Theory of Antiferromagnetic Resonance

The spin resonance condition ωγ=H₀±[HA(2HE+HA)]1/2 previously given by Kittel for a disk-shaped single-domain uniaxial or cubic antiferromagnetic crystal at 0^∘{}K with H₀ parallel to the domain axis is extended by classical calculations to cover finite temperature, ellipsoidal shape, orthorhombic symmetry, generalized two-lattice anisotropy, and arbitrary static field direction. The normal precessional modes are discussed. A quantum-mechanical derivation of the resonance equations is carried out by the method developed by Van Vleck for ferromagnetic resonance; no new features are introduced by the quantum-mechanical calculation. Several factors contributing to the line width are considered. Existing experimental data on antiferromagnetic resonance are reviewed; the data are scanty and taken in circumstances not closely related to the situation envisaged by the theory.