Enhancement of ferromagnetism of ZnO:Co nanocrystals by post-annealing treatment: The role of oxygen interstitials and zinc vacancies

Co-doped ZnO thin films were prepared by polymer-assisted deposition (PAD), and then treated by post-annealing in different atmospheres. XRD, SEM, TEM and SQUID were employed to characterize the samples, which demonstrate that all the samples are composed of nanocrystals and the annealing shows a robust effect on the enhancement of ferromagnetism at room temperature. PL measurements show that the improvement of ferromagnetism upon annealing is related to the decrease of oxygen interstitials (Oi) and the increase of Zn1− vacancies. XPS measurements indicate that post-annealing also induces oxygen vacancies, which have no positive contribution to the ferromagnetic ordering. Our results may suggest an effective way for modulation of structure and magnetic properties of Co-doped ZnO nanocrystal thin films.