Half-metallic ferromagnets

The experimental results, band structure calculations, and theoretical investigations carried out within the framework of the many-electron models of half-metallic ferromagnets (HMFs) are reviewed. These materials have an energy gap for one of the spin projections at the Fermi level and represent a separate class of strong itinerant magnetic substances, which includes some of the Heusler alloys (for example, PtMnSb), CrO₂, etc. Some HMFs and related systems are promising magnetic materials which have, in particular, unique magneto-optic properties. The theoretical interest in HMFs arises from a striking manifestation of nonquasiparticle (spin-polaron) effects; for example, in the spin polarisation of charge carriers and in the nuclear magnetic relaxation rate. Concepts developed in the theory of HMFs are shown to be applicable also to 'conventional' strong itinerant (band) magnetic materials, including the iron-group metals.