Magnetic hyperthermia is an oncological therapy that exploits magnetic nanoparticles activated by radiofrequency magnetic fields to produce a controlled temperature increase in a diseased tissue. The specific loss power (SLP) of magnetic nanoparticles or the capability to release heat can be improved using surface treatments, which can reduce agglomeration effects, thus impacting on local magnetostatic interactions. In this work, Fe₃O₄ nanoparticles are synthesized via a coprecipitation reaction and fully characterized in terms of structural, morphological, dimensional, magnetic, and hyperthermia properties (under the Hergt-Dutz limit). Different types of surface coatings are tested, comparing their impact on the heating efficacy and colloidal stability, resulting that sodium citrate leads to a doubling of the SLP with a substantial improvement in dispersion and stability in solution over time; an SLP value of around 170 W/g is obtained in this case for a 100 kHz and 48 kA/m magnetic field.

The magnetic properties of various families of nanocomposite materials containing nanoparticles of transition metals or transition-metal compounds are reviewed here. The investigated magnetic nanocomposites include materials produced either by dissolving a ferrofluid containing pre-formed nanoparticles of desired composition and size in a fluid resin submitted to subsequent curing treatment, or by generating the nanoparticles during the very synthesis of the embedding matrix. Two typical examples of these production methods are polymer nanocomposites and ceramic nanocomposites. The resulting magnetic properties turn out to be markedly different in these two classes of nanomaterials. The control of nanoparticle size, distribution, and aggregation degree is easier in polymer nanocomposites, where the interparticle interactions can either be minimized or exploited to create magnetic mesostructures characterized by anisotropic magnetic properties; the ensuing applications of polymer nanocomposites as sensors and in devices for Information and Communication Technologies (ICT) are highlighted. On the other hand, ceramic nanocomposites obtained from transition-metal loaded zeolite precursors exhibit a remarkably complex magnetic behavior originating from the simultaneous presence of zerovalent transition-metal nanoparticles and transition-metal ions dissolved in the matrix; the applications of these nanocomposites in biomedicine and for pollutant remediation are briefly discussed.

Magnetite nanoparticles with a size of 5–6 nm with potential impact on biomedicine and information/communication technologies were synthesized by thermal decomposition of Fe(acac)₃ and subsequently coated with a silica shell exploiting a water-in-oil synthetic procedure. The as-produced powders (comprised of either Fe₃O₄ or Fe₃O₄@silica nanoparticles) were mixed with a photocurable resin obtaining two magnetic nanocomposites with the same nominal amount of magnetic material. The static magnetic properties of the two nanopowders and the corresponding nanocomposites were measured in the 10 K–300 K temperature range. Magnetic measurements are shown here to be able to give unambiguous information on single-particle properties such as particle size and magnetic anisotropy as well as on nanoparticle aggregation and interparticle interaction. A comparison between the size distribution functions obtained from magnetic measurements and from TEM images shows that figures estimated from properly analyzed magnetic measurements are very close to the actual values. In addition, the present analysis allows us to determine the value of the effective magnetic anisotropy and to estimate the anisotropy contribution from the surface. The Field-cooled/zero field cooled curves reveal a high degree of particle aggregation in the Fe₃O₄ nanopowder, which is partially reduced by silica coating and strongly decreased by dissolution in the host polymer. In all considered materials, the nanoparticles are magnetically interacting, the interaction strength being a function of nanoparticle environment and being the lowest in the nanocomposite containing bare, well-separate Fe₃O₄ particles. All samples behave as interacting superparamagnetic materials instead of ideal superparamagnets and follow the corresponding scaling law.

Rate equations are used to study the dynamic magnetic properties of interacting magnetite nanoparticles viewed as double well systems (DWS) subjected to a driving field in the radio-frequency range. Dipole–dipole interaction among particles is modeled by inserting an ad-hoc term in the energy barrier to simulate the dependence of the interaction on both the interparticle distance and degree of dipole collinearity. The effective magnetic power released by an assembly of interacting nanoparticles dispersed in a diamagnetic host is shown to be a complex function of nanoparticle diameter, mean particle interdistance and frequency. Dipolar interaction markedly modifies the way a host material is heated by an assembly of embedded nanoparticles in magnetic hyperthermia treatments. Nanoparticle fraction and strength of the interaction can dramatically influence the amplitude and shape of the heating curves of the host material; the heating ability of interacting nanoparticles is shown to be either improved or reduced by their concentration in the host material. A frequency-dependent cut-off length of dipolar interactions is determined and explained. Particle polydispersity entailing a distribution of particle sizes brings about non-trivial effects on the heating curves depending on the strength of dipolar interaction.