Interparticle Interactions and Magnetic Anisotropy in Cobalt Ferrite Nanoparticles: Influence of Molecular Coating

Molecular coating of nanoparticles represents probably the most important and, at the same time, critical step to design new nanostructured magnetic materials. The interaction between molecules and surface atoms leads to a strong modification of surface magnetic properties, that are one of the key points in the physics of magnetic nanoparticles. In this paper the magnetic properties of CoFe₂O₄ nanoparticles (⟨D⟩ ≅ 4–8 nm) coated with oleic acid have been investigated in order to clarify the role of the molecular coating on the interparticle interactions and surface anisotropy. An increase of magnetic anisotropy (i.e., coercive field and anisotropy constant) with particle size is observed in coated nanoparticles, indicating that the magnetic anisotropy is governed mainly by its magneto-crystalline component. The removal of molecular coating induces a strong increase of anisotropy, because of the increase of its surface component, as indicated by the increase of exchange bias field

Morpho-Structural and Magnetic Properties of CoFe₂O₄/SiO₂ Nanocomposites: The Effect of the Molecular Coating

The use of magnetic nanoarchitecture in several applications is often limited by the lack of noninteracting particles, due to the frequent presence of clusters and aggregates of particles. Here, we report an investigation of the interparticle interactions by changing the molecular coating on ∼5 nm CoFe2O4 nanoparticles embedded in a silica structure. The magnetic investigation at a low temperature allows revealing the key role of organic ligands in tuning the morpho-structural properties of hybrid materials. Cobalt ferrite-coated nanoparticles were prepared by the polyol method using triethylene glycol as a co-reagent (CFOT) and by the exchange ligand process using dihydroxyhydrocinnamic acid (CFOH). Then, magnetic mesoporous silica nanocomposites have been prepared starting from CFOT (CFOTS) and CFOH (CFOHS). For the CFOTS sample, the interparticle distance did not change after coating, whereas the CFOHS sample showed an increase in the interparticle distance by 23%. This value has been obtained by investigating interparticle interactions by remanence techniques, which represent a good approach to determine the approximated values of interparticle distances in complex systems. The measurements showed that the silica coating produces a reduction of 47% in the dipolar interaction strength for the CFOHS sample, whereas no significant change was observed for the CFOTS sample. The differences in magnetic response upon varying the molecular coating of nanoparticles are due to the different interactions of the molecular ligands with silica, resulting in a change of interparticle distances and then magnetic interactions

Exchange Bias in Fe@Cr Core-Shell Nanoparticles

We have used X-ray magnetic circular dichroism and magnetometry to study isolated Fe@Cr core−shell nanoparticles with an Fe core diameter of 2.7 nm (850 atoms) and a Cr shell thickness varying between 1 and 2 monolayers. The addition of Cr shells significantly reduces the spin moment but does not change the orbital moment. At least two Cr atomic layers are required to stabilize a ferromagnetic/antiferromagnetic interface and generate the associated exchange bias and increase in coercivity