Nonreciprocal Phonon Propagation in a Metallic Chiral Magnet

Area of Science:

• Condensed Matter Physics
• Materials Science
• Acoustics

Background:

• The phonon magnetochiral effect (MChE) describes nonreciprocal phonon transport due to broken mirror and time-reversal symmetries.
• Previously, MChE was only observed in the ferrimagnetic insulator Cu$_{2}$OSeO$_{3}$, disappearing above 58 K.

Purpose of the Study:

• To investigate the phonon MChE in a room-temperature metallic ferromagnet, Co$_{9}$Zn$_{9}$Mn$_{2}$.
• To explore the temperature dependence of MChE in metallic magnets and compare it to insulators.

Main Methods:

• Ultrasound spectroscopy
• Microwave spectroscopy
• Temperature-dependent measurements of acoustic nonreciprocity.

Main Results:

• The phonon MChE was observed in Co$_{9}$Zn$_{9}$Mn$_{2}$ up to 250 K, significantly higher than previously reported.
• Nonreciprocity in this metallic compound was enhanced at higher temperatures.
• The magnitude of MChE was found to depend on Gilbert damping, which affects magnon-phonon hybridization.

Conclusions:

• Metallic magnets exhibit enhanced phonon nonreciprocity at higher temperatures, contrasting with insulating materials.
• Magnon-phonon hybridization, influenced by Gilbert damping, plays a key role in the observed MChE.
• Engineering magnon bands offers a potential route to further enhance phonon nonreciprocity.