Magnetic properties of different temperature treated Co- and Ni-doped ZnO hollow nanospheres

The effect of different annealing temperatures on magnetic properties of Co- and Ni-doped ZnO hollow nanospheres was investigated. It was found that the hollow structures and room-temperature ferromagnetism were kept when the Co- and Ni-doped ZnO samples were annealed at low temperature of 550 °C. When the temperature was elevated to 700 °C, the hollow structures partially collapsed and the samples still exhibited ferromagnetic behavior. The hollow structures were completely broken with annealing temperature above 1200 °C. The ferromagnetic behavior of Co-doped ZnO disappeared, while the Ni-doped ZnO still exhibited reduced ferromagnetism. However, the ferromagnetism in high-temperature annealed Ni-doped ZnO nanospheres was extrinsic and probably originated from secondary phases.