On the Influence of Chemical Heterogeneity and Shape Anisotropy on Magnetic Behavior in MnFe ₂ O ₄ Nanocubes

Manganese ferrite nanocubes are attracting increasing interest due to their potential applications in nanomedicine, particularly in magnetic resonance imaging. However, it has been observed that these nanocubes can spontaneously adopt a core–shell structure. The influence of shape anisotropy and chemical heterogeneity on the magnetic behavior remains poorly understood. In this study, we use high‐spatial resolution electron energy loss spectroscopy to characterize individual nanocubes to shed light on this influence. High‐angle annular dark‐field scanning transmission electron microscopy clearly reveals the perfect epitaxial relationship between the core and shell phases. Quantification of the chemical composition indicates a core phase close to Mn _0.75 Fe _2.25 O ₄ , while the shell is close to Mn _1.50 Fe _1.50 O ₄ . Regarding oxidation states, manganese is primarily present as Mn ²⁺ in tetrahedral sites, while iron exhibits a predominant contribution of Fe ³⁺ in octahedral coordination. The core–shell structure likely results from differential thermal decomposition and phase separation driven by the minimization of the total free energy due to shape anisotropy. This chemical heterogeneity directly influences the magnetic properties. Estimates of the magnetic moment, as deduced from the empirical ratio of the white lines I(L ₃ )/I(L ₂ ), show a significant reduction, from 4.04 µB per formula unit in the iron‐rich core to only 0.83 µB in the manganese‐rich shell. This weaker magnetism in the shell is attributed to lattice distortions due to epitaxial strain and cation substitution effects that disrupt spin alignment and reduce the overall magnetic response. This work highlights the importance of nanoscale chemical and magnetic mapping for the development of ferrites. By correlating the structure, composition, and magnetic properties at the single‐nanocube level, this approach allows for the tailoring of MnFe ₂ O ₄ nanocubes for high‐performance biomedical applications. The combination of atomic‐scale characterization and functional testing could thus make MnFe ₂ O ₄ nanocubes a versatile class of nanomaterials for nanomedicine.