Neutron scattering is a powerful experimental tool in condensed matter research, which can provide useful information regarding spatial and temporal correlations among atomic motions and/or their magnetic moments inside solid materials. While it is a complementary method to more popular x-ray scattering, neutron scattering is particularly suited for observing spin orderings and their excitations.
In this talk, we will review the recent investigations of spin excitations in various honeycomb magnets using neutron scattering method. Two-dimensional honeycomb lattices are an intriguing structure where 120-degree bonds provide not only structural stability but also competing interactions responsible for unconventional behaviors. For instance, graphene, a single honeycomb layer of carbon atoms, is widely known to be the strongest material ever tested in the lab but also exhibits exotic quantum Hall effect of fermionic Dirac particles evading the wave-particle duality. Our recent neutron scattering works revealed that ferromagnets on honeycomb lattices exhibit spin excitations analogous to the electronic bands in graphene. For instance, spin waves in honeycomb ferromagnets behave as bosonic Dirac particles, which also become chiral in zero field behaving like electrons subject to Lorentz force. As a result, spin waves in honeycomb ferromagnets become topological with “insulating” interior and “conducting” edges. We will discuss how these novel spin properties emerge as the consequence of time-reversal symmetry breaking on honeycomb lattices.