Prediction of transition metal carbonitride monolayers MNC (M = Cr, Mn, Fe, and Co) made up of a benzene ring and a planar MN moiety.

Based on first-principles calculations, we predict a class of graphene-like magnetic materials, transition metal carbonitrides MNC (M = Cr, Mn, Fe, and Co), which are made up of a benzene ring and an MN moiety, two common planar units in the compounds. The structural stability is demonstrated by the phonon and molecular dynamics calculations, and the formation mechanism of the planar geometry of MNC is ascribed to the synergistic effect of sp hybridization, M-N coordination bond, and π-d conjugation. The MNC materials consist of only one layer of atoms and the transition metal atom is located in the planar crystal field, which is markedly different from most two-dimensional materials. The calculations indicate that MnNC, FeNC, and CoNC are ferromagnetic while CrNC has an antiferromagnetic ground state. The Curie temperatures are estimated by solving the anisotropic Heisenberg model with the Monte Carlo method.