ROTATIONAL MAGNETIC PROCESSES IN YBA2CU3O7

A polycrystalline superconducting thin-disk sample was rotated in a fixed field (H) at 4.2 K, and the vector magnitude and orientation of the total magnetization (M) were measured as the sample-rotation angle (θ) was slowly cycled between 0°and 360°. The measured M was decomposed into a penetrating (vortex) flux component Mp that rotates rigidly with the sample at very small θ and a diamagnetic (shielding) component M d that stays antiparallel to H. As θ is raised, Mp deviates from rigid rotation, and the deviations are very different for the zero-field-cooled (ZFC) and field-cooled (FC) states at the same H, indicating different distributions in the strengths of the initial pinning forces. However, Md is exactly the same for both states and closely equals χ0H, where χ0 is the low-H (ZFC) diamagnetic susceptibility. At still higher θ, Mp rotates up to some critical angle relative to H, where it remains as the sample continues to turn. This frictional motion of Mp relative to the sample presumably derives from a sequential hopping of vortices between pinning centers.