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, CeRu2, are reported. The mixed state is described in terms of aligned rigid bundl es of vortices. The bundle diameters decrease above 1/2H(c2) (consiste nt with collective weak pinning theory) and become comparable with the penetration length at a field at which a 'peak effect' is seen in mag netisation measurements. A clear memory of field histories that pass t hrough the 'peak effect' region is also found; however, some of the in duced disorder can be removed by subsequently cycling the field.