Vortex behavior in a rotated YBa₂Cu₃O_7-δ polycrystal

Magnetic measurements on a rotated polycrystalline YBa₂Cu₃O_7-δ (YBCO) disk at 4.2 K yield polar plots of the longitudinal-vs.-transverse components of the vortex flux density (B) relative to the fixed applied field (H) for various angles of rotation (θ) up to 360°. For the sample initially in a field-cooled (FC) state, detailed analyses of these plots reveal that B at any θ consists of a B_R component, which rotates rigidly with the sample, plus a _BF component, which stays at a constant frictional angle relative to H. At each H, B_R and B_F undergo large characteristic changes with θ until a rotational steady state is reached for θ above 180°. Strikingly, their numerical sum B_R + B_F is constant for θ up to ∼ 90°, then decreases linearly up to ∼ 180°, above which it remains constant at its reduced value. This reduction in B_R + B_F is attributed to an exiting of vortices from the sample, primarily those associated with the weakly pinned B_F component. With increasing H, the relative vortex reduction rises to a broad maximum (of nearly 40%) and then slowly decreases, which reflects a competition between the intervortex repulsion and the confining forces produced by H. For the sample initially in a zero-field-cooled (ZFC) state, B_R goes to zero while B_F reaches the same steady-state value as in the corresponding FC case but without any vortex exiting from the sample.