# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 3 Results and Discussions
The signals from the two samples were fitted using four components, coming from the two final states of Fe and Co ions, in which the 3s core electrons are coupled parallel and antiparallel to the majority spin 3\(d\) electrons after photoemission. The results are presented in Figure 8 and listed in Table 1. The binding energies obtained for the two Co 3s peaks indicate the presence of both Co\({}^{3+}\) and Co\({}^{2+}\) ions in both samples [64; 65]. Figure 8: Fitting results of the Co 3s and Fe 3s core levels (open circles correspond to the experimental spectrum and the continuous curves to the fitting results, after background subtraction). Figure 7: The Co 3s and Fe 3s core-level XPS spectra. A high presence of Co\({}^{3+}\) ions is suggested by the fitted values for the main peak, which are closer to 101.2 eV reported for Co\({}^{3+}\) ions in the octahedral sites of Co\({}_{3}\)O\({}_{4}\) than to 103 eV assigned to Co\({}^{2+}\) in the tetrahedral sites of Co\({}_{3}\)O\({}_{4}\)[65]. The exchange splitting for the Co 3s level in Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) is slightly lower than in Co\({}_{0.95}\)Ho\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\), suggesting a lower Co\({}^{3+}\) content. The higher value of the Fe 3s exchange splitting for Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) than for Co\({}_{0.95}\)Ho\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) points to a higher Fe\({}^{3+}\)/Fe\({}^{2+}\) ratio. The magnetization isotherms were measured at two temperatures, 4.2 K and 300 K, under external magnetic fields up to 10 T and are presented in Figure 3:0 It is easy to see that the saturation was not reached in fields up to 10 T. The magnetization isotherms measured at 4.2 K show inflexion points around 1T external magnetic applied field. This behavior can be explained by the presence of a smaller number of superparamagnetic nanoparticles combined with the ferrimagnetic ordering of most of them. The transition temperature for the ferrimagnetic nanoparticle is higher than room temperature for the studied compounds. The nanoparticles with low dimensions, under 10 nm (less than 2 percent of the total number of nanoparticles), show superparamagnetic behavior in the whole studied temperature range and do not show any magnetic transition. The measured magnetization is the sum of these two contributions--from the superparamagnetic nanoparticle and from the ferrimagnetic ones. The saturation magnetization, \(M_{\rm S}\), at 4.2 K respectively 300 K, of Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles calculated considering the approach to saturation law, are presented in Figure 3:1 The obtained values are smaller for the doped samples, being somewhat higher or comparable with previous reports [39; 40; 41; 44; 45; 46; 47]. It was reported that doping with rare-earth in nanocrystalline cobalt ferrite could affect the magnetic properties due to changes \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Co 3s**} & \multicolumn{2}{c}{**Fe 3s**} \\ \hline \multicolumn{2}{c}{**Binding Energy (eV)**} & \(\Delta_{\rm Co}\) & \multicolumn{2}{c}{**Binding Energy**} & \(\Delta_{\rm Fe}\) \\ \multicolumn{2}{c}{} & \multicolumn{1}{c}{} & **(eV)** & **(eV)** & **(eV)** \\ \hline Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) & 106.2 & 4.7 & 98.3 & 5.1 \\  & 101.5 & 4.8 & 98.0 & 4.8 \\ Co\({}_{0.95}\)Ho\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) & 101.3 & 4.8 & 93.2 & 4.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fe 3s and Co 3s XPS peak binding energies were obtained from curve fitting with an accuracy of about 0.1 eV, and the calculated values for the exchange splitting \(\Delta\). Figure 9: Magnetization isotherms of Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles recorded at 4.2 K (**left**) respectively 300 K (**right**). in the cation distribution between the tetrahedral and octahedral sublattices [34; 35; 36; 37; 38]. As examples, values of 54 emu/g [41] and 53 emu/g [39] were reported for a similar compound with Ho.