Irreversibility limits of the Abrikosov and Josephson flux dynamics in homogeneous and granular high-T-C superconductors - art. no. 094516

The magnetic irreversibility of a high-quality YBa2Cu3O7-delta single cryst al and of the granular superconductors YBa1.5Sr0.5Cu3O7-delta and YBa1.75Sr 0.25Cu3O7-delta single crystals and the polycrystalline YBa1.75Sr0.25Cu3O7- delta sample in two different oxygen states was determined in great detail, as a function of applied field up to 5 T from zero-field-cooled and field- cooled dc magnetization. While the T-irr data of the pure single crystal ar e well described by the power law, predicted by the flux-creep models, in t he whole field range, those of the granular superconductors adhere to this power law only in the high-field region. In a low-field region two quite di fferent regimes take place: In the lowest fields the data obey a de Almeida -Thouless-like power law and above a sharp crossover field they follow a Ga bay-Toulouse-like power law. These low-field features are acknowledged as t he signature of a frustrated system.