Investigation of the Chemical Structure of Core–Shell Fe₃O₄@Ni1-xCoxFe₂O₄ Nanoparticles and Its Influence on Their Magnetic Properties

Abstract

Core-shell ferrite nanoparticles offer a promising route toward high-performance, rare-earth-free magnetic nanomaterials, yet fine control over interfacial exchange coupling remains a critical challenge. Here, we report the synthesis of two series of Fe₃O₄@MFe₂O₄ (M = Co, Ni, or mixed Co/Ni) nanoparticles via a two-step seed-mediated thermal decomposition process in organic media, enabling systematic tuning of the shell composition (first series) and size of the Fe₃O₄ seed nanoparticles (second series). High-resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) analyses confirm the formation of an epitaxial spinel shell, with excellent crystallographic continuity and homogeneous spatial cation distribution. X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) reveal that Co²⁺ and Ni²⁺ cations preferentially occupy octahedral sites while a minor -but significant-Ni₁₋ₓCoₓO wüstite phase forms at the periphery, as also suggested by microscopy, introducing small exchange-bias effects. Alongside the exchange bias effect, a comprehensive magnetic characterization using SQUID magnetometry revealed the role of both the shell composition and the surface anisotropy, the latter being dominant over the shell composition when the core nanoparticle size was reduced. This multi-technique study unveils the intricate interplay between composition, interface structure, and magnetic behavior, providing a rational framework for designing chemically engineered, anisotropy-tailored magnetic nanostructures for spintronic, biomedical, and high-density data storage applications.