Validity of classical statistical mechanics for ferromagnetic Heisenberg chains with an easy plane

Quantum corrections to the classical statistical mechanics of quasi-one-dimensional easy-plane ferromagnets (e.g. CsNiF3) in the presence of an easy-plane magnetic field are outlined. Criteria for the validity of a classical description are suggested on the basis of complete quantisation of strongly non-linear excitations. One of these criteria agrees well with one suggested by Mikeska and Patzak (1977) on the basis of a low-order quantum perturbation theory for essentially linear modes in the absence of a magnetic field. It is confirmed that CsNiF3 can be treated substantially classically in typical experimental circumstances.