Absence of Altermagnetic Magnon Band Splitting in MnF2

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 MnF₂. Here, we report inelastic neutron scattering results on an MnF₂ 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 J₁, J₂, J₃ 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 MnF₂ is indistinguishable from a classical Néel antiferromagnet. Application of a magnetic field causes a Zeeman splitting of the magnon modes close to the Γ point. Even if chiral magnon modes are allowed by altermagnetic symmetry, the splitting in real materials such as MnF₂ can be negligibly small.