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 degrees). T he critical current density, the normalized creep rates, the vortex activat ion energies, and the irreversibility lines (IRL) were found to be a sensit ive function of defect anole, In a temperature range from 50 to 70 K, a cry stal with 20 degrees showed the highest enhancement in J(c) as well as the lowest relaxation rare near the matching field of the defect density. We fo und a change in vortex dynamics from the half-loop excitation to the double kink creep with increasing magnetic field in the cross-defect crystals. Th e temperature dependence of IRL as a function of defect angle suggested tha t the crossed defects may promote the breaking of vortex lines into vortex segments near T-c. (C) 1999 Published by Elsevier Science B.V. All rights r eserved.