The classical-spin Bethe-Peierls-Weiss method—I. Simple ferro- and antiferromagnetics

Abstract An approximate Bethe-Peierls-Weiss calculation is presented in which the spin operators are replaced by classical vectors. The resulting thermodynamic quantities, such as the transition temperature, susceptibility, specific heat, short range order, etc. are found to agree well with the quantum-mechanical results for simple ferromagnetics and antiferromagnetics, and with experiment where possible. The simplicity of the method suggests that it would be a useful substitute for the quantum-mechanical model, as an alternative to the Ising model or the molecular field theory, which appear to be less accurate.