Magnetic Excitations of the Frustrated Triangular Ising Magnet FeI2*

We present a detailed investigation of the spin dynamics in single-crystals of the layered spin-one triangular-lattice compound FeI2. Previous thermo-magnetic measurements revealed a strong Ising single-ion anisotropy for the Fe2+ ions in FeI2 and a magnetically long-range ordered state below 9.3K, which can be understood from the competition between nearest neighbor ferromagnetic interactions and a complex set of further-neighbor interactions. Early neutron scattering, far-infrared and ESR measurements, revealed the emergence of a two-magnon bound state (TMBS) as the lowest energy mode from this ordered state. The TMBS carries an apparent g-factor that is doubled compared to that of single magnon excitations, which can be explained by a change of 2 units in spin angular momentum, at odds with the dipole selection rule. We revisit the spin excitations of FeI2 using modern neutron-scattering instrumentation and map out the magnetic structure, diffuse scattering and low-energy magnetic excitation spectrum. We extract a model Hamiltonian for FeI2 and elucidate a novel hybridization mechanism that quantitatively explains current and previous spectroscopic experiments on this enigmatic compound.