Thermodynamics of Low-Dimensional Heisenberg Ferromagnets by the Green's Function Method

The Green's function formalism proposed by Kondo and Yamaji for low-dimensional spin systems with S=1/2 has been extended to the cases of S>1/2. We apply this formalism to the study of one- and two-dimensional Heisenberg ferromagnets over the whole temperature range. It is shown that our theory can reproduce the correct results obtained by the high-temperature expansion method. On the other hand, the results at low temperatures are similar to those of the modified spin-wave theory, which is considered to predict the low-temperature properties of such systems rather correctly. The gross behaviors of the calculated thermodynamic quantities of ferromagnetic Heisenberg chains agree with those obtained by the exact diagonalization method.