Magnon-phonon interactions from first principles

TL;DR

This paper presents a first-principles approach to quantify magnon-phonon interactions in 2D ferromagnets, revealing their impact on magnon relaxation and transport properties relevant for spintronics.

Contribution

It introduces a novel ab initio method to compute magnon-phonon coupling matrices and analyzes their effects in hydrogenated graphene and CrI3 monolayers.

Findings

Strong mag-ph coupling occurs in modes with weak e-ph coupling.

Magnon relaxation time drops sharply above phonon emission threshold.

Temperature-dependent magnon mean-free path is computed from first principles.

Abstract

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 ab initio Bethe-Salpeter equation. We derive expressions for mag-ph coupling matrices and compute them in 2D ferromagnets, focusing on hydrogenated graphene and monolayer CrI3. 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 time is found to decrease abruptly above the threshold for emission of strongly coupled phonons, thereby defining a low-energy window for efficient magnon transport. By averaging in 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 work enable studies of magnon interactions, scattering, and dynamics in generic materials, advancing the design of magnetic systems and magnon- and spin-based devices.