Effect of thermal treatment on room-temperature ferromagnetism in Co-doped ZnO powders

The Co-doped ZnO powders were synthesized by sol–gel method, and treated at different temperatures (673–873 K) in the presence or absence of NH3 atmosphere for 0.5 and 2 h, respectively. X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) show that better crystal structure can cause larger ferromagnetism and the second phase (Co3O4) is the reason for saturation magnetization decrease of the sample sintered at higher temperature in air. XPS and nuclear magnetic resonance (NMR) prove the existence of Co2+ ions in the Zn0.9Co0.1O and the absence of Co clusters, indicating intrinsic ferromagnetism of the samples treated in air. However, strong ferromagnetism of the samples annealed in NH3 is ascribed to cobalt nitride formed during annealing.