Finite-temperature depolarization in half metals

Abstract

The temperature dependence of the spin polarization of type-I half-metallic ferromagnets isinvestigated and compared with that of other magnetic materials, such as semimetals,strong and weak ferromagnets, and exchange-enhanced Pauli paramagnets. Stable atomicmoments, as realized by strong intra-atomic exchange, exhibit a nonzero spin-down densityof states (DOS) at finite temperatures. This thermal spin mixing means that theconductivity of the ‘insulating’ spin channel is always nonzero and that half-metallicferromagnetism is an idealized limit. At zero temperature, similar effects are caused byintersublattice interactions, spin–orbit coupling and crystal imperfections. With increasinginteratomic hopping, the moment becomes unstable, and Stoner-type thermal excitationsyield an additional reduction of the spin polarization. In the Stoner limit, the hybridizationgap closes far below the Curie temperature, and the corresponding transition temperatureT* increases with increasing hybridization gap and decreasing band width. Correlations areanalysed by a version of the Kondo model and by an unrestricted Hartree–Fockapproximation, and it is argued that correlations are less important than the leadingone-electron contributions.