Mg²⁺ Diffusion-Induced Structural and Property Evolution in Epitaxial Fe₃O₄ Thin Films

Epitaxial Fe₃O₄ thin films grown on single crystal MgO(001) present well-defined model systems to study fundamental multivalent ion diffusion and associated phase transition processes in transition-metal-oxide-based cathodes. In this work, we show at an atomic scale the Mg²⁺ diffusion pathways, kinetics, and reaction products at the Fe₃O₄/MgO heterostructures under different oxygen partial pressures but with the same thermal annealing conditions. Combining microscopic, optical, and spectroscopic techniques, we demonstrate that an oxygen-rich environment promotes facile Mg²⁺ incorporation into the Fe²⁺ sites, leading to the formation of Mg_1-Fe₂₊O₄ spinel structures, where the corresponding portion of the Fe²⁺ ions are oxidized to Fe³⁺. Conversely, annealing in vacuum results in the formation of a thin interfacial rocksalt layer (Mg_1-FeO), which serves as a blocking layer leading to significantly reduced Mg²⁺ diffusion to the bulk Fe₃O₄. The observed changes in transport and optical properties as a result of Mg diffusion are interpreted in light of the electronic structures determined by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Our results reveal the critical role of available anions in governing cation diffusion in the spinel structures and the need to prevent formation of unwanted reaction intermediates for the promotion of facile cation diffusion.