The effect of the magnetization direction m on the dielectric tensor of uniaxial crystals is described by a simple dependence of the gyration vector g(ω) on m. It is shown that the vectors g(ω) and m as well as the orbital magnetic moment 〈L^〉 and m are generally aligned noncollinearly in contrast to an isotropic case. Formulas describing the polar Kerr effect are derived for crystals with their principal axis c⊥m and for polycrystals having c randomly oriented in the sample plane. Using these analytical results and performing ab initio calculations, we correctly reproduce anisotropy in optical spectra of ${\mathrm{CrO}}_2$ and the main features in magneto-optical spectra of polycrystalline films of ${\mathrm{CrO}}_2$. The maximal optical anisotropy and orientation dependence of g(ω) of 100% are found in the energy interval ħω≤ 2.1 eV coinciding with the direct half-metallic ferromagnetic gap of ${\mathrm{CrO}}_2$. The noncollinearity effects in this interval are also very large. The obtained results correlate well with strong orientation dependence of 〈L^〉 found in our calculations. © 1996 The American Physical Society.

• Received 10 January 1996

DOI:https://doi.org/10.1103/PhysRevB.54.474