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

摘要

Systematic ac susceptibility measurements have been performed on a MgB$_2$bulk sample. We demonstrate that the flux creep activation energy is anonlinear function of the current density $U(j)\propto j^{-0.2}$, indicating anonlogarithmic relaxation of the current density in this material. Thedependence of the activation energy on the magnetic field is determined to be apower law $U(B)\propto B^{-1.33}$, showing a steep decline in the activationenergy with the magnetic field, which accounts for the steep drop in thecritical current density with magnetic field that is observed in MgB$_2$. Theirreversibility field is also found to be rather low, therefore, the pinningproperties of this new material will need to be enhanced for practicalapplications.