Abstract By measuring magnetic torque moment in a field-sweeping process, the temperature and field dependence of the critical current density j (with a criterion of electric field) and the normalized relaxation rate Q = d lnj/d ln E of a YBa2Cu3O7-$\delta$ thin film were obtained. With a minimum current density (jmin = 10A·cm-2) the irreversibility lines at different sweeping rates were determined. It was found that these irreversibility lines cannot be fitted to either the melting line or the vortex-glass transition. All the data can be interpreted by the thermally-assisted-flux-flow model. Further investigation shows that, at irreversible tem-perature and field, Uc is much smaller than kBT, which indicates that the thermal depinning is the real origin of the irreversibility line.
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