Origin of the polar Kerr effect in PtMnSb and NiMnSb

The polar Ken effect for red light reflected from PtMnSb or NiMnSb crystals is shown to be independent of the half-metallic-ferromagnetic property of these compounds. At low photon energies the interband contribution to the Ken rotation is attributed to transitions between parallel sheets of energy bands that project onto the metallic spin channel. A previously proposed mechanism in which spin-orbit coupling creates an imbalance in optical transitions between states in the vicinity of the Gamma point, in the semiconducting minority spin channel, does not apply to either PtMnSb or NiMnSb. Only upon inclusion of intraband effects is reasonable agreement with experiment achieved.