Ferromagnetism induced by donor-related defects in Co-doped ZnO thin films

Abstract

Co-monodoped ZnO (ZnCoO), Co–Ga codoped ZnO (Zn(Co,Ga)O), and Co–Na codoped ZnO (Zn(Co,Na)O) thin films were prepared on sapphire substrates by pulsed laser deposition. Their structural, optical, electrical, and magnetic properties were studied to clarify the origin of ferromagnetism (FM) in the ZnCoO system. Both the ZnCoO and Zn(Co,Ga)O films showed an n-type conductivity with a similar room-temperature ferromagnetism (RTFM), although the electron concentration of the Zn(Co,Ga)O film (∼10 21 cm −3) was evidently higher than that of the ZnCoO film. The increasing donor density did not affect the FM greatly. The Zn(Co,Na)O film showed a p-type conductivity, but it did not show RTFM. To explore the mechanism of FM, the samples were annealed after growth. The ZnCoO and Zn(Co,Ga)O films annealed in the Ar ambient displayed similar FM features to the as-grown ones, but all of them lost their original RTFM when annealed in the O 2 ambient. The Zn(Co,Na)O film annealed in Ar exhibited a high resistivity, while a better p-type behavior than the as-grown one was observed for the Zn(Co,Na)O film annealed in O 2, however, both of them showed paramagnetism. No clusters, precipitates, or secondary phases were found from the X-ray diffraction pattern and the photoelectron K-edge X-ray absorption near-edge structure measurements. Together with the X-ray spectroscopy and photoluminescence results, it was found that the oxygen vacancy (V o), a donor-related defect, played a vital role in determining the RTFM in the ZnCoO system.