National Institute Of Materials Physics - Romania

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).

An extensive investigation of the gold ores in the South Carpathians (Costesti, Valea lui Stan si Jidostita) has performed, using different chemical and physics experimental techniques. The used structural techniques have been the electron microscopy, X-ray diffraction, Mossbauer effect and electron spin resonance. The investigation of the representative samples by the structural techniques relieved new minerals in the gold ores, the presence of the "coral like" aggregates of nanometric gold solid bubbles.

The cobalt oxide superconductor NaxCoO2 center dot yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E-F) in the normal state of NaxCoO2 center dot yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to NaxCoO2. The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within similar to 30 meV below E-F, closer to E-F than in NaxCoO2. We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.

Magnetic properties of FePt/Ag/Fe and FePt/Pd/Fe layer systems, prepared by magnetron sputtering, were investigated using the nuclear resonant forward scattering of synchrotron radiation. This technique allows the accurate determination of magnetic hyperfine field orientations by using an extremely thin 57 Fe probe layer suitably embedded within the soft magnetic layer. From an evaluation of these measurements within a one-dimensional micromagnetic model, the interlayer exchange coupling constants between the magnetically hard (FePt) and soft (Fe) layers were determined as function of the Ag and I'd interlayer thickness. The interlayer thickness dependence of the bilinear coupling constants provides evidence for the superposition of Ruderman-Kittel-Kasuya-Yoshida coupling and a magnetostatic interaction between the magnetic layers. (c) 2006 Elsevier B.V. All rights reserved.

Fabrication by colloidal chemistry and an extended structural and magnetic investigation are reported for AgCo bimetallic colloidal nanoparticles using X-ray diffraction, transmission electron microscopy and superconducting quantum interference device magnetometry. The AgCo nanoparticles exhibit a core-shell structure, with a face-centred cubic Ag core and a hexagonal close-packed Co (either complete or partial) shell. A bimodal size distribution of superparamagnetic (SPNI) nanoparticles together with a small fraction of nanoparticles ferrornagnetic at room temperature has been determined and their influence on magnetic properties is discussed. The arrays of self-assembled nanoparticles exhibit a lack of saturation of magnetisation and typical SPM behaviour. The annealing of the arrays greatly enhances the ferromagnetic character of the samples. Finally, the observed magnetic behaviour of the AgCo nanoparticles is correlated with surface spin disorder induced by the particular structural features of the sample, high fraction of magnetic atoms in surface states and finite-size effects. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Using the on-set of third-harmonic susceptibility we have measured vortex melting lines of high-pressure synthesized (Cu0.6C0.4)Ba2Ca3Cu4Oy [(Cu,C):1234] and (Cu0.5C0.5)Ba2Ca2Cu3Oy [(Cu,C):1223] with preferentially oriented crystallites and having various carrier concentrations. Vortex melting lines of all (Cu,C):1223 samples and of optimum-doped (Cu,C):1234 were very well described by the commonly accepted theory of melting lines within the two-fluid model. Overdoped (Cu,C):1234 proved to have an anomalous melting line, that was phenomenologically explained by the rather special gaps evolution, that is the significant opening of a second superconducting gap due to CuO2 outer planes at a temperature lower than the critical one, where the first superconducting gap due to inner planes opens. The experimental melting lines of overdoped (Cu,C):1234 were modelled theoretically by assuming an empirical formula for the temperature dependence of anisotropy factor.

To eliminate precipitates-segregates that can easily occur on the thin film surfaces of the multicomponent materials for electronics, a new approach is proposed, consisting of the following aspects: first, on the substrates, artificial steps of predefined height and width are produced, and second, films are grown on such substrates. The width of the step is taken equal to the 'double of the migration length' of the atomic species depositing on the substrate. In these conditions, precipitates migrate and gather at the step edges where the free energy is lowest and the resulting totally precipitate-free surface of the film on the step is suitable for device applications or integration purposes. The method has several other important advantages and they are discussed in the text. Using this new approach we present successful fabrication of a mesa structure showing intrinsic Josephson effect. We have used thin films of Bi-2212/Bi-2223 superstructure grown by MOCVD on (001) SrTiO3 single crystal substrates with artificial steps of about 20 mu m width. (c) 2006 Elsevier B.V. All rights reserved.

NdFeB-based bulk nanocomposite permanent magnets (with Dy and Co partially substituting Nd and Fe, respectively) with additions of Early Transition Metals (ETM = Mo, Ti, Zr, Cu, Nb, V, W), in form of rods with diameters ranging from 0.5 to 0.8 mm, have been prepared by devitrification annealing of amorphous and partly-amorphous precursors produced by injection die casting. A fully amorphous structure was obtained for rods with the diameter as large as 0.6 mm. The as-cast rod samples are magnetically soft and the magnetic hardness develops as the samples are devitrified to the optimum nanostructure. The best-achieved hard magnetic properties have been obtained for the optimum devitrified (15 min at 630 degrees C) Nd(3)Dy(1)F(3)e(66)Co(10)B(20) samples of 0.6 mm in diameter: H-i(c) = 296 kA/m, mu M-0(r) = 0.86 T, Mr/M-max = 0.65 and (BH)(max), = 74 kJ/m. The Mossbauer spectra indicate the coexistence of 4 magnetic phases: 2:14:1, alpha-Fe, Fe3B and a short-range ordered intermetallic Fe-B-type phase.