Characterization of magnon band splitting in altermagnetic FeF2

The rutile structure material FeF₂ is a well-known classical antiferromagnet which we reexamine in the context of altermagnetism. FeF₂ has a simple collinear magnetic structure which meets the symmetry requirements for altermagnetic band splitting. While the material is highly insulating and not amenable to electronic band structure measurements, altermagnetism can also manifest as a characteristic splitting in the magnon bands. Here, we present high resolution polarized and unpolarized inelastic neutron measurements characterizing the energy, momentum and polarization dependence of magnons in FeF₂. Our measurements reveal the presence of magnon band splitting and quantify the effects of altermagnetic interactions on the magnetic excitations in this material.