# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 4 Conclusions  
Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) with R = Gd or Ho nanoparticles were synthesized using a sol-gel approach incorporating bio-based agents. Although the prepared compounds are single phases and have a cubic \(Fd\)-\(3m\) structure, XRD and TEM measurements showed that the particles' average sizes are between 17.24 nm and 20.3 nm. Elemental EDS analysis confirmed the presence of Co, R, Zn, and O elements in all studied samples. XPS was employed to investigate the oxidation state of the ions in Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) and Co\({}_{0.95}\)Ho\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles. The Gd \(4d\) and Ho \(4d\) XPS spectra reflect the presence of Gd\({}^{3+}\) and Ho\({}^{3+}\) ions. The Fe \(2p\) and Co \(2p\) core-level XPS spectra recorded for the two samples are very similar and consist of the two \(2p_{3/2}\) and \(2p_{1/2}\) main peaks and satellite structures. The spin-orbit splitting of about 15.4 eV observed in Co \(2p\) core-level spectra is an indication that Co is predominantly present as Co\({}^{3+}\) state, while the satellite structures located at about 6 eV higher energies than the main lines confirm the existence of divalent Co in Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\). The binding energies of Fe \(2p_{3/2}\) and \(2p_{1/2}\) states, at about 710.7 eV and 724.4 eV, are close to the characteristic values for Fe\({}^{3+}\)[60]. The strong presence of Fe\({}^{3+}\) species is also supported by the satellite peaks observed at approximately 8 eV higher binding energy than the main signals. The multiplet splitting of Co 3s and Fe 3s core-level XPS spectra, generated by the exchange interaction between the valence electrons and the unpaired electron, was analyzed. The positions of the main peaks obtained by curve fitting and the values of the exchange splitting for Co 3s and Fe 3s levels point to the presence of Co\({}^{3+}\), Co\({}^{2+}\), Fe\({}^{3+}\), and Fe\({}^{2+}\) ions and to high Co\({}^{3+}\)/Co\({}^{2+}\) and Fe\({}^{3+}\)/Fe\({}^{2+}\) ratios in both samples. The saturation magnetizations are smaller for the doped samples compared with the pristine one, being somewhat higher or comparable with previous reports due to \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**R** & \(M_{\rm s}\) **(emu/g)** & \(\mu_{0}\)**\(H_{\rm c}\) (T)** & **\(K\) (kJ/m\({}^{3}\))** & \(d\) (nm)** & \(T_{B}\) (K) & \(d\) (nm) & \(T_{B}\) (K) \\  & & & & TEM & TEM & XRD & XRD \\ \hline Gd & 61.35 & 0.069 & 20.53 & 17.24 \(\pm\) 1.19 & 160 & 20.3 \(\pm\) 0.3 & 261 \\ \hline Ho & 57.69 & 0.069 & 19.31 & 18.84 \(\pm\) 1.81 & 195 & 19.3 \(\pm\) 0.4 & 210 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Saturation magnetization, coercive field, anisotropy constant, XRD and TEM average nanoparticle diameters, and calculated blocking temperatures for Co\({}_{0.095}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) samples. interchanges in the cation distribution between the two sublattices. For theoretical magnetization calculation, we considered that the heavy rare earths are in octahedral sites and their magnetic moments are aligned antiparallel with 3d transition magnetic moments. The experimental and theoretical values match very well. ZFC-FC curves show that some nanoparticles remain superparamagnetic, while the rest are ferrimagnetic ordered at room temperature, showing interparticle interactions. The \(M_{\rm S}/M_{\rm S}\) ratio at room temperature is below 0.5, indicating the predominance of magnetostatic interactions. Having in view the green synthesis method and the magnetic properties of these compounds, our objective is to develop applications in cancer therapy--specifically for hyperthermia treatment and the fabrication of magnetoelectric core-shell structures. These structures are designed to locally generate electric charges within the body, controllable by relatively low external magnetic fields, with the aim of targeting and affecting only cancerous cells.