# Magnetic Properties of Nanocomposites
## 3 Polymer Nanocomposites
### Polymer Nanocomposites Containing Weakly Interacting Magnetic Nanoparticles  
An example of polymer NC containing a homogeneous distribution of almost monodisperse magnetite NPs (Figure 1, top left) is shown in Figure 1 (top center). The NPs have sizes distributed according to a Gaussian curve around a mean value of 5.6 nm with a root mean square (r.m.s.) value of 0.1 nm [27]. In this case, the synthesized NPs (entirely made of pure Fe\({}_{3}\)O\({}_{4}\)) were dispersed into a hyperbranched photocurable matrix (HBP) as a reference resin. TEM micrographs of the UV-cured films containing 0.7 vol.% of bare Fe\({}_{3}\)O\({}_{4}\) NPs show a uniform dispersion of the magnetic filler in the polymeric matrix (Figure 1, top center). The FC/ZFC curves of the Fe\({}_{3}\)O\({}_{4}\) nanopowder and of the corresponding NC containing 0.7 vol.% of bare Fe\({}_{3}\)O\({}_{4}\) NPs taken under a field of 200 Oe are shown in Figure 1 (bottom left). A broad maximum of the ZFC curve is observed for bare Fe\({}_{3}\)O\({}_{4}\) NPs around 60 K; above which the FC and ZFC curves closely approached each other, actually merging above about 250 K. The features would be compatible with a large average size and a broad size distribution for these NPs, which is in contrast with the evidence from TEM images [11]. On the other hand, the shape of the curves taken on the NC is notably different; the ZFC curve maximum shifts down to about 20 K and both curves become remarkably steeper. In the NC, the merging of FC and ZFC curves immediately follows the maximum of the ZFC  
Figure 1: **(Top left)** transmission electron microscopy (TEM) image of bare magnetite nanoparticles (NPs) characterized by a narrow size distribution. Reprinted from the work of [11] with permission from Elsevier; **(top center)** TEM image of a magnetic nanocomposite (NC) obtained from the bare NPs shown in Figure 1. Reprinted from the work of [11] with permission from Elsevier; **(top right)** TEM image of a polymer NC containing Fe\({}_{3}\)O\({}_{4}\)@silica NPs. Reprinted from the work of [11] with permission from Elsevier; **(bottom left)** field-cooled (FC)/zero-field-cooled (ZFC) curves measured from T = 10 K to T = 300 K in an almost monodisperse magnetite nanopowder **(top left)** and in the corresponding NC **(top center)**; **(bottom right)** temperature behavior of the coercive field of NCs containing bare magnetite NPs and Fe\({}_{3}\)O\({}_{4}\)@silica NPs. curve. The difference between nanopowder and NC is clearly related to a different aggregation state of magnetic NPs: the powder is clearly characterized by a high degree of interactions (either dipolar or contact); on the other hand, in the NC, the degree of aggregation of bare NPs has been significantly reduced by polymer mixing and stirring; this is confirmed by the TEM image (Figure 1, top center), which shows a uniform distribution of well-separated particles. It should be remarked that this material does not show any trace of the Verwey transition. As a matter of fact, this transition is a rather elusive effect in systems of magnetite nanoparticles: for instance, the recent literature indicates a strong reduction [28; 29; 30; 31] or even a complete disappearance [32; 33] of the Verwey transition temperature \(T_{V}\) in small particles. Moreover, the particle shape plays an important role in the Verwey transition, at least in ultrasmall nanoparticles [34]. As recently pointed out [35], the most important role in determining the existence of the Verwey transition is, however, played by the quality (both structural and chemical) of magnetite particles; in fact, the observation of \(T_{V}\) in nanometer-sized magnetite has recently been proven to be directly connected to the high quality of the sample [36]. In the present case, the particles are so small that the intrinsic defectiveness related to the high surface-to-volume ratio is effective in suppressing the transition.