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 columnar defects are either perpendicular or at 45°to the Cu-O planes. Magnetization curves confirm an anisotropic, unidirectional enhancement of the critical current Jc. 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 the field to the direction of the defect. From this feature we determine the pinning energy of a flux line in a columnar defect.