R A de Groot

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

Electronic band-structure calculations are presented for the ferromagnetic compounds CrTe, Cr3Te4 and Cr2Te4 and Cr2Te3. In these compounds the Cr3d-Te5p covalency and the Cr3d(z2)-Cr3d(z2)-overlap along the c axis are the most important interactions. The magnetic polarisation of Te is parallel to the Cr local moment in CrTe, antiparallel to it in Cr2Te3 and about zero in Cr2Te4. Measurements of electronic transport properties (resistivity, Hall effect and thermo-electric power) and magnetic properties of Cr1- delta Te( delta =0.1) and Cr3+xTe4(x=0.2) indicate that these chromium tellurides are p-type metals, with strong interaction between the holes in the Te5p band and the Cr magnetic moments. In the literature the variation of the magnetic properties of Cr3.2Te4 near Ts approximately=100 K has been attributed to a change from a canted antiferromagnetic to a collinear ferromagnetic structure. However, our Hall-effect measurements indicate that the spin structure is not collinear ferromagnetic above Ts.

The electronic structure of MnBi

The first self-consistent spin-polarised band-structure calculation of the ferromagnetic compound MnBi in its low-temperature phase has been performed. The spin-orbit interaction was treated as a perturbation using scalar-relativistic wavefunctions. On the Mn atoms an effective 3d5.5 configuration is found, with a magnetic moment of 3.6 mu B. A part of the d electrons is rather delocalised and strongly hybridised with Bi 6p states. This leads to a low density of states at the Fermi level in agreement with experiment. The calculated band structure is very similar to that of MnSb. The physical properties of both compounds are compared, and the dependence of the band structure on the direction of the spin magnetic moment is discussed in relation to the magnetic anisotropy and the magneto-optical polar Kerr effect.

Electronic band-structure calculations of some magnetic chromium compounds

Band-structure calculations of CrS, CrSe, Cr3Se4 and CrSb are presented. Together with the accompanying results for the chromium tellurides, these calculations give a coherent picture of the changes in the electronic structure caused by anion substitution and by introduction of cation vacancies. The importance of the Cr-X covalency and the 3d(z2)-3d(z2) overlap of Cr neighbours along the c axis is stressed. Further, the bandstructure calculations shed some light on the formation, the variation in magnitude and the coupling of the Cr magnetic moments and the indirect magnetic polarisation of the anion bands.

The electronic structure of ordered binary Co-Pt compounds

In this article the authors present the results of spin-polarized band-structure calculations on the ordered binary compounds of Co-Pr. Comparison is made with experimental values for the magnetic moments. The results of these calculations show a nonlocal magnetic moment behaviour in these systems. Furthermore the authors find an almost constant number of holes in the Pt 5d band for the different compounds. The nature of the magnetic interactions is investigated. The coupling between the Co atoms can be described with the Heisenberg Hamiltonian and the exchange coupling parameters are evaluated. With these exchange parameters the authors are able to calculate the Curie temperature of ordered and disordered Co-Pt alloys.

Optical properties of some half-metallic ferromagnets

The authors report ellipsometry measurements on polycrystalline samples of NiMnSb and PtMnSb at room temperature under ultra-high vacuum conditions. From these measurements the optical constants of the two materials were determined in the energy range 0.5-5.3 eV. The measured optical conductivities are in reasonable agreement with the optical conductivities calculated from the band structures by de Groot (1983). The band gap for the minority-spin direction is clearly observed and can be brought into agreement with the band-structure calculations by consideration of spin-orbit splitting.

The electronic structure and magnetism of the Al/Fe interfaces

The electronic structure and magnetic properties of the Fe(001) surface with Al overlayers, vice versa, are studied by means of ab initio band structure calculations using the LSW method. An Fe–Al(001) multilayer system is calculated for comparison. The calculations show that Al reduces the magnetic moments of the interface iron. The Al interface layer has a small negative magnetic moment. The interaction between the interface Al and Fe layers, and the intra-atomic exchange splitting are discussed.

Electronic structures of ReS2, ReSe2 and TcS2 in the real and the hypothetical undistorted structures

The transition-metal dichalcogenides (X = S or Se) and with a electron configuration have distorted and structures, respectively, with the Re(Tc) atoms in each layer forming parallelogram-shaped connected clusters (diamond chain). Ab-initio band-structure calculations were performed for and , and the hypothetical undistorted 1T- and 3R- structures. The calculations show that , and are semiconductors with energy gaps of about 1.0 eV, 0.5 eV and 0.7 eV, respectively, while for the undistorted structures the Fermi level is in the partly filled band of and orbitals of the manifold. X-ray photoemission spectra for the core levels and valence band of and are presented. The valence x-ray photoemission spectra showed that is a p-type semiconductor with an energy gap of about 1.5 eV, while is an n-type semiconductor. The experimental results are in good agreement with the band-structure calculations.

The electronic structure of the mixed valence compound Pb3O4

Abstract Ab initio self-consistent calculations of the electronic structure of Pb 3 O 4 are presented. The calculations show that Pb 3 O 4 is a semiconductor. The calculated bandgap of 1.1 eV is smaller than the observed gap of 2.1–2.2 eV. The calculations show strong hybridization between Pb(6s) and O(2p) states. For one type of lead atom, this leads to a distribution of Pb(6s) states over two occupied energy bands, similar to the situation in PbO. For the other type of lead atom in Pb 3 O 4 the Pb(6s) states are distributed over an occupied and an unoccupied band, similar to the situation in β-PbO 2 . This clearly demonstrates that Pb 3 O 4 is a mixed valence compound, i.e. Pb(II) 2 Pb(IV)O 4 . According to the calculations, the conductivity of p-type Pb 3 O 4 is due to holes in a Pb(II)(6s) valence band and the conductivity in n -type Pb 3 O 4 is due to electrons in a Pb(IV)(6s) conduction band.

Differences between LaB6 and CeB6 by means of spectroscopic ellipsometry

The optical properties of LaB6 and CeB6 have been investigated using spectroscopic ellipsometry. The results were compared with band-structure calculations. The main features of the spectra of both materials originate from optical transitions starting at boron-derived levels of similar character. A sharp peak in the joint density of states for CeB6 around 0.5 eV is observed.

RESISTIVITIES AND BAND STRUCTURES OF ALKALINE-EARTH-PNICTIDE SYSTEMS

The electrical resistivities rho of the liquid Sr-Bi and SrSb alloys have been determined for compositions covering the whole concentration range. Plotted as a function of composition the results resemble those obtained previously for Mg-Bi by other authors. There is a very sharp resistivity maximum at 60 at.% Sr, well up into the semiconducting range. The band structure of solid Mg3Bi2 has been calculated using the augmented spherical wave ASW) method. No gap has been found and it is concluded that solid Mg3Bi2 is metallic, in agreement with measurements found in the literature dating from the thirties and before. These measurements were checked and extended through the melting point. Finally, a band-structure calculation was carried out for Mg3Sb2, which is a semiconductor in the crystalline as well as in the liquid state.