Optical study of the antiferromagnetic-paramagnetic phase transition in chromium oxide Cr₂O₃

Optical effects of the first and second order with respect to the order parameter (1 is antiferromagnetic vector) have been studied in Cr2O3 around its phase transition at TN = 306 K from the antiferromagnetic to paramagnetic state. The magnetic linear birefringence is characterized by a rather large magnitude Δn sp ≃ 10−3 and by a large contribution of fluctuations of the order parameter to the birefringence. The study of the nonreciprocal optical rotation induced by an electric field has shown that the phase transition has a well defined first-order character. This result is also supported by the observation of a nonlinear (quadratic in the electric field) nonreciprocal rotation in a narrow temperature region ΔT = 0.15 K around TN . The temperature variation of the order parameter l(T) below TN is well described by a power low l where τβ = (TN - T)/TN and β = 0.355. We also observed a very reproducible effect of the rotation of the optical indicatrix in opposite directions for two types of antiferromagnetic domains. The possible explanation of this effect could be related to the so-called gyrotropic birefringence, an effect related to kili terms in the dielectric permeability.