Frequency dependence of the depinning and irreversibility lines in BSCCO H€ useyin S€ ozeri * , Lev Dorosinskii TUBITAK-UME, National Metrology Institute, P.O. Box 54, 41470 Gebze-Kocaeli, Turkey Abstract Penetration of AC magnetic field in Bi 2 Sr 2 CaCu 2 O 8þd single crystals was studied using the magnetooptic technique. It was found that the apparent depinning line shifts towards higher ðH ; T Þ 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 irrevers- ibility 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. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Depinning and irreversibility lines; Magnetooptics; AC susceptibility measurements; Geometrical barrier 1. Introduction There are three main sources of irreversibility in magnetization of superconductors: pinning, Bean–Liv- ingston surface barrier and geometrical barrier. In glo- bal measurements it is difficult to separate these effects. However, they can be separated by polishing samples into a prism shape thus eliminating the effect of the geometrical barrier [1]. In this way, it was shown in [1] that the geometrical barrier is the main source of irre- versibility in Bi 2 Sr 2 CaCu 2 O d at high temperatures. It was also reported in [1] that depinning line crosses the flux-line-lattice (FLL) melting line, so that in low fields the vortex liquid is unpinned everywhere down to the melting temperature, and even there is a region of un- pinned vortex solid in a region of the H –T phase dia- gram just below melting line. Thus, [1] corrected the previously accepted notion that the depinning line co- incides with the FLL melting line. However, most of the works on irreversible properties of BSCCO, were done using slow measurement techniques. This may lead to wrong conclusions because of the giant flux creep, which is especially strong in BSCCO samples at elevated tem- peratures. In order to study the flux pinning, the ex- perimental time window should be smaller than the characteristic time of magnetization relaxation. This is achieved by studying the penetration of AC field, then the effective time window will be equal to the inverse frequency of the field. In the present work we measured profiles of the amplitude of AC field in BSCCO single crystals using the magnetooptic (MO) technique. From the slope of the profiles we could estimate the critical current in the sample, and from the amplitude jump at the sample edge the magnitude of the surface or geo- metrical barrier could be estimated. Thus, the depinning line and the line, where the barrier vanishes, were plotted on the H –T phase diagram. 2. Experimental High-quality BSCCO crystals with T c  91–92 K and grown by the self-flux method [2] were used. * Corresponding author. E-mail address: hbocuk@ume.tubitak.gov.tr (H. S€ ozeri). 0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02560-1 Physica C 388–389 (2003) 745–746 www.elsevier.com/locate/physc