Marie Roussel

Superconducting and microstructural properties of two types of MgB/sub 2/ films prepared by pulsed laser deposition

Significant differences in superconducting and microstructural properties between two types of MgB/sub 2/ films prepared by pulsed laser deposition were determined. A very high H/sub c2/-T slope of 1.1 T/K was achieved in the in situ film. The J/sub c/-H curves of the in situ film also show a much weaker field dependence than that of the ex situ film. The magneto-optical (MO) images show that at 4 K the flux penetrates the in situ MgB/sub 2/ film through random paths, while for the ex situ film, the flux penetration pattern is mostly repeatable, indicating a defect-controlled flux penetration. Microstructural study (transmission electron microscopy and atomic force microscopy) revealed a relatively big grain size in the ex situ film. The correlation between the superconducting properties, microstructure and preparation conditions is discussed with regard to the two types of films.

Flux Jumps in Magnesium Diboride

Magneto-optical imaging was used to study flux penetration into MgB2 films in a slowly increasing perpendicular applied field. A variety of flux jumps and avalanches have been observed at temperatures below 10 K. At small fields, jumps with typical size of 20 μm and regular shape occur at random locations along the flux front. Above some threshold field of 2–10 mT, big dendritic jumps with dimensions comparable to the sample size (mm scale) take place. The jumps are developing extremely fast, result in highly-branching irreproducible flux patterns, and effectively suppress the apparent critical current. Both types of jumps are believed to result from thermo-magnetic instability.

Influence of the iron sheath on the local supercurrent distribution in MgB2 wires

The magnetic behavior of iron sheathed magnesium diboride (MgB2) wires was investigated. Global magnetization measurements have shown unusual critical current density variations compare to a superconductor with no magnetic environment. Local quantitative studies, through MO images, are linked to these global magnetization measurements in order to better understand the observed results. It is shown that the current distribution is affected by the soft magnetic sheath interacting with the superconducting core.

Structure, pinning and supercurrent in YBa2Cu307 films and ReBa2Cu307 multilayers

High quality YBa2Cu3O7 (YBCO) films and multilayers of ReBa2Cu3O7 superconductors, where Re is rare earth elements (Y and Nd), have been prepared by pulsed laser deposition. Pinning characteristics of the structures obtained have been analysed and attributed to growth conditions and corresponding structural peculiarities. Relatively thick (~1 µm) multilayers exhibit better performance than mono-layer YBCO films having arbitrary thickness. differences in the films and multilayers are discussed in terms of their structure homogeneity and defects induced by the growth of the layers.

Optimization of Bi-2223 Tape Fabrication Procedure With Help Of Magneto-Optical Imaging

A set of Ag-sheathed Bi-2223 (Bi1.72Pb0.34Sr1.85Ca1.99Cu3Ox) tapes was produced using the powder in tube technique and two heat treatments separated by a mechanical deformation. For the second heat treatment, a two step annealing process was used with the first step at 825°C and the second at different temperatures from 725 to 800 ° C, an additional tape was produced by a slow cooling ramp after annealing at 825 ° C. These tapes were studied by the way of magneto-optical imaging (MOI). This “local’ technique provides results which correlate and clarify the results obtained by other “global” techniques (XRD, magnetization, critical current Jc, critical temperature Tc). The MO results show better superconducting properties (higher Jc, good shielding properties) for the slow cooled sample and for the sample which was annealed at 750 ° C during the second step.