Interplay between Magnetic and Orbital Ordering in the Strongly Correlated Cobalt Oxide: A DFT plus U Study

The strongly correlated CoO is investigated by means of DFT + U calculations to elucidate the origin of the cubic-to-monoclinic distortion observed below T-N for that system. An effective U-eff value between 4 and 5 eV is required to reproduce, in the meanwhile, the monoclinic symmetry, the band gap, and the type-II antiferromagnetic ordering of CoO. The extraction of the exchange coupling constants combined with full structural relaxations allows the interpretation of the low-temperature structure of CoO. An interplay between magnetic and orbital ordering is shown to be responsible for the peculiar structural behavior of CoO. For U-eff < 4 eV, the magnetic driving force dominates over the Jahn-Teller effect, leading to a rhombohedral structure. In opposition, the Jahn-Teller effect is predominant for U-eff > 5 eV, leading to a tetragonal structure.