Nutation Wave as a Platform for Ultrafast Spin Dynamics in Ferromagnets

At short time scales the inertia term becomes relevant for the magnetization dynamics of ferromagnets and leads to nutation for the magnetization vector. For the case of spatially extended magnetic systems, for instance Heisenberg spin chains with isotropic spin-exchange interaction, this leads to the appearance of a novel collective excitation, the "nutation wave", whose properties are elucidated by analytic arguments and numerical studies. The one--particle excitations can be identified as relativistic massive particles. These particles, the "nutatons", acquire their mass via the Brout-Englert-Higgs mechanism, through the interaction of the wave with an emergent topological gauge field. This spin excitation would appear as a peak in the spectrum of the scattering structure factor in inelastic neutron scattering experiments. The high frequency and speed of the nutation wave can open new paths for realizing ultrafast spin dynamics.