Soliton Features in Easy-Plane Ferromagnets?

Numerical transfer-integral and molecular-dynamics calculations are used to demonstrate that the mapping of an easy-plane ferromagnet in a field to the sine-Gordon system fails. The crucial implication for CsNi${\mathrm{F}}_3$ is that the central peak observed by Kjems and Steiner at $T=10\mathrm{}$ K is more likely to be due to multimagnon difference processes, or pulse-soliton features of the nearly isotropic model rather than due to kink or breather solitons.