Nature of the Ordered Phase in a Hexagonal Ising Antiferromagnet

Abstract

A Monte Carlo study of a hexagonal Ising Antiferromagnet reveals the occurrence of a new type of modulated phase in which a spin structure described by using three sublattices is randomly modulated in the lattice. The order parameter of this phase is not individual sublattice magnetizations but a symmetric function of them which corresponds to the magnetic structure factor F(1/3, 1/3, 0). As the temperature is decreased, this phase gradually changes into a usual homogeneous ferrimagnetic phase. This process is discussed in detail. It is suggested that this modulated phase occurs in CsCoCl₃ and CsCoBr₃.