Effects of atmosphere and Cu doping on the magnetic properties of ZnO powders

Pure ZnO and Cu-doped ZnO powders (i.e. Cu = 0, 0.5, 1, 1.5 and 2 at.%) have been prepared by a coprecipitation method and annealed in O2, N2 and air. The role of atmosphere and doping in structure and magnetism has been investigated by x-ray diffraction (XRD), Raman spectrum and magnetic measurement. The XRD and Raman results indicate that the undoped and Cu-doped samples below 1 at.% are pure phase with the ZnO wurtzite structure. The undoped samples show unexpected room-temperature ferromagnetism (RTFM) with a strong diamagnetic background. The unsaturation hysteresis loops of the samples prepared in oxygen-poor atmospheres such air and N2 indicates that oxygen vacancy (VO) prefers to increase paramagnetic component instead of stabilizing the ferromagnetic ordering. This result demonstrates that VO does not play an importance role to inducing the observed RTFM, which is very different from recent experimental and theoretical results. Cu doping has suppressed the diamagnetic background and caused a significant increase of saturation magnetization (MS), i.e. as the x increases from 0 to 1.5 at.%, the MS of the samples annealed in O2 increases from 5 × 10−4emu/g to 6.5 × 10−3emu/g in an applied field of 5000 Oe. It strongly indicates that Cu doping can enhance the RTFM, which could be ascribed to the additional magnetic interaction associated to the unpaired d electrons of Cu ions. •The ZnO powder samples without dopant can be ferromagnetism at room temperature.•The oxygen vacancy does not play an essential role to generating the observed RTFM in the pure ZnO samples.•The solid solubility of Cu in ZnO matrix is less than 1.5 at.%.•Cu doping can suppress the diamagnetism of ZnO matrix and enhance the ferromagnetism.