Half-metallic Heusler alloys are amongst the most promising materials for future magneto-electronic applications. We review some recent results on the electronic properties of these compounds. The origin of the gap in these half-metallic alloys and its connection to the magnetic properties are well understood. Changing the lattice parameter slightly shifts the Fermi level. Spin-orbit coupling induces states within the gap but the alloys keep a very high degree of spin polarization at the Fermi level. Small degrees of doping and disorder as well as defects with low formation energy have little effect on the properties of the gap, while temperature effects can lead to a quick loss of half-metallicity. Finally, we discuss two special issues: the case of quaternary Heusler alloys and the half-metallic ferrimagnets.

The emergence of the field of spintronics has brought half-metallic ferromagnets to the centre of scientific research. A lot of interest was focused on newly created transition-metal pnictides (such as CrAs) and chalcogenides (such as CrTe) in the metastable zinc-blende lattice structure. These compounds were found to have the advantage of high Curie temperatures in addition to their structural similarity to semiconductors. Significant theoretical activity has been devoted to the study of the electronic and magnetic properties of these compounds in an effort to achieve a better control of their experimental behaviour in realistic applications. This paper is devoted to an overview of the studies of these compounds, with emphasis on theoretical results, covering their bulk properties (electronic structure, magnetism, stability of the zinc-blende phase, stability of ferromagnetism) as well as low-dimensional structures (surfaces, interfaces, nanodots and transition-metal delta-doped semiconductors) and phenomena that can possibly destroy the half-metallic property, like structural distortions or defects.

The Korringa-Kohn-Rostoker Green-function method for noncollinear magnetic structures was applied on Mn and Cr nanoclusters deposited on the Ni(111) surface. We consider various dimers, trimers, and tetramers. We obtain collinear and noncollinear magnetic solutions, brought about by the competition of antiferromagnetic interactions. It is found that the triangular geometry of the Ni(111) substrate, together with the intracluster antiferromagnetic interactions, is the main cause of the noncollinear states, which are secondarily affected by the cluster-substrate exchange interactions. The stabilization energy of the noncollinear, compared to the collinear, states is calculated to be typically of the order of 100 meV/atom, while multiple local-energy minima are found, corresponding to different noncollinear states, differing typically by 1-10 meV/atom. Open structures exhibit sizable total moments, while compact clusters tend to have very small total moments, resulting from the complex frustration mechanisms in these systems.

{The authors determine from first principles the Curie temperature T-C for bulk Co in the hcp, fcc, bcc, and body-centered-tetragonal (bct) phases, for FeCo alloys, and for bcc and bct Fe. For bcc Co