Tunneling density of states as a function of thickness in superconductor/ strong ferromagnet bilayers

TL;DR

This study investigates how the tunneling density of states in superconductor/ferromagnet bilayers varies with ferromagnetic layer thickness, revealing a universal exponential decay in DOS deviations without expected oscillations.

Contribution

It provides experimental evidence of a universal exponential decay in DOS deviations in superconductor/ferromagnet bilayers, contrasting with theoretical predictions of oscillations.

Findings

DOS deviations decrease exponentially with ferromagnetic thickness

No oscillations observed in DOS as a function of thickness

Universal scaling behavior identified in the DOS

Abstract

We have made an experimental study of the tunneling density of states (DOS) in strong ferromagnetic thin films (CoFe) in proximity with a thick superconducting film (Nb) as a function of $d_F$, the ferromagnetic thickness. Remarkably, 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 that decreases exponentially in amplitude with characteristic length $d^*\approx 0.4$ nm. We do not see oscillations in the DOS as a function of $d_F$, as expected from predictions based on the Usadel equations, although an oscillation in $T_c(d_F)$ has been seen in the same materials.