Defect pairing and magnetism in C or N-doped MgO and ZnO: a density-functional study

It is demonstrated that C or N doping recently proposed as a way to create magnetism in otherwise nonmagnetic oxide insulators is curtailed by formation of defect pairs. Our density-functional calculations show that N-N pairing in MgO lowers the energy by 0.4 eV, leading to a nonmagnetic state. C-C pairing is even exothermic by more than 3 eV, and the resultant (C-C)$^{4-}$ molecules with spin=1 couple antiferromagnetically in MgO. However, calculations for C-doped ZnO, when properly treated using the PBE0 hybrid functional, show that the spin-polarized $pp\pi^*$ levels resonate with the host conduction band, which could possibly mediate a long-range ferromagnetic order. Magnetism of open-shell {\it impurity molecules} is proposed as a possible route to $d^0$-ferromagnetism in oxide spintronic materials.