Dependence of the flux-creep activation energy on current density and magnetic field for the MgB2 superconductor - art. no. 060505

Systematic ac susceptibility measurements have been performed on a MgB2 bul k sample. We demonstrate that the flux creep activation energy is a nonline ar function of the current density U(j) proportional to j(-0.2), indicating a nonlogarithmic relaxation of the current density in this material. The d ependence of the activation energy on the magnetic field is determined to b e a power law U(B) proportional to B-1.33, showing a steep decline in the a ctivation energy with the magnetic field, which accounts for the steep drop in the critical current density with magnetic field that is observed in Mg B2. The irreversibility field is also found to be rather low, therefore, th e pinning properties of this material will need to be enhanced for practica l applications.