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.