Interfacial Atomic and Electronic Structures of Fe 3 O 4 with MgO(111), SrTiO 3 (111), or Al 2 O 3 (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 FeB 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.