Anomalous magnetization due to the vortex lattice melting transition in YBa2Cu3O y

Magnetization measurements in YBa₂Cu₃O_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_irr(T) as compared with the vortex lattice melting line H_m(T), the vortex lattice melting is observed in the reversible magnetization region and the entropy change per vortex per CuO₂ layer at the transition is estimated to be s0.5–1.0 k _B . For samples with a slightly larger value of H_irr(T) comparable to H_m(T), on the other hand, the jump height and the value of s is enhanced. Thus, the value of s is affected significantly by the relation between H _irr (T) which varies from sample to sample and H_m(T) which is an universal line.