FLUX FLOP IN Y-BA-CU-O CRYSTALS IRRADIATED WITH 5.3-GEV PB IONS

Y-Ba-Cu-O crystals were irradiated with 5.3-GeV Pb ions. The induced c olumnar defects are either perpendicular or at 45-degrees to the Cu-O planes. Magnetization curves confirm an anisotropic, unidirectional en hancement of the critical current J(c). However, a sharp crossover to isotropic enhancement is observed in the low-field limit. We interpret these results in terms of flux flop from a direction determined by th e field to the direction of the defect. From this feature we determine the pinning energy of a flux line in a columnar defect.