Distorted hysteresis loops and flux creep in YBa₂Cu₂O_7-4 ceramics

The relaxation of magnetic moment of high T_c superconductors in an applied field is several orders of magnitude larger than that of conventional ones, so it is worthwhile to discuss the relationship between the hysteresis loops and the magnetic relaxation in high T_c superconductors. Experimentally we used a vibrating sample magnetometer measure the hysteresis loops, and then at different points of this loop, the field sweep was stopped while the external field was maintained constant. The signals of magnetic moment were recorded at 10s and 100s after each stop, forming several distorted hysteresis curves. The dependence of their shapes on the magnetic relaxation, the pinning potential and other parameters in ceramic YBa₂Cu₂O_7-4 samples was discussed.