Extensive calculations based on density functional theory were carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism was confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arose from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740µB. Moreover, the study of vacancy-vacancy interactions indicated that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44meV higher in energy. Since this was larger than the thermal energy at room temperature, the ferromagnetic state could be stable against thermal fluctuations. Calculations and analyses were also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies were found to lead to the ferromagnetic state too. The present theoretical study not only demonstrated that ZnO samples could be magnetic even without transition-metal doping but also suggested that introducing Zn vacancy was a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

Vacancy-Induced Magnetism in ZnO Thin Films and Nanowires. Wang, Q., Sun, Q., Chen, G., Kawazoe, Y., Jena, P.: Physical Review B, 2008, 77[20], 205411