A combined first principles study of the structural, magnetic, and phonon properties of monolayer CrI.

The first magnetic 2D material discovered, monolayer (ML) CrI, is particularly fascinating due to its ground state ferromagnetism. However, because ML materials are difficult to probe experimentally, much remains unresolved about ML CrI's structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin-phonon and spin-lattice coupling of ML CrI. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI's ML geometry with DMC accuracy, yielding lattice parameters in good agreement with recently published STM measurements-an accomplishment given the ∼10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find that previous DFT predictions of ML CrI's magnetic spin moments are correct on average across a unit cell but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts that magnetic moments in ML CrI are 3.62 μ per chromium and -0.145 μ per iodine, both larger than previous DFT predictions. The large disparate moments together with the large spin-orbit coupling of CrI's I-p orbital suggest a ligand superexchange-dominated magnetic anisotropy in ML CrI, corroborating recent observations of magnons in its 2D limit. We also find that ML CrI exhibits a substantial spin-phonon coupling of ∼3.32 cm. Our work, thus, establishes many of ML CrI's key properties, while also continuing to demonstrate the pivotal role that DMC can assume in the study of magnetic and other 2D materials.