Second Harmonic Generation and Magnetic-Dipole-Electric-Dipole Interference in AntiferromagneticCr2O3

The second harmonic spectrum of ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ is studied in the 1.7-2.9 eV spectral range as a function of temperature. Below the Néel temperature $T_N=307.5\mathrm{}$ K a nonlinear electric susceptibility ${\chi }_{\mathrm{ijk}}^e\mathrm{}\left(c\right)\mathrm{}$ appears, which changes sign under the time-reversal operation. The interference of this susceptibility with the time-invariant magnetic susceptibility ${\chi }_{\mathrm{ijk}}^m\mathrm{}\left(i\right)\mathrm{}$ leads to a pronounced polarization dependence for circularly polarized light propagating along the optical axis. This gives a novel tool to study antiferromagnetic domains with opposite orientation of the order parameter.