LOW-TEMPERATURE SPIN-WAVE AND DOMAIN-WALL THERMODYNAMICS AND FORM-FACTORS FOR THE CLASSICAL EASY-PLANE FERROMAGNETIC CHAIN

Using a steepest descent approach, equivalent to the static limit of the ideal gas phenomenology, we derive low temperature expressions for the free energy and the static spin correlation functions of the classical easy-plane ferromagnetic chain in an in-plane field. At low field the system forms a stable gas of spin waves and domain walls. Approaching the critical field for domain wall transition, the domain wall free energy, the longitudinal and transverse in-plane domain wall form factors increase in strength. The transverse in-plane spin wave correlations display damped oscillations and the out-of-plane spin correlations decay exponentially with a growing correlation length. Correspondingly, the out-of-plane form factor increases in intensity. We propose to look for this effect in CsNiF//3 by means of (polarized) neutrons.