Absence of altermagnetic magnon band splitting in MnF$_2$

TL;DR

This study investigates the presence of altermagnetic magnon band splitting in MnF$_2$ using inelastic neutron scattering, finding no observable splitting and suggesting that MnF$_2$ behaves like a classical antiferromagnet.

Contribution

The paper provides experimental evidence that MnF$_2$ does not exhibit the predicted altermagnetic magnon splitting, challenging recent theoretical predictions.

Findings

No magnon splitting observed within measurement resolution.

Magnon modes are degenerate across the Brillouin zone.

Magnetic field causes Zeeman splitting near the $ ext{ extGamma}$ point.

Abstract

Altermagnets are collinear compensated magnets in which the magnetic sublattices are related by rotation rather than translation or inversion. One of the quintessential properties of altermagnets is the presence of split chiral magnon modes. Recently, such modes have been predicted in MnF$_2$. Here, we report inelastic neutron scattering results on an MnF$_2$ single-crystal along high-symmetry Brillouin zone paths for which the magnon splitting is expected. Within the resolution of our measurement, we do not observe the predicted splitting. The inelastic spectrum is well-modeled using $J_1, ~J_2, ~J_3$ nearest-neighbor exchange interactions with weak uniaxial anisotropy. These interactions have higher symmetry than the crystal lattice, while the interactions predicted to produce the altermagnetic splitting are negligibly small. Therefore, the two magnon modes appear to be degenerate over the entire Brillouin zone and the spin dynamics of MnF$_2$ is indistinguishable from a classical N\'eel antiferromagnet. Application of magnetic field causes a Zeeman splitting of the magnon modes close to the $\mathrm{\Gamma}$ point. Even if chiral magnon modes are allowed by altermagnetic symmetry, the splitting in real materials such as MnF$_2$ can be negligibly small.