2 mol% La2O3 comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La2O3. The presence of LaH3 after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La2O3 gives a lower hysteresis factor compared with sample with 2 mol%. (C) 2006 Elsevier B.V. All rights reserved.