Defect-induced magnetic order in pure ZnO films

We have investigated the magnetic properties of pure ZnO thin films grown under N₂ pressure on a-, c-, and r-plane Al₂O₃ substrates by pulsed-laser deposition. The substrate temperature and the N₂ pressure were varied from room temperature to 570 ^∘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 mbar N₂ pressure. Positron annihilation spectroscopy measurements as well as density-functional theory calculations suggest that the ferromagnetism in ZnO films is related to Zn vacancies.