Evidence for different regimes of collective flux-line pinning in YBa2Cu3O7-δ single crystals

YBa2Cu3O7-δ 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. Pinning force volume density Fp as a function of the component Bz of the flux density parallel to the c-axis direction revealed different regimes of collective pinning: For very low Bz, in the single vortex pinning regime, current density is independent of Bz. With increasing Bz 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 flux bundles. In both cases an increase of the current density with increasing Bz (i.e., fishtail effect) can be observed, which is characterized, at low Bz, by a power-law behavior Fp∼Bzp 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-δ 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. copyright 1997 The American Physical Society