ZnO thin films made by sputtering: room temperature ferromagnetism due to Zn defects/vacancies?

Abstract

Unlike TiO₂ and SnO₂, room temperature ferromagnetism in pristine ZnO films does not appear to originate from oxygen vacancies. In this study, we investigated thin films of ZnO deposited on R-cut Al₂O₃ by sputtering. The ZnO films were ferromagnetic, with a very high T_C of about 800 K and were quite magnetically homogenous. Our experiments were complemented by quantum-mechanical calculations of both bulk wurtzite-structure ZnO and its (0001) surfaces, with and without Zn vacancies. While the bulk ground state and the bulk-terminated, vacancy-free (0001) surfaces were non-magnetic, a higher concentration of Zn vacancies deep beneath the surface was shown to contribute magnetic moments to the ferromagnetic state of ZnO.