halfmetal

Half-metal is a term used in solid-state physics and materials science to describe a class of ferromagnetic materials in which electrons of one spin orientation have metallic conductivity while electrons of the opposite spin face a band gap at the Fermi level. In practice, this yields a highly spin-polarized current, ideally with 100% spin polarization at the Fermi energy.

Electronic structure in half-metals is characterized by a finite density of states at the Fermi level for one spin channel and a vanishing density of states for the opposite spin. The spin polarization at the Fermi level is defined as P = (N↑(EF) − N↓(EF)) / (N↑(EF) + N↓(EF)); in an ideal half-metal P = ±1. This arrangement allows conduction to occur predominantly through electrons of one spin orientation.

Common examples of candidate or confirmed half-metals include intermetallic Heusler alloys such as Co2MnSi and Co2FeAl, the oxide CrO2, and magnetite Fe3O4, with manganites like La0.7Sr0.3MnO3 also studied for high spin polarization. Real materials often approach, but do not reach, the ideal behavior, particularly at room temperature, due to disorder, surface states, and other perturbations.

Applications of half-metals are mainly in spintronics, where high spin polarization can enhance spin injection into semiconductors, improve performance of magnetic tunnel junctions and spin valves, and contribute to advances in MRAM and spin-based electronics. Measurement of spin polarization employs techniques such as spin-resolved photoemission, Andreev reflection, and tunneling spectroscopy, each with its own sensitivities to surface and interface effects.

Challenges include preserving polarization at room temperature, controlling defects and interfaces, and achieving consistent synthesis. Ongoing research seeks robust materials with high polarization, compatible interfaces, and scalable fabrication for device integration.