Antiferromagnet-ferromagnet transition in the one-dimensional frustrated spin model

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Abstract
The one-dimensional s=1/2 quantum spin model with nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic interactions is considered. The behavior of the system in the vicinity of the transition point from the antiferromagnetic ground state to the ferromagnetic one is studied. The consideration is based on the classical approximation and the regular procedure is proposed to find the corrections to the classical energy. The dependence of the energy on spin S is calculated. We obtain evidence of a phase separation for the state with intermediate S. © 1996 The American Physical Society.