Anomalous magnetization behavior of single-crystalline CeRu2

Dc- and ac-magnetic measurements have been performed in the superconducting state of single-crystalline CeRu2 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, Birr(T), above which it becomes reversible completely. It is also observed that the hysteresis loop abruptly opens up again at Bp(T) near Bc2(T). Since the onset field of this anomalous peak effect with ascending field Bp,u does not match the ending field with descending field Bp,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 CeRu2 is argued in terms of the possible pinning mechanisms in this compound