Onset of plasticity and hardening of the hysteretic response in the vortex system of YBa2Cu3O7-δ

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