Ordered Phases and Phase Transitions in The Stacked Triangular Ising Antiferromagnet CsCoCl₃ and CsCoBr₃

We study the ordered phases and the phase transitions in the stacked triangular antiferromagnetic Ising (STAFI) model with strong interplane coupling modeling CsCoCl$_3$ and CsCoBr$_3$. We find that there exists an intermediate phase which consists of a single phase of so-called partial disordered (PD) type, and confirm the stability of this phase. The low temperature phase of this model is so-called two-sublattice ferri magnetic phase. The phase transition between the PD phase and two-sublattice ferri magnetic phase is of the first order. This sequence of the phases is homomorphic as that in the three-dimensional generalized six-state clock model which have the same symmetry of the STAFI model. By studying distributions of domain walls in one dimensional chains connecting layered triangular lattices, we clarify the nature of the phase transition and give an interpretation of little anomaly of the specific heat.