​Ferromagnetism in polycrystalline ZnO doped with Co has been observed sustainable in recent experiments. We use first-principle calculations to show that Co impurities favorably substitute at the grain boundary (GB) rather than in the bulk-like region. We reveal that room-temperature ferromagnetism (RTFM) at the Co-doped ZnO GB in the presence of Zn vacancies is due ferromagnetic exchange coupling of a pair of closely associated Co atoms in the GB, with a ferromagnetic exchange coupling energy of ~ 300 meV, which is in contrast to a previous study that suggested the O vacancy−Co complex induced ferromagnetism. Electronic structure analysis used to predict the exchange coupling mechanism, showing that the hybridization of O p states with Co and Zn d states enhances the magnetic polarization originating from the GB. Our results indicate that RTFM originates from Co clusters at interfaces or in GBs.

DOI: 10.1039/C6RA11607H