It is shown that substitution of C or N for O recently proposed as a way to create ferromagnetism in otherwise nonmagnetic oxide insulators is curtailed by formation of impurity pairs, and the resultant ${\mathrm{C}}_2$ $\mathrm{\text{spin}}=1$ dimers as well as the isoelectronic $\mathrm{N}_2^{2+}$ interact antiferromagnetically in $p$-type MgO. For C-doped ZnO, however, we demonstrate using the Heyd-Scuseria-Ernzerhof hybrid functional that a resonance of the spin-polarized ${\mathrm{C}}_2$ $pp{\pi }^{*}$ states with the host conduction band results in a long-range ferromagnetic interaction. Magnetism of open-shell impurity molecules is proposed as a possible route to $d^0$-ferromagnetism in oxide spintronic materials.

• Received 15 March 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.267203

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© 2010 The American Physical Society