EVIDENCE FOR A TIME-DEPENDENT CROSSOVER FROM SURFACELIKE TO BULK-LIKE FLUX RELAXATION IN YBA2CU3O7-DELTA

dc flux-relaxation experiments were performed on single crystals of YBa2Cu3O7-delta at low magnetic fields. A sharp, time-dependent crossover in the logarithmic magnetization decay rate was observed in two different crystals. Consideration of the size dependence of the relaxation rate in one of the YBa2Cu3O7-delta crystals suggests that the crossover in the relaxation rate is due to a transition from surface-controlled to bulk-controlled relaxation behavior. Our observations seem to indicate that, over a certain range of temperatures and magnetic fields, surface pinning may play a significant role in the flux-relaxation behavior of YBa2Cu3O7-delta.