Vortex-lattice phase diagram of Nb/NbZr multilayers

Multilayer of Nb/NbZr shows an upturn behavior in the parallel upper critical field Hc2parallel (T). In this work, we studied the critical current density Jc as a function of magnetic field H and of the angle between H and film plane at various temperatures. From the non-monotonic behavior Jc(H), we found two characteristic fields, HP and HL, above and below the upturn temperature. An analysis with the angular dependence shows that a dimensional crossover in the mixed state and a shift of the order parameter from Nb to NbZr occur at HP and HL, respectively. (orig.)