Anisotropy of the Spin-Wave Spectrum

The frequency of a spin wave depends on its propagation vector and on the effective internal field in which it propagates. In an isotropic medium this implies that all the spin waves of a given wavelength lying on a θk cone about the effective field are degenerate. In anisotropic media this degeneracy is removed in those directions in which the minimum of the energy surface is not axially symmetric. This effect can be observed by measuring the contribution to the ferromagnetic resonance linewidth from scattering into moderate-kspin waves as a function of frequency and orientation in single crystal samples. Theoretical predictions and experimental results are given for a sphere of yttrium iron garnet.