Evidence for different regimes of collective flux-line pinning in YBa2CU3O7-delta single crystals

YBa2Cu3O7-delta Single crystals with different microstructure of the twin pattern were studied by means of torque magnetometry. Current densities and pinning force volume densities were calculated from torque hysteresis data. Pinning force volume density F-p in dependence on the component B-z of the flux density parallel to the c-axis direction revealed different regimes of collective pinning. For very low B-z, in the single vortex pinning regime, current density is independent on B-z. With increasing B-z a transition towards the regime of collective pinning of small flux bundles occurs. Here two different cases can be distinguished. For pinning at randomly distributed point pinning centers a strong influence of thermal oscillations of flux lines within the pinning barriers can be observed. In contrast, for single crystals having a ''mosaic'' twin structure, the influence of thermal oscillations can be neglected due to a fraction of flux lines which are strongly correlated pinned by twin planes and hinder the thermal oscillations of the other weakly pinned flux lines in the Aux bundles. In both cases an increase of the current density with increasing B-z (i.e., ''fishtail'' effect) can be observed, which is characterized, at low B-z, by a power-law behavior F-p similar to B-z(p) with p = 7/4 for collective random point pinning of small flux bundles and p = 2 for pinning of small flux bundles with additional correlated pinning contributions from the mosaic twin structures, respectively. For YBa2Cu3O7-delta Single crystals, with only medium current density but with a mosaic twin structure, a ''lock-in'' transition from pure collective random point pinning of large flux bundles, which is strongly influenced by thermal oscillations, towards collective pinning of small bundles with contributions of correlated pinning can be observed.