Undoped ZnO nano-particles were synthesized by a solution route and annealed in N2, O2, and Ar, respectively. X-ray diffraction and X-ray photoelectron spectroscopy measurements show that the samples possess typical wurtzite structure and have no other impurity phases. Magnetization loops for the ZnO samples were measured and clearly show typical ferromagnetic saturation behavior. With the combination of defect analysis based on photoluminescence spectroscopy and first-principles calculations of the possible magnetic defect centers in ZnO, the effect of defects on the nature and origin of ferromagnetism was investigated. The results suggest oxygen defects, especially singly ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the undoped ZnO system.

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