Nonlinear ac susceptibility of high temperature superconducting rings

We have studied the ac response of structured thin, narrow rings of high-T-c superconducting films at ac and de fields less than or equal to 1 mT. The ac susceptibility as a function of reduced ac field h/h(m) (h(m) being the ac field at the dissipation maximum) shows no temperature dependence and can be well described by a model presented in this work. We show that the measurement of the magnetic moment of samples with ring geometry enables the identification of regions with degraded superconducting properties. The response of the ring shows up to similar to 1000 times smaller critical current density than the bulk. The magnetic moment shows a crossover from the ring to strip response upon increasing the applied ac field. (C) 1997 American Institute of Physics.