Defect Generation and Evolution in Irradiated Epitaxial Films and Heterostructures of Fe₃O₄ and Cr₂O₃

The functionality of nuclear structural materials, sensors, and microelectronics in harsh environments such as radiation relies on understanding defect generation and evolution processes in oxide layers. The initial radiation response of epitaxial thin films of Fe₃O₄(111), Cr₂O₃(0001), and Fe₃O₄(111)/Cr₂O₃(0001) heterostructures deposited on Al₂O₃(0001) by oxygen-assisted molecular beam epitaxy and irradiated with 200 keV He⁺ is characterized. X-ray diffraction and X-ray absorption near edge spectroscopy showed that the Cr₂O₃ layers underwent significant lattice expansion and disordering under irradiation, whereas the Fe₃O₄ layers do not exhibit noticeable changes. In contrast, positron annihilation spectroscopy revealed an evolution of cation vacancy point defects in the Fe₃O₄ layers into larger vacancy clusters with increasing irradiation, while the cation vacancies in Cr₂O₃ remained primarily as single vacancies and small clusters. The results suggest that the Fe₃O₄ lattice can utilize the free volume of the larger vacancy clusters to relax but the small vacancies in the Cr₂O₃ lattice do not facilitate relaxation. Comparing defect concentrations in the single layer films versus the heterostructure suggests that point defects may cross the interface from Fe₃O₄ into Cr₂O₃. Together, these results enhance the understanding of the initial defect evolution mechanisms in oxide layers in harsh irradiation environments.