The design of novel multifunctional materials based on nanoparticles requires tuning of their magnetic properties, which are strongly dependent on the surface structure. The organic coating represents a unique tool to significantly modify the surface structure trough the bonds between the ligands of the organic molecule and the surface metal atoms. This work presents a critical overview of the effects of the organic coating on the magnetic properties of nanoparticles trough a selection of papers focused on different approaches to control the surface structure and the morphology of nanoparticles’ assemblies.

The effect of the annealing temperature T_ann on the magnetic properties of cobalt ferrite nanoparticles embedded in an amorphous silica matrix (CoFe₂O₄/SiO₂), synthesized by a sol-gel auto-combustion method, was investigated by magnetization and AC susceptibility measurements. For samples with 15% w/w nanoparticle concentration, the particle size increases from ~2.5 to ~7 nm, increasing T_ann from 700 to 900 °C. The effective magnetic anisotropy constant (K_eff) increases with decreasing T_ann, due to the increase in the surface contribution. For a 5% w/w sample annealed at 900 °C, K_eff is much larger (1.7 × 10⁶ J/m³) than that of the 15% w/w sample (7.5 × 10⁵ J/m³) annealed at 700 °C and showing comparable particle size. This indicates that the effect of the annealing temperature on the anisotropy is not only the control of the particle size but also on the core structure (i.e., cation distribution between the two spinel sublattices and degree of spin canting), strongly affecting the magnetocrystalline anisotropy. The results provide evidence that the magnetic anisotropy comes from a complex balance between core and surface contributions that can be controlled by thermal treatments.

In this work, we investigate the flexibility of the high temperature thermal decomposition (HTD) synthesis to design magnetic nanoparticles with engineered properties. We prepared spinel iron oxide particles with desired composition ( ${\rm{Fe_{3}O_{4}}}$ and ${\rm{CoFe_{2}O_{4}}}$ ) and well-controlled average size (5-8 nm) by tuning the synthesis procedure. We show that the substitution of Fe by Co produces a dramatic increment of the magnetocrystalline anisotropy: for 5-nm particles, the anisotropy constant increased from ${\rm{\sim 3.9\cdot 10^{4}\ to \sim 7.5\cdot 105\ J/m^{3}}}$ . The magnetocrystalline anisotropy has shown to be the main player in the magnetic behavior of high crystalline particles, except for the smallest iron oxide sample, where the surface anisotropy plays a dominant role. Finally, we observed that the presence of surfactant (oleic acid) on the nanoparticles prevents direct exchange coupling among them while the dipolar interaction was predominated with an estimated strength in the range of 4—60 K.

Today, public health is one of the most important challenges in society. Cancer is the leading cause of death, so early diagnosis and localized treatments that minimize side effects are a priority. Magnetic nanoparticles have shown great potential as magnetic resonance imaging contrast agents, detection tags for in vitro biosensing, and mediators of heating in magnetic hyperthermia. One of the critical characteristics of nanoparticles to adjust to the biomedical needs of each application is their polymeric coating. Fatty acid coatings are known to contribute to colloidal stability and good surface crystalline quality. While monolayer coatings make the particles hydrophobic, a fatty acid double-layer renders them hydrophilic, and therefore suitable for use in body fluids. In addition, they provide the particles with functional chemical groups that allow their bioconjugation. This work analyzes three types of self-assembled bilayer fatty acid coatings of superparamagnetic iron oxide nanoparticles: oleic, lauric, and myristic acids. We characterize the particles magnetically and structurally and study their potential for resonance imaging, magnetic hyperthermia, and labeling for biosensing in lateral flow immunoassays. We found that the myristic acid sample reported a large $r_2$ relaxivity, superior to existing iron-based commercial agents. For magnetic hyperthermia, a significant specific absorption rate value was obtained for the oleic sample. Finally, the lauric acid sample showed promising results for nanolabeling.

We have studied the magnetic properties of a sample obtained by high-energy mechanical milling from a ferromagnetic FeSiB amorphous ribbon. The milled material mainly consists of a Fe-based amorphous matrix embedding a minor fraction of α-Fe nanocrystallites (∼23%), and magnetization dynamics effects characterize the magnetic behavior. In particular, a magnetic transition occurs at T ∼ 50 K, from a low temperature disordered collective frozen state, similar to a spin-cluster-glass, to a high temperature regime where ferromagnetism predominates. The phenomenon has been ultimately ascribed to the local modification of the interatomic distance distribution in the amorphous matrix, induced by milling.