Non-quasiparticle effects in half-metallic ferromagnets

The unusual electronic structure of the half-metallic ferromagnets (HMF) is analysed taking account of correlation effects (electron-magnon interaction, in particular spin-polaron effects). Special attention is paid to the so-called non-quasiparticle (NQP) (incoherent) states which arise in the minority- (majority-) spin gap above (below) the Fermi level and which may make considerable contributions to the electronic properties. First-principles calculations of the NQP states in HMF within the local-density approximation plus dynamical mean field theory (LDA+DMFT) are reviewed. These states can be probed, in particular, by spin-polarized scanning tunnelling microscopy. They also lead to observable effects in core-hole spectroscopy, nuclear magnetic relaxation and contribute to the temperature dependence of impurity resistivity, etc. The peculiarities of the transport properties of three- and two-dimensional HMF are discussed, which are connected with the absence of one-magnon spin-flip scattering processes.