Decoding magnetization and magnetic anisotropy in core@shell ferrite nanoparticles: interplay between cation distribution and spin disorder.

The tailoring of magnetic properties in spinel ferrite nanoparticles for theranostic applications relies heavily on the precise control of their chemical composition and crystalline structure. In this work, we investigate the complex interplay between cation distribution, spin disorder, and magnetic anisotropy in core@shell nanoparticles composed of non-stoichiometric Zn-Mn mixed ferrite cores (ZnδMnβFeεO4) protected by a maghemite (γ-Fe2O3) shell. A set of samples with varying Zn/Mn ratios was synthesized via coprecipitation and characterized by complementary techniques, including Selected Area Electron Diffraction and Neutron Powder Diffraction. Rietveld refinement of the neutron data revealed a non-equilibrium cation distribution where the occupancy of tetrahedral and octahedral sites by Zn2+, Mn2+/3+, and Fe3+ ions does not follow a monotonous trend with increasing Zinc content. We demonstrate that this specific cationic arrangement directly governs the saturation magnetization and magnetic anisotropy constants. By proposing a theoretical model based on the experimental cation distribution, we successfully predicted the saturation magnetization and magnetocrystalline anisotropy of the cores, finding excellent agreement with macroscopic magnetic measurements at 5 K. Furthermore, the discrepancy between the intrinsic core anisotropy and the effective anisotropy highlights the critical role of surface spin disorder and symmetry breaking in these confined nanostructures. These findings provide a robust pathway for decoding and tuning the magnetic performance of complex core@shell nanoarchitectures through structural design.