Exploring Critical Synthetic Parameters for Nanoscale ε-Fe₂O₃ and Their Influence on Magnetic Behaviors

An intermediate polymorph of iron oxide, ε-Fe₂O₃, has attracted significant attention due to its giant coercive field (H_c) and potential applications in high-frequency millimeter-wave absorption and high-density magnetic recording. However, the fabrication of ε-Fe₂O₃ with high phase purity is still a challenge due to complicated synthetic procedures and a large variety of reaction parameters. Here, we have identified critical reaction parameters to improve the phase purity of ε-Fe₂O₃, and the effects of all possible reaction parameters have been tested through systematic studies. A combination of structural and magnetic characterization techniques provides us with an accurate and reliable phase purity analysis of the ε-Fe₂O₃ phase. Specifically, we observed that (1) the reaction temperature and time and (2) the addition of Ba are critical parameters to improve the phase purity. We identified the optimal conditions that maximize the coercivity and phase purity, giving insight into the effects of each parameter on the γ- to ε- to α-phase-transition pathway. We obtained nearly single-phase ε-Fe₂O₃ (∼87 wt % with ∼13 wt % α-phase) with a large coercivity of H_c = 20.6 kOe, enabling us to obtain pure ε-Fe₂O₃ by a simple magnetic separation protocol, and tuned the H_c of the ε-Fe₂O₃ nanoparticles in the range of 4.0–20.6 kOe by controlling the reaction parameters. Furthermore, the structural properties of the resulting ε-Fe₂O₃ nanoparticles are confirmed by characterizing their chemical and magnetic properties using X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements.