Spin waves in the spin-flop phase of RbMnF₃

The spin-wave excitations of the near-ideal 3D Heisenberg antiferromagnet RbMnF₃ have been measured in the presence of an applied fled using neutron scattering. Detailed measurements of both the wave vector and field dependence of the excitations were made for fields up to 5.7 T. For applied fields >or=0.26 T, the spins undergo a spin-flop transition. In this phase the twofold degeneracy of the zero-field spin waves is lifted. A rigorous analysis of the data, allowing for the effects of instrumental resolution, shows that the excitations are well explained by linear spin-wave theory. No evidence is found for higher-order processes, such as have been found in systems of lower dimensionality.