antiferromagnetism

Antiferromagnetism is a type of magnetic order in which adjacent atomic magnetic moments align in opposite directions, resulting in little or no net magnetization in the absence of an external field. In most antiferromagnets the exchange interaction favors antiparallel alignment of neighboring spins, producing a regular checkerboard or other ordered pattern. This contrasts with ferromagnetism, where moments align parallel and yield a macroscopic magnetization, and with paramagnetism, where moments are randomly oriented above the magnetic ordering temperature.

Antiferromagnets exhibit a phase transition at the Neel temperature (T_N). Below T_N, spins order into an antiferromagnetic structure; above T_N, thermal fluctuations destroy long-range order and the material becomes paramagnetic. The magnetic susceptibility typically decreases with decreasing temperature and follows Curie-Weiss behavior with a negative Weiss constant, reflecting the dominant antiferromagnetic exchange.

Common models describe antiferromagnetism with the quantum Heisenberg exchange interaction between localized spins. Collinear AFM arrangements (opposite spins on neighboring sites) occur in many insulators such as NiO and MnO, often stabilized by superexchange via intervening anions. Noncollinear or incommensurate orders are found in some materials (e.g., spin-density waves in chromium). The lattice geometry and frustration can lead to complex or degenerate ground states.

Because the net magnetization is small or zero, antiferromagnets are less responsive to external magnetic fields but can interact with ferromagnets through exchange bias, which is exploited in magnetic sensors and memory devices. Antiferromagnetic spintronics seeks to harness fast, robust spin dynamics in these materials for future information technologies.