A nonmagnetic metal whose electrical conduction is 100% spin polarized seems like an oxymoron. Yet this is only one of the peculiar properties of a half-metallic (HM) antiferromagnet (AFM).

A HM AFM is also a metal with zero spin susceptibility, a property normally associated with insulators. Due to mixing of atomic orbitals with neighboring non-magnetic ions such as oxygen, magnetic ions commonly take on non-integral values of magnetic moment. Yet in a HM AFM distinct ions have antialigned moments that conspire to cancel exactly.

The prospect of getting a fully magnetized current from a metal that has no magnetic field provides not only fertile ground for research but conceivable new ``spin electronics" devices that rely on the spin polarization of the carriers.

Recently the likelihood of a novel form of superconductivity in HM AFMs has been proposed, which further intensifies theoretical interest in these unique systems. To date, there is no known example of a HM AFM. Is there a real likelihood of discovering, or even predicting, new HM AFMs, or are they destined to remain a theoretical curiosity?

W. E. Pickett, Phys. Rev. B 57, 10613 (1998)

Computational Search for Half-Metallic Antiferromagnets

We have conducted a preliminary computational search for HM AFMs that indicates that double perovskite structure oxides (see Figure at right) La₂M'M"O₆ incorporating open shell 3d (or 4d) transition metal ions M', M" such as V, Cr, Fe, Cu, Ru, etc. provide excellent possibilites for realizing the first example of such novel materials.

The ordered double perovskite crystal structure, also called the elpasolite or cryolite structure. Often the MO₆ octrahedra are rotated around a <100>, <110> or <111> axis.