## 3 Results
The saturation magnetization at 5 K reached values of \(\sim\)203 Am\({}^{2}\)/kg and \(\sim\)215 Am\({}^{2}\)/kg for the Fe\({}_{50}\)Co\({}_{50}\) and Fe\({}_{66}\)Co\({}_{34}\) MNPs, respectively, which were slightly lower than the values of FeCo bulk alloys of similar compositions [20]. This reduction was probably due to the formation of a thin oxide shell, which may have arisen at the intermediate step when the sample was transferred from the furnace to the glow box. Additionally, the magnetic properties of both metallic MNPs were much less temperature-dependent because of the higher size of these particles compared with the oxide ones. To additionally prove the effectiveness of the reduction, the reverse mechanism (i.e., the oxidation of metallic MNPs) was investigated by performing DTA/TG in an O\({}_{2}\) atmosphere (Figure 3). The mass gain during oxidation for both samples was \(\sim\)36%, consistent with the value expected from the difference between the molar mass (M\({}_{\text{W}}\)) of Fe\({}_{66}\)Co\({}_{34}\) (\(\sim\)71 g/mol) and CoFe\({}_{2}\)O\({}_{4}\) (\(\sim\)235 g/mol) (i.e., (M\({}_{\text{W}}\)[CoFe\({}_{2}\)O\({}_{4}\)]/M\({}_{\text{W}}\)[Fe\({}_{66}\)Co\({}_{34}\)] \(-1\)) \(\times\) 100% = 37.5%). A similar value of 37.2% was measured for the Fe\({}_{50}\)Co\({}_{50}\) sample. Oxidation of the Fe\({}_{66}\)Co\({}_{34}\) sample occurred in a single step at a temperature of \(\sim\)410 \({}^{\circ}\)C (exothermic peak of DTA curve), while the oxidation of the Fe\({}_{50}\)Co\({}_{50}\) MNPs occurred at higher temperatures [45], and it had a two-step character. This may have been due to the heterogeneity of the starting  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample:** & **CoFe\({}_{2}\)O\({}_{4}\)** & **Fe\({}_{66}\)Co\({}_{34}\)** & **(CoFe)\({}_{3}\)O\({}_{4}\)** & **Fe\({}_{50}\)Co\({}_{50  
material, which may have contained two different oxide phases. TG measurements of the Fe\({}_{50}\)Co\({}_{50}\) MNPs under an inert atmosphere (Ar) showed no significant mass variations. The DTA clearly revealed the bcc (\(\alpha\)-phase) to fcc (\(\gamma\)-phase) transition (endothermal peak at \(\sim\)982 \({}^{\circ}\)C). According to the Fe-Co phase diagram, the position of this peak depended on the alloy composition and, for the Fe\({}_{55}\)Co\({}_{45}\) composition, reached a maximum value of \(\sim\)985 \({}^{\circ}\)C [46]. The sharpness of the peak indirectly indicated the compositional homogeneity of the Fe\({}_{50}\)Co\({}_{50}\) sample, despite the suspicion of the heterogeneity of the parent oxide sample (CoFe)\({}_{3}\)O\({}_{4}\). It is also interesting to note that after oxidation at 1000 \({}^{\circ}\)C, the size of the crystallites, evaluated from the XRD pattern, remained almost unchanged compared with the metallic particles. To study in detail the reduction process and the effect of an external magnetic field, the temperature dependence of the magnetization was measured during the reduction of Co-ferrite powders under a reductive H\({}_{2}\) atmosphere (Figure 4). Magnetization was recorded as a function of the temperature under a magnetic field of 10 mT and 0.5 T. Since the magnetic properties of the Co-ferrite oxides and FeCo metals were very different, the sudden increase in magnetization around \(\sim\)350 \({}^{\circ}\)C may have been associated with the reduction of oxides to metals. The rate of reduction process could be estimated from the derivative of magnetization versus temperature dM(T)/dT. The maxima of these curves depended on the applied magnetic field. In the higher field, this peak shifted toward a lower temperature of about 20-25 \({}^{\circ}\)C, thus suggesting that the magnetic field promoted the reduction process. Moreover, analysis of the M(T) curves recorded during reduction allowed for disclosing the topochemical reaction mechanisms.