INTERGRANULAR AND INTRAGRANULAR CRITICAL CURRENTS IN SILVER-SHEATHED PB-BI-SR-CA-CU-O TAPES

The role of the intergranular and intragranular critical current densities in silver-sheathed Pb-Bi-Sr-Ca-Cu-O tapes is investigated by means of magnetic-moment measurements in perpendicular field at different temperatures using a superconducting quantum interference device magnetometer. We find that low-field magnetic-moment loops are mainly caused by the intergranular (transport) critical current density J(c)J but that in high fields the contribution from the intragranular critical current density J(c)G becomes significant. The low-field data can be described well in terms of a critical-state model for a thin strip of a homogeneous type-II superconductor in a perpendicular field where J(c)J flows over the entire tape. The high-field data require in addition a description in terms of a critical-state model for the grains where J(c)G circulates in each grain. Strong mechanical bending of the tape causes J(c)J to disappear while J(c)G is unaffected. The experimental data reveal the hysteretic nature of J(c)J which is caused by the trapping of flux in grains. In all the tapes measured, J(c)J is found to be much smaller than J(c)G. Comparing the remanent J(c)J with J(c)G in different tapes, we find that strong pinning in grains does not imply a high remanent intergranular critical current density.