Impact of oxygen doping and oxidation state of iron on the electronic and magnetic properties of BaFeO3-delta
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We studied the structural, electronic, and magnetic properties of a cubic perovskite BaFeO3-delta (0 <= delta <= 0.5) within the density-functional theory using a generalized gradient approximation (GGA) and a GGA+ U method. According to our calculations, BaFeO3 in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with an extremely high Curie temperature (T-C) of 700-900 K. However, such an estimate of TC disagrees with all available experiments, which report that T-C of the BaFeO3 and undoped BaFeO3-delta films varies between 111 and 235 K, or, alternatively, that no ferromagnetic order was detected there. Fitting the calculated x-ray magnetic circular dichroism spectra to the experimental features seen for BaFeO3, we concluded that good agreement can be obtained when oxygen vacancies are included in our model. Thus, the relatively low T-C measured in BaFeO3 can be explained by oxygen vacancies intrinsically present in the material. Since iron species near the O vacancy change their oxidation state from 4+ to 3+, the interaction between Fe4+ and Fe3+, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing delta in BaFeO3-delta, its T-C decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of BaFeO3-delta illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above room temperature.