Critical Current Densities of Nb-Ti Alloy Multilayers

We sputter deposited 11 period Nb_0.37Ti_0.63/Nb and Nb_0.37Ti_0.63/Ti multilayers onto heated Si substrates to examine the roles composition and geometry play in determining the critical current density J_c in commercially important superconductors. Nb_0.37Ti_0.63 thicknesses were 15-30 nm; Nb and Ti thicknesses were 5-15 nm. The samples were characterized by electrical transport between 4.2 K and T_c in magnetic fields up to 6 T. For applied fields near parallel with the layers, many of the multilayers showed a strong non-monotonic field dependence of J_c in the range 0.2-0.5 H_c2(T). At 5 T and 4.2 K, the maximum J_c = 2.4 MA/cm^2 for the Ti multilayers and 1.3 MA/cm^2 for the Nb multilayers. These J_c values are among the highest achieved in the NbTi system: 3 to 10 times the magnitude of those of conventional heat-processed NbTi wire across the entire field range. We present a model of the competition between the vortex-vortex and vortex-pin interactions which accounts for the qualitative features of this peak effect. ^*supported by IGC and State of CT