Phys. Rev. B 80, 035331 (2009)
We have investigated the magnetic properties of pure ZnO thin films grown under N2 pressure on a-, c-, and r-plane Al2O3substrates by pulsed-laser deposition. The substrate temperature and the N2 pressure were varied from room temperature to570 °C and from 0.007 to 1.0 mbar, respectively. The magnetic properties of bare substrates and ZnO films were investigated by SQUID magnetometry. ZnO films grown on c- and a-plane Al2O3 substrates did not show significant ferromagnetism. However, ZnO films grown on r-plane Al2O3 showed reproducible ferromagnetism at 300 K when grown at 300–400 °C and 0.1–1.0 mbarN2 pressure. Positron annihilation spectroscopy measurements as well as density-functional theory calculations suggest that the ferromagnetism in ZnO films is related to Zn vacancies.