Beyond the Effect of Particle Size: Influence of CoFe₂O₄ Nanoparticle Arrangements on Magnetic Properties

Abstract

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.

Colloidal magnetic nanocrystal clusters: variable length-scale interaction mechanisms, synergetic functionalities and technological advantages

“…In addition, a minimal exchange bias field has been measured in maghemite nanoclusters stabilized with polyacrylic acid [21]. Contrary to that, an exchange bias associated with a possible surface spin structure of clusters where amine [31] or oleic acid [115] were the capping agents could not be resolved.…”

“…The blocking temperatures were found almost equal for the two samples and larger than that for the more organized superparamagnetic 2D assembly ( Figure 12A, D), indicating the existence of dipolar interactions in the former two cases. Furthermore, a reduced magnetic anisotropy has been claimed in the case of the 3D nanoclusters in which the incorporated nanocrystals were crystallographically oriented ( Figure 12C, F) compared to those of randomly oriented nanocrystals in the randomly packed 2D particle assembly ( Figure 12B, E) [115].…”

“…In order to evaluate the strength of dipolar interactions, Peddis et al studied the magnetic behavior of ensembles made of multiple cobalt-ferrite nanocrystals arranged in different topological manners [115]. The nanocrystals assumed to be organized in 2D superlattices (of two different nanocrystal concentrations) ( Figure 12A, B, D, E) or self-assembled in 3D nanoclusters ( Figure 12C, F).…”

“…Additionally, these spectra showed narrower lines with higher hyperfine magnetic fields compared to the corresponding values measured for their individual superparamagnetic counterparts (~13 nm) ( Figure 13D). The possible emergence of exchange interactions among the surface spins of neighboring nanocrystals in CoFe 2 O 4 clusters have been investigated by the analysis of the remanent magnetization curves, measured by isothermal remanent magnetization (IRM) and direct current demagnetization (DCD) protocols [115]. The two types of magnetization are related according to the Wohlfarth equation:…”

Assembly-mediated interplay of dipolar interactions and surface spin disorder in colloidal maghemite nanoclusters

“…strong magnetic anisotropy and high magnetization) as it has been reported for 40-60 nm CoFe 2 O 4 nanocrystal aggregates. 55 …”