## 3 Results
It was recently demonstrated that the reduction of Fe\({}_{3}\)O\({}_{4}\) particles in H\({}_{2}\) can occur in one- or two-step processes [31], the latter being due to the formation of an intermediate antiferromagnetic FeO phase (paramagnetic above the Neel temperature T\({}_{\rm N}\) = 198 K [41]). In the case of Co-ferrites, in a hypothetical two-step process, the Me\({}_{3}\)O\({}_{4}\) spinel phase (where Me\({}^{2+/3+}\) is a Fe\({}^{2+/3+}\) or Co\({}^{2+/3+}\) metal ion) may first transform to a MeO rock salt phase and finally to a pure Me phase. In a one-step process, the Co ferrites directly transform to a pure metal phase. Since, in both samples, no significant change in magnetization was detected after the sudden increase of the moment, one can conclude that the mechanism of the topochemical process occurred in one step in both the low and high magnetic fields, or a two-step process occurred smoothly at the same temperature. Figure 3: DTA (red or blue) and TG (black) curves recorded in the O\({}_{2}\) atmosphere for (**a**) Fe\({}_{66}\)Co\({}_{34}\) and (**b**) Fe\({}_{50}\)Co\({}_{50}\) MNPs. The inset of (**b**) is the DTA curve of the Fe\({}_{50}\)Co\({}_{50}\) MNPs measured in an Ar atmosphere. After the samples were reduced, the M(T) curves were recorded during cooling (Figure 5). Considering that at a field of 0.5 T the system was close to the saturation state, and the studied temperature range was still relatively far from the Curie temperatures (T\({}_{\rm C}\)(Fe\({}_{67}\)Co\({}_{33}\)) = 985 \({}^{\circ}\)C, T\({}_{\rm C}\)(Fe\({}_{51}\)Co\({}_{49}\)) = 964 \({}^{\circ}\)C [24]), [PERSON]'s law could be applied to describe the reduction in magnetization:  
\[\rm M(T)=M_{0}\left(1-BT^{\beta}\right), \tag{2}\]  
where B is the Bloch constant, \(\beta\) is the Bloch exponent (which is usually in the range of 1.5-2), and M\({}_{0}\) is the magnetization at 0 K [47, 48]. By fitting the low-temperature region of the experimental curves reported in the T\({}^{3/2}\) scale, the magnetization at 0 K (M\({}_{0}\)) was estimated to be 263 \(\pm\) 2 Am\({}^{2}\)/kg and 267 \(\pm\) 4 Am\({}^{2}\)/kg for the Fe\({}_{66}\)Co\({}_{34}\) and Fe\({}_{50}\)Co\({}_{50}\) samples, respectively. It is worth mentioning that these values were obtained after correcting the mass for the losses measured during the reduction process (see the section with TGA). The M\({}_{0}\) values were significantly higher than the saturation magnetization determined by SQUID measurements, probably because in this latter case, the samples were partially oxidized during the quick exposition to air before SQUID measurements. Moreover, the estimated M\({}_{0}\) values were significantly higher than those expected for bulk materials (more than about 20% because at 0.5 T, the saturation was not completely reached). Similar to what was already observed in the \(\alpha\)-Fe particles obtained after high-pressure hydrogenation treatments [25], the enhancement of the saturation magnetization may be explained by the local changes of the electronic configurations due to the incorporation into the bcc FeCo structure of residual C atoms present on the surface of the oxide particles after the combustion of citric acid. Further investigations of this issue should be conducted with a wide range of samples. Figure 4: Temperature dependence of magnetization under an external magnetic field of 10 mT and 0.5 T, measured during the H\({}_{2}\) reduction of (**a**) CoFe\({}_{2}\)O\({}_{4}\) and (**b**) (CoFe)\({}_{3}\)O\({}_{4}\) MNPs. Bottom panels: normalized first derivative of magnetization vs. temperature curve.