The origin of the 90 degree magneto-optical Kerr rotation in CeSb

We calculate the linear magneto-optical Kerr rotation for CeSb in the near-infrared spectral range. Using an exact formula for large Kerr rotation angles and a simplified electronic structure of CeSb we find at \hbar \omega = 0.46 eV a Kerr rotation of 90 degree which then for decreasing \omega jumps to -90 degree as recently observed. We identify the general origin of possible 180 degree polarization rotations as resulting from mainly nonmagnetic optical properties, in particular from the ratio of the dominant interband resonance frequency to the plasma frequency. The dependence of the Kerr rotation on moments and magnetization is discussed.