Engineering Graded Magnetic Anisotropy via Cation Interdiffusion in Core/Shell Nanoparticles

Spinel ferrite nanoparticles (NPs) with controlled composition, size, and morphology have offered a wide range of functional properties, but engineering nonhomogeneous composition profiles remains elusive. Here, we use Fe₃O₄/CoFe₂O₄ core/shell NPs as precursors to prepare compositionally graded NPs via controlled interfacial diffusion of metal cations. Electron-microscopy-based elemental mapping reveals that thermal annealing above 200 °C in an oxygen-rich atmosphere transforms the initially sharp core/shell interface into a compositionally graded spinel structure with a Co-rich outer layer. This cation redistribution results in pronounced changes in the magnetic properties, including a remarkable increase in coercivity and high-field susceptibility. Enhancement in the effective anisotropy is quantitatively described by modeling the time-dependent Co diffusion, enabling estimation of the cation diffusivity. These findings demonstrate a robust strategy for preparing anisotropy-graded spinel ferrite NPs, offering a broadly applicable approach to tailoring the properties of complex metal oxide nanostructures.