Effect of Dipolar Interactions on the Spin-Wave Spectrum of a Cubic Antiferromagnet

The effect of dipolar interactions on the spin-wave spectrum of a cubic antiferromagnet is studied. The spin-wave spectrum is found to consist of two branches whose frequencies are (ℏω)2=(gβ)2[HA+HEa2k2][HA+HE(2−a2k2)−8πM/3+8πM sin2∂k] and (ℏω)2=(gβ)2[HA+HEa2k2][HA+HE(2−a2k2)−8πM/3]. Here a is the lattice constant, M the magnetic moment per unit volume, k the wave vector, θk the angle between the magnetization and the wave vector, and HA and HE the anisotropy and exchange fields. The reasons for discrepancies between these formulas and those given previously by other authors are discussed.