Altermagnetic and dipolar splitting of magnons in FeF$_2$

TL;DR

This study uses high-resolution neutron scattering to show that dipolar interactions, rather than altermagnetic exchange, primarily cause magnon splitting in FeF$_2$, revealing the significant role of dipolar effects on magnon chirality.

Contribution

It demonstrates that dipolar interactions dominate magnon splitting in FeF$_2$, challenging the previous assumption that altermagnetic exchange was the main cause.

Findings

Dipolar interactions are the main source of magnon splitting in FeF$_2$.

Altermagnetic chiral splitting is estimated at approximately 35 μeV.

Chiral magnon modes become mixed and predominantly linearly polarized where dipolar splitting occurs.

Abstract

FeF$_2$ is a prototypical rutile antiferromagnet recently proposed as an altermagnet, with a magnetic symmetry that permits spin-split electronic bands and chiral magnons. Using very-high-resolution inelastic neutron scattering on a single crystal of FeF$_2$, we show that the dominant source of magnon splitting is in fact the long-range dipolar interaction rather than altermagnetic exchange terms. At momenta where the dipolar splitting vanishes, we observe additional broadening due to altermagnetic chiral splitting and estimate this splitting to be $\sim$35 $\mu$eV. Polarized measurements further reveal that, where dipolar splitting is present, the chiral magnon modes become mixed and the resulting modes are predominantly linearly polarized, with at most a small chiral component. These findings highlight the significant effect of dipolar interactions on magnon chirality, particularly when altermagnetic interactions are weak.