## 3 Results  
Thermal decomposition of Fe(acac)\({}_{3}\) and Co(acac)\({}_{2}\) was used to synthesize spherical Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles; the specifics are mentioned in the Materials and Methods section. In the thermal decomposition reaction, the decomposition of Fe(acac)\({}_{3}\) and Co(acac)\({}_{2}\), in the presence of 1,2 hexadecanediol, oleic acid, and oylamine, to cationic metals resulted in the formation of the nanoparticle. In the current case, at 200 \({}^{\circ}\)C, the nucleation process occurred, followed by the growth of the nucleus at 280 \({}^{\circ}\)C. The segregation of nucleation and the growth process directed the formation of the monodisperse nanoparticles [42,43]. The size and morphology of the particles are dependent on the initial ratio of the precursors, surfactants and solvent. TEM images of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles are displayed in Figure 1. The analysis of the TEM images shows that along with having a nearly monodisperse size distribution, the particles are nearly spherical in shape. It is worth mentioning that the particles are well separated from each other, which indicates the uniform surfactant coating of the nanoparticles' surface. The mean diameter (\(D\)) and a standard deviation (\(\sigma\)) from the fitting of the size histogram to a lognormal distribution are presented in Table 1. The mean diameter of the particles for all compositions is in the range of 6.6 to 7.8 nm. This control over the size of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles allowed us to understand the effect of Co substitution on their magnetic and hyperthermia responses. Figure 1: TEM images of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) nanoparticles with cobalt content of (**a**) x = 0, (**b**) x = 0.07, (**c**) x = 0.10, (**d**) x = 0.50, (**e**) x = 0.75, and (**f**) x = 1. The inset of each image shows the corresponding diameter histograms. Scale bar is 20 nm in all the images. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization vs. temperature in an applied field of 50 Oe for Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles is shown in Figure 3a. Monodisperse, non-interacting, and superparamagnetic nanoparticles, which are ferromagnetic in the ground state, are known to show a maximum in the ZFC magnetization curve corresponding to the blocking temperature (\(T_{\rm B}\)) when the thermal energy is equivalent to the activation energy. The ZFC magnetization (M\({}_{\rm ZFC}\)(T)) curves for all the samples clearly showed maxima in the measured temperature range. It is evident that the M\({}_{\rm ZFC}\)(T) peak or the \(T_{\rm B}\) shifts to a higher temperature with an increase in the Co content. \(T_{\rm B}\) vs. Co content is plotted in Figure 3b and Table 2. The sharp increase in \(T_{\rm B}\), with an increase in Co content until x = 0.75, can be attributed to the increase in the magnetocrystalline anisotropy of the nanocrystals [37]. The nearly constant value of \(T_{\rm B}\) from x = 0.75 to x = 1 indicates that the magnetocrystalline anisotropy of the nanocrystals does not increase with a further increase in Co content. This is in agreement with previous reports, where a decrease in magnetocrystalline anisotropy in Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (0.75 \(\leq\) x \(\leq\)1) was reported [40; 46; 47; 48; 49]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x)**} & \multicolumn{1}{c}{**Mean Diameter (\(D\)) in nm**} & \multicolumn{1}{c}{**Standard Deviation (\(\sigma\)) in nm**} \\ \hline
0 & 7.8 & 0.1 \\
0.07 & 6.7 & 0.1 \\
0.10 & 6.6 & 0.1 \\
0.50 & 7.2 & 0.4 \\
0.75 & 7.1 & 0.2 \\
1 & 6.5 & 0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Size of nanoparticles corresponding to the Co content in Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0–1). Figure 2: XRD pattern of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0.10) nanoparticles. The lower pattern corresponds to the standard cubic Fe\({}_{3}\)O\({}_{4}\).