Anomalous magnetization and field-driven disordering transition of a vortex lattice in untwinned YBa2Cu3Oy

Magnetization measurements on untwinned YBa2Cu3Oy are performed in magnetic fields up to 27 T using the Hall probe magnetometry and the anomalous second peak is found near the multicritical point. Below the second peak, the vortex pinning force shows a steep increase at a characteristic field H*(T), which is connected both with the first-order vortex lattice melting line T-m(H) and with the second-order vortex glass transition line T-g(H) at the multicritical point. The field-driven transition from the ordered Bragg glass to the disordered vortex glass phase is discussed as a possible origin of H*(T). [S0163-1829(98)00841-8].