  The search for two-dimensional (2D) magnetic materials has attracted a great
deal of attention because of the experimental synthesis of 2D CrI$_3$, which
has a measured Curie temperature of 45 K. Often times, these monolayers have a
higher degree of electron correlation and require more sophisticated methods
beyond density functional theory (DFT). Diffusion Monte Carlo (DMC) is a
correlated electronic structure method that has been demonstrated successful
for calculating the electronic and magnetic properties of a wide variety of 2D
and bulk systems, since it has a weaker dependence on the Hubbard parameter (U)
and density functional. In this study we designed a workflow that combines
DFT+U and DMC in order to treat 2D correlated magnetic systems. We chose
monolayer CrX$_3$ (X = I, Br, Cl, F), with a stronger focus on CrI$_3$ and
CrBr$_3$, as a case study due to the fact that they have been experimentally
realized and have a finite critical temperature. With this DFT+U and DMC
workflow and the analytical method of Torelli and Olsen, we estimated an upper
bound of 43.56 K for the T$_c$ of CrI$_3$ and 20.78 K for the T$_c$ of
CrBr$_3$, in addition to analyzing the spin densities and magnetic properties
with DMC and DFT+U. We expect that running this workflow for a well-known
material class will aid in the future discovery and characterization of lesser
known and more complex correlated 2D magnetic materials.
