Surface pinning in niobium and a high-T_c superconductor

We present magnetization-hysteresis measurements on a well annealed thin niobium sample. We observe that the magnetization hysteresis is inversely proportional to the applied magnetic field and the reverse magnetization is constant over a large range of the applied field; both are well known features associated with surface-barrier effects. We see a "shoulder"-like structure in the hysteresis loop. This behavior is also observed in the hysteresis loops of Bi₂Sr₂CaCu₂O₈ single crystal, between 30 K and 40 K. We analyze our results of magnetization measurements, in both niobium and Bi₂Sr₂CaCu₂O₈, in the light of the surface-pinning effect.