EDGE-SHAPE BARRIER IRREVERSIBILITY AND DECOMPOSITION OF VORTICES IN BI2SR2CACU2O8

Magnetic flux dynamics is studied in Bi2Sr2CaCu2O8 single crystals by means of the magneto-optical technique. It is clearly demonstrated tha t, the magnetic irreversibility of these crystals in a magnetic field perpendicular to the basal plane at temperatures higher than approxima tely 35 K is governed by an edge-shape barrier and its disappearance d etermines the high temperature part of the magnetic irreversibility li ne which is commonly associated in the literature with Vortex lattice melting. We argue that, this barrier exists because of the non ellipso idal shape of the samples and can disappear only when the flux lines l ose their rigidity decomposing into pancakes, which is the only true m agnetic phase transition on the B-T diagram for Bi2Sr2CaCu2O8.