Beyond the Effect of Particle Size: Influence of CoFe 2 O 4 Nanoparticle Arrangements on Magnetic Properties

This paper focuses on the magnetic properties of CoFe 2 O 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 2 O 4 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 (ACoFe HTD ). The DM method results to be appropriate to prepare either irregular arrangements (IACoFe DM ) or spherical iso-oriented nanoporous assemblies (SACoFe DM ) of primary CoFe 2 O 4 nanocrystals. Despite the same particle size, magnetization measurements of the HTD sample show a tendency toward cubic anisotropy ( M r / M s ≈ 0.7), while in DM samples, a uniaxial anisotropy ( M r / M s ≈ 0.4) is observed. The comparison between IACoFe DM and SACoFe DM samples indicates that the ordering of nanocrystals at the mesoscopic scale induces an increase of the coercive field (μ 0 H c ≈ 1.17 T → μ 0 H c ≈ 1.45 T) and of the reduced remanent magnetization ( M r / M s ≈ 0.4 → M r / 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.