Irreversibility limits of the Abrikosov and Josephson flux dynamics in homogeneous and granular high-T/sub c/ superconductors

The magnetic irreversibility of a high-quality YBa₂Cu₃O₇₋₈ single crystal and of the granular superconductors YBa₁.₅Sr₀.₅Cu₃O₇₋₈ and YBa₁.₇₅Sr₀.₂₅Cu₃O₇₋₈ single crystals and the polycrystalline YBa₁.₇₅Sr₀.₂₅Cu₃O₇₋₈ 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 Tirr data of the pure single crystal are well described by the power law, predicted by the flux-creep models, in the 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 different 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 Gabay-Toulouse-like power law. These low-field features are acknowledged as the signature of a frustrated system