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Resolution exacte du modele generalise en incluant des interactions spin-pseudospin supplementaires des plus proches voisins suivant la direction x. Ce nouveau modele peut etre interprete en termes de couplage magneto-elastique

On montre notamment que les transitions de phase vers la phase paramagnetique sont discontinues pour toutes les valeurs des parametres, qu'il n'y a pas de transitions ordre-ordre entre les phases Ai et Fi, et que, pour certaines surfaces tres petites dans l'espace du parametre, la transition a la phase paramagnetique est accompagnee d'un changement de signe de la fonction de correlation nn

A new model of metamagnetic phase transition with temperature in Mn3GaC is proposed. The magnetoelastic Hamiltonian takes into account two independent strains (trigonal and isotropic ones) as functions of temperature, calculated similarly to the microscopic theory of magnetostriction. Necessary conditions for the transition are established within the molecular field approximation and the fit with experiment is discussed.

A new model of the temperature-induced metamagnetic phase transitions is considered for the class of the two-sublattice magnets with axial symmetry. The model Hamiltonian contains the usual Ising term as well as the coupling of the uniform strain with both exchange interaction and single ion anisotropy. Within the MFA all these couplings are found to be necessary to produce the metamagnetic phase change. The results are compared with experiment. [Russian Text Ignored].

Calcul au moyen d'un modele de type Heisenberg, deja decrit, dans l'approximation de phase aleatoire