antiferromagnet

An antiferromagnet is a material in which the magnetic moments of atoms or ions align in an ordered pattern in which neighboring moments point in opposite directions. In the simplest case the material comprises two interpenetrating sublattices with opposite net moments, giving a vanishing total magnetization in zero field. The ordering is described by the Néel vector and is usually the result of antiferromagnetic exchange coupling, typically modeled with a positive Heisenberg exchange J>0.

The transition to the disordered state occurs at the Néel temperature TN. Below TN long-range antiferromagnetic order exists; above TN thermal agitation destroys it and the material becomes paramagnetic. The magnetic susceptibility of an antiferromagnet often follows a Curie-Weiss law at high temperature with a negative Weiss constant.

Under an external magnetic field the spins can cant slightly, yielding a small net moment in some materials (weak ferromagnetism). At higher fields many antiferromagnets undergo a spin-flop transition, where spins reorient roughly perpendicular to the field. Spin dynamics include two magnon branches due to the two-sublattice structure.

Common antiferromagnets include MnO, NiO, FeF2 and Cr2O3. These materials have applications in basic research, and in devices such as exchange-biased spin valves and magnetoelectric materials; antiferromagnets can also serve as active components in spintronic devices for fast, low-dissipation control and as pinning layers in magnetic memories.