We employ the transfer-integral operator method to calculate various classical thermodynamic functions and static spin-spin correlation functions for a ferromagnetic chain of spins intended to model the magnetic material CsNi${\mathrm{F}}_3$. In addition to including spin tipping out of the easy plane, we explicitly take into account the restricted phase space allowed to each spin due to the periodic nature of the nearest-neighbor exchange interaction. For parameter values in the range of interest for CsNi${\mathrm{F}}_3$, we identify contributions from spin waves and nonlinear "soliton" excitations. We find that the sine-Gordon approximation with a renormalized exchange constant provides an adequate representation for the classical statistical mechanics of CsNi${\mathrm{F}}_3$. In addition, we find qualitative agreement between the theoretical static structure factor and experimental data for integrated neutron scattering intensities for small wave vectors, assuming a bare soliton energy of 34 K in contrast to the 69-K value implied by a naive ideal soliton-gas phenomenology.

• Received 21 March 1980

DOI:https://doi.org/10.1103/PhysRevB.22.4389