Defect-induced magnetic order in pure ZnO films

Abstract

We have investigated the magnetic properties of pure ZnO thin films grown under ${\text{N}}_{2}$ pressure on $a$-, $c$-, and $r$-plane ${\text{Al}}_{2}{\text{O}}_{3}$ substrates by pulsed-laser deposition. The substrate temperature and the ${\text{N}}_{2}$ pressure were varied from room temperature to $570\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ 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 ${\text{Al}}_{2}{\text{O}}_{3}$ substrates did not show significant ferromagnetism. However, ZnO films grown on $r$-plane ${\text{Al}}_{2}{\text{O}}_{3}$ showed reproducible ferromagnetism at 300 K when grown at $300--400\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and 0.1--1.0 mbar ${\text{N}}_{2}$ pressure. Positron annihilation spectroscopy measurements as well as density-functional theory calculations suggest that the ferromagnetism in ZnO films is related to Zn vacancies.