## 3 Results
The relatively high values of \(M_{\rm S}\) for the Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles confirm the high crystalline quality. Owing to the enhanced magnetocrystalline anisotropy with the Co addition, the Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles are expected to show SAR enhancement. AC hyperthermia experiments via the calorimetric method were conducted on the Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles to demonstrate this. An investigation and comparison of the SAR values of the Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles was conducted for the purpose of examining the effect of Co content on the heating efficiency of the Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles. The study involved the measurement of temperature vs. time at different strengths of the external AC magnetic field using a nanoparticle concentration of 1 mg/mL in water. The heating rate was directly proportional to the increasing magnetic field, and it was found that an external AC magnetic field H\({}_{\rm AC}\)\(\geq\) 600 Oe is required to attain the temperature range of 40-44 \({}^{\circ}\)C, where tumor cells are deactivated or destroyed. The heating efficiency of the nanoparticles, SAR can be calculated from the initial slope of the T vs. time curves (\(\Delta\)T/\(\Delta\)t) [51]:  
\[\rm SAR=\ \frac{\Delta T}{\Delta t}\ \frac{C_{P}}{\varphi} \tag{1}\]  
where C\({}_{P}\) is the heat capacity of the dispersion medium (4.186 J/g K for water) and \(\varphi\) = 1 \(\times\) 10\({}^{-3}\) is the mass of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles per unit mass of water. It was established that the SAR values decrease drastically upon reducing the applied external AC magnetic field (Figure 6a). It is generally known that the agglomeration of the nanoparticles, which results in sedimentation, has a negative effect on the SAR value. Figure 4: (**a**) M–H curve of 7 nm series of Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0–1) nanoparticles at 10 K. (**b**) Variation of coercivity (\(H_{\rm C}\)) at 10 K as a function of Co content. Figure 5: (**a**) M–H curve of 7 nm Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0–1) nanoparticles at 300 K. (**b**) Variation of saturation magnetization (\(M_{\rm S}\)) at 300 K as a function of Co content, x. However, in the present study, the oleic acid and dylamine functionalized hydrophobic nanoparticles were phase transferred to the aqueous phase using TMAH as a capping agent. The TMAH-coated particles remained well dispersed in water during the hyperthermia measurement. Therefore, the effect of particle sedimentation on the hyperthermia response was negligible. The variation of the SAR at external AC applied magnetic fields of 600 and 800 Oe as a function of Co content (Figure 6b). Pure magnetite Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0) showed poor heating efficiency even at an external AC applied magnetic field of 800 Oe; however, a reasonable value of the SAR was observed with a small amount of Co content. Similar to the \(T_{\rm B}\) and \(H_{\rm C}\) at 10 K, the SAR value showed an increase with an increase in the Co content until x = 0.75, above which it showed a decreasing trend (Table 2). This indicates that the SAR value of nanoparticles depends primarily on the magnetic anisotropy of the particles. As discussed above, the magnetocrystalline anisotropy in Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (x = 0-1) nanoparticles shows a peak at x = 0.75; therefore, the peak in the SAR value at x = 0.75 indicates an important role of the magnetic anisotropy in hyperthermia heating of the present nanoparticles. The relatively smaller values of the SAR for Fe\({}_{3-x}\)Co\({}_{x}\)O\({}_{4}\) (0.75 \(\leq\) x \(\leq\)1) nanoparticles, as compared to 20 nm nanocubes [40], could be attributed to the shape and small size of the nanoparticles. The SAR values are dependent on the size and crystallinity of the nanoparticles and are relatively smaller when the size of the nanoparticles is below 10 nm [52].