Impact of oxygen doping and oxidation state of iron on the electronic and magnetic properties of BaFeO₃₋δ

الأصل · EN

We studied structural, electronic and magnetic properties of a cubic perovskite BaFeO₃₋δ (0 ≤ δ≤ 0.5) within the density functional theory using a generalized gradient approximation and a GGA+U method. According to our calculations, BaFeO₃ in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with extremely high Curie temperature (TC) of 700 - 900 K. However, a such estimate of TC disagrees with all available experiments, which report that TC of the BaFeO₃ and undoped BaFeO₃₋δ films varies between 111 K 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 the presence of oxygen vacancies in our model enables a good agreement. Thus, the relatively low TC measured in BaFeO₃ can be explained by oxygen vacancies intrinsically presented 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 δ in BaFeO₃₋δ, its TC decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of BaFeO₃₋δ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above the room temperature.