Research: Nonreciprocal Phonon Propagation in a Metallic Chiral Magnet

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, below the Curie temperature of 58 K [T. Nomura et al., Phys. Rev. Lett. 122, 145901 (2019)].

In a recent work, a team of researchers from Japan and Germany has observed the phonon MChE in metallic ferromagnet Co₉Zn₉Mn₂ close to room temperature. The results suggest a mechanism to enhance the nonreciprocity in metallic magnets. The enhanced nonreciprocity is related to the magnon-phonon hybridization and reduced magnon bandwidth. The magnitude of the phonon MChE of Co₉Zn₉Mn₂ mostly depends on the Gilbert damping, which increases at low temperatures and hinders the magnon-phonon hybridization. Here a magnon-electron scattering plays an important role. At lower temperature, the electron scattering time increases and the larger momentum transfer at the intraband relaxation results in the enhanced damping.

The obtained results suggest that the phonon nonreciprocity could be further enhanced by engineering the magnon band of materials.

T. Nomura, X.-X. Zhang, R. Takagi, K. Karube, A. Kikkawa, Y. Taguchi, Y. Tokura, S. Zherlitsyn, Y. Kohama, and S. Seki,
Nonreciprocal Phonon Propagation in a Metallic Chiral Magnet,
Phys. Rev. Lett. 130, 176301 (2023) (arXiv)