Critical-state scaling and weak links in Ag-sheathed Bi2Sr2Ca2Cu3Oz.

Abstract

Transport and magnetization measurements are compared for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{z}}$/Ag tapes with critical current densities approaching ${\mathit{J}}_{\mathit{c}}$=${10}^{4}$ A/${\mathrm{cm}}^{2}$ at 77 K, and at zero field. Upon subdivision of the sample, the magnetic hysteresis is found to scale approximately with the sample dimension, indicating that persistent currents flow throughout the superconducting core. The ${\mathit{J}}_{\mathit{c}}$ determined using the critical-state analysis is a factor of \ensuremath{\sim}2--3 higher than that obtained in transport. A weak magnetic-field dependence of ${\mathit{J}}_{\mathit{c}}$ is demonstrated at low temperatures, and yet, a comparison with data for powder samples indicates that the transport ${\mathit{J}}_{\mathit{c}}$ is limited by weak intergranular coupling. At low temperature, the transport ${\mathit{J}}_{\mathit{c}}$ is 2 orders of magnitude below the intragranular ${\mathit{J}}_{\mathit{c}}$. At high temperatures, weak intragranular flux pinning appears to be the dominant limiting mechanism. The degree to which transport properties might be enhanced through the introduction of pinning centers and better intergranular coupling is explored.