Description of ferromagnetic properties of a system with S=1 allowing for the non-Heisenberg isotropic exchange

The effect of non-Heisenberg isotropic exchange interactions on the ferromagnetic properties of a system with localized magnetic moments s=1 is studied. It is shown that two types of magnetic order, determined by the magnitude of the magnetic moment at a site, are possible in the ground state. It is found that the effective spin at a site may depend on temperature and on an external magnetic field and, therefore, a second-order phase transition may take place at T¬=;0 from a corresponding to an effective spin smaller than unity to a state with effective spin equal to unity