Percolative supercurrent in superconductor-ferromagnetic insulator bilayers

TL;DR

This study investigates how superconducting and ferromagnetic insulator layers interact, revealing that spin-splitting and supercurrent flow are influenced by the ratio of coherence length to magnetic domain size, leading to percolative supercurrent behavior.

Contribution

It introduces a theoretical model linking coherence length and magnetic domain size to supercurrent percolation in superconductor-ferromagnetic insulator bilayers.

Findings

Remanent spin-splitting observed in bilayers.

Supercurrent persists above the paramagnetic limit.

Spin-averaging causes local suppression of superconductivity.

Abstract

We report tunneling spectroscopy and transport measurements in superconducting Al and ferromagnetic-insulator EuS bilayers. The samples display remanent spin-splitting, roughly half the superconducting gap, and supercurrent transport above the average paramagnetic limit. We interpret this behavior as arising from the interplay between two characteristic length scales: the superconducting coherence length, $\xi$, and the magnetic domain size, $d$. By comparing experimental results to a theoretical model, we find $\xi/d \approx 10$. In this regime, spin-averaging across the micromagnetic configuration can locally suppress superconductivity, resulting in percolative supercurrent flow.