Directional dependence of vortex pinning in crossed-defect Y-Ba-Cu-O single crystals

We used a micro Hall array to measure the magnetic field profiles in heavy-ion irradiated Y-Ba-Cu-O single crystals with crossed-defect configurations of ±20° and ±40° for a temperature range from 30 to 80 K. The Hall array was aligned either parallel or perpendicular to the inclination plane of the columnar defects. The critical current density, J_c, determined by the slope of the field profile was higher for vortex motion across the defect plane than that for motion in the plane for ±20° crystal, but no significant difference in J_c between two directions of vortex motion was observed for ±40° crystal. On the other hand, the normalized relaxation rate and the thermal activation energy did not show any clear anisotropy in all crystals. Comparing the lack of obvious directional dependence in the relaxation rate and the activation energy with the J_c anisotropy, we suggest that the J_c anisotropy is mainly due to the difference in the flux bundle size between two directions of vortex motion