Dependence of the flux-creep activation energy on current density and magnetic field for the MgB₂ superconductor

Systematic ac susceptibility measurements have been performed on a MgB₂ bulk sample. We demonstrate that the flux creep activation energy is a nonlinear function of the current density U(j)∝j^-0.2, indicating a nonlogarithmic relaxation of the current density in this material. The dependence of the activation energy on the magnetic field is determined to be a power law U(B)∝B^-1.33, showing a steep decline in the activation energy with the magnetic field, which accounts for the steep drop in the critical current density with magnetic field that is observed in MgB₂. The irreversibility field is also found to be rather low, therefore, the pinning properties of this material will need to be enhanced for practical applications.