Beyond the effect of particle size: influence of CoFe_2O_4 nanoparticle arrangements on magnetic properties

This paper focuses on the magnetic properties of CoFe2O 4 nanoparticles, discussing the influence of nanoparticles arrangements obtained by different synthesis methods. Using high thermal decomposition (HTD) and direct micellar (DM) routes, three samples of CoFe 2O4 nanoparticles with equal primary particle size (∼5 nm) were prepared. The HTD method allows one to obtain highly crystalline primary nanoparticles coated by oleic acid organized in a self-assembling arrangement (ACoFeHTD). The DM method results to be appropriate to prepare either irregular arrangements (IACoFeDM) or spherical iso-oriented nanoporous assemblies (SACoFeDM) of primary CoFe 2O4 nanocrystals. Despite the same particle size, magnetization measurements of the HTD sample show a tendency toward cubic anisotropy (Mr/Ms ≈ 0.7), while in DM samples, a uniaxial anisotropy (Mr/Ms ≈ 0.4) is observed. The comparison between IACoFeDM and SACoFeDM samples indicates that the ordering of nanocrystals at the mesoscopic scale induces an increase of the coercive field (μ0Hc ≈ 1.17 T → μ0Hc ≈ 1.45 T) and of the reduced remanent magnetization (Mr/Ms ≈ 0.4 → Mr/M s ≈ 0.5). The reason for these differences is discussed. In particular, a detailed study on interparticle interactions is carried out, highlighting the influence of the molecular coating and the formation of spherical iso-oriented assemblies