Finite-temperature depolarization in half metals

The temperature dependence of the spin polarization of type-I half-metallic ferromagnets is investigated and compared with that of other magnetic materials, such as semimetals, strong and weak ferromagnets, and exchange-enhanced Pauli paramagnets. Stable atomic moments, as realized by strong intra-atomic exchange, exhibit a nonzero spin-down density of states (DOS) at finite temperatures. This thermal spin mixing means that the conductivity of the 'insulating' spin channel is always nonzero and that half-metallic ferromagnetism is an idealized limit. At zero temperature, similar effects are caused by intersublattice interactions, spin-orbit coupling and crystal imperfections. With increasing interatomic hopping, the moment becomes unstable, and Stoner-type thermal excitations yield an additional reduction of the spin polarization. In the Stoner limit, the hybridization gap closes far below the Curie temperature, and the corresponding transition temperature T* increases with increasing hybridization gap and decreasing band width. Correlations are analysed by a version of the Kondo model and by an unrestricted Hartree-Fock approximation, and it is argued that correlations are less important than the leading one-electron contributions.