Ni doped ZnO nanoparticles were synthesized by a solution route and annealed in O2, air, 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 ZnO samples were measured and clearly show typical ferromagnetic saturation behavior. With the defect analysis based on photoluminescence spectroscopy, the effect of defects on the nature and origin of ferromagnetism was investigated. The results suggest that oxygen vacancies, especially single ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the Ni doped ZnO. 1. Introduction Diluted magnetic oxide semiconductors (DMOSs) have gained much attention recently due to the possibility to control spin and charge simultaneously for future spintronics [1, 2]. These oxide semiconductors with a wide band gap are optically transparent in visible region and important for the development of spin related optoelectronic devices. After the theoretical prediction by Dietl et al. suggesting the existence of room temperature ferromagnetism (FM) in doped ZnO, the system has been extensively studied [3, 4]. Remarkable progress has been made in the realization of transition metal (TM) doped ZnO with Curie temperature (Tc) at or above room temperature (RT). It was also reported that even doping “nonmagnetic” atoms such as Cu or Bi leads to RT FM in ZnO [5, 6]. More recently, FM had been observed in undoped ZnO, which opened an extensive debate on the origin of FM [7, 8]. Therefore, some researches suggested that induced FM is due to oxygen defects in ZnO, not TM ions or secondary phases. Although the origin of FM in TM doped and undoped ZnO has not been fully understood so far, oxygen (or Zn) defects are broadly recognized as an important reason for the FM behavior of doped and undoped ZnO, which is also consistent with our previous work on FM of undoped ZnO [9]. In the present work, we have focused our attention on further research of the role of oxygen (or Zn) defects by investigating the effect of different annealing atmospheres on the FM properties of Ni doped ZnO nanoparticles based on an analysis of photoluminescence (PL) spectroscopy. 2. Experimental 2.1. Preparation of Ni Doped ZnO Nanoparticles zinc acetate (Zn(AC)2·2H2O) and Nickel acetate (Ni(AC)2·4H2O) in the chosen atomic rations were dissolved in 200ml deionized water as the precursor together with 0.01？mol methenamine (C6H12N4, HMT). Then they were mixed under vigorous stirring for 30？min in a
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