Tuning magnons in 2D materials

Two-dimensional honeycomb ferromagnets, such as the monolayer chromium-trihalides, are predicted to behave as topological magnon insulators - characterized by an insulating bulk and topologically protected edge states, giving rise to a thermal magnon Hall effect. We demonstrate that the topological magnons in the chromium-trihalides are susceptible to tuning under a variety of external stimuli including the application of biaxial and uniaxial strain, introduction of an external in-plane or out-of-plane magnetic field, the adsorption of non-magnetic atoms on the surface, heterostructuring using other van der Waals monolayers, or changing the stacking order in a bilayer, among others. We reveal that through this external tuning, we can both directly tailor the size and the topology of the magnonic bandgap in the ferromagnetic state, as well as induce magnetic phase transitions to other states with significantly different magnonic behavior.