Tunneling Studies of Superconductor/Strong Ferromagnet Bilayers

Thin-film heterostructures composed of superconductors and ferromagnets have recently received much interest, as they provide a unique opportunity to study the proximity effect between superconductivity and magnetism. We report systematic tunneling density of states (DOS) measurements on superconductor (Nb) /strong ferromagnet (CoFe, Ni) bilayers made with high quality aluminum-oxide planar tunnel junctions as a function of ferromagnetic thickness, $d_F$. In CoFe, we find that as $d_F$ increases, the superconducting DOS exhibits a scaling behavior in which the deviations from the normal-state conductance have a universal shape, which decreases exponentially in amplitude. The decay length, $d_1$, is approximately $0.4$~nm. We do not see oscillations in the DOS as a function of $d_F$, as one would expect from predictions based on the Usadel equations using reasonable parameters, although an oscillation in $T_c(d_F)$ has been seen in the same materials. Measurements on Nb/Ni bilayers will also be presented. This work is supported by AFOSR, DOE, and KOSEF through CSCMR.