Crossover from elastic to plastic vortex creep across the second magnetization peak of high-temperature superconductors

We investigated the relaxation of the irreversible magnetization of YBa₂Cu₃O_7-δ, Pb₂Sr₂Y_0.53Ca_0.47Cu₃ O₈, and Bi₂Sr₂CaCu₂O_8+δ a single crystals with significant quenched disorder in the region of the second magnetization peak. It was found that for an applied magnetic field between the onset field and the peak field the relevant current-density dependence of the activation energy exhibits a sudden change, which can be interpreted as a crossover from elastic to plastic vortex creep. The evolution of this change with magnetic field illustrates the increase of the collective pinning barrier between the onset field and the peak field. The observed increase of the collective pinning barrier is limited by the plastic barrier at the peak field. This appears to be a general behavior, and may have important consequences on the interpretation of the thermally induced vortex phase transition at high magnetic fields.