A detailed ab initio (DFT/GGA and $\text{DFT}/\text{GGA}+U$) investigation of codoping ZnO with Co and Cu ions reveals the role of Cu and the existence of optimum structural dopant configurations which could lead to the enhancement of ferromagnetism. The role of the Cu ions in Zn(Co,Cu)O is to act as superexchange mediators while causing a remote delocalization through the hybridization of the $\text{Cu}{d}_{3z^2-r^2}$ spin-majority states with the $\text{O}p$ states, thereby enhancing the ferromagnetism. The ferromagnetic state is found to be more favorable than the antiferromagnetic one if the $\text{Cu}\text{−}d$ band is spin splitted. Such a process is reminiscent of the conventional RKKY interaction among magnetic impurities embedded in a metal.

• Received 8 April 2008

DOI:https://doi.org/10.1103/PhysRevB.78.193311