MO-Imaging of Granular And Structured High-T _c Superconductors

Abstract

Peculiarities observed during the magnetic studies of silver-sheathed (Pb,Bi)2223 tapes revived the interest in the properties of granular superconductors. In this contribution, several observations on granular superconducting materials will be presented, showing also the unique advantages of magneto-optic imaging (MOI) over most other methods for field distribution imaging, and, of course, over integral measurements. The MOI technique enables the direct measurements of critical current densities within individual grains of a granular superconductor. The properties of various high-T_c superconductors are discussed. Several different regimes of flux penetration into granular superconductors can be distinguished. The (Pb,Bi)-2223 tapes are found to be an unique system due to the specific temperature dependence of the inter- and intragranular current densities, being “granular” at low temperatures. This is the reason for most of the pecularities observed in integral magnetic measurements. In order to model a granular superconductor, model samples were prepared starting from YBCO thin films, where a hexagonally close-packed structure was patterned. MOI of this type of samples allows to distinguish between the inter- and intragranular properties, and hence, this sample exhibited the desired properties in integral magnetization measurements.