Model for Vacancy-Induced d0 Ferromagnetism in Oxide Compounds

We propose a model with few parameters for vacancy-induced ferromagnetism based on a correlated model for oxygen orbitals with random potentials representing cation vacancies. For certain potentials, moments appear on oxygen sites near defects. Treating the randomness exactly, we calculate the magnetic couplings between moments, the Curie temperature and spin and charge densities as a function of the potential, the density of vacancies, and correlation strength. For physically reasonable parameters this predicts Curie temperatures well above room temperature for small concentrations of vacancies. We discuss our results in relation to questions of stability and reproducibility raised in experiments. To circumvent the difficulties of controlling intrinsic defects, we propose specific nonmagnetic host doping that could be, for example, substituted for cations in HfO2 or ZrO2.