First-principles insights on the magnetism of cubic SrTi1−<i>x</i>Co<i>x</i>O3−<i>δ</i>

We present hybrid density functional calculations suggesting that magnetism in cubic SrTi1−xCoxO3−δ (STCO) with x = 0.25 is sensitive to the nearest neighbor arrangement of the Co and the presence of oxygen vacancies. Spin polarized calculations for x = 0.25 in which the nearest neighbor (nn) Co spacing is a, 2a or 3a with a the lattice parameter predict lowest energies for the 2a nn separation and favor the ferromagnetic state. Oxygen deficiency (δ=0.125) lowers the average Co valence state and favors mixed valence and spin states (high spin for the Co adjacent to the vacancy and low for the non-adjacent Co), an increase of the band gap and an expansion of the lattice parameter compared to stoichiometric STCO in which both Co ions are low spin. Predicted configurations of the two neighboring Co ions are (t2g5eg0, t2g5eg0) and (t2g4eg2, t2g6eg0) with average 1.0 and 1.6 μB/Co for stoichiometric and 1-O-vacancy systems, respectively.