Antiferromagnets are promising for magneto-optical light control that could be performed at THz frequencies via excitation of the quasi-antiferromagnetic spin modes. However, most of the antiferromagnetic crystals possess optical anisotropy that is usually treated as an unfavorable condition for the magneto-optical measurements: optical anisotropy is known to diminish the Faraday rotation with respect to the case of the isotropic medium. Here we show that the situation could be quite opposite: a phenomenon of birefringence mediated enhancement of the magneto-optical activity appears if orientation of the incident light linear polarization is chosen properly. The present study relies on the experimental, analytical and numerical studies of iron borate FeBO$_3$ crystals. We demonstrate a significant increase of the magneto-optical activity by more than 10 times for 70$^circ$ angle between light polarization and incidence plane instead of commonly-used p- or s-polarizations. It provides a unique sensitivity to the in-plane magnetization of FeBO$_3$ that is crucial for the pump-probe studies, magneto-optical microscopy and other. The most important practical application of the observed phenomenon is the light modulation with up to 100$%$ efficiency at THz frequencies. The approach is applicable to other types of the birefringent crystals with the magneto-optical response.
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