Interfacial Atomic and Electronic Structures of Fe₃O₄ with MgO(111), SrTiO₃(111), or Al₂O₃(0001)

The functional performance of thin-film materials is fundamentally influenced by interfacial phenomena, which regulate charge transfer, lattice strain relaxation, and defect formation. In this study, Fe₃O₄ thin films were epitaxially deposited on MgO(111), SrTiO₃(111), and Al₂O₃(0001) substrates. Utilizing a combination of scanning transmission electron microscopy (STEM) and first-principles calculations, we elucidated the atomic structures, electronic structures, and changes in the magnetic properties at the interfaces. Our findings reveal that the interfacial characteristics of Fe₃O₄ are strongly dependent on the nature of the termination layer. Specifically, on MgO substrates, oxygen-terminated epitaxial Fe₃O₄ preserves its half-metallic behavior at the interface with the coordination environment of Fe ions remaining unaltered. Conversely, on Al₂O₃ substrates, oxygen-terminated Fe₃O₄ displays an almost closed band gap attributed to significant lattice mismatch, yet it maintains a half-metallicity. This observation indicates that strain can modulate the band gap without compromising the half-metallic nature of Fe₃O₄, which is also linked to the octahedral coordination of Fe ions at the interface. In contrast, Fe₃O₄ films grown on SrTiO₃ substrates exhibit Fe-terminated interfaces, where Fe_B ions transition from octahedral to trigonal prismatic coordination. The change in the crystal field modifies the electronic structure, thereby weakening the local magnetism of Fe₃O₄ and leading to metallic behavior. The stabilization of these high-energy Fe-ion states may be facilitated by compensation through a periodic dislocation network. Therefore, controlling the interfacial strain or changing the coordination environment of Fe ions at the interface is essential for tailoring the magnetoelectric properties of Fe₃O₄.