Anomalous magnetization due to the vortex lattice melting transition in YBa sub(2)Cu sub(3)O sub(y)

Magnetization measurements in YBa sub(2)Cu sub(3)O sub(y) single crystals are performed as a function of the temperature T and the magnetic field H. It is found that an anomalous magnetization step which provides the thermodynamic evidence of the first-order vortex lattice melting transition and the resistive kink with the hysteresis are observed in the same phase boundary in H-T plane. For samples with a small value of the irreversibility field H sub(irr)(T) as compared with the vortex lattice melting line H sub(m)(T), the vortex lattice melting is observed in the reversible magnetization region and the entropy change per vortex per CuO sub(2) layer at the transition is estimated to be Delta s) 0.5-1.0 k sub(B). For samples with a slightly larger value of H sub(irr)(T) comparable to H sub(m)(T), on the other hand, the jump height and the value of Delta s is enhanced. Thus, the value of Delta s is affected significantly by the relation between H sub(irr)(T) which varies from sample to sample and H sub(m)(T) which is an universal line.