Kinetics and intermediate phases in epitaxial growth of Fe₃O₄ films from deposition and thermal reduction

We have studied the kinetics of the transitions between the Fe₂O₃ and Fe₃O₄ phases as thin epilayers (∼2.5 nm) on Al₂O₃ (001) substrates using time-resolved reflection high energy electron diffraction. The different iron oxide phases were identified using a combination of in-situ and ex-situ characterizations. The transition from an α-Fe₂O₃ (001) epilayer to a Fe₃O₄ (111) epilayer through thermal reduction was found to be determined by the Fe-O bonding energy, resulting in a long time scale. The oxidation at high temperature converts a Fe₃O₄ (111) epilayer to an α-Fe₂O₃ (001) epilayer quickly; at low temperature, a γ-Fe₂O₃ (111) epilayer was slowly generated instead. By repeating the deposition/thermal reduction processes, a thicker Fe₃O₄ (111) film was obtained, which exhibit high crystallinity and moderate magnetic coercivity.