# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 3 Results and Discussions
For Gd samples, 69 emu/g [40] and 71 emu/g [39], respectively, were reported. In our compounds, the \(M_{\mathrm{s}}\) values at 4.2 K are 71.36 emu/g for R = Gd and 73.54 emu/g for R = Ho. The obtained values are promising for future applications in Magnetic Resonance Imaging as a contrast agent for early diagnosis in several diseases, as was reported earlier for Fe\({}_{3}\)O\({}_{4}\) nanoparticles [66; 67]. The calculated total magnetic moment using our proposition of cation distribution is very close to the experimentally observed values determined at 4.2 K--see Table 2. The considered magnetic moments per ion are 3 \(\upmu_{\mathrm{B}}\) for Co\({}^{2+}\), 4 \(\upmu_{\mathrm{B}}\) for Fe\({}^{2+}\), 5 \(\upmu_{\mathrm{B}}\) for Fe\({}^{3+}\), 7 \(\upmu_{\mathrm{B}}\) for Gd\({}^{3+}\), and 10 \(\upmu_{\mathrm{B}}\) for Ho\({}^{3+}\). The magnetic moments of the ions located in octahedral and tetrahedral sites are oriented parallel in each sublattice and are oriented antiparallel between them. On the other hand, the magnetic moments of heavy rare earths are antiparallel with the 3\(d\) metals' magnetic moments. In our calculations, we considered that the Gd and Ho magnetic moments located in octahedral sites are oriented antiparallel with Co and Fe magnetic moments located in the same sites, keeping in view that they are heavy rare-earth elements. One can see that the best match with the experimental values was obtained if we considered the majority of the cobalt ions having a 3+ valence, as was demonstrated by XPS measurements. Further understanding of the magnetic behavior of ferrite magnetic nanoparticles (MNPs) can be obtained by examining the temperature dependence of magnetization under two distinct measurement protocols: zero-field cooling (ZFC) and field cooling (FC). In the ZFC procedure, the samples were initially cooled to 5 K in the absence of an external magnetic field. Upon reaching this temperature, a magnetic field of 50 Oe was applied, and magnetization was recorded during subsequent heating. For FC measurements, the samples were cooled under the same external magnetic field. A characteristic feature of the ZFC magnetization curves for is the presence of a peak, which indicates the average transition from a ferromagnetic to a superparamagnetic state. The temperature at which this peak occurs is called the blocking temperature (\(T_{B}\)). Additionally, the temperature at which the ZFC and FC curves begin to diverge--known as the irreversibility temperature--is typically higher than \(T_{B}\). For ideal, monodisperse, and non-interacting single-domain nanoparticles exhibiting superparamagnetic behavior, \(T_{B}\) coincides with the irreversibility temperature. In the case of our samples, the ZFC magnetization peak begins to appear near room temperature and is extended at higher temperatures (Figure 10). The FC curve and the ZFC curve converge slightly above room temperature. The magnetization follows a plateau in the case of the FC curve, while a decrease is observed for the ZFC curve with decreasing temperature. This behavior can be explained considering that some nanoparticles remain superparamagnetic, while the rest are ferrimagnetic ordered at room temperature, showing interparticle interactions. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**R**} & \multicolumn{6}{c}{**Cation Distribution**} & \multirow{2}{*}{\(M_{\mathrm{S}}\) (\(\upmu_{\mathrm{B}}\)) **Calculated**} & \multirow{2}{*}{\(M_{\mathrm{S}}\) (\(\upmu_{\mathrm{B}}\)) **Calculated**} & \multirow{2}{*}{\(M_{\mathrm{S}}\) (\(\upmu_{\mathrm{B}}\)) **Experimental**} & \multirow{2}{*}{\(M_{\mathrm{S}}\) (\(\upmu_{\mathrm{B}}\)) **Experimental**} \\ \cline{2-2} \cline{6-9}  & **Fe\({}^{3+}\)** & & & & & & & & & \\ \hline Gd & 0.7 & 0.3 & 0.05 & 0.75 & 0.2 & 0.5 & 0.5 & 3.05 & 3.06 & 2.63 \\ \hline Ho & 0.6 & 0.4 & 0.05 & 0.8 & 0.15 & 0.55 & 0.45 & 3.1 & 3.16 & 2.48 \\ \hline \hline \end{tabular}
\end{table}