Nonreciprocal Phonon Propagation in a Metallic Chiral Magnet

TL;DR

This study demonstrates room-temperature nonreciprocal phonon transport in a metallic chiral magnet, Co9Zn9Mn2, revealing enhanced phonon magnetochiral effects at higher temperatures compared to insulators.

Contribution

It uncovers the temperature-dependent nonreciprocal phonon transport in a metallic magnet, contrasting with insulators, and links the effect to magnon bandwidth and hybridization.

Findings

Nonreciprocal phonon transport observed up to 250 K.

Enhanced nonreciprocity correlates with increased magnon bandwidth.

Mechanism differs from insulating counterparts, suggesting new engineering avenues.

Abstract

The phonon magnetochiral effect (MChE) is the nonreciprocal acoustic and thermal transports of phonons caused by the simultaneous breaking of the mirror and time-reversal symmetries. So far, the phonon MChE has been observed only in a ferrimagnetic insulator Cu2OSeO3, where the nonreciprocal response disappears above the Curie temperature of 58 K. Here, we study the nonreciprocal acoustic properties of a room-temperature ferromagnet Co9Zn9Mn2 for unveiling the phonon MChE close to the room temperature. Surprisingly, the nonreciprocity in this metallic compound is enhanced at higher temperatures and observed up to 250 K. This clear contrast between insulating Cu2OSeO3 and metallic Co9Zn9Mn2 suggests that metallic magnets have a mechanism to enhance the nonreciprocity at higher temperatures. From the ultrasound and microwave-spectroscopy experiments, we conclude that the magnitude of the phonon MChE of Co9Zn9Mn2 mostly depends on the magnon bandwidth, which increases at low temperatures and hinders the magnon-phonon hybridization. Our results suggest that the phonon nonreciprocity could be further enhanced by engineering the magnon band of materials.