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. Types of half-metallic compounds theoretically predicted so far include some Heusler alloys, such as Co 2 FeSi, 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 CrO 2 ; some perovskites, such as LaMnO 3 and SeMnO 3 ; and a few more simply structured zincblende (ZB) compounds, including CrAs and superlattices. NiMnSb and CrO 2 have been experimentally determined to be half-metals at very low temperatures.