Conversion of Chiral Phonons into Magnons in Magnets

Spin-rotation coupling, which refers to the coupling between mechanical rotations and electron spins, leads to various phenomena, such as the Einstein-de Haas effect and the Barnett effect, in which the mechanical rotation of a crystal acts on the electron spins as an effective magnetic field. Recently, surface acoustic waves and chiral phonons involving the rotational motion of atoms have attracted considerable attention in spintronics. Surface acoustic waves refer to classical surface waves in continuous elastic media, whereas chiral phonons represent the microscopic local rotation of atoms with a phonon angular momentum.

Previous studies have indicated a phenomenon involving the conversion of phonon angular momentum into electron spins and charges in nonmagnetic materials and shown that chiral phonons behave as an effective magnetic field. Chiral phonons accompanying atomic rotations modulate electron hopping and spin-orbital coupling because the distance between atoms periodically changes with time. In the case of magnets, spin-spin interactions, such as exchange interactions and Dzyaloshinskii-Moriya interaction, always exist. These spin-spin interactions govern the collective propagation of precessions of electron spins, which are known as magnons. However, the interplay between atomic rotation and magnons in magnets is not fully understood.

In this study, we investigate a new conversion of chiral phonons into magnons in both ferromagnets and antiferromagnets and show that chiral phonons modulate spin-spin interactions. Because the masses of atoms are much larger than those of electrons, the behavior of magnon dynamics in an adiabatic response to atomic rotations leads to a geometric effect. We demonstrate that this effect requires breaking the spin-rotation symmetry. Consequently, the geometric effect due to chiral phonons results in a nontrivial change in magnon excitations. Specifically, chiral phonons with clockwise and counterclockwise rotational modes induce a change in magnon numbers with opposite signs, which corresponds to increasing and decreasing spin magnetization due to the chiral nature of atomic rotations.

The proposed effect is universal for a wide range of magnets. Measurement in an antiferromagnet is easier than in a ferromagnet because the net spin magnetization becomes nonzero owing to the geometric effect induced by chiral phonons. Our theory can generally be applied to real materials and is expected to be experimentally realized in some candidate materials, such as antiferromagnets XPS3 (X =Fe, Mn, and Ni), with a strong magnetic anisotropy.

(Written by D. Yao and S. Murakami)