Cavity-mediated superconductor$\unicode{x2013}$ferromagnetic insulator coupling

TL;DR

This paper develops a microscopic theory for cavity-mediated coupling between superconductors and ferromagnetic insulators, revealing an induced anisotropy field that can influence FI spins and has potential applications in superconducting spintronics.

Contribution

The paper introduces a detailed microscopic model for long-distance superconductor–ferromagnetic insulator interactions mediated by cavity photons, focusing on the induced anisotropy field.

Findings

Induced anisotropy field of 14–16 μT on FI spins.

Potential for experimentally observable spin tilts in low-coercivity FIs.

Implications for superconducting spintronics applications.

Abstract

A recent proof of concept showed that cavity photons can mediate superconducting (SC) signatures to a ferromagnetic insulator (FI) over a macroscopic distance [Phys. Rev. B, 102, 180506(R) (2020)]. In contrast with conventional proximity systems, this facilitates long-distance FI$\unicode{x2013}$SC coupling, local subjection to different drives and temperatures, and studies of their mutual interactions without proximal disruption of their orders. Here we derive a microscopic theory for these interactions, with an emphasis on the leading effect on the FI, namely, an induced anisotropy field. In an arbitrary practical example, we find an anisotropy field of $14 \unicode{x2013} 16$ $\mu$T, which is expected to yield an experimentally appreciable tilt of the FI spins for low-coercivity FIs such as Bi-YIG. We discuss the implications and potential applications of such a system in the context of superconducting spintronics.