A Combined First Principles Study of the Structural, Magnetic, and Phonon Properties of Monolayer CrI₃

In recent years, 2D materials have garnered a wealth of interest because of their unique low-dimensional physics, exotic magnetism, and the relative ease with which their properties can be tuned via doping, crinkling, strain, and stacking. The first magnetic 2D material to be discovered, monolayer (ML) CrI₃, is particularly fascinating due to its ground state ferromagnetism, which can be employed to design spintronic materials. Yet, because monolayer materials are notoriously difficult to probe experimentally, much remains unresolved about CrI₃’s properties. Here, we report predictions of the atomic magnetic moments, lattice parameters, and geometry of ML CrI₃ using highly accurate fixed-node Diffusion Monte Carlo (DMC) calculations.  Alongside Density Functional Theory (DFT) benchmarked by DMC, we also predict its spin-phonon/lattice couplings. Notably, we find that the atomic magnetic moments in CrI₃ are 3.62 μB per chromium and -0.145 μB per iodine, which are both quite large, supporting a potentially large ligand superexchange-dominated magnetic anisotropy. Our DMC-predicted lattice constant (a₀) of 6.87 Å is also very close to the experimental a₀ = 6.84 Å, demonstrating the predictive power of DMC for geometry and magnetism in 2D materials.