Valence-change-driven reduction of antiphase boundaries in spinel ferrite epitaxial films

Abstract

Antiphase boundaries (APBs) formed in thin films sometimes cause severe degradation of their physical properties. In particular, a high density of APBs in spinel ferrite films generates a non-negligible magnetic dead layer near the interface. In this study, we examined the effect of post-oxidation annealing in an oxygen plasma atmosphere on Co$_{0.125}$Fe$_{2.875}$O$_4$(001) thin films grown on MgO(001) as a model system. The thickness of the magnetic dead layer was found to be significantly reduced after post-oxidation, resulting in an increase in the saturation magnetization and an improved squareness ratio. Dark-field transmission electron microscopy analysis revealed that the post-oxidation process increased the antiphase domain size, indicating a substantial reduction in APB density. Furthermore, reflection high-energy electron diffraction and x-ray diffraction measurements confirmed that the spinel crystal structure and epitaxial strain were preserved after post-oxidation. These results suggest that post-oxidation proceeds through a topotactic solid-state reaction in which Fe$^{2.5+}$ ions are oxidized to Fe$^{3+}$, accompanied by cation rearrangement across APBs, thereby reducing APB density without degrading crystallinity and leading to improved magnetic properties in spinel ferrite epitaxial films.