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

Abstract

Spinel ferrite nanoparticles (NPs) with controlled composition, size and morphology have offered a wide range of functional properties but engineering non-homogeneous 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. The enhancement in effective anisotropy is quantitatively described by modeling the time-dependent Co diffusion, enabling the estimation of cation diffusivity. These findings demonstrate a robust strategy for preparing anisotropy-graded spinel ferrite NPs, offering a broadly applicable approach to tailor the properties of complex metal oxide nanostructures.