Phase transition of a triangular lattice Ising antiferromagnet FeI2

The phase transition of FeI2 is revisited. This material has the hexagonal crystal structure and shows an antiferromagnetic order at T-N=9.3 K in zero field. When an external magnetic field is applied along the c axis below T-N, successive metamagnetic transitions occur. The magnetism of FeI2 has been interpreted based on the further neighbor interaction model on the triangular net. However, the exchange interaction constants derived from the analysis do not seem realistic. We have measured the magnetic susceptibility, magnetization, and specific heat on single crystals of FeI2. From the specific-heat measurement, the temperature versus magnetic field phase diagram is constructed, in which five distinct magnetic phases, namely, the antiferromagnetic and four ferrimagnetic ones, exist. The magnetization measurement reveals that magnetization steps appear at 1/3, 12/25, 13/25 and between 16/25 and 17/25 of the saturation magnetization, M-s=3.5 mu(B)/Fe. Based on an experimental evidence, we argue that a lattice distortion occurs below T-N and the exchange interaction between spins on a triangle becomes anisotropic. We discuss the phase transition using the anisotropic triangular lattice model with J(1), J(2), and J(3). From the analysis of the metamagnetic transition fields, we obtain, 2.5