Two-dimensional to three-dimensional crossover and magnetic penetration depth study in NbTi/Co multilayers

A series of NbTi single thin films, Co/NbTi/Co trilayers, and multilayers are made by dc sputtering to investigate the superconductive dimensional crossover between two-dimensions (2D) and three dimensions. From the temperature dependence of the upper critical fields measured resistively, we see a nonlinear changing to linear dependence of Hc2(T) from 60 to 80 nm of NbTi thickness for trilayers. Insertion of 0.6 nm thick Co between 30 nm NbTi films also shows 2D behavior, which suggests confinement of the superconducting wave function within individual layers. Magnetic field response is measured to study the interplay between the superconductor and ferromagnet. We inferred a magnetic penetration depth of ∼35 nm into NbTi. [copyright] 2001 American Institute of Physics