Measurements of the magneto-optical rotation in CdCr2Se4 at wavelengths between 0.9 and 1.3 µm are reported. In the paramagnetic state, the rotation passes a minimum with decreasing wavelength and changes sign at the absorption edge. The Verdet constant is proportional to the paramagnetic susceptibility (V = −18 deg/kOe-cm at 300°K and λ = 1.05 µm). In the ferromagnetic state the Faraday effect is large and the sign reversal very pronounced; e.g., the rotation changes by 6.5 × 104 deg/cm in a wavelength interval of 400 Å in saturated material at 93°K. The photon energy at which the sign reversal occurs shows a strong temperature-dependent red shift; the temperature dependence is similar to that of the absorption edge. At low temperatures an additional rotational minimum appears that is subject to the same red shift.
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