Transition from elastic to plastic vortex phase and its evolution with quenched disorder in YBa2Cu3Oy single crystals

We report on a detailed investigation of history effects in the magnetic hy steresis of pure YBa2Cu3Oy single crystals with various types and densities of pinning sites. A partial magnetization loop technique is employed that enables us to detect the point where topological disorder first invades the vortex system. Accordingly, studies of detwinned single crystals with very low densities of point defects reveal a transition in the mixed state of t he superconductor that separates a dislocation-free Bragg glass from a high ly disordered vortex phase. The transition line is identified in the field- temperature phase diagram and found to lie in the vicinity of the onset of the second magnetization peak. The effect of decreasing oxygen stoichiometr y in the studied region (6.550 less than or equal to y less than or equal t o 6.999) is the shift of this boundary to lower fields, and its final disap pearance for high values of the oxygen deficiency in agreement with theoret ical predictions. Above the transition metastable topological defects proli ferate in the vortex lattice resulting in prominent history effects in the critical current. The last diminish at high enough fields for low oxygen co ncentrations near optimal doping, an effect not seen for samples close to t he stoichiometric state. We also study the influence of extended defects an d find that a low density of twin boundaries does not affect the Bragg glas s significantly. However, a high concentration of twins as well as low dens ities of columnar defects are shown to suppress the transition and eliminat e the memory effects.