The introduction of ferromagnetic order in ZnO results in a transparent piezoelectric ferromagnet and further expands its already wide range of applications into the emerging field of spintronics. Through an analysis of density functional calculations we determine the nature of magnetic interactions for transition metals doped ZnO and develop a physical picture based on hybridization, superexchange, and double exchange that captures chemical trends. We identify a crucial role of defects in the observed weak and preparation sensitive ferromagnetism in $\mathrm{ZnO}:\mathrm{Mn}$ and $\mathrm{ZnO}:\mathrm{Co}$. We predict and explain codoping of Li and Zn interstitials to both yield ferromagnetism in $\mathrm{ZnO}:\mathrm{Co}$, in contrast with earlier insights, and verify it experimentally.

• Received 26 October 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.187204