National Institute Of Materials Physics - Romania

We review the current status of the growth, characterization and applications of Bi-Sr-Ca-Cu-O (BSCCO) whiskers from the materials science point of view. Our analysis leads to the conclusion that reconsideration of the importance of the whisker studies is necessary; whiskers can play a major role in studies of the growth mechanism and defects, properties control and their understanding, and new devices and applications. Arguments favourable to the idea that BSCCO whiskers can possibly have a significant impact on future synthesis of nano-objects in this system or other high-T-c superconducting systems, as well as in investigation and understanding of the growth processes under elevated magnetic fields, are discussed. Emphasis is made on superconducting whiskers, but non-superconducting ones are also briefly addressed.

Ceria-zirconia supported (10 and 20 wt.% EuCoO3) versus bulk EuCoO3 perovskite were prepared via citrate decomposition at 700 degrees C and tested for toluene complete oxidation. S-BET, XRD, XPS, H-2-TPD and in situ Eu-151 Mossbauer investigations were performed. Electron delocalization processes around the Eu cations are induced by the reaction, as proved via the evolution of the Mossbauer parameters. All characterization data indicate the formation of a well-dispersed EuCoO3 at the surface of Ce0.9Zr0.1O2 for the 10 wt.% loading and larger crystallite formation for 20 wt.% loading. In terms of intrinsic activity for toluene combustion, supported catalysts were more active than bulk EuCoO3. All structural changes during operation time are reversible. (C) 2006 Elsevier B.V. All rights reserved.

The crystallization processes that occur in amorphous melt-spun ribbons of nominal composition Fe73.5Cu1Nb3Si13.5B9 with Gd addition in different concentrations are studied by differential scanning calorimetry (DSC). The aim of Gd addition is to study the changes induced in the calorimetric behavior of Finemet alloys, the crystallization sequences at intermediate stages of annealing as well as the nature of obtained crystallization products. The nature and structural properties of the phases formed at the end of the calorimetric process are investigated using X-ray diffraction (XRD) and Mossbauer spectrometry (MS). The crystallization sequence and also the phase composition at different stages of annealing were investigated for the Fe68.5Gd5Cu1Nb3Si13.5B9 alloy. Structure and magnetic properties of resulting crystallized samples have been thoroughly investigated. It is shown that in the above-mentioned composition, the decomposition at around 690 degrees C of the metastable Gd3Fe62B14 phase formed at incipient stages of crystallization leads to the co-existence of Gd2Fe14B, Fe2B and alpha-Fe(Si) phases. Magnetic measurements of as-cast and annealed samples are in good agreement with the XRD and MS results and with the proposed crystallization sequences and decomposition of intermediate metastable phases. (c) 2005 Elsevier B.V. All rights reserved.

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.

Fe-Ni-Co-Ti shape memory alloys were prepared by various techniques. A comparison between samples prepared by classical metallurgy and by melt spinning technique is performed in respect to the microscopic mechanisms responding for the shape memory effects. X-ray diffraction, thermomagnetic measurements and Mossbauer spectroscopy were applied for a complete structural and magnetic characterization. Only samples supporting an aging treatment gave evidence for shape memory effects and correlated structural transformations.

Three novel mu(3)-oxo trinuclear carboxylates [Fe2CaO(CCl3COO),(THF)(4)] center dot THF (1), [Fe2SrO(CCl3COO)(6)(THF)(6)] center dot 0.5H(2)O center dot 0.5THF (2) and [Fe2BaO(CCl3COO)(6)(THF)(6)] center dot 0.5H(2)O center dot 0.5THF (3) have been synthesised and subsequently characterised by X-ray structure analysis, thermal gravimetry (TG), infra red (IR) spectroscopy and magnetic measurements. Compounds I and 3 are molecular complexes with s- and d-metals displayed in the apexes of isosceles triangles with a triply bridging oxygen atom at its centre. The Ca center dot center dot center dot Fe and Ba center dot center dot center dot Fe separations have the values 3.750(1) and 4.177(1) angstrom, respectively. In comparison with these. Fe center dot center dot center dot Fe separations, 3.229(2) angstrom in 1 and 3.201(1) angstrom in 3, appear much shorter. The molecular complexes obey C-2 symmetry with the twofold axis passing through s metal and mu(3)-oxo atom. The coordination numbers are 6, 7 and 9 for iron. calcium and barium correspondingly. Magnetic studies reveal the presence of an antiferromagnetic exchange in the isosceles triangular skeletons of the trinuclear species. Using the spin Hamiltonian H = -2J(12)[S1S2], the fitting parameters g(Fe) = 2.00. J(Fe-Fe) = -58.90 cm(-1) for 1 and -60.40 cm(-1) for 3; g(Fe) = 2.14, J(Fe-Fe) = -75.40 cm(-1) for 2 have been estimated. (c) 2006 Elsevier Ltd. All rights reserved.

