Investigation of the chemical structure of core–shell Fe$_3$O$_4$@Ni$_1$−$_x$Co$_x$Fe$_ 2$O$_4$ nanoparticles and its influence on their magnetic properties

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 Fe$_3$O$_4$@Ni$_1$−$_x$Co$_x$Fe$_ 2$O$_4$ nanoparticles via a two-step seed-mediated thermal decomposition process in organic media, enabling systematic tuning of the shell composition and size of the pristine Fe$_3$O$_ 4$ nanoparticles. 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 Co$^{2+}$ and Ni$^{2+}$ cations preferentially occupy octahedral sites while a minor -but significant-Ni$_{1−x}$Co$_x$O 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. ... mehrThis multitechnique study unveils the formation mechanism of such core–shell bimagnetic nanoparticles and the intricate interplay between composition, interface structure, and magnetic behavior. It provides a potential framework for designing chemically engineered, anisotropy-tailored magnetic nanostructures for spintronic, biomedical, and high-density data storage applications.