Orbital Magnetic Moment of Magnons

TL;DR

This paper reveals that magnons possess an orbital magnetic moment due to spin-orbit coupling, which is independent of the spin magnetic moment and can lead to orbital weak ferromagnetism in certain antiferromagnets.

Contribution

It introduces the concept of an orbital magnetic moment of magnons and demonstrates its existence in kagome antiferromagnets with Dzyaloshinskii-Moriya interaction.

Findings

Magnons have an independent orbital magnetic moment.

Orbital magnetization can be nonzero even when spin magnetization is zero.

Magnonic orbital moments are widespread in certain magnetic materials.

Abstract

In experiments and applications usually the spin magnetic moment of magnons is considered. In this Paper we identify an additional degree of freedom of magnons: an \emph{orbital} magnetic moment brought about by spin-orbit coupling.Our microscopic theory uncovers that spin magnetization $\vec{M}^\mathrm{S}$ and orbital magnetization $\vec{M}^\mathrm{O}$ are independent quantities. They are not necessarily collinear; thus, even when the total spin moment is compensated due to antiferromagnetism ($\vec{M}^\mathrm{S} = \vec{0}$), $\vec{M}^\mathrm{O}$ may be nonzero. This scenario of orbital weak ferromagnetism is realized in paradigmatic kagome antiferromagnets with Dzyaloshinskii-Moriya interaction. We demonstrate that magnets exhibiting a magnonic orbital moment are omnipresent and propose transport experiments for probing it.