ROTATIONAL MAGNETIC MEASUREMENTS OF VORTEX PINNING IN POLYCRYSTALLINE TYPE-II SUPERCONDUCTORS

Rotational magnetization-vector measurements were carried out on polycrystalline samples of YBa2Cu3O7 and (Ba, K)BiO3 at various fields (H) and temperatures. For both samples upon rotation in fixed H at each temperature, the vortex flux density vector was seen to bifurcate into B-R, which rotates rigidly with the sample, and B-F, which stays at fixed angle theta(F) relative to H and thus turns frictionally relative to the sample. With increasing H, B-R diminishes and eventually vanishes 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 average pinning torque per vortex and mu the vortex moment), decreases with increasingly high H at each temperature for both samples. The broad distribution of vortex pinning torques and the field dependence of their average value are shown to be consistent with the collective-pinning process of vortex bundling. At fixed H, tau(P) decreases rapidly with increasing temperature (T), varying approximately as T--beta, where beta approximate to 0.8 for both samples. References are made to related properties deduced from conventional data via the critical-state model.