Polarization of the spin-<i>S</i>exchange-interaction model

Abstract

The spin-S exchange interaction model of ferromagnetism is studied by the mean-field approximation. This model has 4S(S+1) order parameters, and has infinitely many ground-state configurations. We define a single polarization parameter to describe the ordering of the system. Thermal variation of the polarization q(T) is determined for various spins. The phase transition is first order (except for S=1/2) with phase transition temperature ${\mathit{kT}}_{\mathit{c}}$/Jz=(2S-1)/(4S ln2S), where z is the coordination number, and the discontinuity of q at ${\mathit{T}}_{\mathit{c}}$ is ${\mathit{q}}_{\mathit{c}}$=(2S-1)/2S.