On the origin of the ferromagnetism in Zn0.8Mn0.2O having a higher Curie temperature than Zn0.8Co0.2O

► It is found that Zn0.8Mn0.2O has a higher Curie temperature than Zn0.8Co0.2O. ► Grain boundaries and related defects are the intrinsic origin for the ferromagnetism. ► The ferromagnetic properties may be related to the formation of acceptors.