## 3 Results
To further verify the variation of magnetocrystalline anisotropy in Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles, we measured low temperature (10 K) magnetization as a function magnetic field (M-H) loops (Figure 4a). At 10 K, Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0) nanoparticles show coercivity of 330 Oe which increases to 1265 with Co content of x = 0.07 and show a further increase with an increase in the Co content (Table 2). The variation of \(H_{\rm C}\) at 10 K as a function of Co content (x = 0-1) is shown in Figure 4b and Table 2. The \(H_{\rm C}\) at 10 K increases with an increase in the Co content until x = 0.75 and above, at which point it showed a decreasing trend. [PERSON] et al. reported that in the series of 20 nm Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) nanocrystals (0.1 \(\leq\) x \(\leq\) 1), with the increase in cobalt content from 0.1 to 0.5, the \(H_{\rm C}\) at 5 and 298 K increased; additionally, with a further increase to 1, \(H_{\rm C}\) showed a decreasing trend. [PERSON] et al. also observed similar results for 35 nm Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles where the maximum in the \(H_{\rm C}\) was observed at x = 0.6, which then decreased with a further increase of Co content [50]. These results convey that the size and crystallinity of the nanoparticles determine the maximum of the \(H_{\rm C}\) with Co content in Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) nanocrystals (x = 0-1). This is in corroboration with previous experimental and theoretical results [40, 46, 47, 48, 49]. It should be noted that although high magnetic anisotropy results in high coercivity, high coercivity does not always mean high magnetic anisotropy as there could be other factors that can account for magnetic anisotropy, such as surface, shape, etc. Therefore, the coercivity trend observed in the present case cannot account for only magnetic anisotropy but, together with other results, it points toward the increase in magnetic anisotropy with the small replacement of Fe\({}^{+2}\) with Co\({}^{+2}\) ions in \(B\)-sites. Figure 5a displays the M-H loops at 300 K for Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles. For all the nanoparticles, the magnetization saturate is \(\sim\)9000 Oe, without notable coercivity, \(H_{\rm C}\), and remanence (Figure 5a). The lack of notable \(H_{\rm C}\) and remanence at 300 K (above the \(T_{\rm B}\)) confirms the superparamagnetic-like nature of the nanoparticles. The saturation magnetization (\(M_{\rm S}\)) at 300 K as a function of Co content (x = 0-1) is shown in Figure 5b and Table 2. The values of \(M_{\rm S}\) remained almost constant with increasing Co content. These \(M_{\rm S}\) values (\(\sim\)60 emu/g) are smaller than those of bulk CoFe\({}_{2}\)O\({}_{4}\) and bulk Fe\({}_{3}\)O\({}_{4}\). The little variation in \(M_{\rm S}\) with Co content could be due to the small difference in size and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1)} & \multirow{2}{*}{**Blocking Temperature**} & \multirow{2}{*}{**Anisotropy Energy**} & \multirow{2}{*}{**Saturation**} & \multirow{2}{*}{**Coercivity**\(H_{\rm C}\) at 10 K**} & \multirow{2}{*}{**SAR at 800 Oe (W/g)**} \\  & & & & & \\ \hline
0 & 53 & 0.0045 & 58.0 & 330 & \(5\pm 4\) \\
0.07 & 57 & 0.0049 & 54.3 & 1265 & \(12\pm 5\) \\
0.10 & 75 & 0.0065 & 54.0 & 2230 & \(26\pm 3\) \\
0.50 & 157 & 0.0135 & 57.5 & 8126 & \(29\pm 3\) \\
0.75 & 215 & 0.0185 & 60.6 & 17761 & \(38\pm 2\) \\
1 & 218 & 0.0187 & 60.7 & 7649 & \(21\pm 4\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters corresponding to the Co content in Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0–1). Figure 3: (a) Zero-field-cooling (ZFC) and field-cooling (FC) magnetization curve of 7 nm series of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0–1) nanoparticles under 50 Oe magnetic field. (b) The Co content dependence of blocking temperatures of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0–1) nanoparticles. crystallinity of the nanoparticles.