Phase stability and structural evolution of core/shell iron oxide nanoparticles due to oxidative diffusion: Implications for spintronic applications

Core/shell iron oxide nanoparticles are promising candidates for spintronic applications due to their tunable magnetic properties and interfacial exchange interactions that allow the modulation of spin-polarized conduction. However, their performance depends critically on phase stability and structural integrity under fabrication and operating conditions involving thermal and oxidative diffusion. This study examines the transformation of iron oxide nanoparticles induced by oxidative diffusion in thermal annealing from wüstite to hematite through an intermediate (wüstite)-core/(magnetite–maghemite)-shell structure. The nanoparticles exhibit a rounded cubic morphology, with a distorted C2/m FeO phase at the core under compressive strain along (010), while the Fe₃O₄ shell exhibits tensile strain along (110). Oxygen diffusion occurs preferentially along [100] from the cube faces, influencing shape evolution. The system exhibits an exchange bias field of up to 3 kOe and enhanced magnetic hardening of up to 4 kOe, attributed to interfacial exchange interactions. Higher annealing temperature promotes the formation of γ-Fe₂O₃ with ordered vacancies. The exchange bias effect persists, even when the FeO core is smaller than 1 nm in size, indicating that the strain stabilizes the antiferromagnetic (AFM) order and enhances core/shell magnetic coupling. As oxidation proceeds, strain is gradually relaxed, and at 873 K, the oxidation to hematite is promoted, characterized by a Morin transition at 245 K. These findings reveal the intricate relationship between oxidation-driven structural evolution and magnetic behavior in engineered nanoparticle systems, underscoring the critical importance of material selection tailored to specific fabrication processes, operating conditions, and device performance requirements.