Magnetic excitations in 2D van der Waals honeycomb ferromagnets

The recent discovery of 2D ferromagnets opens up a vast field of research in nanomaterials and spintronics. Probing the magnetic excitations and their interplay with other degrees of freedom (electronic, lattice, etc.) is crucial in order to understand the underlying interactions and potential applications to future spintronic devices. Furthermore, the nontrivial topology in their magnon bands could be used to realize dissipationless spin transport. In this talk, I will present the inelastic neutron scattering (INS) measurement of magnetic excitations in honeycomb ferromagnet insulators CrI3, CrGeTe3, and their isostructural compounds. Using INS, we identified topological gaps in the spin excitations in CrI3, then confirmed that the topological gap in CrI3 is mainly a result of the Dzyaloshinskii-Moriya (DM) interaction rather than the Kitaev interaction. We also investigated the strong spin-lattice coupling in CrGeTe3, and observed anharmonic non-linear magnon-phonon scattering in this compound. Such a process severely shortens magnon lifetime and renormalizes magnon energy. Our results provide crucial insights into the spin-driven phenomena in these 2D van der Waals ferromagnetic systems.