Theory of Antiferromagnetic Resonance

The spin resonance condition $\frac{\omega }{\gamma }=H_0\pm {\left[H_A\right(2\mathrm{}{H}_E+H_A\left)\right]}^{\frac{1}{2}}$ previously given by Kittel for a disk-shaped single-domain uniaxial or cubic antiferromagnetic crystal at 0°K with $H_0$ 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.