Ab Initio Study of the β-Fe₂O₃ Phase

We present first-principles results on the electronic and magnetic properties of the cubic bulk β-phase of Fe2O3. Given that all Fe-Fe magnetic couplings are expected to be antiferromagnetic within this high-symmetry crystal structure, the system may exhibit some signature of magnetic frustration, making it challenging to identify its magnetic ground state. We have analyzed the possible magnetic phases of the β-phase, among which there are ferrimagnets, altermagnets, and Kramers antiferromagnets. While the α-phase is an altermagnet and the γ-phase is a ferrimagnet, we conclude that the magnetic ground state for the bulk β-phase of Fe2O3 is a Kramers antiferromagnet. Moreover, we find that close in energy, there is a bulk d-wave altermagnetic phase. We report the density of states and the evolution band gap as a function of the electronic correlations. For suitable values of the Coulomb repulsion, the system is a charge-transfer insulator with an indirect band gap of 1.5 eV. More in detail, the unit cell of the β-phase is composed of 8Fea atoms and 24Feb atoms. The 8Fea atoms lie on the corner of a cube, and their magnetic ground state is a G-type. This structural phase is composed of zig-zag chains Fea-Feb-Fea-Feb with spin configuration ↑-↑-↓-↓ along the 3 directions such that for every Fea atoms there are 3Feb atoms. As the opposite to the γ-phase, the magnetic configuration between the first neighbor of the same kind is always antiferromagnetic while the magnetic configuration between Fea and Feb is ferro or antiferro. In this magnetic arrangement, first-neighbor interactions cancel out in the mean-field estimation of the Néel temperature, leaving second-neighbor magnetic exchanges as the primary contributors, resulting in a Néel temperature lower than that of other phases. Our work paves the way toward the ab initio study of nanoparticles and alloys for the β-phase of Fe2O3.