Anomalous magnetization behavior of single-crystallineCeRu2

Dc- and ac-magnetic measurements have been performed in the superconducting state of single-crystalline ${\mathrm{CeRu}}_2$ grown by the Czochralski method in order to understand the anomalous pinning mechanism associated with the peak effect. It is found that with increasing magnetic field the dc-magnetization hysteresis loop closes at a relatively low magnetic field, ${\mathbf{B}}_{\mathrm{irr}}$(T), above which it becomes reversible completely. It is also observed that the hysteresis loop abruptly opens up again at ${\mathbf{B}}_{\mathit{p}}$(T) near ${\mathbf{B}}_{\mathit{c}2}$(T). Since the onset field of this anomalous peak effect with ascending field ${\mathbf{B}}_{\mathit{p},\mathit{u}}$ does not match the ending field with descending field ${\mathbf{B}}_{\mathit{p},\mathit{d}}$, this sudden appearance (disappearance) of magnetic hysteresis at the different magnetic field is considered to be the first-order transition. The vortex phase diagram in ${\mathrm{CeRu}}_2$ is argued in terms of the possible pinning mechanisms in this compound. © 1996 The American Physical Society.