Examining the Rich Magnetic Phases of FeI₂ with Monte Carlo Simulation

The ferrous halide, FeI₂, exhibits a rich variety of magnetic phases, which have not been fully understood despite decades of experimental¹ and theoretical² study. A common modeling strategy is to seek the simplest model that captures qualitative features, e.g., as observed from magnetization and neutron diffraction data. The magnetic dipoles of this system can be well modeled as Ising spins with both ferromagnetic and antiferromagnetic interactions on a triangular lattice. We use replica-exchange Monte Carlo simulations to map the magnetic phase diagram in temperature and external magnetic field, and report a number of exotic magnetic orderings. We find evidence for a 3Q magnetic structure that could relax into a skyrmion lattice upon softening the easy-axis anisotropy, and a “floating” phase where the ordering wave vectors move continuously.

¹ A. Wiedenmann et. al. Journal of Magnetism and Magnetic Materials 74(1), (1988).

² X. Bai et al. Nature Physics 17, (2021).