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 kTc/Jz=(2S-1)/(4S ln2S), where z is the coordination number, and the discontinuity of q at Tc is qc=(2S-1)/2S.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics