National Institute Of Materials Physics - Romania

There are four topologically different paths to interchange two single-component winding vortices in a two-band superconductor having a weak inter-band interaction, where each band has its own superconducting component and its own phase, theta(v), other than the ordinary quantum phase, Theta (v is the index of the band and Theta is defined at the region of theta(1) = theta(2).). The acquired phase for the interchange of two single-winding vortices depends on the path, and three values Theta(soliton),-Theta(soliton) and 0, are observed. Theta(soliton) is the phase-slip accompanying the inter-band phase difference soliton (i-soliton). The interchange requires the creation and annihilation of the i-soliton and gives rise to an ambiguity in the statistics. (C) 2008 Elsevier B.V. All rights reserved.

We report on the pulsed laser deposition and characterization of thick (1.27 mu m) films ablated from a nanocrystalline YBa2Cu3O7-delta (YBCO) target doped with 4 wt% BaZrO3 (BZO), nYBCO/BZO. DC magnetization measurements have shown a four-to five-fold increase in the critical current density J(c) and a three- to four-fold increase in the maximum pinning force in comparison with 1 mu m thick films grown from a commercial microcrystalline YBCO target, at both 77.3 and 65 K. We have found that the frequency-dependent critical current densities J(c)(f(0)/f) estimated from susceptibility measurements are linear in a double-logarithmic plot ( where f is the frequency of the AC field and f(0) is a macroscopic attempt frequency of about 10(6) Hz), consistent with a logarithmic dependence of the pinning potential on the current density, and are again several times higher in nYBCO/BZO thick films. From the slopes of the J(c)(f(0)/f) dependences we have also estimated the values of pinning potentials in fields between 3 and 5 T.

Mossbauer spectroscopy studies of Fe-Co/MgO catalysts proved that a high dispersion degree of Fe may be achieved for catalysts containing Fe/Co in a ratio close to 1. During materials processing. the presence of hydrogen at high temperatures, even for short time, induced an increased particle size of the catalytic Fe. The evaluation of various iron species from their areas in the Mossbauer spectra revealed the dependence of the nanotube amount on (FeCo alloy + Fe2+) content in the catalyst, in good agreement with literature mechanisms. The Raman spectra show that carbon nanotubes synthesized from ethanol in argon flow on 4.5%Fe:4.5%Co/MgO are richer in low diameter (0.7-1.0nm) tubes than those obtained at low ethanol pressure, in agreement with the effects of hydrogen reported in literature. The close similarity with the Raman spectra of Aldrich double-walled carbon nanotubes and from literature, recommends the syntheses in argon flow as better conditions for double-walled carbon nanotube growth.

We report on the fabrication of MgO nanorods and nanowires by a novel, all-self-assembled method, using Pulsed Laser Deposition. The seeds for the formation of these one-dimensional nanostructures consisted of self-assembled gold nanodots. Gold was used due to its well-known catalytic effect. Dimensions and surface density of the Au nano-dots, investigated by Atomic Force Microscopy, depend on the deposition conditions and the number of laser pulses. Morphology of the MgO nanorods and nanowires was investigated by SEM, and was found to depend strongly on the deposition conditions and on the architecture of the Au nanodots on the decorated substrates.

Artificial pinning centres produced by Gd2Ba4CuWOy (2411W) nanophase inclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO target containing 1 mol% 2411W have been investigated. For comparison, YBCO thin films have been grown under the same conditions using a pure (commercial) YBCO target. The resulting films were characterized by AC susceptibility (superconducting transition) and SQUID magnetometry (critical current density, J(c)), and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition). J(c) of the film grown from the composite target is practically the same as that measured for the reference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatures higher than 50 K, however, the presence of the nano-inclusions leads to increased J(c). The relative enhancement in J(c) increases with further increase in temperature. J(c) of the film containing nanophase inclusions is between two and three times higher than that of the reference sample at 77.3 K and 1.5 T. This can be explained by considering the dimensions of nanophase inclusions compared with the temperature-dependent coherence length. Susceptibility measurements reveal that, apart from a large and sharp transition at about 91 K, there is a secondary, smaller and broader transition between 88 and 84 K in the YBCO film containing nanophase inclusions. This can be explained by substitutions of compatible atoms between YBCO and the 2411W phase (Gd with Y), which might also create additional pinning centres of delta T-c-type.

