Unraveling magnetic physics in monolayer CrI3 using diffusion Monte Carlo

The extensive list of new and useful physics exhibited by 2D materials and their heterostructures is paving the way towards dissipationless electronics and spintronics, next-generation sensors, and vastly improved electrical grid technologies. One material at the forefront of this revolution is 2D chromium triiodide (CrI3), the monolayer of which was confirmed as the first 2D ferromagnet only several years ago. Since then, mono-, bi-, and few-layer CrI3 have emerged as playgrounds for exploring everything from pressure-tunable magnetism and magnetic polarons to spin-polarized localized excitons, topological behavior, and domain wall magnons. Despite these exciting advances, information regarding the microscopic details of the energy, magnetic moment, and lattice parameter of monolayer CrI3 is either conflicting or altogether missing. To address quantitative disagreements regarding these properties, we have predicted the energetics, spin densities, and atomic magnetic moments of monolayer CrI3 using highly accurate and highly parallelizable fixed-node Diffusion Monte Carlo (DMC) calculations as implemented in the code QMCPACK. We find that the atomic magnetic moments in CrI3 are 3.9 μB per chromium and -0.14 μB per iodine which are both larger than, but qualitatively consistent with, previous predictions based on mean-field orbital moments. Many models of magnetism in ML CrI3 assume that chromium has a moment of 3.0 μB and iodine has a moment 0.0 μB; thus, our findings can be used to improve future magnetic models of CrI3 by more accurately representing this material’s magnetic interactions.