Multiple peaks in the angular and field dependence of the critical currents in Nb-47wt%Ti/Cu multilayers

The effect of Nb-Ti layer thickness d/sub s/ on the angular and field dependencies of J/sub c/ in Nb-47wt.% Ti/Cu multilayers was investigated. J/sub c/ exhibits several peaks in the field and angular dependencies for superconductor and pin layer thickness comparable to the coherence and proximity lengths. The shape of the J/sub c/(B) curve strongly depends on the thickness of Nb-Ti layer. A single peak is present for d/sub s/>10 nm and multiple peaks (two and three) are observed in a sample with d/sub s/=10 nm. The low field peak is temperature independent, while the high-field peaks have a pronounced temperature dependence. We suggest that a crossover between dominant pinning mechanisms and orientational transitions in the vortex lattice is responsible for the observed behavior of J/sub c/(B).