Magnon-phonon relaxation in yttrium iron garnet from first principles

TL;DR

This paper presents a first-principles calculation of magnon-phonon relaxation times in YIG, revealing how magnon lifetime broadening arises from exchange interaction fluctuations and providing quantitative relaxation times relevant for spintronics.

Contribution

The study introduces a first-principles method to calculate magnon-phonon relaxation times in YIG, linking microscopic exchange fluctuations to macroscopic relaxation phenomena.

Findings

Magnon spectra exhibit significant broadening at room temperature.

Optical magnon relaxation time is constant; acoustic magnon relaxation time scales as 1/k^2.

Magnon-phonon relaxation times are quantitatively extracted from spectral broadening.

Abstract

We combine the theoretical method of calculating spin wave excitation with the finite-temperature modeling and calculate the magnon-phonon relaxation time in the technologically important material Yttrium iron garnet (YIG) from first principles. The finite lifetime of magnon excitation is found to arise from the fluctuation of the exchange interaction of magnetic atoms in YIG. At room temperature, the magnon spectra have significant broadening that is used to extract the magnon-phonon relaxation time quantitatively. The latter is a phenomenological parameter of great importance in YIG-based spintronics research. We find that the magnon-phonon relaxation time for the optical magnon is a constant while that for the acoustic magnon is proportional to $1/k^2$ in the long-wavelength regime.