A neutron study of the flux lattice in the superconductor CeRu2

Small-angle neutron diffraction measurements from the flux lattice in a single crystal of the cubic Laves' phase superconductor, CeRu(2), are reported. The mixed state is described in terms of aligned rigid bundles of vortices. The bundle diameters decrease above 1/2H(c2) (consistent with collective weak pinning theory) and become comparable with the penetration length at a field at which a 'peak effect' is seen in magnetisation measurements. A clear memory of field histories that pass through the 'peak effect' region is also found; however, some of the induced disorder can be removed by subsequently cycling the field.