The Hall effect has been studied in a series of AuFe samples in the re-entrant concentration range, as well as in part of the spin glass range. An anomalous Hall contribution linked to the tilting of the local spins can be identified, confirming theoretical predictions of a novel topological Hall term induced when chirality is present. This effect can be understood in terms of Aharonov-Bohm-like intrinsic current loops arising from successive scatterings by canted local spins. The experimental measurements indicate that the chiral signal persists, meaning scattering within the nanoscopic loops remains coherent, up to temperatures of the order of 150K.

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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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Very detailed magnetic irreversibility data for fields applied along the c axis or the ab plane of a pure and untwinned YBa 2 Cu 3 O 7Ϫ␦ single crystal and Sr-doped and heavily twinned YBa 2Ϫx Sr x Cu 3 O 7Ϫ␦ (xϭ0.25, 0.37, and 0.5͒ single crystals are reported. The irreversibility lines T irr (H) of the pure single crystal show a considerable planar anisotropy but follow the same power-law regime, for both field orientations, arising within the conventional flux-creep theories in the whole field range. Very differently, however, the T irr (H) lines of the doped superconductors exhibit besides large anisotropies, several different regimes. In fields lower than 8 kOe the T irr (H) data of the doped samples display the de Almeida-Thouless ͑AT͒ and Gabay-Toulouse ͑GT͒-like power-law behaviors, the signature of a frustrated superconductor. For higher-field values, and in particular for Hʈc, flux dynamics seems to be conventional. However, for Hʈab and field values above 30 kOe, the flux dynamics displays sharp directional properties along the twinning planes ͑TP's͒ for rotations about the c axis. This behavior is cusplike, comparable to that caused by columnar defects, which characterizes a Bose-flux-glass phase. We appoint the superconducting granularity and frustration as responsible for the AT and GT behaviors below 8 kOe and the strong anisotropic pinning for H parallel to the TP's as the cause of the Bose-glass features. On the other hand, the isotropic pinning for large angular displacements or for any angle in fields below 30 kOe is the most probable cause of the vortex-glass features.