Neutron-scattering studies of spin waves in rare-earth orthoferrites

Iron sublattice spin waves have been studied in TmFeO₃ and ErFeO₃ by inelastic neutron scattering. In TmFeO₃ magnon dispersion curves were measured along several symmetry directions and the results fitted to an isotropic Heisenberg Hamiltonian with near-neighbor exchange constant J^'=(-1.9±0.2) ^∘{}K and next-nearest-neighbor exchange J=(-29.1±0.6) ^∘{}K. A detailed high-resolution study of the long-wavelength spin waves was performed near the moment reorientation transformation. In TmFeO₃, the lowest magnon frequency renormalizes in qualitative agreement with the "soft"-mode theory of the transformation. In ErFeO₃ mode softening was observed only at the lower transformation temperature. Improved estimates are made of the twofold and fourfold anisotropy energies in TmFeO₃.