Effect of Dipolar Fields on the Antiferromagnetic Spin-Wave Spectrum

The spin-wave dispersion relation for a simple uniaxial antiferromagnet is calculated, with inclusion of volume dipolar fields. The spin-wave spectrum is anisotropic and a degeneracy exists between the uniform antiferromagnetic-resonance modes and manifolds of short-wavelength spin waves, similar to the situation in a ferromagnet. The densities of states of these degenerate spin waves are calculated, and are then used to estimate antiferromagnetic-resonance linewidths caused by paramagnetic impurities having a different $g$ value or exchange coupling to the host spins. Finally, the linewidth caused by surface pits in the sample is estimated.