MAGNETIC-FIELD DEPENDENCE OF THE CRITICAL CURRENT-DENSITY FOR THE BISMUTH-BASED BULK HIGH-TC SUPERCONDUCTORS

Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T(c) = 0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density, J(c), drops 80% from 124 (0 T) to 29 A cm-2 (5 mT) for the cold-pressed pellet, 80% from 582 (0 T) to 126 A cm-2 (30 mT) for the HIPed sample and 50% from 6500 (0 T) to 2850 A cm-2 (30 mT) for the rolled tape. In the flux flow regime, where B is perpendicular to the c-axis a modified Kim's model J(c) = (alpha/B0)/[(1 + B/B0)]n can be used to describe the field dependence of the critical current density, J(c), in the field range 0.2-1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range of B(c2) for bismuth-based bulk high T(c) superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. When B is parallel to the c-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.