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We studied structural, electronic and magnetic properties of a cubicperovskite BaFeO$_{3-\delta}$ ($0 \le \delta \le 0.5$) within the densityfunctional theory using a generalized gradient approximation and a GGA+Umethod. According to our calculations, BaFeO$_3$ in its stoichiometric cubicstructure should be half-metallic and strongly ferromagnetic, with extremelyhigh Curie temperature ($T_C$) of 700 - 900 K. However, a such estimate of$T_C$ disagrees with all available experiments, which report that $T_C$ of theBaFeO$_3$ and undoped BaFeO$_{3-\delta}$ films varies between 111 K and 235 Kor, alternatively, that no ferromagnetic order was detected there. Fitting thecalculated x-ray magnetic circular dichroism spectra to the experimentalfeatures seen for BaFeO$_3$, we concluded that the presence of oxygen vacanciesin our model enables a good agreement. Thus, the relatively low $T_C$ measuredin BaFeO$_3$ can be explained by oxygen vacancies intrinsically presented inthe material. Since iron species near the O vacancy change their oxidationstate from $4+$ to $3+$, the interaction between Fe$^{4+}$ and Fe$^{3+}$, whichis antiferromagnetic, weakens the effective magnetic interaction in the system,which is predominantly ferromagnetic. With increasing $\delta$ inBaFeO$_{3-\delta}$, its $T_C$ decreases down to the critical value when themagnetic order becomes antiferromagnetic. Our calculations of the electronicstructure of BaFeO$_{3-\delta}$ illustrate how the ferromagnetism originatesand also how one can keep this cubic perovskite robustly ferromagnetic farabove the room temperature.

Impact of oxygen doping and oxidation state of iron on the electronic and magnetic properties of BaFeO$_{3-\delta}