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 Zn_0.9Co_0.1O 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.