  In the paper we obtain equations for large-scale fluctuations of the mean
field (the field of magnetization and quadrupole moments) in a magnetic system
realized by a square (cubic) lattice of atoms with spin s >= 1 at each site. We
use the generalized Heisenberg Hamiltonian with biquadratic exchange as a
quantum model. A quantum thermodynamical averaging gives classical effective
models, which are interpreted as Hamiltonian systems on coadjoint orbits of Lie
group SU(3).
