Low-temperature theories of the classical one-dimensional easy-plane ferromagnet: success and limitations

Abstract
Computer simulation results for the dynamic structure factors of a ferromagnetically coupled classical one-dimensional easy-plane spin system are compared with calculations based on a linear spin wave theory for the case where a magnetic field is applied in the easy plane. It is found that the spin wave theory gives an adequate representation of the simulation results at sufficiently low temperature. At higher temperatures the authors demonstrate the failure of linear spin wave theory evinced by the appearance of a central-peak structure and a crossover to the isotropic Heisenberg behaviour. This central-peak feature is analogous to the behaviour of Ising-Heisenberg chains recently interpreted in terms of thermally induced bound-state transitions, corresponding in the classical limit to envelope solitons. The case of zero applied field is also considered.