Terahertz study of the frustrated triangular Ising magnet FeI₂

2D antiferromagnetic (AF) triangular lattices are of great interest due to their geometric frustration, which may lead to the absence of a long-range magnetic order at T=0. FeI₂ is part of the ferrous halides family, in which the Fe²⁺ ions are distributed on hexagonal planes and spontaneously order in an AF phase below T_N∼9K [1]. Contrary to other ferrous halides, the spins in the ordered phase of FeI₂ form triangular AF sheets (instead of antiparallel ferromagnetic sheets), giving rise to a more complex magnetic structure. In this peculiar ground state, the spins (S=1) generate several types of magnetic excitations, including a mysterious two-magnon bound state that should be prevented by selection rules [2]. Moreover, when an external magnetic field is applied along the c-axis at T<T_N, successive magnetic transitions occur, including a phase without evidence of long-range order [3]. We present the results from in-field time-domain terahertz spectroscopy on FeI₂. These experiments give insight into the diverse magnetic excitations of this triangular AF lattice.
[1] Bertrand et al., J. Phys. 35, 385 (1974)
[2] Petitgrand et al., JMMM 15-18, 381 (1980)
[3] Fert et al., Sol. St. Comm. 13, 1219 (1973)