Sweep-Rate Dependent Reversal of Magnetic Moment in Superconducting Films

We report on a dynamical effect in YBa 2 Cu 3 O x (YBCO) films and YBCO/CMR (Colossal Magnetoresistive) multilayers, in which variations in the magnetic field sweep rate can change, or reverse the sign of, the magnetic moment. The effect occurs in thin films having a low critical current density, either intrinsically or as a result of operation near the critical temperature. We attribute this effect to the large demagnetizing field present in the films, leading to a relaxation in the non-equilibrium vortex distribution, in which current near the edge of the film changes direction. We have investigated this effect in a variety of systems by magnetometery, and provide some magneto-optical evidence of its mechanism.