Thermodynamics of the classical, easy-plane ferromagnetic chain with in-plane magnetic field

A path-integral evaluation of the thermodynamics for a continuous, easy-plane, classical ferromagnetic Heisenberg chains with in-plane magnetic field is presented. The technique is restricted to low densities of non-linear (soliton) excitations dressed by spin waves. However, important corrections due to strong out-of-plane spin components are explicitly included and lead to incipient divergences in thermodynamic quantities such as specific heat and magnetic susceptibility. It is suggested that these should be looked for experimentally in materials such as CsNiF₃.