ANTIFERROMAGNET FERROMAGNET TRANSITION IN THE ONE-DIMENSIONAL FRUSTRATED SPIN MODEL

The one dimensional s = 1/2 quantum spin model with nearest-neighbor f erromagnetic and next-nearest-neighbor antiferromagnetic interactions is considered. The neighbor antiferromagnetic interactions is consider ed. The behavior of the system in the vicinity of the transition point from the antiferromagnetic ground state to the ferromagnetic one is s tudied. The consideration is based on the classical approximation and the regular procedure is proposed to find the corrections to the class ical energy. The dependence of the energy on spin S is calculated. We obtain evidence of a phase separation for the state with intermediate S.