Prediction of transition metal carbonitride monolayers MN 4 C 6 (M = Cr, Mn, Fe, and Co) made up of a benzene ring and a planar MN 4 moiety.

Based on first-principles calculations, we predict a class of graphene-like magnetic materials, transition metal carbonitrides MN 4 C 6 (M = Cr, Mn, Fe, and Co), which are made up of a benzene ring and an MN 4 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 MN 4 C 6 is ascribed to the synergistic effect of sp 2 hybridization, M-N coordination bond, and π-d conjugation. The MN 4 C 6 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 MnN 4 C 6 , FeN 4 C 6 , and CoN 4 C 6 are ferromagnetic while CrN 4 C 6 has an antiferromagnetic ground state. The Curie temperatures are estimated by solving the anisotropic Heisenberg model with the Monte Carlo method.