Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

We investigate the spatial distribution of spin orientation in magnetic nanoparticles consisting of hard and soft magnetic layers. The nanoparticles are synthesized in a core–shell spherical morphology where the target stoichiometry of the magnetically hard, high anisotropy layer is CoFe 2 O 4 (CFO), while the synthesis protocol of the lower anisotropy material is known to produce Fe 3 O 4 . The nanoparticles have a mean diameter of ∼9.2–9.6 nm and are synthesized as two variants: a conventional hard/soft core–shell structure with a CFO core/FO shell (CFO@FO) and the inverted structure FO core/CFO shell (FO@CFO). High-resolution electron microscopy confirms the coherent spinel structure across the core–shell boundary in both variants, while magnetometry indicates the nanoparticles are superparamagnetic at 300 K and develop a considerable anisotropy at reduced temperatures. Low-temperature M vs H loops suggest a multistep reversal process. Small angle neutron scattering (SANS) with full polarization analysis reveals a considerable alignment of the spins perpendicular to the field even at fields approaching saturation. The perpendicular magnetization is surprisingly correlated from one nanoparticle to the next, though the interaction is of limited range. More significantly, the SANS data reveal a pronounced difference in the reversal process of the magnetization parallel to the field for the two nanoparticle variants. For the CFO@FO nanoparticles, the core and shell magnetizations appear to track each other through the coercive region, while in the FO@CFO variant, the softer Fe 3 O 4 core reverses before the higher anisotropy CoFe 2 O 4 shell, consistent with expectations from mesoscale magnetic modeling. These results highlight the interplay between interfacial exchange coupling and anisotropy as a means to tune the composite properties of the nanoparticles for tailored applications including biomedical/theranostic uses.