Local fields in YBa2Cu3O7-delta measured on Yb-170(3+) Mossbauer probes. II. Vortex alignment in the field-cooled and zero field-cooled configurations

We examine the directional properties of the vortex lattice in grain-orient ed YBa2Cu3O7 as a function of the direction of the applied field relative t o the c-axis. The measurements are made by following the direction of the f ield produced by the vortex lattice on non-perturbing paramagnetic Yb-170(3 +) Mossbauer probes. In the field-cooled (FC) configuration, where the vort ex density is uniform, the average vortex direction remains parallel to the applied field whatever the temperature and the direction of the applied fi eld relative to the c-axis may be. We relate this behaviour to the competin g influence of the effective mass anisotropy and the pinning to the twin pl anes. In the zero field-cooled (ZFC) configuration, where the vortex densit y is non-uniform, the average vortex direction may no longer be parallel to the applied field and its direction depends on the size and direction of t he field and on the temperature. The critical currents now also play a role . (C) 1999 Elsevier Science B.V. All rights reserved.