Phase transition of a triangular lattice Ising antiferromagnet FeI ₂

The phase transition of FeI₂ is revisited. This material has the hexagonal crystal structure and shows an antiferromagnetic order at TN =9.3K in zero field. When an external magnetic field is applied along the c axis below TN, 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, Ms =3.5 μB /Fe. Based on an experimental evidence, we argue that a lattice distortion occurs below TN and the exchange interaction between spins on a triangle becomes anisotropic. We discuss the phase transition using the anisotropic triangular lattice model with J1, J2, and J3. From the analysis of the metamagnetic transition fields, we obtain, 2.5< J1 / kB <3.0K, -16.2< J2 / kB <-15.2K, and -16.4< J3 / kB <-14.4K.