First order phase transition at the irreversibility line of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

Magnetization and susceptibility measurements show a first order phase transition in the magnetic flux structure at the irreversibility line of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ single crystals, in the field range 0<H<360 Oe. The resistivity in the c direction drops 5 orders of magnitude in less than 0.1 K, and there is a discontiuous change in the magnetization at the same temperature. The change in magnetization together with the Clausius-Clapeyron equation gives a variation of entropy of 0.06${\mathit{k}}_{\mathit{B}}$ per vortex layer, at the transition. Above 360 Oe the irreversibility line has the known frequency dependence.