In this paper, we study the Ising model with general spin S in presence of an external magnetic field by means of the equations of motion method and of the Green's function formalism. First, the model is shown to be isomorphic to a fermionic one constituted of 2S species of localized particles...

The spin-1 Ising model with bilinear and quadrupolar short-range interactions under magnetic field is investigated within the two-particle cluster approximation. It is shown that for those values of the quadrupolar interaction when at zero magnetic field the system undergoes a temperature phase...

Generalized xy lattice spin models consist of three-component unit vectors, associated with a D-dimensional lattice (say <InlineEquation ID="Equ1"> <EquationSource Format="TEX">${\mathbb{Z}}^D$</EquationSource> </InlineEquation>), parameterized by usual spherical angles (θ<Subscript>k</Subscript>,φ<Subscript>k</Subscript>), and interacting via a ferromagnetic potential restricted to nearest neighbours, of the form <InlineEquation ID="Equ2"> <EquationSource...</equationsource></inlineequation></subscript></subscript></equationsource></inlineequation>

An model of La<Subscript>2-x</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Sub script> explaining the features of incommensurate spin textures without any assumption of stripe formation is proposed. The foundations of this model are the mechanism of negative-U center formation proposed earlier and the concept of specific ordering of doped ions. It is shown...<//sub></subscript></subscript></subscript>

A large deviation technique is used to calculate the microcanonical entropy function s(v,m) of the mean-field ϕ<Superscript>4</Superscript>-model as a function of the potential energy v and the magnetization m. As in the canonical ensemble, a continuous phase transition is found. An analytical expression is obtained for...</superscript>