Surface barriers and two-dimensional-collective pinning in single crystal Nd_1.85Ce_0.15CuO_4-δ superconductors

We report detailed magnetization studies on a single crystal Nd _1.85Ce_0.15CuO_4-δ superconductor in the magnetic field parallel to the c-axis direction. Two characteristic peaks are observed in the magnetization data, corresponding to surface barriers and two-dimensional (2-D) collective pinnings. The critical currents corresponding to the maximum and minimum widths in the hysteresis loops increases with decreasing temperature exponentially. We propose that the fishtail magnetization in this compound is due to the presence of a surface barrier and the pinning of collective 2-D vortices. The initial peak in magnetization is due to the presence of Bean-Livingston surface barriers and the larger peak in M at higher H corresponds to 2-D pinning.