Field Dependence of Critical Current of $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ Film in Low Magnetic Field

The dependence of magnetic moment relaxation of YBa
₂
Cu
₃
O
_7-δ
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 U was calculated from macroscopic electrodynamics. Field dependence of the critical current density Jc was obtained from the analysis of U. 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 Jc expected in low fields for single vortex pinning in the collective pinning model.