We investigate the order parameter of noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B via the behavior of the penetration depth lambda(T). The low-temperature penetration depth shows BCS-like behavior in Li2Pd3B, while in Li2Pt3B it follows a linear temperature dependence. We propose that broken inversion symmetry and the accompanying antisymmetric spin-orbit coupling, which admix spin-singlet and spin-triplet pairing, are responsible for this behavior. The triplet contribution is weak in Li2Pd3B, leading to a wholly open but anisotropic gap. The significantly larger spin-orbit coupling in Li2Pt3B allows the spin-triplet component to be larger in Li2Pt3B, producing line nodes in the energy gap as evidenced by the linear temperature dependence of lambda(T). The experimental data are in quantitative agreement with theory.

We report successful growth of high-quality thin films with clean and completely precipitate-free surface, suitable for device applications, by applying a new concept and method to the substrates. The concept consists in generation of artificial steps of controlled height and width, and desired shape on the surface of the substrate. The width of the step is chosen so that it is equal to the double of the migration (surface diffusion) length of the atomic species in the growth process of the film. If precipitates occur, they will be selectively gathered to the step edge where the free energy is lowest. Using this new method, we have successfully obtained by MOCVD high-quality precipitate-free Bi-2223 and Bi-2223/Bi-2212-superlattice thin films on (100) SrTiO3 substrates with artificial steps of controlled width and height. These as-grown films have been further used to fabricate patterned intrinsic Josephson junctions. Completely precipitate-free films offer a strong advantage for integration, and generate new possibilities for the device fabrication.

The series of compounds RFe11.3W0.7, where R = Dy, Ho, Er, and Lu, has been measured by Mossbauer spectroscopy between 15 and 295 K. A model which takes into account the Wigner-Seitz cell local environment of each iron site, the distribution of tungsten atoms on the 8i site, and the temperature-dependent magnetic structures of the compounds has been used to analyse the spectra. The assignment and the temperature dependencies of the hyperfine fields and the isomer shifts are in complete agreement with the Wigner-Seitz; cell analysis of the three iron sites in RFe11.3W0.7 compounds. The plausible orientations of the iron magnetic moments in the canted magnetic phase of ErFe11.3W0.7 compound have been established by a thorough Mossbauer spectral analysis. The Mossbauer spectra of DyFe11.3W0.7 clearly show the influence of the two spin re-orientations occurring in this compound and indicate the directions of the iron magnetic moments in both canted and planar magnetic arrangements. (c) 2005 Elsevier B.V. All rights reserved.

We investigated the superconducting parameters of iodine-intercalated BiSrCaCuO high-temperature superconducting materials. While for samples containing a significant amount of Bi2Sr2CaCu2O8+x, (Bi-2212) iodine intercalation results in the dramatic decrease of the superconducting parameters, it was found that for samples with a nearly single Bi2Sr2Ca2Cu3O10-y (Bi-2223) phase these parameters can increase. We argue that this different behavior of the superconducting parameters is the result of a different doping regime, and can be explained based on the parabolic dependence between T, and the number of holes per CuO2 layer. Iodine intercalation appears to be an efficient way to bring the usually under-doped superconductor to its optimal T-c and flux pinning properties.