Temperature-sensitive spin-wave nonreciprocity induced by interlayer dipolar coupling in ferromagnet/X (X=paramagnet, superconductor) hybrid systems

TL;DR

This paper theoretically investigates how interlayer dipolar coupling in ferromagnet/hybrid systems induces temperature-sensitive nonreciprocity in spin-wave spectra, enabling control of SW propagation via temperature near critical points.

Contribution

It introduces a dipolar mechanism for spin-wave nonreciprocity in FM/PM and FM/SC systems, highlighting temperature dependence and substrate effects, which is novel.

Findings

SW spectrum becomes nonreciprocal depending on the wave vector sign.

The nonreciprocity is tunable by temperature near the critical temperature.

The sign of the nonreciprocity depends on the substrate type.

Abstract

Spin-wave (SW) spectra have theoretically been studied in a thin film of a ferromagnet (FM) on a substrate from a paramagnet (PM) (an FM above the critical temperature) or from superconductor (SC). A spin wave propagating in the FM induces the dynamic magnetization and superconducting current in the underlying PM and SC, respectively, which affect the SW propagation by their magnetic fields. As a result of this interaction, the SW spectrum becomes nonreciprocal to depend on the sign of the SW wave vector q. We show that the nonreciprocal contribution to the SW spectra in FM/PM and FM/SC systems is given by the frequency shift of {\Delta}{\omega}(q)={\omega}(q)-{\omega}(-q)=a(T)({\tau}{\cdot}q) with {\tau}=(n{\times}M) being the toroidal magnetic moment, M the FM magnetization, n the unit vector normal to the FM/PM(SC) interface, and a(T) the temperature-dependent constant of a dipole nature, whose sign depends on the substrate type. As the {\Delta}{\omega}(T) dependence is strong at temperatures T close to the critical temperature Tc for the FM-PM or normal metal-SC transition, one gets a possibility to control the frequency SW nonreciprocity with temperature variation near Tc. The dipolar mechanism we propose for SW frequency nonreciprocity is promising for introducing this property of SW propagation into functional devices.