Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors

Abstract

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/Tc=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,Jc, drops 80% from 124 (OT) to 29 A cm−2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm−2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm−2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJc=(α/B0)/[(1+B/B0)]n can be used to describe the field dependence of the critical current density, Jc, 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 ofBc2 for bismuth-based bulk highTc superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.