Frequency dependence of the depinning and irreversibility lines in BSCCO

Penetration of AC magnetic field in Bi2Sr2CaCu2O8+delta single crystals was studied using the magnetooptic technique. It was found that the apparent depinning line shifts towards higher (H, T) values with increasing field frequency. At higher frequencies irreversibility in magnetization is controlled by pinning but not by the geometrical or surface barrier. Comparison with the melting line has shown that the depinning line lies completely in the vortex liquid region. Moreover, at least in low DC fields, the vortex liquid does not become unpinned up to the critical temperature of superconductor. Therefore, contrary to general understanding, we show that pinning is the main reason of irreversibility in magnetization at higher frequencies. It was also shown that the reason why this pinning is not observed at lower frequencies (i.e., using slow measurement technique) is the giant flux creep in BSCCO. (C) 2003 Elsevier Science B.V. All rights reserved.