Intrinsic Ferromagnetism of Two-Dimensional (2D) MnO2 Revisited: A Many-Body Quantum Monte Carlo and DFT plus U Study

Monolayer MnO2is one of the few predicted two-dimensional (2D) ferromagnets that has been experimentallysynthesized and is commercially available. The Mermin-Wagnertheorem states that magnetic order in a 2D material cannot persistunless magnetic anisotropy (MA) is present and perpendicular tothe plane, which permits afinite critical temperature. Previouscomputational studies have predicted the magnetic ordering andCurie temperature of 2D MnO2with DFT+U(Density FunctionalTheory + HubbardUcorrection), with the results having a strongdependence on the HubbardUparameter. Diffusion Monte Carlo(DMC) is a correlated electronic structure method that has haddemonstrated success for the electronic and magnetic properties ofa variety of 2D and bulk systems since it has a weaker dependenceon the starting Hubbard parameter and density functional. In this study, we used DMC and DFT+Uto calculate the magneticproperties of monolayer MnO2. We found that the ferromagnetic ordering is more favorable than antiferromagnetic and determineda statistical bound on the magnetic exchange parameter (J). In addition, we performed spin-orbit MA energy calculations usingDFT+U, and using our DMC and DFT+Uparameters along with the analytical model of Torelli and Olsen, we estimated an upperbound of 28.8 K for the critical temperature of MnO2. These QMC results intend to serve as an accurate theoretical benchmark,necessary for the realization and development of future 2D magnetic devices. These results also demonstrate the need for accuratemethodologies to predict magnetic properties of correlated 2D materials.

Automated summary

In this study, different electronic structure methods were used to calculate the magnetic properties of monolayer MnO2. It was found that ferromagnetic ordering is more favorable than antiferromagnetic ordering, and a statistical bound on the magnetic exchange parameter was determined. Using an analytical model, an upper bound for the critical temperature of MnO2 was estimated, providing an accurate theoretical benchmark for the realization and development of future 2D magnetic devices.