Nonreciprocal Directional Dichroism and Toroidalmagnons in Helical Magnets

TL;DR

This paper explores the discovery of toroidalmagnons in helical magnets, revealing their unique dynamical properties and their role in nonreciprocal directional dichroism caused by the interference of magnetic and electric responses.

Contribution

It introduces the concept of toroidalmagnons, demonstrating their existence in conical spin structures and their impact on electromagnetic wave absorption.

Findings

Toroidalmagnons possess both magnetic and electric dynamical moments.

Their excitation induces nonreciprocal directional dichroism.

The phenomenon is due to interference between magnetic and electric responses.

Abstract

We investigate a dynamical magnetoelectric effect due to a magnetic resonance in helical spin structures through the coupling between magnetization and electric polarization via a spin current mechanism. We show that the magnon has both the dynamical magnetic moment $\Delta M^\omega$ and the electric moment $\Delta P^\omega$ ($\perp \Delta M^\omega$), i.e., a dynamical toroidal moment, under external magnetic fields, and thus it is named the {\em toroidalmagnon}. The toroidalmagnon exists in most conical spin structures owing to the generality of the spin current mechanism. In the absorption of electromagnetic waves, the toroidalmagnon excitation process generally induces a nonreciprocal directional dichroism as a consequence of an interference of the magnetic and electric responses.