Topological excitations in 2D spin system with high spin s>=1s>= 1

We construct a class of topological excitations of a mean field in a two-dimensional spin system represented by a quantum Heisenberg model with high powers of exchange interaction. The quantum model is associated with a classical one (the continuous classical analogue) that is based on a Landau-Lifshitz like equation, and describes large-scale fluctuations of the mean field. On the other hand, the classical model is a Hamiltonian system on a coadjoint orbit of the unitary group SU($2s {+} 1$) in the case of spin $s$. We have found a class of mean field configurations that can be interpreted as topological excitations, because they have fixed topological charges. Such excitations change their shapes and grow preserving an energy.Comment: 10 pages, 1 figur

Magnetic and quadrupolar order in a one-dimensional ferromagnet with cubic crystal-field anisotropy

The zero temperature phase diagram of a one-dimensional S=2 Heisenberg ferromagnet with single-ion cubic anisotropy is studied numerically using the density-matrix renormalization group method. Evidence is found that although the model does not involve quadrupolar couplings, there is a purely quadrupolar phase for large values of the anisotropy. The phase transition between the magnetic and quadrupolar phases is continuous and it seems to be characterized by Ising critical exponents.Comment: 11 pages, 7 figures, REVTeX, accepted in Phys. Rev. B (scheduled on June 99

ÉTUDE DE QUELQUES PROPRIÉTÉS MAGNÉTIQUES DU THIOSPINELLE MnCr2S4

On donne une interprétation théorique des courbes d'aimantation du thiospinelle MnCr2S4 obtenues en champs intenses. On met en évidence l'importance de la striction d'échange pour les interactions JAA et JAB simultanément à l'introduction d'un terme biquadratique intrinsèque pour les interactions AA.A theoretical explanation of the magnetization curves obtained at high fields for the sulfur spinel MnCr2S4 is given. The important part played by magnetostrictive effects on both JAA and JAB together with necessity of an intrinsic biquadratic term for the AA interactions is noticed