The ferromagnet-antiferroraagnet transition in frustrated quantum spin models

The character of the transition between the ferromagnetic and the antiferromagnetic ground states in systems of strongly correlated electrons is studied in a framework of frustrated quantum spin models. A perturbation theory in a small parameter characterizing the transition and new variational approach have been used. It is shown that the transition from the state with maximal spin to the state with S=0 occurs by passing the states with intermediate spins though the character of the transition depends on parameters of models.