Percolative supercurrent in superconductor–ferromagnetic insulator bilayers

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>ξ</a:mi></a:math> and the magnetic domain size <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>d</b:mi></b:math>. By comparing experimental results to a theoretical model, we find <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>ξ</c:mi><c:mo>/</c:mo><c:mi>d</c:mi><c:mo>≈</c:mo><c:mn>10</c:mn></c:mrow></c:math>. In this regime, spin averaging across the micromagnetic configuration can locally suppress superconductivity, resulting in percolative supercurrent flow.