Superfluid density of superconductor-ferromagnet bilayers

TL;DR

This study measures the superfluid density in superconductor-ferromagnet bilayers, revealing that ferromagnetic layers can host significant superfluid density even when the critical temperature stabilizes.

Contribution

First experimental measurement of superfluid density in SC/FM bilayers, showing superfluid presence within ferromagnetic layers beyond T_C behavior.

Findings

Superfluid density decreases with Ni thickness initially

Minimum superfluid density at Ni thickness around 25 Å

Superfluid density inside Ni persists even after T_C stabilizes

Abstract

We report the first measurements of the effective superfluid density n_S(T) \propto \lambda^{-2}(T) of Superconductor-Ferromagnet (SC/FM) bilayers, where \lambda is the effective magnetic field penetration depth. Thin Nb/Ni bilayers were sputtered in ultrahigh vacuum in quick succession onto oxidized Si substrates. Nb layers are 102 A thick for all samples, while Ni thicknesses vary from 0 to 100 A. T_C determined from \lambda^{-2}(T) decreases rapidly as Ni thickness d_Ni increases from zero to 15 A, then it has a shallow minimum at d_Ni \approx 25 A. \lambda^{-2}(0) behaves similarly, but has a minimum several times deeper. In fact, \lambda^{-2}(0) continues to increase with increasing Ni thickness long after T_C has stopped changing. We argue that this indicates a substantial superfluid density inside the ferromagnetic Ni films.