Spin-exchange Hamiltonian and topological degeneracies in elemental gadolinium

Elemental Gd is a textbook example of a spin-only ferromagnet, but due to its complex magnon dispersion, its magnetic exchange Hamiltonian has never been fully determined. Here, the authors use modern inelastic time-of-flight neutron scattering to fully map and fit the Gd spectrum to a spin wave model. The results show long-range magnetic interactions, in accord with RKKY conduction-mediated exchange. The authors also identify symmetry-protected topological degeneracies in the magnon dispersion, which should appear in other ferromagnetic hcp metals.