Bernatska, JuliaHolod, Petro2009-10-122009-10-122008Bernatska J. Ordered States and Nonlinear Large-Scale Excitations in a Planar Magnet of Spin s=1 / J. M. Bernatska, P. I. Holod // Український фізичний журнал = Ukrainian Journal of Physics. - 2008. - Vol. 53, No. 12. - P. 1208-1218.https://ekmair.ukma.edu.ua/handle/123456789/335We study ordered states and topological excitations in a quasitwo-dimensional magnet modeled by a square lattice with spins s=1 at all sites, and a Hamiltonian with the biquadratic exchange interaction between the nearest neighbor sites. We propose two effective Hamiltonians for the description of large-scale excitations in the strictly two-dimensional case. They describe excitations of the mean field in the nematic phase and the mixed ferromagneticnematic phase. It is shown that the effective Hamiltonians are minimized on configurations with fixed topological charge. These topological excitations can arise at low temperatures and cause the destruction of a long-range order in the strictly two-dimensional system.enArticle