Modulating the Structure and Magnetic Properties of ε-FeO Nanoparticles via Electrochemical Li Insertion.

ε-FeO, a metastable phase of iron oxide, is widely known as a room-temperature multiferroic material or as a superhard magnet. Element substitution into ε-FeO 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-FeO in the Fe/Fe coexisting states, Li was electrochemically inserted into ε-FeO to reduce Fe. The discharge and charge of Li into/from ε-FeO 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-FeO 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-FeO is a useful tool for characterizing crystal structure, lithiation limit, and magnetic properties in the coexistence of Fe/Fe.