Magnetization jumps and irreversibility in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

Using torque magnetometry, the bulk magnetization of single-crystal ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ has been investigated for temperatures close to the superconducting transition temperature. The results establish that the irreversibility field coincides with the field at which the vortex solid melts. It is also reported that the magnitude of the magnetization jump that can appear at the melting transformation is strongly correlated with the irreversibility displayed by the vortex solid. The accepted interpretation of the magnetization jumps observed in both local and bulk magnetic experiments on high-${\mathit{T}}_{\mathit{c}}$ superconductors is that they provide evidence for an entropy jump at the melting transformation. The results reported here suggest that the magnetization jumps may have an artifactual origin. © 1996 The American Physical Society.