Magnetic flux creep in YBa2Cu3O7-δ high-Tc superconducting thin films near the critical temperature

The temperature dependence of the dynamic relaxation rate Q for epitaxial thin films of YBa2Cu3O7-delta is obtained from measurements of the ac magnetic susceptibility at different frequencies in the temperature range from 77 K to the critical temperature T-c. The critical current density is determined from the measurements according to two known methods: using the "loss maximum" and a "high constant amplitude" of the ac exciting field. The results obtained by these two methods are compared to each other and with the theoretical predictions of the critical state model as modified by Clem and Sanchez. It is found that for the temperature and field conditions used in the loss maximum measurements, the deviations from that model are small and independent of temperature right up to Tc. Under these conditions the value of Q is determined correctly and is found to be independent of temperature. The results obtained from the measurements by the method of high constant amplitude of the ac field cannot be interpreted correctly because of practical limitations encountered in the whole range of temperatures studied. With increasing distance from Tc the approximate formulas used in that method diverge rapidly from the exact formulas of the critical state model. On approach to Tc the amplitude of the ac field becomes much greater than the corresponding loss maximum, and it is shown that under those conditions the relations among the parameters obtained diverge from the predictions of the Clem-Sanchez theory. It is concluded that this last circumstance is due to the deviations from the critical state model at such high amplitudes of the ac field. As a result, the use of this theory to obtain data on Q becomes incorrect. At high amplitudes of the ac field there is a region about 1 K wide below Tc in which one observes a regime where the real part of the complex magnetic susceptibility falls off practically to zero and only the imaginary part remains. This corresponds to a flux-flow-induced resistive state of the superconductor. (c) 2006 American Institute of Physics.