Two-step transition towards the reversibility region inBi2Sr2CaCu2O8−δsingle crystals

Abstract

We have performed magnetization measurements on ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8\mathrm{-}\mathrm{\delta }}$ single crystals in the c^ crystallographic direction for fields from 2 Oe up to 700 Oe. The results strongly suggest that the reversible thermodynamic region is achieved after the vortex flux structure shows an abrupt transition at a temperature lower than that determined by the irreversibility line.