Topological Spin Excitations in Honeycomb Ferromagnet CrI3

Abstract

In two-dimensional honeycomb ferromagnets, bosonic magnon quasiparticles (spin waves) may either behave as massless Dirac fermions or form topologically protected edge states. The key ingredient defining their nature is the next-nearest-neighbor Dzyaloshinskii-Moriya interaction that breaks the inversion symmetry of the lattice and discriminates chirality of the associated spin-wave excitations. Using inelastic neutron scattering, we find that spin waves of the insulating honeycomb ferromagnet CrI3 (T-C = 61 K) have two distinctive bands of ferromagnetic excitations separated by a similar to 4 meV gap at the Dirac points. These results can only be understood by considering a Heisenberg Hamiltonian with DzyaloshinskiiMoriya interaction, thus providing experimental evidence that spin waves in CrI3 can have robust topological properties potentially useful for dissipationless spintronic applications.