Acoustical Faraday Effect in AntiferromagneticCr2O3

The rotation of the plane of polarization of an ultrasonic wave in both the antiferromagnetic and flopped phases of the antiferromagnetic crystal ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ has been calculated and measured at 4.2°K using 9-GHz sound waves. The Faraday effect is found to arise from a resonant interaction between phonons and low-frequency magnon modes, by single-ion magnetostriction. The values of magneto-elastic coupling constants $G_{44}$ and $G_{46}$ of ${\mathrm{Cr}}^{3+}$ in ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ are determined: $\left|G_{44}\right|\propto 4.5$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\left|G_{46}\right|\simeq 2.5$ ${\mathrm{cm}}^{-1\mathrm{}}$.