Spin one exchange interaction model of ferromagnetism

Schrodinger's spin one exchange interaction, P ij = (S i S j ) 2 + (S i S j ) -1, is introduced to form a Hamiltonian for a nearest-neighbour lattice model of ferromagnetism. The physical relevance of this Hamiltonian is discussed. Group theoretical methods for obtaining the high-temperature partition function and initial susceptibility series are introduced. The first eight terms of the susceptibility series are obtained for the face-centred cubic lattice. Analysis by the ratio and Padé approximate methods yields the critical temperature and the critical index γ which is found to be very nearly 5/4 and thus certainly different from the spin one Heisenberg model index.