Magnetic properties of Co-doped ZnO nanoparticles

The magnetism in nanoparticulate powders of Zn1-xCoxO with 0 <= x <= 0.09 synthesized by a combustion reaction technique is investigated in a broad range of temperatures (5 <= T <= 750K) for applied magnetic fields up to 85 kOe. The hysteresis loops indicated the presence of both ferromagnetic and paramagnetic ordering at room temperature. An additional antiferromagnetic phase was observed for temperatures below 260 K. A particle model that can account for the results is that the some of doping Co2+ ions are not interagent among themselves, a small quantity form clusters, leading to the ferromagnetic ordering with some of the particles in the superparamagnetic state, and few others Co2+ ions form CoO at the grain boundary yielding the antiferromagnetic phase. It was also found that a modified Langevin function can be used for describing the H- dependence for magnetization data.