Mean Field Analysis for Successive Phase Transitions of Heisenberg-Ising Antiferromagnets on the Triangular Lattices

Phase transitions in Heisenberg-Ising antiferromagnets with nearest neighbour ( nn ) and next nearest neighbour ( nnn ) interactions on the triangular or on the hexagonal lattice are investigated by a mean field approximation. Four phases are found, namely the paramagnetic phase and three relevant types of ordered phases; (i) partially disordered phase where z -component is ordered on sublattices but the uniform susceptibility is finite, (ii) ferrimagnetic phase where the spontaneous magnetization appears and (iii) non-collinear ferrimagnetic phase where x y -components are also ordered.