Thin films of (001) Bi2Sr2Ca2Cu3O10 With high zero-resistance critical temperature T(c)0 = 75-95.1 K and low roughness up to three half-c-axis unit cells were grown by metal-organic chemical vapor deposition (MOCVD) on substrates with large film-substrate lattice mismatch and different film-substrate mismatch anisotropy. Comparative analysis of the films on (001) and (110) MgO and NdGaO3 suggests that Bi-2223 films can easily accommodate large mismatch film-substrate differences, while mismatch on different directions, that is, mismatch anisotropy, has a strong influence on the quality of the film. The highest quality (low roughness, high uniformity, and high T-c0(R=O)) is obtained when mismatch anisotropy, taken as the mismatch ratio (r), is given only by compressive or only by tensile mismatch stress, and it is around 1, that is, it is as for the (001) MgO substrate. Mismatch anisotropy (value and sign) is an important parameter and can be used to tune superconducting properties of the film.

Four-cation nanograined strontium and magnesium doped lanthanum gallate (La0.8Sr0.2) (Ga0.9Mg0.1)O3-delta (LSGM) and its composite with 2 wt% of ceria (LSGM-Ce) were prepared. Morphologcally homogeneous nanoreactors, i.e., complex intermediate metastable aggregates of desired composition were assembled by spray atomization technique, and subsequently loaded with nanoparticles of highly energetic C3H6N6O6. Rapid nanoblast calcination technique was applied and the final composition was synthesized within the preliminary localized volumes of each single nanoreactor on the first step of spark plasma treatment. Subsequent SPS consolidations of nanostructured extremely active LSGM and LSGM-Ce powders were achieved by rapid treatment under pressures of 90-110 MPa. This technique provided the heredity of the final structure of nanosize multimetal oxide, allowed the prevention of the uncontrolled agglomeration during multicomponent aggregates assembling, subsequent nanoblast calcination, and final ultra-rapid low-temperature SPS consolidation of nanostructured ceramics. LaSrGaMgCeO3-delta nanocrystalline powder consisting of similar to 11 nm crystallites was consolidated to LSGM-Ce nanoceramic with average grain size of similar to 14 nm by low-temperature SPS at 1250 degrees C. Our preliminary results indicate that nanostructured samples of (La0.8Sr0.2)(Ga0.9Mg0.1)O3-delta with 2 wt% of ceria composed of similar to 14 nm grains can exhibit giant magnetoresistive effect in contrast to the usual paramagnetic properties measured on the samples with larger grain size.

Commercial wires of Nb3Sn and NIgB2 were inspected through X-ray cone-beam micro-tomography (mu CT). Details of the architecture of the wires of different topologies were visualized. In the case of MgB2 wires this allowed to compare the geometrical perfection of the component elements between the wires with 18 and 7 superconducting sub-elements heat treated for different conditions. Defects such as 3D voids and interruption of the diffusion barriers were also visualized. High resolution 3D-mu CT observations of the Nb3Sn wire architecture allowed also direct and non-invasive determination of the twist-pitch parameter. 3D tomography shows important advantages over 2D traditional microscopy methods.

Fe / Fe3O4 (magnetite) powders obtained by ball milling at room temperature, undergo an incomplete redox reaction with formation of FeO. This reaction is favoured due to the high energy developed during the milling and alloying. Concurrent effects of the milling, such as grain refinement down to the nanometric scale lead at the end of the milling processes to a mixed multiphased nanopowder, with a homogeneous dispersion of Fe and Fe oxide grains. Such ferromagnetic - antiferromagnetic systems are extensively studied due to their exchange bias properties, extremely useful in technological applications. We study the phase transformation that leads to a multiphased metal / oxide microstructure with an energy-dispersive in-situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray spectra after few minutes exposure, at selected temperatures, ranging between 20 degrees C and 1000 degrees C. Magnetic behavior has been studied for as-milled and annealed samples and the obtained magnetic parameters are correlated to the microstructure and phase composition at each stage of annealing. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanogranular powder.

Exchange bias and spin valve structures with Fe as ferromagnetic layers selectively enriched in Fe-57 and Ir-Mn of different compositions as antiferromagnetic pinning layers have been prepared by r.f. sputtering. Conversion Electron Mossbauer Spectroscopy and Magneto Optic Kerr Effect have shown that the local structure and interactions in the whole ferromagnetic layers, with direct influence on their magnetic reversal processes, depend on growing order of the layers and composition of the AF layer. Interfacial atomic diffusion is the main reason of perturbing the local structure of the ferromagnet. Mossbauer Spectroscopy is a powerfool tool which allows a detailed analysis of local configurations, in order to optimize the magnetic and giant magnetoresistive parameters of such multilayers of important technological applications.