# Magnetic Properties of Nanocomposites
## 3 Polymer Nanocomposites
### Anisotropic Magnetic Structures in Polymer Nanocomposites  
NCs containing a fine, even dispersion of magnetic NPs, such as the ones discussed so far, are intrinsically isotropic. On the contrary, NCs exhibiting remarkable anisotropic magnetic properties can be obtained by means of magnetic field-assisted curing of precursor resins. Anisotropic magnetic properties are expedient, for example, in the realization of direction-sensitive magnetic sensors and devices [41; 42; 43; 44]. A typical example consists of core-shell particles of titania (TiO\({}_{2}\)) encompassing a large number of aggregate magnetite NPs, which are first dispersed in an epoxy monomer; subsequent curing under a strong inhomogeneous magnetic field produces stable chains constituted of magnetite-charged titania particles (Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\)) formed in the matrix by field-assisted self-assembly, and is responsible for a distinctly anisotropic magnetic behavior. The TEM images of bare Fe\({}_{3}\)O\({}_{4}\) NPs and of a typical titania particle containing a number of magnetite NPs are given in Figure 4 (top panels). In this case, the bare NPs have mean diameter of 19.2 nm with standard deviation of 3.3 nm (Figure 4, top left); a typical particle of magnetite-charged titania contains large aggregates of Fe\({}_{3}\)O  
Figure 4: TEM images of bare magnetite NPs (**top left**) and Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) NPs (**top right**) used to prepare anisotropic NCs. Reprinted from the work of [41] with permission from Elsevier; (**bottom left**) sketch of the setup used to produce anisotropic magnetic NCs; (**bottom right**) micrometer-sized chains, which have been aligned along the direction of magnetic flux lines. Reprinted in modified form from the work of [41] with permission from Elsevier. NPs (tens to hundreds of NPs); these Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) particles are characterized by highly irregular shapes (Figure 4, top right) and by a wide distribution of sizes. NC slabs containing Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) particles obtained by applying a suitable magnetic field gradient show the formation of high-aspect ratio structure (micrometer-size chains aligned along the direction of the magnetic flux lines and along the short axis of the slab); here, we focus on a 0.5-mm thick sample kept adjacent to the magnet during the curing process (sample M; Figure 4, bottom left). The exploited geometry ensures that the magnetic force on NPs is high throughout the slab. This NC displays a high chain length and density (Figure 4, bottom right). In particular, the average chain length is estimated to be 70 \(\upmu\)m.  
According to a simple model, these chains can be thought of as prolate rotation ellipsoids made up of a large number of titania particles, each of them containing in turn a number of magnetite NPs with crystal axes at random with respect to the major ellipsoid's axis. The chain structure and substructure are depicted in Figure 5. Contact interaction among NPs is neglected because they do not touch each other, being enveloped by the titania layer. Within each chain, titania particles self-assemble by effect of electrostatic and magnetostatic interactions under the strong magnetic force applied during the process of monomer curing; however, the irregular shape of titania particles (see Figure 4, bottom right) implies that their degree of compaction is not very high, and the hosting polymer fills the interstices among a given titania particle and the adjacent ones. Therefore, a single chain is composed mostly of diamagnetic media, with a low overall magnetic fraction. A strong dipolar interaction is expected to arise among NPs treated as magnetic dipoles; the density of magnetic units is large enough to predict long-range directional magnetic order of NP moments sustained by dipolar interaction.