Antiferromagnetic

Antiferromagnetism is a type of magnetic order in which the atomic magnetic moments, or spins, align in opposite directions to one another. In an antiferromagnet the net magnetization is effectively zero in the absence of an external magnetic field, distinguishing it from ferromagnetism, where spins align parallel, and from ferrimagnetism, where unequal opposing moments yield a nonzero net moment.

The phenomenon arises from exchange interactions among electrons, often described by the Heisenberg model. In a simple two-sublattice antiferromagnet, spins on one sublattice point up while those on the other point down, creating a staggered magnetization as the order parameter. At high temperature thermal fluctuations disrupt long-range order and the material becomes paramagnetic; below a characteristic Néel temperature, T_N, antiferromagnetic order develops.

Properties include a vanishing net moment in zero field and usually modest, anisotropic magnetic response. External magnetic fields can induce spin rearrangements, such as spin-flop or spin-flip transitions, depending on the material’s anisotropy. The magnetic susceptibility often shows a cusp near T_N. Collective excitations, or magnons, arise from spin waves within the ordered state.

Common antiferromagnetic materials include transition metal oxides such as MnO, NiO, FeO, and Cr2O3. Antiferromagnetism also appears in itinerant systems, notably chromium, where a spin-density wave describes the ordered state.

Applications of antiferromagnets appear in spintronics and magnetic sensing, where exchange bias—a coupling between an antiferromagnet and a neighboring ferromagnet—stabilizes magnetic configurations. As model systems they illuminate quantum magnetism and phase transitions.