Effect of dipolar interaction on exceptional points in synthetic layered magnets

Abstract

Within both synthetic ferromagnets and antiferromagnets, exceptional points, where optical and acoustic magnons coalesce into a single branch, can be used to control the magnon energy spectra. To date, exceptional point phenomena in magnon systems have been predominantly predicted and understood within the framework of macrospin models that are based upon coupled Landau–Lifshitz–Gilbert equations of motion. Although these equations can be readily linearized and solved, they do not necessarily incorporate all of the physical effects that are present in a real synthetic magnetic structure such as dipolar interactions. We have used micromagnetic simulations to model Permalloy based synthetic magnets that include both dipolar interactions, as well as the interlayer exchange coupling which determines whether or not the material is ferromagnetic or antiferromagnetic. For the material parameters considered in this work, we predict that only a single exceptional point is present when the system is ferromagnetic, and that no exceptional point appears when the material is antiferromagnetic. These results suggest that when calculating exceptional points within layered magnetic materials, interactions other than the interlayer exchange field must be accounted to accurately predict the existence of exceptional points, and that micromagnetic simulations are a useful tool to perform this task.