Phase diagram of the triangular Ising antiferromagnet

The phase boundaries of the antiferromagnetic Ising model on the triangular lattice are determined. Using the interface method of Müller-Hartmann and Zittartz we obtain closed-form expressions for the transition temperature as function of the magnetic field. For isotropic interactions the resulting phase diagram is in fair agreement with the Monte-Carlo results. For anisotropic interactions we find two antiferromagnetic phases with no common boundary, in disagreement with a previous conjecture by Lin and Wu.