Flux Creep Determination of Field Dependence of Magnetic J_c at Low Temperatures

The effective activation energy of one HgBaCaCuO 1223 superconductor as a function of current density was extracted from the magnetic relaxation measurements below 30K, and found to obey a power law. We also obtained effective activation energy as a function of current density from the magnetic hysteresis loops (M-H). The data sets for the M-H loops at different temperatures overlap each other, and all fall on the smooth U_eff( J,0 ) curve based solely on the magnetization relaxation measurements. This indicates that the field dependence of magnetic critical current density is determined by the flux creep via nonlinear effective activation energy at low temperatures.