A half-metal is any substance that acts as a conductor to electrons of one spin orientation, but as an insulator or semiconductor to those of the opposite orientation. Although all half-metals are ferromagnetic (or ferrimagnetic), most ferromagnets are not half-metals. Many of the known examples of half-metals are oxides, sulfides, or Heusler alloys.^[1] Types of half-metallic compounds theoretically predicted so far include some Heusler alloys, such as Co₂FeSi, NiMnSb, and PtMnSb; some Si-containing half–Heusler alloys with Curie temperatures over 600 K, such as NiCrSi and PdCrSi; some transition-metal oxides, including rutile structured CrO₂; some perovskites, such as LaMnO₃ and SeMnO₃; and a few more simply structured zincblende (ZB) compounds, including CrAs and superlattices. NiMnSb and CrO₂ have been experimentally determined to be half-metals at very low temperatures.

In half-metals, the valence band for one spin orientation is partially filled while there is a gap in the density of states for the other spin orientation. This results in conducting behavior for only electrons in the first spin orientation. In some half-metals, the majority spin channel is the conducting one while in others the minority channel is.^[2]

Half-metals were first described in 1983, as an explanation for the electrical properties of manganese-based Heusler alloys.^[3]

Some notable half-metals are chromium(IV) oxide, magnetite, and lanthanum strontium manganite (LSMO),^[1] as well as chromium arsenide. Half-metals have attracted some interest for their potential use in spintronics.