Superconducting properties and critical current density of Nb-Ti/Ti multilayers

Multilayers of Nb47Ti superconductor (S) and titanium pins (N) have been made with a critical temperature T-c and an upper critical field H-c2 approaching bulk values, There is no proximity effect suppression of T-c and H-c2 for layer thickness d(S) = d(N) = 10 nm, in contrast to the strong suppression for comparable multilayers made with Cu, Cu-alloy, or Nb pins. This may be because the proximity length of the Ti layers, similar to 10 nm, is less than or equal to their thickness. The critical current density J(c) exhibits multiple peaks in J(c)(H), which suggests that matching effects contribute to the overall pinning, J(c) for 20 nm bilayers is comparable to that of Nb47Ti tapes, which have a higher number density of pins but a lower pin volume fraction. The weak proximity coupling suggests that much higher J(c) can be obtained if smaller bilayer periods can be made with good adhesion. The results are compared to other Nb-Ti/Ti multilayer experiments, and stability limitations are also discussed.