Studies on Anisotropy and Vortex Motion in Melt Texture Grown Superconductors

Abstract

The focus of this project was to prepare and study the magnetic and transport properties of oriented, polycrystalline high T, superconductors, with special emphasis on their anisotropy. it had further been planned that the behavior of the magnetic flux, (so called vortices) would be analyzed in terms of the dynamics of these entities, subjected to the thermal and anisotropic conditions. During the period of this grant proposal, we have been able to achieve almost all the goals envisaged in the proposal. These are illustrated by the 10 publications directly related to the project, which have been published in quality international journals during this period. (List attached). We have also produced two Ph.D. theses in superconductivity during this period, one of which was submitted by the Research Officer, supported by this project. We have prepared very good quality melt textured high TT superconductors, which were required for performing the studies on anisotropy of the magnetization of these materials. We have also conducted and published the dc and ac magnetization studies of these -materials focusing on the properties of the vortices and obtained an understanding of the factors limiting the critical currents in various crystallographic directions. We have also studied in detail the dynamics of vortex motion in these materials by the methods of magnetic relaxation, field sweep rate dependence and ac susceptibility in superposed dc fields. Our studies have enabled us to draw a comprehensive picture that the barriers to vortex motion, and subsequent dissipation, are lowered in dc fields and in particular in crossed ac and dc fields the effects are drastic. We have also found that the decrease in critical currents with increasing fields can be explained on the basis of the so-called Critical state model for fields applied parallel to the c-axis, while the behavior perpendicular to the same is quite different. We have conducted transport measurements and identified the role of the (JxB) force as well as an angle independent, phase slip contribution to the dissipation. Our studies therefore constitute part of the major international effort to determine the electrodynamics of layered, anisotropic superconductors, the mechanisms determining the losses, and the anisotropy of their response.