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

We investigate the spatial distribution of spin orientationinmagnetic nanoparticles consisting of hard and soft magnetic layers.The nanoparticles are synthesized in a core-shell sphericalmorphology where the target stoichiometry of the magnetically hard,high anisotropy layer is CoFe2O4 (CFO), whilethe synthesis protocol of the lower anisotropy material is known toproduce Fe3O4. The nanoparticles have a meandiameter of similar to 9.2-9.6 nm and are synthesized as two variants:a conventional hard/soft core-shell structure with a CFO core/FOshell (CFO@FO) and the inverted structure FO core/CFO shell (FO@CFO).High-resolution electron microscopy confirms the coherent spinel structureacross the core-shell boundary in both variants, while magnetometryindicates the nanoparticles are superparamagnetic at 300 K and developa considerable anisotropy at reduced temperatures. Low-temperature M vs H loops suggest a multistep reversal process. Smallangle neutron scattering (SANS) with full polarization analysis revealsa considerable alignment of the spins perpendicular to the field evenat fields approaching saturation. The perpendicular magnetizationis surprisingly correlated from one nanoparticle to the next, thoughthe interaction is of limited range. More significantly, the SANSdata reveal a pronounced difference in the reversal process of themagnetization parallel to the field for the two nanoparticle variants.For the CFO@FO nanoparticles, the core and shell magnetizations appearto track each other through the coercive region, while in the FO@CFOvariant, the softer Fe3O4 core reverses beforethe higher anisotropy CoFe2O4 shell, consistentwith expectations from mesoscale magnetic modeling. These resultshighlight the interplay between interfacial exchange coupling andanisotropy as a means to tune the composite properties of the nanoparticlesfor tailored applications including biomedical/theranostic uses.