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

TL;DR

This study uses density functional theory to explore how oxygen vacancies and iron oxidation states influence the electronic and magnetic properties of BaFeO$_{3-\delta}$, revealing the role of vacancies in reducing Curie temperature and magnetic order.

Contribution

It demonstrates that oxygen vacancies explain the discrepancy between theoretical high Curie temperature and experimental lower values in BaFeO$_{3-\delta}$, highlighting the impact of vacancy-induced oxidation state changes.

Findings

Oxygen vacancies lower the Curie temperature of BaFeO$_{3-\delta}$.

Iron oxidation state shifts from 4+ to 3+ near vacancies.

Magnetic order transitions from ferromagnetic to antiferromagnetic with increasing vacancies.

Abstract

We studied structural, electronic and magnetic properties of a cubic perovskite BaFeO$_{3-\delta}$ ($0 \le \delta \le 0.5$) within the density functional theory using a generalized gradient approximation and a GGA+U method. According to our calculations, BaFeO$_3$ in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with extremely high 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 the BaFeO$_3$ and undoped BaFeO$_{3-\delta}$ 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 BaFeO$_3$, we concluded that the presence of oxygen vacancies in our model enables a good agreement. Thus, the relatively low $T_C$ measured in BaFeO$_3$ 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 Fe$^{4+}$ and Fe$^{3+}$, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing $\delta$ in BaFeO$_{3-\delta}$, its $T_C$ decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of BaFeO$_{3-\delta}$ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above the room temperature.