Strain-induced ZnO spinterfaces

Abstract

A series of undoped ZnO films of different thicknesses was grown on different substrates over a range of different temperatures and oxygen partial pressures. Notably, ferromagnetism was detected in very thin ZnO films (∼20 nm), and its magnetic ordering was also found to be thermally stable up to 800 °C. To our surprise, magnetic ordering was destroyed as the ZnO overlayer grew thicker, just as its in-plane compressive strain was released and the ZnO/substrate interface damaged by misfit dislocations. The source of magnetism was found to be due to neither defects in the bulk of the ZnO overlayer nor the bulk of the substrate. Experimental results showed that strain at the ZnO/substrate interface led to a strain-induced magnetic effect. Using first-principles ab intio calculation, we confirmed that strain at the ZnO/substrate interface stabilizes zinc vacancy defects, which are magnetic. Ferromagnetic ordering is a result of the coupling of unpaired electron spins originating from the oxygen 2p orbitals surrounding the zinc vacancies. © 2011 American Chemical Society.