Abstract

We use density functional theory (DFT) calculations to show that oxygen vacancies (vO) and mobility induce noncentrosymmetric polar structures in SrTi1-x-yFexCoyO3-delta (STFC, x=y=0.125) with delta={0.125,0.25}, enhance the saturation magnetization, and give rise to large changes in the electric polarization |Delta P|. We present an intuitive set of rules to describe the properties of STFC, which are based on the interplay between (Co/Fe)-vO defects, magnetic cation coordination, and topological vacancy disorder. STFC structures consist of layered crystals with sheets of linearly organized O4,5,6-coordinated Fe-Co pairs, sandwiched with layers of O5-coordinated Ti. (Co/Fe)-vO defects are the source of crystal distortions, cation off-centering and bending of the oxygen octahedra which, considering the charge redistribution mediated by vO and the cations' electronegativity and valence states, triggers an effective electric polarization. Oxygen migration for delta=0.125 leads to vertical bar Delta P vertical bar|>similar to 10 mu C/cm(2) due to quantum-of-polarization differences between delta=0.125 structures. Increasing the oxygen deficiency to delta=0.25 yields vertical bar Delta P vertical bar, the O migration of which resolved polarization for delta=0.25 is >similar to 3 mu C/cm(2). Magnetism is dominated by the Fe,Co spin states for delta=0.125, and there is a contribution from Ti magnetic moments (similar to 1 mu B) for delta=0.25. Magnetic and electric order parameters change for variations of delta or oxygen migration for a given oxygen deficiency. Our results capture characteristics observed in the end members of the series SrTi(Co,Fe)O-3, and suggest the existence of a broader set of rules for oxygen-deficient multiferroic oxides.