Vortex behavior in a rotated YBa2Cu3O7-delta polycrystal

Magnetic measurements on a rotated polycrystalline YBa2Cu3O7-delta (YBCO) d isk at 4.2 K yield polar plots of the longitudinal-vs.-transverse component s of the vortex flux density (B) relative to the fixed applied field (H) fo r various angles of rotation (theta) up to 360 degrees. For the sample init ially in a field-cooled (FC) state, detailed analyses of these plots reveal that B at any theta consists of a B-R component, which rotates rigidly wit h the sample, plus a B-F component, which stays at a constant frictional an gle relative to H. At each H, B-R and B-F undergo large characteristic chan ges with theta until a rotational steady state is reached for theta above 1 80 degrees. Strikingly, their numerical sum B-R + B-F is constant for theta up to similar to 90 degrees, then decreases Linearly up to similar to 180 degrees, above which it remains constant at its reduced value. This reducti on in B-R + B-F is attributed to an exiting of vortices from the sample, pr imarily those associated with the weakly pinned B-F component. With increas ing H, the relative vortex reduction rises to a broad maximum (of nearly 40 %) and then slowly decreases, which reflects a competition between the inte rvortex repulsion and the confining forces produced by H. For the sample in itially in a zero-field-cooled (ZFC) state, B-R goes to zero while B-F reac hes the same steady-state value as in the corresponding FC case but without any vortex exiting from the sample. (C) 1998 Elsevier Science B.V. AU righ ts reserved.