First-principles study of the giant magneto-optical Kerr effect in MnBi and related compounds

First-principles band-structure calculations of the magneto-optical Kerr spectra of MnBi and related compounds are reported. We find that band-structure theory, based on density-functional theory in the local spin-density approximation, explains the measured Kerr effect of MnBi very well. A giant Kerr rotation of about - 1.75 degrees at 1.8 eV photon energy is given by our ab initio calculations, in accordance with recent experiments. A second peak at 3.4 eV in the Kerr rotation spectrum, however, comes out smaller in our calculations than what was recently measured. It is discussed that this can be due to the Mn-Bi stoichiometry. The microscopic origin of the giant Kerr effect in MnBi is analyzed in detail. We find that the huge Kerr effect in MnBi is caused by the combination of a sizeable magnetic moment of 3.7 mu(B) on manganese, the large spin-orbit coupling of bismuth, and a strong hybridization between the manganese d bands and the bismuth p states. The magneto-optically active states are mainly the p states of Bi. We pay further attention to the experimentally observed unusual temperature dependence of the MnBi Kerr spectra. We show that the observed temperature dependence can be explained by the reduction of the magnetic moment and the average lifetime with increasing temperature. The ab initio calculated Kerr effect in MnBi is furthermore compared to that calculated for the isoelectronic compounds MnAs and MnSb, and that of CrBi, CrTe, and Mn2Bi. (C) 1996 American Institute of Physics.