Rotational magnetic processes in YBa sub 2 Cu sub 3 O sub 7

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