Dimensional Reduction and Commensurate-Incommensurate Magnetic Order Transition in FeI₂

Iron iodide (FeI₂) exhibits a rich variety of magnetic phases, which have not been fully understood despite decades of experimental¹ and theoretical² study. The system includes a strong effective easy-axis anisotropy, such that the magnetic dipoles of this system are Ising-like. These spins interact via competing ferromagnetic and antiferromagnetic interactions on triangular lattice layers. By constructing a simplistic spin model from the observed magnetization and neutron diffraction data and performing replica-exchange Monte Carlo simulations, we obtain the magnetic phase diagram in temperature and external magnetic field in agreement with previous studies at low temperature. At higher temperature and external field, we observe a nontrivial dimensional reduction onto an effective 2D model that is selected dynamically via spontaneous symmetry breaking, with only weak correlations that decay exponentially in the third dimension. The system also exhibits a commensurate to incommensurate transition with increasing applied field.

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

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