  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.
