Higher Order Spin Coupling in Complex Compounds

Abstract

From the analyses of the magnetic susceptibility and/or the magnetic heat capacity of the polynuclear complex compounds of iron group ions, it has been suggested that there exists an additional higher order spin coupling besides the usual Heisenberg exchange interaction. It is shown that the higher order spin coupling can be derived from the minimum condition of the free energy of the cluster system. For that purpose, a magnetoelastic term is introduced in the free energy and a dependence of the exchange coupling on the distance between interacting ions is assumed. In an alternative way, it is also shown that the higher order spin coupling can be derived from the perturbation procedure in which the spin-orbit interaction and the usual exchange interaction are taken as the perturbation. A quantitative comparison of the present theory with the values obtained from data analyses makes it clear that the magneto-striction is an important origin for the appearance of the higher order spin coupling.