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 new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma