Morphology and Magnetic Coupling in ZnO:Co and ZnO:Ni Co-Doped with Li

$Zn_{0.95}Co_{0.05}O$ and $Zn_{0.97}Ni_{0.03}O$ nanorods, prepared by a solvothermal method, show intriguing morphology and magnetic properties when co-doped with Li. At low and moderate Li incorporation (below 10 and 3 at.% Li in the Co- and Ni-doped samples, respectively) the rod aspect ratio is increased and room temperature ferromagnetic properties are enhanced, whereas the ferromagnetic coupling in $Zn_{0.97}Ni_{0.03}O$ is decreased for Li concentrations < 3 at.%. First-principles theoretical analyses demonstrate that Li co-doping has primarily two effects in bulk $Zn_{1- x}M_{x}O$ (with M = Co or Ni). First, the Li-on-Zn acceptors increase the local magnetic moment by depopulating the M 3d minority spin-states. The magnetic coupling is Ruderman-Kittel-Kasuya-Yosida-like both without and with Li co-doping. Second, Li-on-Zn prefer to be close to the M atoms to compensate the M-O bonds and to locally depopulate the 3d states, and this will help forming high aspect nanostructures. The observed room temperature ferromagnetism in Li co-doped $Zn_{1- x}M_{x}O$ nanorods can therefore be explained by the better rod morphology in combination with ionizing the magnetic M atoms.