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 clean 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 field of the peak strongly depends on previous field-history, resulting in an apparent asymmetry of the peak structure. No trace of paramagnetic suppression of superconductivity in the vortex-penetrated state is observed from the superconducting equilibrium magnetization curves. The peak position, although highly anisotropic. can be interrelated to the peak strength, regardless of the applied field direction. It is likely that the irreversibility peak is due to a synchronization pinning of the defective flux line lattice, rather than due to a first-order transition from the mixed state to the Fulde-Ferrell-Larkin-Ovchinnikov state.