ROTATIONAL MAGNETIC MEASUREMENTS OF VORTEX PINNING IN POLYCRYSTALLINETYPE-II SUPERCONDUCTORS

Rotational magnetization-vector measurements were carried out on polyc rystalline samples of YBa2Cu3O7 and (Ba, K)BiO3 at various fields (H) and temperatures. For both samples upon rotation in fixed H at each te mperature, the vortex flux density vector was seen to bifurcate into B -R, which rotates rigidly with the sample, and B-F, which stays at fix ed angle theta(F) relative to H and thus turns frictionally relative t o the sample. With increasing H, B-R diminishes and eventually vanishe s while B-F grows monotonically, demonstrating that the vortex pinning torques have a broad range of values. The experimental quantity Hsin theta(F), which at equilibrium equals tau(P)/mu (tau(P) being the aver age pinning torque per vortex and mu the vortex moment), decreases wit h increasingly high H at each temperature for both samples. The broad distribution of vortex pinning torques and the field dependence of the ir average value are shown to be consistent with the collective-pinnin g process of vortex bundling. At fixed H, tau(P) decreases rapidly wit h increasing temperature (T), varying approximately as T--beta, where beta approximate to 0.8 for both samples. References are made to relat ed properties deduced from conventional data via the critical-state mo del.