Ferromagnetic Epitaxial μ-Fe₂O₃ on β-Ga₂O₃: A New Monoclinic Form of Fe₂O₃

Here we demonstrate a new monoclinic iron oxide phase (μ-Fe₂O₃), epitaxially stabilized by growth on (010) β-Ga₂O₃. Density functional theory (DFT) calculations find that the lattice parameters of freestanding μ-Fe₂O₃ are within ∼1% of those of β-Ga₂O₃, and that its energy of formation is comparable to that of naturally abundant Fe₂O₃ polytypes. A superlattice of μ-Fe₂O₃/β-Ga₂O₃ is grown by plasma assisted molecular beam epitaxy, with resulting high-resolution X-ray diffraction (XRD) measurements indicating that the μ-Fe₂O₃ layers are lattice-matched to the substrate. The measured out-of-plane (b) lattice parameter of 3.12 ± 0.4 Å is in agreement with the predicted lattice constants and atomic-resolution scanning transmission electron microscopy (STEM) images confirm complete registry of the μ-Fe₂O₃ layers with β-Ga₂O₃. Finally, DFT modeling predicts that bulk μ-Fe₂O₃ is antiferromagnetic, while the interface region between μ-Fe₂O₃ and β-Ga₂O₃ leads to ferromagnetic coupling between interface Fe³⁺ cations selectively occupying tetrahedral positions. Magnetic hysteresis persisting to room temperature is observed via SQUID measurements, consistent with the computationally predicted interface magnetism.