# Magnetic Properties of Nanocomposites
## 3 Polymer Nanocomposites  
Magnetic NPs dispersed within polymer matrices can be exploited for a number of applications, thanks to the interesting properties that result from the combination of inorganic components and polymers. These NCs exhibit interesting magnetic [11; 18; 19; 20] properties for use in micro- and nanoelectronics. On the other hand, significant contributions to fundamental studies in nanoscale physics have been made by studying these materials, including quantum tunneling of magnetization, spin-reversal mechanism in single-domain particles, and quantum size effects [21; 22; 23]. Polymer NCs can be prepared by curing of a ferrofluid obtained by mixing a dispersion of magnetic NPs in a resin, which is polymerized by thermal or ultraviolet-light treatment, forming a solid film. Magnetite is a cheap material with a lower degree of toxicity when compared with other metallic ferromagnets, and it can be easily synthesized through a variety of low-cost techniques, such as the thermochemical route and the non-hydrolytic sol-gel process [24; 25]. Thermal/UV curing are also cheap techniques (in particular, UV curing is presently appealing for use with coatings) [26]. In this process, UV light induces the polymer formation, allowing one to obtain a fast transformation of the liquid monomer into the solid film with tailored physico-chemical and mechanical properties. It is an environmental friendly technique because it is a solvent free process. The substrate does not need to be heated as in the traditional thermal curing, therefore it guarantees energy saving. The magnetic response of a polymer NC is strongly affected by the properties of the embedded NPs (density in the matrix, size distribution degree of dispersion, aggregation) [26]. Data analysis is much easier in magnetic nanocomposites containing particles uniform in size. Such a condition would be beneficial to many practical applications of NP systems too. However, the existing preparation techniques are not able to produce strictly monodisperse nanoparticle systems; as a consequence, real particle systems are characterized by a particle size distribution \(p(D)\), which is typically estimated by measuring the particle size \(D\) from TEM images of different parts of the material; the \(p(D)\) distribution in granular systems often follows either a gaussian or a log-normal distribution function. For each particle size distribution, there is a corresponding distribution of particle volumes \(p(V)\), leading to a range of particle moments and of blocking temperatures (both quantities being proportional to volume \(V\)). The latter effect may bring about a difficulty in interpreting experimental data because superparamagnetic and blocked particles may be simultaneously present in the same material at a given temperature. Typical values of static magnetic quantities measured in polymer nanocomposites are reported in Table 1 (top). It should be noted that in polymer nanocomposites, particle aggregation effects are particularly apparent only in the case of low magnetite content; in fact, a better dispersion of magnetite nanoparticles is typically observed in materials with a higher filler content. Such counterintuitive behavior has been explained by invoking the formation of a particle surface with a relatively low surface energy and with better compatibility with the surrounding matrix. As a matter of fact, the magnetite content of a nanocomposite is typically increased by synthesizing the nanoparticles with an increasingly higher precursor concentration, keeping constant the amount of reactive solvent. The higher amount of precursor results in a change of the reaction rate and in the variation of nanoparticle growth, in turn affecting particle size, shape, and surface. In particular, the increased amount of precursor produces a higher quantity of carboxylate species bound to the nanoparticle surface [13],