Flux creep and critcal-current anisotropy in Bi2Sr2CaCu2O8+δ

Abstract

We have studied magnetic relaxation, temperature dependence of the critical currents, magnetic irreversibility, and lower critical fields in single crystals of the high-$T_c$ superconductor ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ (idealized composition) through magnetization measurements. The results are analyzed within the framework of the critical state and a thermally activated flux-creep model. The results indicate three different critical curents and flux-creep rates depending on the relative orientation of the current, the field, and the Cu-O planes. The flux-creep rate for flux lines moving perpendicular to the planes, was found to be unobservably low. Similarly, the critical current density perpendicular to the planes was very low. However the critical current density in the plane and the flux-pinning well depth $U_0$, for fluxon motion parallel to the planes, are similar to the values found in Y-Ba-Cu-O. Implications of insulating layers between the Cu-O planes are discussed.