Features of vortex pinning and magnetic flux creep in epitaxial thin films of high-Tc superconductor YBa2Cu3O7−δ near the critical temperature

Abstract

The temperature dependence of the dynamic relaxation rate in YBa2Cu3O7−δ epitaxial thin films is investigated in the temperature range from 77K to the critical temperature Tc with and without an applied dc magnetic field, and the dependence on the dc field at 77K is measured at fields up to 45mT. It is shown that the experimental results are in good agreement with the vortex lattice pinning model proposed previously which considers the main pinning centers in thin films to be threading dislocations on a network of low-angle grain boundaries. From the results of the experiment it is concluded that the influence of thermal fluctuations on the vortex pinning by threading dislocations and on the magnetic flux creep near Tc is not so crucial as in the case of pinning by extended linear defects in thick films or single crystals. Estimates are made which show that this fact can be explained by a transformation of the Abrikosov vortices into Pearl vortices when the magnetic field penetration depth becomes greater than the film thickness as the temperature approaches a critical value. Because of this, the mechanisms of pinning and flux creep in thin films are of a substantially collective character even in extremely weak magnetic fields and at temperatures very close to Tc.