The magnetic irreversibility of a high-quality YBa 2 Cu 3 O 7Ϫ␦ single crystal and of the granular superconductors YBa 1.5 Sr 0.5 Cu 3 O 7Ϫ␦ and YBa 1.75 Sr 0.25 Cu 3 O 7Ϫ␦ single crystals and the polycrystalline YBa 1.75 Sr 0.25 Cu 3 O 7Ϫ␦ 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 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.
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