Vortex pinning in YBa₂Cu₃O₇ single crystals with crossed columnar defects

We have studied the vortex pinning properties in YBa₂Cu₃O₇ single crystals with crossed columnar defects of various defect angles (0-±40°). The critical current density, the normalized creep rates, the vortex activation energies, and the irreversibility lines (IRL) were found to be a sensitive function of defect angle. In a temperature range from 50 to 70 K, a crystal with 20° showed the highest enhancement in J_c as well as the lowest relaxation rate near the matching field of the defect density. We found a change in vortex dynamics from the half-loop excitation to the double kink creep with increasing magnetic field in the cross-defect crystals. The temperature dependence of IRL as a function of defect angle suggested that the crossed defects may promote the breaking of vortex lines into vortex segments near T_c.