Static form factors for the classical easy-plane ferromagnetic chain with an in-plane magnetic field: Application to CsNiF3

We present low-temperature expressions for the correlation functions of the classical easy-plane Heisenberg chain in a field. Approaching the critical field for domain-wall transition, the in-plane domain-wall form factors increase in strength and the in-plane spin-wave correlations display damped oscillations. The out-of-plane spin-wave correlations decay with a growing correlation length and the form factor is increased in intensity. We propose to look for this effect in CsNi${\mathrm{F}}_3$ by means of neutrons.