Vortex dynamics and second magnetization peak in PrFeAsO _0.60F_0.12 superconductor

We have studied the vortex dynamics in the PrFeAsO 0.60 F 0.12 superconducting sample by dc magnetization and dynamic magnetization-relaxation rate (Q) measurements. The field dependence of the superconducting irreversible magnetization M_s reveals a second magnetization peak or fishtail effect. The large value of Q is an indication of moderate vortex motion and relatively weak pinning energy. Data analysis based on the generalized inversion scheme suggests that the vortex dynamics can be described by the collective pinning model. The temperature dependence of the critical current is consistent with the pinning due to the spatial variation in the mean free path near a lattice defect (δ l-pinning). The temperature and field dependence of Q indicate a crossover from elastic to plastic vortex creep with increasing temperature and magnetic field. Finally, we have constructed the vortex phase diagram based on the present data.