# Magnetic Properties of Nanocomposites
## 4 Ceramic Nanocomposites
### Ceramic Nanocomposites Containing Fe  
In order to avoid interpretation problems arising from compositional inhomogeneity of NPs, single TM species can be used to load the precursor zeolite. The room temperature loops of four metal-ceramic NCs obtained by applying different thermal treatments and exchange processes to an A zeolite loaded with Fe\({}^{2+}\) ions are shown in Figure 8.  
These metal-ceramic nanocomposites were obtained as previously described for nanocomposites bearing Co and Fe. In particular, the samples shown in the left-side panel of Figure 8 were obtained by eight iterations the exchange of Na\({}^{+}\) ions with Fe \({}^{2+}\) ions [63]; despite the high number of iterations, the exchange of TM ions for Na\({}^{+}\) did not reach completion; on the contrary, the samples in the right-side panel were obtained by implementing an exchange of NH\({}_{4}\)\({}^{+}\) for Na\({}^{+}\) first, followed by the exchange of Fe\({}^{2+}\) for NH\({}_{4}\)\({}^{+}\) and the residual Na\({}^{+}\), resulting in a more complete substitution of non-magnetic ions with magnetic ions [62]. The shape and hysteretic properties of the loops measured on samples FeA750C--2 h (obtained by keeping the parent materials at 750 \({}^{\circ}\)C for two hours [63]) and FeA800C--0 min (obtained by heating the parent materials up to 800 \({}^{\circ}\)C with subsequent  
Figure 7: **(Left panel)** magnetic hysteresis loops measured at room temperature on two CoFe ceramic NCs obtained starting from A zeolite; **(right panel)** normalized magnetic susceptibility of the same samples. immediate cooling [63]) are typical of a magnetization process involving both magnetization rotation and domain-wall motion, usually observed in Fe particles with a size of several tens of nanometers, which are in a magnetic multi-domain state. The coercive field is of the order of 170 Oe in both samples. Sample FeA800C--0 min displays a much weaker magnetic signal and clear unsaturating behavior of the M(H) curve at high fields. Such behavior can be explained by the presence of very small iron NPs or clusters, although the contribution of a paramagnetic signal (either from isolated Fe ions or from a compound such as fayalite, Fe\({}_{2}\)SiO\({}_{4}\)) cannot be excluded. This implies that a non-negligible fraction of the starting TM ions has not completely coalesced to form zerovalent Fe NPs. The same effect is observed in all other samples, although the efficiency of the TM ion exchange is generally improved by adding the preliminary NH\({}_{4}\)\({}^{+}\) exchange. The different amount and morphology of the magnetic phases present in the final products result in a very different magnetic behavior, as shown in Figure 8, right panel [62]. The NC (Fe,H)A600C--90 min exhibits an overall magnetic response stronger than (Fe,H)A800C--0 min, as expected from the estimated amounts of ferro/ferrimagnetic phases present there (magnetite = 7.1 and metallic Fe = 4.8 wt.% in the former sample; magnetite = 5.4 and metallic Fe = 0.2 wt.% in the latter). The NC (Fe,H)A800C--0 min, particularly rich in magnetite, has a coercivity typical of Fe\({}_{3}\)O\({}_{4}\) NPs (\(\cong\)45 Oe), while the coercive field of magnetic NC (Fe,H)A600C--90 min (\(H_{c}\cong\) 375 Oe) is fully compatible with the presence of higher-anisotropy NPs of metallic Fe. Once again, it is found that although the process is effective in producing zerovalent Fe NPs dispersed in the ceramic matrix, a fraction of the starting TM ions is not included and either takes part in magnetic compounds or remains dissolved in the host.