Unanticipated proximity behavior in ferromagnet-superconductor heterostructures with controlled magnetic noncollinearity

TL;DR

This study investigates the proximity effect in ferromagnet-superconductor heterostructures with controlled magnetic noncollinearity, revealing unexpected nonmonotonic superconductivity enhancement that challenges existing theories.

Contribution

It demonstrates unanticipated proximity behavior in F/S heterostructures with controllable magnetic noncollinearity, indicating the need for new theoretical models.

Findings

Superconductivity enhancement is nonmonotonic with Py domain wall twisting.

Magnetic fields and vortex motion are excluded as causes.

Results suggest new physics in F/S systems with noncollinear magnetization.

Abstract

Magnetization noncollinearity in ferromagnet-superconductor (F/S) heterostructures is expected to enhance the superconducting transition temperature (Tc) according to the domain-wall superconductivity theory, or to suppress Tc when spin-triplet Cooper pairs are explicitly considered. We study the proximity effect in F/S structures where the F layer is a Sm-Co/Py exchange-spring bilayer and the S layer is Nb. The exchange-spring contains a single, controllable and quantifiable domain wall in the Py layer. We observe an enhancement of superconductivity that is nonmonotonic as the Py domain wall is increasingly twisted via rotating a magnetic field, different from theoretical predictions. We have excluded magnetic fields and vortex motion as the source of the nonmonotonic behavior. This unanticipated proximity behavior suggests that new physics is yet to be captured in the theoretical treatments of F/S systems containing noncollinear magnetization.