Importance of electronic correlations for the magnetic properties of the two-dimensional ferromagnet CoBr2

We investigate the emergence of ferromagnetism in the two-dimensional metal halide CoBr2, with a special focus on the role of electronic correlations. The calculated phonon spectrum shows that the system is thermodynamically stable, unlike other Co halides. We apply two well-known methods for the estimation of the Curie temperature. First, we do density-functional theory +U calculations to calculate exchange couplings, which are subsequently used in a classical Monte Carlo simulation of the resulting Ising spin model. The transition temperature calculated in this way is of the order of 100 K but shows a strong dependence on the choice of interaction parameters. Second, we apply dynamical mean-field theory to calculate the correlated electronic structure and estimate the transition temperature. This results in a similar estimate for a noticeable transition temperature of approximately 100 K, but without the strong dependence on the interaction parameters. The effect of electron-electron interactions are strongly orbital selective, with only moderate correlations in the three low-lying orbitals (one doublet plus one singlet) and strong correlations in the doublet at higher energy. This can be traced back to the electronic occupation in DMFT, with five electrons in the three low-lying orbitals and two electrons in the high-energy doublet, making the latter one half filled. Nevertheless, the overall spectral gap is governed by the small gap originating from the low-lying doublet+singlet orbitals, which changes very weakly with interaction U. In that sense, the system is close to a Mott metal-to-insulator transition, which was shown previously to be a hot spot for strong magnetism.