Neutron-Scattering Determination of Spin-Wave Dispersion Relations in FeF2

Abstract

Spin‐wave dispersion in antiferromagnetic FeF₂ has been investigated by inelastic neutron scattering at 4.2°K. Incident neutrons of 122 cm⁻¹ energy were used to excite spin waves in a {010} plane, and the energies of the scattered neutrons determined by time‐of‐flight techniques. The single mode observed has a rather flat dispersion lying between 53 and 79 cm⁻¹. The measured energies were fitted to a theoretical expression to determine values for the single ion anisotropy constant D, $({\mathcal{H}}_s^i{\mathrm{=-DS}}_{\mathrm{iz}}^2)$ , and the exchange interactions between the ions J_l, $({\mathcal{H}}_{\mathrm{ex}}^{12}{\mathrm{=J}}_l{\mathrm{S}}_1·{\mathrm{S}}_2)$ . We find D = 6.46(+0.29, −0.10) cm⁻¹, J₁ (coupling neighboring ions along the c axis) = −0.048 (±0.060) cm⁻¹, J₂ (coupling neighboring ions at the corner and body center of the cell) = +3.64(±0.10) cm⁻¹, and J₃ (coupling neighboring ions along the a axis) = +0.194(±0.060) cm⁻¹.