Cooperative phenomena in non-Heisenberg spin model with two- and three-atom interactions

In this paper we present a detailed study of the physical properties of a non-Heisenberg spin model described by a Hamiltonian which consists of an usual bilinear coupling with the nearest-neighbour coupling constant J and a three-site interaction measured by a coupling constant L. This study is based on irreducible Green’s function theory with appropriate refinements. The results for spontaneous magnetization, Curie temperature and the zero-field susceptibility have been derived. The agreement of some of these theoretical results with those obtained from exact high-temperature series expansion is satisfactory. The observed susceptibility data for a quadrinuclear cluster of chromium has been examined on the basis of the present theory.