Magnetization study on the history-dependent peak effect in the superconducting mixed state of CeRu2

A huge peak of irreversible magnetization in CeRu2 single crystal, together with the equilibrium magnetization. was investigated by means of a static magnetization measurement using a Faraday force magnetometer. The residual resistance ratio of the used crystal exceeds 250, indicating that it is a c lean superconductor. Contrary to the previous reports, there is no pinning- free region in the magnetization curves below the upper critical field H-c2 . The detailed minor-hysteresis-loop measurement has revealed that the peak structure in the increasing-field magnetization curve around the onset fie ld of the peak strongly depends on previous field-history, resulting in an apparent asymmetry of the peak structure. No trace of paramagnetic suppress ion of superconductivity in the vortex-penetrated state is observed from th e superconducting equilibrium magnetization curves. The peak position, alth ough highly anisotropic, can be interrelated to the peak strength, regardle ss of the applied field direction. It is likely that the irreversibility pe ak is due to a synchronization pinning of the defective nux line lattice, r ather than due to a first-order transition from the mixed state to the Fuld e-Ferrell-Larkin-Ovchinnikov state.