National Institute Of Materials Physics - Romania

We have measured the AC susceptibility response in field-cooling conditions of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites. In a certain but large range of the applied DC magnetic field, the out-of-phase susceptibility has two dissipation peaks. Due to the characteristics of the sample, the shape of the second peak and its position's field-dependence, we ruled out the possibility that the anomalous second dissipation peak could be related to intergrain dissipation. Because this multilayered cuprate is known to be a two-gap superconductor, we believe that our results are an experimental indication of a two-component vortex matter in high-critical temperature superconductors.

Sr2FeMoO6 was synthesizes by soft chemistry route using citric and/or oxalic acid as complexing agents. This method provides good compositional control with promising results as a base for further improvements. The material shows complex temperature- and field(magnitude and orientation) dependent electro-magnetic behavior as probed by magnetic and transport measurements. The zero-field cooling curves of magnetization versus temperature taken at different magnetic fields, show two temperature regions separated by a transition temperature T-cups(ZFC). When H increases, T-cups(ZFC) values decrease. At 5 K and 5 T the saturation moment is 3.55 JIB per formula unit, and the coercitive force is 0.021 T. The relative value of the magnetoresistive effect (i.e. 100 (rho(H)-rho(0))/rho(H), %, with rho being resistivity) at 5 K is 68% and 36% when applied field H is parallel and perpendicular the measuring current I, respectively. The results are discussed in relation to magnetic sublattices and the interaction that produces anti-site defects and spin-dependent electron anisotropy. (C) 2006 Elsevier B.V. All rights reserved.

The critical temperature, T-c, for (Bi,Pb)Sr(Ca2-x, Y-x)CuO/2223, with x between 0 and 0.45, polycrystalline samples have been investigated. T-c of (Bi,Pb)(2)Sr2Ca2-xYxCu3O10+delta phase, 2223, found in samples with x < 0.05, decreases whereas Tc of (Bi,Pb)(2)Sr2Ca1-xYxCu2O8+delta phase, 2212, found in samples with x > 0.05, displays a parabolic dependence with the increase of Y content. The change of T-c for 2223 originates mainly from the increase of x. The variation of both x and oxygen content, delta, influences the T-c of 2212. The doping state of 2212 and 2223 in Y substituted samples suggests that, generally, in the samples consisting of both superconducting phases, 2212 is in over-doped state whereas 2223 is in under-doped state. The last affirmation suggests that 2212 could bring a significant contribution in the transport parameters of (Bi,Pb)SrCaCuO/2223 samples.

First-order branched tree structures of MoO3 belts on sillimanite fibers (refractor wool) were grown by a vapor transport method. At longer growth times, MoO3 micro-belts on micro-belts form a flower-like structure. Most belts are of alpha-MoO3, and they grow in the two opposite directions along the c-axis. They show a high level of crystal quality, clean surfaces, sharp edges, and a triangular-shape tip with certain angles. The most frequent are angles of 47 degrees and 94 degrees. The growth mechanism is of vapor-solid type. In the initial stages of growth, elements specific for spontaneous spread and island formation on the surface of the sillimanite fibers described in the literature are observed. Significant enhancement of the thickness suggests that a layer-by-layer 2D growth is also probable. It is thought that responsible for such growth is direct vapor deposition of the Mo-O vapors on the already formed belts, this being similar to thin film growth. Curved surfaces of the substrate (in our case of the sillimanite fibers), on which the MoO3 belts are growing, are important and can be used for the directional growth control of the belts and formation of hierarchical structures. The vapor transport method allows the formation of beta-MoO3 belts that grow in the b-axis direction.

The superconducting properties of iodine-intercalated high-temperature superconducting Bi2Sr2Ca2Cu3O10+x phase (Bi-2223) were systematically studied. It was found that for samples containing a significant amount of Bi2Sr2CaCu2O8+x , iodine intercalation results in the dramatic decrease of the inter-granular critical current density, as well as a significant decrease of the critical temperature (T-c), the critical current density in the grains (J(cg)), and of the amount of Bi-2223. For samples with a large amount of Bi-2223, T-c changes insignificantly, whereas J(cg) can even increase. We argue that the different behavior of the superconducting parameters is the result of various oxygen concentrations, and we explain the effect of iodine intercalation based on the parabolic dependence between T-c and the number of holes per CuO2 layer. The H(T) curves (determined from the peak position in the loss signal of ac susceptibility) for intercalated samples deviate significantly from the quasi 2D-like behavior, pointing toward an enhancement of the 3D fluctuations of vortices. For the change in the values and dimensionality of the flux pinning in the process of the intercalation, we attempted a qualitative explanation based on the models proposed in literature.

