Spin Inertia as a Source of Topological Magnons: Chiral Edge States from Coupled Precession and Nutation

Abstract

Spin inertia has been demonstrated to give rise to high-frequency nutational excitations beyond the conventional low-frequency precessional modes. Here, we demonstrate that the hybridization between precessional and nutational magnons may give rise to topological phenomena in the spin-wave spectrum. This hybridization requires the presence of interactions breaking angular-momentum conservation, such as the pseudodipolar interaction. We show on the example of a honeycomb ferromagnet how topological gaps open between the precessional and nutational bands that host chiral edge states in slab geometries. Our work establishes a theoretical foundation for exploring inertial spin dynamics as a new route to engineer topological phases in magnetic materials.

Figures (4)

• Figure 1: Sketch of the ferromagnet on the honeycomb lattice. Red and blue trajectories denote counterclockwise precessional and clockwise nutational modes, respectively.
• Figure 2: Magnon band structure in momentum space along high-symmetry points. The states are colored according to their ellipticity defined in Eq. \ref{['Eq6']}.
• Figure 3: Magnon band structure in a slab geometry. The system is $40$ unit cells wide along the $x$ direction with open boundary conditions, while it is infinite along the $y$ direction with periodic boundary conditions, meaning that the eigenstates can be indexed by the wave vector component $k_{y}$. $a$ denotes the distance between nearest-neighbor atoms in the lattice, i.e., the lattice constant is $\sqrt{3}a$. Dotted lines show the boundaries of the one-dimensional Brillouin zone. Coloring shows the localization defined in Eq. \ref{['Eq7']}.
• Figure 4: Berry curvature $B^{z}_{n}(\bm{k})$ of the bands calculated from Eq. \ref{['Eq8']}. (a)-(d) show the four magnon bands in increasing order of frequency. The Chern numbers $C_{n}$ for each band are also given.