Room temperature ferromagnetism in as-deposited and post-annealed Co-doped ZnO films

The Co-doped ZnO thin films were prepared on $c$-cut sapphire substrates by magnetron co-sputtering, and then annealed at various temperatures in vacuum. Magnetic measurements indicate that all the films are ferromagnetic at room temperature and the magnetization of the annealed Zn$_{0.88}$Co$_{0.12}$O films is increased about one order of magnitude in comparison with the corresponding as-deposited one. Both X-ray diffraction and TEM results show that there are not any Co and Co oxides secondary phases. Optical spectrometry indicates that Co$^{2+}$enters the tetrahedral sites of the wurtzite structure of ZnO host and substitutes for Zn$^{2+}$.