Modulating the Structure and Magnetic Properties of ε-Fe₂O₃ Nanoparticles via Electrochemical Li⁺ Insertion

ε-Fe₂O₃, a metastable phase of iron oxide, is widely known as a room-temperature multiferroic material or as a superhard magnet. Element substitution into ε-Fe₂O₃ has been reported in the literature; however, the substituted ions have a strong site preference depending on their ionic radii and valence. In this study, in order to characterize the crystal structure and magnetic properties of ε-Fe₂O₃ in the Fe²⁺/Fe³⁺ coexisting states, Li⁺ was electrochemically inserted into ε-Fe₂O₃ to reduce Fe³⁺. The discharge and charge of Li⁺ into/from ε-Fe₂O₃ revealed that Li⁺ insertion was successful. X-ray magnetic circular dichroism results indicated that the reduced Fe did not exhibit site preference. Increasing the Li⁺ content in ε-Fe₂O₃ resulted in decreased saturation magnetization and irregular variation of the coercive field. We present a comprehensive discussion of how magnetic properties are modified with increasing Li⁺ content using transmission electron microscopy images and considering the Li⁺ diffusion coefficient. The results suggest that inserting Li⁺ into crystalline ε-Fe₂O₃ is a useful tool for characterizing crystal structure, lithiation limit, and magnetic properties in the coexistence of Fe²⁺/Fe³⁺.