FIRST-PRINCIPLES STUDY OF THE GIANT MAGNETOOPTICAL KERR-EFFECT IN MNBI AND RELATED-COMPOUNDS

First-principles band-structure calculations of the magneto-optical Ke rr spectra of MnBi and related compounds are reported. We find that ba nd-structure theory, based on density-functional theory in the local s pin-density approximation, explains the measured Kerr effect of MnBi v ery well. A giant Kerr rotation of about - 1.75 degrees at 1.8 eV phot on energy is given by our ab initio calculations, in accordance with r ecent experiments. A second peak at 3.4 eV in the Kerr rotation spectr um, however, comes out smaller in our calculations than what was recen tly measured. It is discussed that this can be due to the Mn-Bi stoich iometry. The microscopic origin of the giant Kerr effect in MnBi is an alyzed 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 manga nese, the large spin-orbit coupling of bismuth, and a strong hybridiza tion between the manganese d bands and the bismuth p states. The magne to-optically active states are mainly the p states of Bi. We pay furth er attention to the experimentally observed unusual temperature depend ence of the MnBi Kerr spectra. We show that the observed temperature d ependence can be explained by the reduction of the magnetic moment and the average lifetime with increasing temperature. The ab initio calcu lated Kerr effect in MnBi is furthermore compared to that calculated f or the isoelectronic compounds MnAs and MnSb, and that of CrBi, CrTe, and Mn2Bi. (C) 1996 American Institute of Physics.