Computationally based explanation of the peculiar magneto-optical properties of PtMnSb and related ternary compounds V. N. Antonov,* P. M. Oppeneer, ² A. N. Yaresko,* A. Ya. Perlov,* and T. Kraft Max-Planck Research Group ‘‘Theory of Complex and Correlated Electron Systems,’’ University of Technology, D-01062 Dresden, Germany Received 5 June 1997 The magneto-optical MO spectra of 15 ternary ferromagnetic compounds are investigated on the basis of local-density band-structure calculations. The key material of interest in this study is PtMnSb, for which the MO Kerr spectra are unusually large and quite different from that of, e.g., the isoelectronic compounds NiMnSb and PdMnSb. First we show that the spectral differences between NiMnSb, PdMnSb, and PtMnSb can be fully understood from their relativistic electronic structure. Further, we investigate the following ternary ferromagnetic compounds: PtMnSn, PtCrSb, PtFeSb, Pt 2 MnSb, Co 2 HfSn, NiMnAs, PdMnAs, PtMnAs, RuMnAs, PtMnBi, BiMnPt, and PtGdBi. Within the total group of alloys, we study the MO spectra of PtMnSb in relationship to the spin-orbit coupling strength; the magnitude of the 3 d -magnetic moment; the degree of hybridization in the bonding; the half-metallic character, or, equivalently, the Fermi-level filling of the band structure; the intraband plasma frequency; and the influence of the crystal structure. We find that these char- acteristics form a unique combination in PtMnSb leading to its outstanding MO Kerr spectra. None of the other compounds can match up to PtMnSb in this respect. For the compounds NiMnAs, PdMnAs, PtMnAs, and, particularly, RuMnAs, whose MO spectra are not measured as yet, we predict Kerr spectra which look promising for ultraviolet laser light recording applications. S0163-18299701544-0 I. INTRODUCTION The Heusler alloys NiMnSb, PdMnSb, and PtMnSb have been the subject of intensive experimental and theoretical investigations since the early 1980s. 1–4 The interest in these compounds arose first from the experimental discovery of an extremely large magneto-optical MO Kerr rotation of -1.27° in PtMnSb at room temperature. 1 This value was for many years the record Kerr rotation observed in a transition- metal compound at room temperature, and therefore called the ‘‘giant’’ Kerr effect see, e.g., the recent surveys in Refs. 5 and 6. Almost simultaneously with the experimental dis- covery, the theoretical finding of the so-called ‘‘half- metallic’’ nature of PtMnSb was reported. 4 Half-metallicity means that, according to semirelativistic band-structure theory, the material is metallic for majority-spin electrons, but insulating for minority-spin electrons. 4 Such a gap for one spin type naturally may give rise to unusual magne- totransport and optical properties. The isoelectronic Heusler alloy NiMnSb also was predicted to be half-metallic, whereas the isoelectronic compound PdMnSb was predicted not to be half-metallic. 4 The MO Kerr rotations in both NiMnSb and PdMnSb, on the other hand, were experimen- tally found to be much smaller than that of PtMnSb, which resulted in a puzzling combination of features. Experimental efforts were undertaken to verify the proposed half-metallic character of NiMnSb and PtMnSb, 7–10 which was subse- quently established in the case of NiMnSb. 7,10 Very recently, experimental evidence in favor of half-metallicity in PtMnSb was also reported. 11 On the theoretical side, several model explanations of the MO spectra of the compounds were proposed. 12–14 One of these was based on a possible loss of the half-metallic char- acter due to spin-orbit SO coupling, which was suggested to lead to a symmetry breaking between the different m states of the Sb p bands in the vicinity of the Fermi energy E F . 12 Another explanation was based on differences of the semirelativistic effects in NiMnSb and PtMnSb, 14 and an- other one on enhancement of the MO Kerr spectra near the plasma resonance. 13 While the proposed models contain in- teresting physical mechanisms in themselves, one of the re- maining major stumbling blocks was to explain the measured differences in the MO spectra of the isoelectronic NiMnSb, PdMnSb, and PtMnSb. Owing only to the development of ab initio calculations of the MO spectra, a detailed quantitative comparison be- tween experiment and first-principles spectra became feasible. 15–20 Heusler compounds are, of course, the most attractive materials for ab initio calculations of their MO spectra on account of the mentioned unusual features. Sev- eral first-principles calculations for these compounds were reported very recently. 21–25 The various calculated MO Kerr spectra, however, spread rather widely. The origin of the differences in the spectra obtained in the various calculations traces back, first, to the fact that the MO Kerr effect is a tiny quantity in calculations, related to the difference of reflection of left- and right-hand circularly polarized light. 6 SO cou- pling in the presence of spontaneous magnetization is re- sponsible for symmetry breaking in the reflection of left- and right-hand circularly polarized light. Second, since the MO Kerr effect is only a tiny quantity in first-principles calcula- tions, numerical accuracy and the influence of approxima- tions made in the evaluation gain an appreciable importance. For this reason, the evaluation of the MO Kerr spectra of the ferromagnetic 3 d transition metals Fe, Co, and Ni have be- come benchmark test cases for MO calculation PHYSICAL REVIEW B 15 NOVEMBER 1997-II VOLUME 56, NUMBER 20 56 0163-1829/97/5620/1301214/$10.00 13 012 © 1997 The American Physical Society