SPIN-WAVE DISTRIBUTION FUNCTION AND THERMODYNAMICS OF HEISENBERG FERROMAGNETIC

The application of the elementary excitation theory to the spin system of a ferromagnetic is investigated on the basis of the Heisenberg model. It is found that the spectrum of elementary excitations considered as a gas of quasi‐particles, and their distribution function are completely determined by the Hamiltonian of the system. The spectrum of the transverse spin waves and the distribution function both of the transverse and the longitudinal spin waves are calculated by the double‐time anticommutator Green's function method. Since the distribution functions have no singularities, it is possible to obtain expressions for the thermodynamic quantities in the form of convergent power series in a small parameter τ T/Tc. Expressions for the specific heat and the spontaneous magnetization of a ferromagnetic are calculated. Copyright © 1968 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim