Surface barrier and irreversibility in niobium and Bi₂Sr₂Cacu₂O₈ single crystal

In hard superconductors the flux motion and the associated hysteresis is mostly controlled by bulk pinning centres. At the same time surface barriers are known to play an important role in the vortex dynamics of different superconductors with a significant contribution to the irreversible properties. Generally the effect is better manifested in systems with weak bulk pinning. In these cases, both in conventional and high temperature superconductors, an appropriate temperature and magnetic fiels regime can be selected where the bulk pinning strength is very mush reduced so that the effect of surface barrier can be seen prominently