Onset of plasticity and hardening of the hysteretic response in the vortexsystem of YBa2Cu3O7-delta

A method based on partial magnetization loops has been used to study memory effects in detwinned YBa2Cu3O7-delta single crystals. These measurements h ave revealed the transition from a dislocation-free Bragg glass to a disord ered vortex phase. We have mapped this boundary in the B-T phase diagram an d have found it to be in proximity to the onset of the second magnetization peak. For fields above the transition Line, metastable topological disorde r invades the vertex system leading to a pronounced dependence of the criti cal current on the formation history of the vortex lattice.