Asymmetric Magnon Excitation by Spontaneous Toroidal Ordering

TL;DR

This paper theoretically investigates how spontaneous toroidal ordering affects magnetic excitations, revealing asymmetric magnon dispersions that could lead to novel nonreciprocal magnetic responses in certain materials.

Contribution

It demonstrates that toroidal order induces asymmetric magnon dispersion patterns, a novel insight into magnetic excitations influenced by spin-orbit coupling and multi-orbital effects.

Findings

Toroidal order causes band-bottom shifts and valley splitting in magnon dispersion.

Asymmetric magnon excitations may lead to nonreciprocal magnon transport.

Theoretical predictions applicable to candidate materials with toroidal order.

Abstract

Effects of spontaneous toroidal ordering on magnetic excitation are theoretically investigated for a localized spin model that includes a staggered Dzyaloshinsky-Moriya interaction and anisotropic exchange interactions, which arise from the antisymmetric spin-orbit coupling and the multi-orbital correlation effect. We show that the model exhibits a Neel-type antiferromagnetic order, which simultaneously accompanies a ferroic toroidal order. We find that the occurrence of toroidal order modulates the magnon dispersion in an asymmetric way with respect to the wave number: a toroidal dipole order on the zigzag chain leads to a band-bottom shift, while a toroidal octupole order on the honeycomb lattice gives rise to a valley splitting. These asymmetric magnon excitations could be a source of unusual magnetic responses, such as nonreciprocal magnon transport. A variety of modulations are discussed while changing the lattice and magnetic symmetries. Implications of candidate materials for asymmetric magnon excitations are presented.