The paper deals with optical and electronic properties of the aluminophosphate glasses containing Fe-Mn and Fe-Cr ion pairs in different concentration. The influence of the mixed alkali ions over the electronic properties has been investigated. The optical behavior (optical transmission) of the glass samples has been studied by UV-VIS spectroscopy and the refractive index dependency on wavelength has been discussed. The transmission spectra show features specific for the doping transition ions (TM), revealing different oxidation states of iron (Fe2+/Fe3+), manganese (Mn2+/Mn3+) and chromium (Cr3+/Cr6+) in the vitreous network. Mossbauer spectroscopy offers information regarding the TM oxidation states, redox processes and the iron coordination symmetry in the vitreous network. In the case of Fe-Mn doped glasses, the percentage of Fe2+ is about 40% and a doubled iron content leads to an increasing of Fe2+ percentage up to 53%. The replacing of lithium ions by natrium ions (mixed alkali effect) provides an increasing of the Fe2+ percentage up to 56%. The occurrence of the tetrahedral or octahedral symmetry of Fe(2+)supercript stop ions bonded by O2- ions depends on the transition ion nature and Li+/Na+ ratio. Infrared absorption spectra of the pair transition ions-doped aluminophosphate glasses reveal optical phonons specific for the phosphate glass matrix.

Two new hetero tri-nuclear oxalates (NH4)(8)[Fe2CO(C2O4)(8)]-6H(2)O and [Fe2Co(C2O4)(2)(OH)(4)]center dot 2H(2)O have been synthesized. The compound presented opposite behaviour in ac-susceptibility measurements, different frustration level and spin-orbit coupling in magnetization ones. The Mossbauer approach pointed to local interactions governed by metal-oxalate strongly coupled unit. The data were interpreted in terms of role of positive (NH4) or negative (OH) bridging ligands on general magnetic properties. (c) 2006 Published by Elsevier B.V.

Six practical composite wires of Nb3Sn (Furukawa Electric Co, Ltd) with different architectures ( design and reinforcement) were compared from the normal state resistance R, critical temperatures T-c(onset), T-c(offset) and Delta T-c, upper critical field B-c2 ( at 4.2 K) and critical current density J(c) points of view. Wires were as follows: three of near-the-edge reinforcement design with Nb reinforcement of 0, 21, 50 vol% in the CuNb region, and two of central reinforcement design with Nb of 21 and 50 vol%. One wire with near the edge 50% vol Nb reinforcement had a different reinforcement/superconductor ratio. As-reacted wires show very different patterns of R, T-c(onset), T-c(offset), Delta T-c, B-c2 and J(c). For the superconducting parameters this is probably due to different 3D thermal residual strains. Data suggest that the architecture of the as-reacted wire can control residual strain values and distribution. During multiple bending of the wires at room temperature ( named pre-bending), introduction of the reinforcement improves relaxation of the 3D residual strain and especially of the lateral components. As a consequence, B-c2 and J(c) versus pre-bending strain, epsilon(pb), are enhanced to values closer to those of the Nb3Sn in the stress-free state. Relative enhancement of these critical parameters for the reinforced wires is higher than for the reinforcement-free wire. Evolution during pre-bending and maximum attained absolute values of the superconducting parameters can be grouped roughly as a function of near-the-edge or central reinforcement design. Variation of the superconducting parameters suggests that the pre-bent state may depend on the as-reacted one ( which is a function of wire architecture and processing history); when J(c) was low as in the case of as-reacted wires with central reinforcement, during pre-bending, despite a relatively high J(c) enhancement, this parameter attained lower absolute maximum values than for the other wires. Depending on the wire, when pre-bending strain epsilon(pb) = 0.8 - 1%, the enhancement of non-Cu J(c) at 4.2 K in the pre-bent wire is by up to 43.5% at 15 T compared to the as-reacted case. The maximum absolute value of J(c) is obtained for the near-the-edge reinforcement pre-bent wires with 21 and 50% vol Nb. B-c2 of these wires was lower than for the other reinforced wires and higher than for the reinforcement-free wire. Pre-bent wires with near-the-edge reinforcement are identified as new and promising candidates for fabrication of react-and-wind coils with improved performance.

Various samples of ferrofluids consisting of colloidal suspensions of surfacted cobalt ferrite or magnetite nanoparticles in water were studied by x-ray diffraction and temperature dependent Mossbauer spectroscopy. Information about the particle mean size, the size dispersion and the effective magnetic anisotropy energy was obtained for each sample. The results are consistent with the formation of a magnetic dead layer at the particle surface, whose thickness depends on the surfactant - ferrite combination. The magnetic relaxation processes are faster in the colloidal suspensions of magnetite particles as compared with the suspensions of cobalt ferrite particles. The type of the surfactant also influences the magnetic relaxation behaviour, and hence the macroscopic properties of the ferrofluid at ambient temperature.

An AC magnetic field can rotate a vortex-inolecule composed of two fractional vortices in a multiband type of multicomponent superconductor, where two components couple with each other through the intercomponent Josephson interaction. It may directly relate to an abnormal AC loss peak, which we have found recently in the multilayer cuprate supercondutor CuxBa2Ca2Cu3Oy (Cu-1223).