Field Dependence of Critical Current of [Formula Omitted] Film in Low Magnetic Field

The dependence of magnetic moment relaxation of [Formula Omitted] films on magnetic field was studied in both partly and fully penetrated critical states. A simple approach was used to account for the influence of the demagnetizing effect on the measured moment. Field and current dependence of the activation energy for flux motion [Formula Omitted] was calculated from macroscopic electrodynamics. Field dependence of the critical current density [Formula Omitted] was obtained from the analysis of [Formula Omitted]. It was shown that the critical current is suppressed by field and that the suppression takes place even in partly penetrated critical state. The observed behavior contradicts the field independence of [Formula Omitted] expected in low fields for single vortex pinning in the collective pinning model.