Half-metals

These oddly named materials are ferromagnets with electrons of only one spin polarization at the Fermi level. Cobalt and nickel are not half-metals because of the presence of the 4s electrons at ε_F which are not fully spin-polarized. Indeed, no ferromagnetic element is a half-metal. It is necessary to form a compound where the 4s electrons can be removed from the vicinity of the Fermi energy by charge transfer or hybridization. Examples include the oxide CrO₂ and the ordered intermetallic compound MnNiSb. The characteristic feature of a half-metal is a spin gap in the ↑ or ↓ density of states at ε_F . Furthermore, the spin moment per formula unit in a stoichiometric half-metallic compound is an integral number of Bohr magnetons. This is because there are an integral number N^↑ + N_↓ of electrons per formula unit, and the band with the spin gap must contain an integral number of N↑ (or N^↓) electrons, hence N^↑ − N^↓ is also an integer. Spin-orbit interaction tends to destroy half-metallicity by mixing ↑ and ↓ states