A review of new developments in theoretical and experimental electronic structure investigations of half-metallic ferromagnets (HMF) is presented. Being semiconductors for one spin projection and metals for another ones, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMF by the peculiarities of their electronic structure and chemical bonding is discussed. Effects of electron-magnon interaction in HMF and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Especial attention is paid to appearance of non-quasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-art electronic calculations for correlated $d$-systems is discussed, and results for specific HMF (Heusler alloys, zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.Comment: to be published in Reviews of Modern Physics, vol 80, issue

Nonquasiparticle states above the Fermi energy are studied by first-principle dynamical mean field calculations for a prototype half-metallic ferromagnet NiMnSb. We present a quantitative evaluation of the spectral weight of this characteristic feature and discuss the possible experimental investigation ͑Bremsstrahlung isohromat spectroscopy, nuclear magnetic resonance, scanning tunneling microscopy, and Andreev reflection͒ to clarify the existence of these states.

Spin-electronics requires an electron source with a spin-polarization as high as possible. For this, halfmetallic materials seem ideally suited as they exhibit 100% spin polarization. Because of its high Curie temperature and compatibility with existing semiconductor technology, NiMnSb is a most desirable half metal. However, using first-principles calculations we find that NiMnSb surfaces are not half metallic, even if they are stoichiometric and perfectly ordered. Moreover, several surface and interface sensitive experiments have reported polarizations far less than 100%. These findings are easily rationalized, as they result from the symmetry breaking at the surface. We show that it is possible to restore half metallicity at interfaces, by a proper engineering at the microscopic level. Therefore the half metal NiMnSb is, in principle, a suitable source material for 100% spin-polarized charge carriers.

Several crystal structures of tungsten trioxide have been studied with a first-principles pseudopotential method. The electronic band gap increases significantly with the distortion of the octahedra that are the building blocks of the various crystal structures. Moreover, the tilting of the octahedra in the more complex structures leads to a strong increase of the gap upon compression. ͓S0163-1829͑99͒04104-1͔

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