Critical state in high Tc superconductors: magnetization and creep of the intergranular region

The intergranular or transport currents in sintered YBCO samples have been analysed using both a.c. magnetization for slab sized samples and d.c. magnetization for tubular samples. The magnetization loops of the transport regime have to be separated from the contribution of the grains and must be properly normalized to obtain meaningful information. The fit of the magnetization is not very sensitive to the form of the field dependence of Jc and does not allow one to distinguish between different theoretical models. A modified Rollins method was therefore used to obtain direct information on the field and current profile inside the slab samples. Creep was measured by the decay of the intergranular Jc in a ring and the data may be interpreted within the framework of the critical state model to obtain a pinning energy, Uo 2∼ 1 eV at very low field, which rapidly decreases by one order of magnitude at a few oersted. A tentative correlation between these two creep regimes, the Jc field dependence, the fluxoid dimensions and the type of pinning centres (holes or junctions) is presented in this paper.