## 3 Results and Discussion  
Characterization of (NiZn)\({}_{1-x}\)Mn\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\)/CNT Nanocomposites--XRD and TEM Studies  
The crystal structure and size of as-synthesized ferrites and ferrite/CNT nanocomposites are presented in Figures 1 and 2, while the chemical and phase compositions studied with X-ray fluorescence and X-ray diffraction are presented in Table 1 and Figure 3, respectively. One can see that the nickel to zinc ratio in all (NiZn)\({}_{1-x}\)Mn\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\) ferrites and (NiZn)\({}_{1-x}\)Mn\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\)/CNT nanocomposites is close to 1, as well as the Ni:Zn:Mn ratio, which confirms the successful synthesis of substituted ferrites. As can be seen in TEM pictures (Figures 1 and 2), both pristine ferrites and \(\rm(NiZn)_{1-x}\)\(\rm Mn_{x}Fe_{2}O_{4}\)/CNT nanocomposites can be characterized by nanometric structure, with a crystallite size below 20 nm. Contrary to \(\rm MnFe_{2}O_{4}\), which crystallizes mostly along the (311) direction, corresponding to an interplanar distance of 0.25-0.26 nm, the \(\rm(NiZn)_{1-x}Mn_{x}Fe_{2}O_{4}\) ferrites crystallize along different directions. As presented in Figure 1b, planes with a distance of 0.23-0.25 nm correspond to the (222) direction, while planes presented in Figure 1e with a distance of 0.22 nm can be assigned to the (400) direction. Illustrated in Figure 1c,f are planes with distances of 0.29-0.32 nm and with the presence of some lattice strain or crystal imperfections, corresponding most likely to the (220) direction. In the case of \(\rm(NiZn)_{1-x}Mn_{x}Fe_{2}O_{4}\)/CNT nanocomposites, three crystallization directions have been  
Figure 1: TEM images of synthesized \(\rm(NiZn)_{1-x}Mn_{x}Fe_{2}O_{4}\) nanoparticles: x = 0 (**a–c**), x = 0.5 (**d–f**) and x = 1 (**g**,**h**). recognized: (311), with a distance of 0.25-0.26 nm (Figure 2g); (220), with a distance of 0.30-0.34 (Figure 2e); and (111), with a distance of 0.44-0.48 (Figure 2b,c,f). It should be mentioned that, according to the ICDD# 00-054-0964 card, the model distance for the (400) plane is 0.208 nm, while for the (222), (311), (220), and (111) planes the distances are 0.241 nm, 0.251 nm, 0.295 nm and 0.481 nm, respectively. Such a big variation from model data might be a result of lattice strain or crystal imperfections, especially due to the large number of defects in the nanometric structures. The results of TEM and XRF techniques stand in good agreement with XRD data and confirm crystallite sizes below 15 nm for x = 0 and x = 0.5, while for x = 1, this parameter is in the range of 19 to 22 nm. It is worth mentioning that usually linear defects, such as dislocations, occur simultaneously with point defects, i.e., Frenkel defects, and both strongly affect lattice strain and crystal imperfections. The calculated dislocation densities  
Figure 2: TEM images of synthesized \(\mathrm{(NiZn)_{1-x}Mn_{x}Fe_{2}O_{4}}/0.05\mathrm{CNT}\) nanocomposites: x = 0 (**a–c**), x = 0.5 (**d–g**) and x = 1 (**h**,**i**). of ferrites, as a representation of crystal imperfections, are 8.26 \(\times\) 10\({}^{15}\)/m\({}^{2}\), 6.94 \(\times\) 10\({}^{15}\)/m\({}^{2}\), and 2.77 \(\times\) 10\({}^{15}\)/m\({}^{2}\) for x = 0, x = 0.5, and x = 1, respectively. The dislocation densities for (NiZn)\({}_{1-x}\)Mn\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\)/CNT nanocomposites are 6.94 \(\times\) 10\({}^{15}\)/m\({}^{2}\), 5.10 \(\times\) 10\({}^{15}\)/m\({}^{2}\), and 2.07 \(\times\) 10\({}^{15}\)/m\({}^{2}\) for x = 0, x = 0.5, and x = 1, respectively. This result shows that with an increase in manganese content, the crystal imperfections decrease as a result of crystal-lite growth. Rietveld refinement analyses also show a minor increase in lattice parameters with a decrease in manganese content. For MnFe\({}_{2}\)O\({}_{4}\), (NiZn)\({}_{0.5}\)Mn\({}_{0.5}\)Fe\({}_{2}\)O\({}_{4}\), and (NiZn)Fe\({}_{2}\)O\({}_{4}\) ferrites, the lattice parameters of the cubic Fd-3m structure are a = 8.400, a = 8.410, and a = 8.416, respectively.