Static properties of ${\mathrm{CsNiF}}_3$: Comparison of numerical results, experimental data, and predictions from soliton-bearing models

The numerical quantum transfer-matrix method has been applied to the s=1 easy-plane ferromagnetic chain system ${\mathrm{CsNiF}}_3$. A detailed analysis of experimental data on the static properties reveals that the generally accepted spin Hamiltonian for this compound gives an accurate description of the individual chains for J/${\mathit{k}}_{\mathit{B}}$=25 K and D/${\mathit{k}}_{\mathit{B}}$=7.7 K. At temperatures up to several degrees Kelvin above the three-dimensional ordering temperature, deviations between theory and experiment are found that are attributed to the coupling between the chains. A comparison of the quantum-transfer-matrix-method results with various analytical results supports the description of solitary excitations in ${\mathrm{CsNiF}}_3$ by the classical sine-Gordon model, extended to include spin fluctuations out of the easy plane and quantum effects.