Critical currents and magnetic relaxation of epitaxial YBa₂Cu₃O_{7 - δ} films

Critical currents of YBa₂Cu₃O_{7 - δ} epitaxial films, obtained by co-evaporation of yttrium, copper and BaF₂ in an ultrahigh vacuum system, were measured using resistometric and magnetic torque methods. Typical values of j_c $ ̌2 × 10¹¹ A m^-2 at 4.2 K and 4 × 10¹⁰ A m^-2 at 77 K were found. In spite of their good quality these films exhibited large magnetic relaxation even at 4.2 K. Using the results of Monte-Carlo simulations of thermally activated flux motion for the analysis of the relaxation data typical values of 25 ± 3 meV were found for the activation energy U of flux motion at 4.2 K. It is shown that the occurrence of highj_c values and small activation energies is not inconsistent in high T_c superconductors because of their short coherence lengths.