Magnon-phonon interactions from first principles

Modeling spin-wave (magnon) dynamics in novel materials is important to advance spintronics and spin-based quantum technologies. The interactions between magnons and lattice vibrations (phonons) limit the length scale for magnon transport. However, quantifying these interactions remains challenging. Here, we show many-body calculations of magnon-phonon (mag-ph) coupling based on the Bethe-Salpeter equation. We derive expressions for mag-ph coupling matrices and compute them in two-dimensional ferromagnets, focusing on hydrogenated graphene and monolayer {CrI}₃ . Our analysis shows that electron-phonon (e−ph) and mag-ph interactions differ significantly, where modes with weak e−ph coupling can exhibit strong mag-ph coupling (and vice versa), and reveals which phonon modes couple more strongly with magnons. In both materials studied here, the inelastic magnon relaxation times decrease abruptly above the threshold for emission of strongly coupled phonons, revealing a low-energy window where magnons are long lived. Averaging over this window, we compute the temperature-dependent magnon mean free path, a key figure of merit for spintronics, entirely from first principles. The theory and computational tools shown in this Letter enable studies of magnon interactions, scattering, and dynamics in generic materials, advancing the design of magnetic systems and magnon- and spin-based devices.