Dependence of the flux-creep activation energy on current density and magnetic field for theMgB2superconductor

Abstract

Systematic ac susceptibility measurements have been performed on a ${\mathrm{MgB}}_{2}$ bulk sample. We demonstrate that the flux creep activation energy is a nonlinear function of the current density $U(j)\ensuremath{\propto}{j}^{\ensuremath{-}0.2},$ 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 $U(B)\ensuremath{\propto}{B}^{\ensuremath{-}1.33},$ 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 ${\mathrm{MgB}}_{2}.$ 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.