Magnetization jump and the vortex-lattice melting transition in YBa2Cu3Oy

Magnetization measurements in high-quality twinned YBa2Cu3Oy single crystals are performed as a function of temperature, magnetic field, and its direction. We find an anomalous magnetization jump just below the irreversibility line, which provides the thermodynamic evidence of the first-order vortex-lattice melting transition in YBa2Cu3Oy system, following a reasonable Lindemann criterion with c(L)=0.13. In addition, we demonstrate that a peak effect in the critical current density just below the melting line is associated with the enhanced vortex pinning due to the premelt softening of the vortex lattice and this peak is different from a so-called second peak.