A Measurement of Spin-Wave Dispersion in MnF2 at 4.2°K

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Abstract
Antiferromagnetic spin waves have been observed in a single crystal of MnF2 at 4.2°K by the inelastic scattering of neutrons. Measurements were carried out using a twin rotor time‐of‐flight spectrometer having a mean incident neutron wavelength of 3.00 Å and a resolution (half‐width at half‐height) of 0.06 Å. Spin waves were excited in the {010} plane of the crystal and the dispersion plotted for propagation in the 〈100〉 and 〈001〉 directions. The observed dispersion curves can be accurately described by spin‐wave theory using the following values for the parameters concerned: J1=0.32°K (ferromagnetic), J2=−1.76°K (antiferromagnetic), |J3|anisotropy field at zero wave vector HA=1.06°K, where J1 is the exchange coupling between nearest neighbours (in the 〈001〉 direction), J2 acts between next nearest neighbours (〈111〉 directions), and J3 refers to third‐neighbour coupling (along the 〈100〉 and 〈010〉 directions). A small effect arising from the wave vector dependence of HA has been calculated and taken into account in the evaluation of these parameters. The values quoted are in excellent agreement with those predicted from measurements of perpendicular susceptibility, spin‐wave resonance, paramagnetic resonance in (Mn, Zn)F2, and from paramagnetic neutron scattering at room temperature.

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