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 YBa2Cu3 O7-delta, Pb2Sr2Y0.53Ca0.47Cu3O8, and Bi2Sr2CaCu2O8+delta single crystals w ith significant quenched disorder in the region of the second magnetization peak. It was found that for an applied magnetic held between the onset fie ld and the peak field the relevant current-density dependence of the activa tion energy exhibits a sudden change, which can be interpreted as a crossov er from elastic to plastic vortex creep. The evolution of this change with magnetic field illustrates the increase of the collective pinning barrier b etween the onset field and the peak field. The observed increase of the col lective pinning barrier is Limited by the plastic barrier at the peak field . This appears to be a general behavior, and may have important consequence s on the interpretation of the thermally induced vortex phase transition at high magnetic fields.