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

Unlike TiO and SnO, 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 -cut AlO by sputtering. The ZnO films were ferromagnetic, with a very high 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.