Thermal Evolution of Dirac Magnons in the Honeycomb Ferromagnet CrBr3

Abstract

CrBr3 is an excellent realization of the two-dimensional honeycomb ferromagnet, which offers a bosonic equivalent of graphene with Dirac magnons and topological character. We perform inelastic neutron scattering measurements using state-of-the-art instrumentation to update 50-year-old data, thereby enabling a definitive comparison both with recent experimental claims of a significant gap at the Dirac point and with theoretical predictions for thermal magnon renormalization. We demonstrate that CrBr3 has next-neighbor J2 and J3 interactions approximately 5% of J1, an ideal Dirac magnon dispersion at the K point, and the associated signature of isospin winding. The magnon lifetime and the thermal band renormalization show the universal T2 evolution expected from an interacting spin-wave treatment, but the measured dispersion lacks the predicted van Hove features, pointing to the need for more sophisticated theoretical analysis.