Flux dynamics in YBCO thin films

Magnetic measurements are reported on YBCO thin films (d < 500 nm) in the perpendicular geometry, H <parallel> c. By analyzing the sweep rate dependence of the magnetization for temperatures 35 less than or equal to T less than or equal to 77 K and H less than or equal to 10 kOe, we find that the moment has a power law dependence on the sweep rate. This dependence is indicative of power law E(J) characteristics for these thin films and from this a logarithmic current dependence of the activation potential U(J) is inferred. We find that our magnetic relaxation measurements are also consistent with the logarithmic U(J) dependence obtained from the sweep rate measurements. Activation barriers for Aux creep are determined for various temperatures and their field dependence is discussed.