## 3 Results  
The XRD patterns of the Co-ferrite particles (Figure 1) confirmed the formation of a phase-pure cubic spinel structure for both samples (COD card no. 1,533,163 [39]). When the Fe:Co molar ratio of the starting salts was set to 2:1 with the stoichiometry CoFe\({}_{2}\)O\({}_{4}\) nanoparticles form, while a mixture of CoFe\({}_{2}\)O\({}_{4}\) and FeCo\({}_{2}\)O\({}_{4}\) phases was likely obtained when the Fe:Co ratio was set to 1:1 [40]. Hereafter, we will use the conditional formula \"(CoFe)\({}_{3}\)O\({}_{4}\)\" to indicate the latter sample. In stoichiometric CoFe\({}_{2}\)O\({}_{4}\), Co\({}^{2+}\) cations preferentially occupy octahedral sites with an inversion degree of ~0.8. In cobalt-rich spinel ferrites, some cobalt ions are in the 3+ low-spin state and preferentially occupy the octahedral sites, pushing some Co\({}^{2+}\) to tetrahedral positions and thus reducing the inversion degree toward \(\sim\)0.5 [40]. As a result, the lattice parameter \(a\) decreased from 0.8381(1) nm for CoFe\({}_{2}\)O\({}_{4}\) to 0.8242 (2) nm for FeCo\({}_{2}\)O\({}_{4}\)[40], and the reflection peaks of the Co-reach spinel ferrites shifted to higher 20 values. In our samples, the lattice parameter of the stoichiometric sample was 0.8392(5) nm, which was very close to the nominal value, and it decreased to 0.8346(6) nm for the (CoFe)\({}_{3}\)O\({}_{4}\) sample, thus confirming the trend of reduction of the lattice parameter while increasing the Co percentage. The crystallite size (d\({}_{\text{XRD}}\)) estimated using [PERSON]'s equation [37] was \(\sim\)12 and 7 nm for the CoFe\({}_{2}\)O\({}_{4}\) and (CoFe)\({}_{3}\)O\({}_{4}\) particles, respectively. The XRD patterns of the samples after H\({}_{2}\) reduction featured drastic changes in the position and width of the diffraction peaks, which could be indexed to the bcc FeCo phase (I m \(-\)3 m structure, COD card no. 1100108). Lattice parameters of 0.2866(1) nm and 0.2859(7) nm, indicating the formation of Fe\({}_{66}\)Co\({}_{34}\) and Fe\({}_{50}\)Co\({}_{50}\) MNPs were estimated for the FeCo particles obtained from CoFe\({}_{2}\)O\({}_{4}\) and (CoFe)\({}_{3}\)O\({}_{4}\) oxides, respectively. Moreover, it is worth noting that the size of the crystallites increased up to 25 nm and 33 nm for the Fe\({}_{66}\)Co\({}_{34}\) and Fe\({}_{50}\)Co\({}_{50}\) samples, respectively. The magnetic properties confirmed the change of the particle structure after H\({}_{2}\) reduction (Figure 2 and Table 1). At low temperatures (5 K), the CoFe\({}_{2}\)O\({}_{4}\) and (CoFe)\({}_{3}\)O\({}_{4}\) samples possessed a large coercivity (H\({}_{\text{C}}\)), resulting from the high magnetic anisotropy of Co-ferrites, and a saturation magnetization (M\({}_{\text{S}}\)) lower than the bulk value (90 Am\({}^{2}\)/kg for stoichiometric cobalt ferrite at 0 K [41]), owing to the large contribution of magnetically frustrated surface spins in small nanoparticles [28; 42; 43]. The coercivity rapidly reduced when measurements were performed at 300 K, due to thermal fluctuation occurring when the temperature approached the blocking temperature (T\({}_{\text{B}}\), which was above room temperature for those samples) according to the following equation:  
\[\text{H}_{\text{C}}(T)=\zeta\frac{2K}{\text{M}_{\text{S}}}\left[1-\left(\frac {T}{\text{T}_{\text{B}}}\right)^{\frac{1}{2}}\right]\,, \tag{1}\]  
where \(K\) is the magnetic anisotropy constant and \(\zeta\) is a coefficient that mainly depends on the anisotropy symmetry [44]. Figure 1: XRD patterns of (**a**) CoFe\({}_{2}\)O\({}_{4}\) and corresponding metallic Fe\({}_{66}\)Co\({}_{94}\) MNPs and (**b**) (CoFe)\({}_{3}\)O\({}_{4}\) and corresponding metallic Fe\({}_{50}\)Co\({}_{50}\) MNPs. Diffraction peaks of oxide and reduced powders are indexed by cubic spinel ferrite and body-centered cubic (bcc) FeCo phase, respectively (the corresponding Miller indexes are reported). The M-H loops for both H\({}_{2}\)-reduced samples were characterized by an increased Ms and reduced H\({}_{\text{C}}\).