Electromagnetic Proximity Effect: Superconducting Magnonics and Beyond

TL;DR

This review analyzes the electromagnetic proximity effect arising from non-contact dipolar interactions between magnons in ferromagnets and superconductors or normal metals. It develops a unified framework linking Maxwell-London electrodynamics, superconducting order parameter dynamics, and magnonic excitations to predict chiral field generation, gate-tunable magnon transport, and ultrastrong magnon–photon and magnon–Cooper-pair couplings. The authors catalog a spectrum of emergent quasiparticles and modes—magnon-Meissner collective modes, magnon cooparons, Josephson plasmonic modes, and nodal magnon–photon polaritons—with experimental evidence from FMR shifts, NV imaging, magnonic crystals, and three-terminal magnon devices. They further show how superconducting and normal-metal gates enable chiral control, nonreciprocal transport, and non-Hermitian phenomena, potentially enabling reconfigurable magnonic circuits and hybrid quantum systems. Collectively, the work highlights the rich physics and practical prospects of electromagnetic proximity for advanced magnonics and superconducting spintronics.

Abstract

The exchange interaction at interfaces between superconductors (SCs) and ferromagnets (FMs) has been a central topic in condensed matter physics for many decades, starting with the prediction of exotic phases such as the Fulde-Ferrell-Larkin-Ovchinnikov states and leading to the discovery of triplet superconductivity. This review focuses on new phenomena in SC$|$FM heterostructures caused by the \textit{non-contact dipolar interaction} between magnons, i.e., the quanta of spin wave excitations in the ferromagnet, and the superconducting order. A universal non-relativistic spin-orbit coupling locks the polarization and momentum of their evanescent stray magnetic fields and leads to chiral screening by proximate superconductors. The interaction-induced hybrid quasiparticles are magnon-Meissner collective modes, magnon-cooparon, Josephson plasmonic modes, and nodal magnon-photon polaritons. Superconducting and normal metallic gates modulate and control the magnetodipolar interaction and thereby magnetization and energy transport at interfaces and in thin films.

Figures (45)

• Figure 1: Overview of the main coupling mechanisms between a superconductor and a ferromagnet, viz. the $s$-$d$ exchange interaction and the dipolar interaction. The exchange interaction either in the magnet or at the interface can lead to triplet Cooper pairing and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The dipolar proximity interaction gives rise to the Meissner and electromagnetic proximity effects.
• Figure 2: Coupling between spin waves and supercurrents via the electromagnetic proximity effect.
• Figure 3: A superconductor$|$ferromagnet heterostructure contains three bulk regions (S, F, Vacuum) separated by three boundaries ($\Gamma_1, \Gamma_2, \Gamma_3$). Inside the ferromagnet, the magnetization ${\bf M}\neq0$, and inside the superconductor, the total current ${\bf J}({\bf r},t)$ includes a normal ${\bf J}_n({\bf r},t)$ and a supercurrent ${\bf J}_s({\bf r},t)$. Maxwell's equations, with proper boundary conditions, govern the electromagnetic field throughout all space.
• Figure 4: Chirality of classical waves of different nature and configurations. (a) Damon-Eshbach surface spin waves; (b) the dipolar magnetic stray field of spin waves; (c) the electric field of surface plasmons; (d) surface acoustic waves; (e) the magnetic field of microwave guides; (f) near magnetic field of microwave striplines in a plane. Sources: Fig. (b) is taken from Ref. Tao_chiral_pumping; Fig. (c), (d), and (f) are taken from Ref. Yu_chirality; Fig. (e) is taken from Ref. Tao_Magnon_Accumulation.
• Figure 5: Right-handed classical waves [(a)] vs. right-handed classical waves [(b)].