Evidence for an angular-dependent contribution from columnar defects to the equilibrium magnetization of YBa2Cu3O7-delta

We have measured an angle-dependent contribution to the equilibrium magneti zation of a YBa2Cu3O7-delta single crystal with columnar defects created by irradiation with 5.8-GeV Pb ions. This contribution manifests itself as a jump in the equilibrium torque signal, when the magnetic-field direction cr osses that of the defects. The magnitude of the jump, which is observed in a narrow temperature interval of less than 2 K wide, for fields up to about twice the dose equivalent field B-phi, is used to estimate the energy gain ed by vortex pinning on the defects. The vanishing of the effective pinning energy at a temperature below T-c is attributed to its renormalization by thermal fluctuations.