History effects and transition to plastic vortex dynamics in YBa2Cu3O7-delta single crystals

Partial magnetization loops have been performed in pure YBa2Cu3O7-delta sin gle crystals with various types and densities of pinning sites. The obtaine d results have demonstrated that the critical current density in detwinned samples depends strongly on the magnetic history of the superconductor. As shown in a recent study [S. Kokkaliaris, P.A.J. de Groot, S.N. Gordeev, A.A . Zhukov, R. Gagnon, L. Taillefer, Phys. Rev. Lett., 82 (1999) 5116], this effect can be attributed to the presence of metastable topological disorder in the vortex system following the transition from a dislocation-free Brag g glass to a disordered vortex phase. Mapping of this transition in the fie ld-temperature (B-T) phase diagram has revealed two important features: the transition line is always located in proximity to the onset of the second magnetization peak and shifts to lower fields with increasing oxygen defici ency. The influence of twin boundaries on the Bragg glass phase has also be en investigated and found to be negligible for low densities of twins. In c ontrast, a high concentration of twin planes is shown to destroy it and eli minate the memory effects. (C) 2000 Elsevier Science B.V. All rights reserv ed.