Date of this Version
Magnetic properties of Mn-doped ZnO strongly depend on sample preparation and annealing. While the samples sintered in air at 500 °C show ferromagnetism at room temperature, those sintered at 900 °C are paramagnetic. The samples sintered in vacuum show a ferromagnetic transition at 45 K, which is attributed to ferrimagnetic Mn3O4. With an increase of annealing temperature, the ferromagnetism observed at room temperature in the low-temperature-sintered samples is gradually suppressed and finally completely removed. These results can be understood by considering a metastable ferromagnetic phase and its stability under varying temperatures and atmospheres.