New Expansion for the Classical Heisenberg Model and its Similarity to theS=12Ising Model

Abstract

The zero-field susceptibility of the classical Heisenberg model is expanded in the new expansion parameter $u\equiv \mathcal{L}\left(\frac{2J}{k\mathrm{T}}\right)$ and a formal similarity with the $S=\frac{1}{2}$ Ising-model expansion is noted. The new Heisenberg-model expansion is seen to provide more reliable extrapolations (especially for one- and two- dimensional lattices) than heretofore, and to permit comparison with Brown's recent work on the Bethe-Peierls approximation.