Field and frequency dependence of Tirr in an YBa2Cu3O7 film

The irreversibility line of an epitaxially grown YBaCuO film is determined from the maximum of the ac losses in a dynamic susceptibility experiment. In this paper it is shown that this irreversibility line depends both upon the measuring frequency and the amplitude of the ac field, which might indicate that it represents a cross over from pinning to flow regimes. However, the powerlaw behavior, which is predicted by flux creep theories, is only found in a limited field range. Furthermore, it appears that, in contrast to predictions by flux creep, the powerlaw behavior depends on the amplitude of the ac field