Energy fluctuations of an isotropic ferromagnetic chain in an applied field

Abstract

The energy fluctuations of a classical Heisenberg ferromagnetic chain are investigated both using a nonperturbative Green function approach and by computer simulation techniques. Particular attention is focused on nonhydrodynamic cases, which can safely be described in terms of undamped boson propagators. As previously found for the magnetisation fluctuations, the interacting spin wave theory can be used to describe the competition between the thermal disorder and the ordering effect of the magnetic field through a thermally induced coupling between magnetisation and energy fluctuations. The spectra of the energy fluctuations are calculated and compared with the corresponding simulation data for different values of fields and wavevectors. Furthermore, thermodynamic quantities such as static energy correlations and specific heat are evaluated and found to be in very good agreement with new transfer matrix calculations. The limits of validity of the theory for very low fields are also analysed.