Axial quadrupole phase of a uniaxial spin-1 magnet
  Uniaxial phase of the axial quadrupole spin-1 magnet in an external magnetic field is investigated. A case where the system includes single-ion anisotropy and anisotropic biquadratic exchange interaction the most general form, is considered. It is proved, that the molecular field approximation of the relative magnetization does not depend on temperature and increases linearly with the external magnetic field. Two branches of the spin excitation spectrum are derived. Condition of mitigation of the spectrum defines the boundary between the axial quadrupole and angular phases. The critical temperature of the corresponding phase transition depends considerably on the anisotropy constants of the biquadratic exchange interaction. 
I.P.Shapovalov and P.A.Sayko
cond-mat.str-el cond-mat.stat-mech