Collinear néel-type ordering in partially frustrated lattices

Abstract

We consider two partially frustrated S=1/2 antiferromagnetic spin systems on the triangular and pentagonal lattices. In an elementary plaquette of the two lattices, one bond has exchange interaction strength a (a≤1) whereas all other bonds have exchange interaction strength unity. We show that for a less than a critical value a_c, collinear Néel-type ordering is possible in the ground state. The ground-state energy and the excitation spectrum have been determined using linear spin-wave theory based on the Holstein-Primakoff transformation.