Inelastic neutron scattering was used to study the magnetic excitations of the triangular antiferromagnets ${\mathrm{Mn}}_3$Sn and ${\mathrm{Mn}}_3$Ge. These compounds have itinerant d electrons and large magnetic moments localized at the Mn sites and may be regarded as materials that lie in the intermediate regime between local-moment and itinerant-electron systems. The spin-wave spectra exhibit steep dispersion and strong damping, which is characteristic behavior of itinerant-electron systems. Nevertheless, it is useful to analyze the data in terms of a local-moment model with anisotropy. We find the data are remarkably well described by this model with exchange parameters extending to fifth-nearest neighbors and with both axial- and basal-plane anisotropy. The axial-anisotropy parameters were determined from the uniform out-of-plane spin fluctuation, and the signs show that the spins are confined to the basal plane. The second-order basal-plane anisotropy constants were determined by satisfying both the magnitude of the weak basal-plane ferromagnetic moments and the observed splitting of a doubly degenerate acoustic-spin-wave branch. The sixth-order basal-plane anisotropy was determined by adjusting to the observed energy gap associated with spin fluctuations within the basal plane. The exchange parameters have the correct signs to stabilize the triangular antiferromagnetic structure but yield Néel temperatures that are higher than those observed by a factor of 3 or 4. This overestimation of the Néel temperature is not an uncommon result when a local moment model is applied to an itinerant-electron system.

• Received 18 March 1993

DOI:https://doi.org/10.1103/PhysRevB.48.6159