# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 1 Introduction
This size reduction introduces surface effects that significantly alter its properties compared to the bulk form. Significant modifications in the properties of cobalt ferrites can be achieved through the substitution or co-substitution of divalent (+2) and/or trivalent (+3) cations--whether magnetic or non-magnetic--into the spinel lattice. Additionally, factors such as synthesis methods, sintering temperature, particle shape and size, size distribution, and chemical stoichiometry also play crucial roles in tailoring the material's characteristics [34; 35; 36; 37; 38]. In recent years, there has been growing interest in exploring the effects of rare earth (RE)element substitution on the spinel structure of cobalt ferrites. Due to their localized 4\(f\) electrons, which are shielded by 5\(p\) and 6\(s\) orbitals, RE ions exhibit unique magnetic behavior. These elements possess a wide range of magnetic moments between 0 and 10.5 \(\upmu_{\text{B}}\) and strong spin-orbit (L-S) coupling. The incorporation of RE ions into the spinel lattice results in apparition of 3_d_-4\(f\) exchange interactions in addition to the intrinsic 3_d_-3\(d\) interactions between transition metals such as Fe, Ni, and Co ([39] and references therein). It was shown that the saturation magnetization decreases with the increase of heavy rare-earth element concentration, this behavior being explained by the weakening of A-B exchange interactions [40; 41]. It was reported that the magnetic interactions between 3\(d\) transition and rare-earth elements can be best described by the 4_f_-5_d_-3\(d\) exchange route [42]. The 4\(f\) electrons of rare-earths (except Ce) are localized, presenting a small spatial extent. The magnetic moments of heavy rare earths are aligned antiparallelly with the 3\(d\) transition metals' magnetic moments. There are many papers reporting the effects of rare-earth substitution at Fe sites on cobalt ferrites' structural, morphological, electrical, magnetic, and chemical properties. It was shown that the saturation magnetization decreases due to substitution with heavy rare earth: Gd [40; 43], Ho [41], Ho, Er [39], Dy, Gd [44], Dy, Er [44], Dy, Tb [45], Yb, Dy, Gd [46]. The magnetic properties in all cases appear to be highly sensitive to the distribution of cations between octahedral and tetrahedral sites. The incorporation of RE\({}^{3+}\) ions--more probably into the octahedral sites due to their larger ionic radii--can trigger a structural rearrangement within the spinel lattice [47; 48]. Previous studies have shown that the original cation distribution in spinel structures can be significantly altered by ion substitution. Accordingly, doping with RE\({}^{3+}\) ions likely disrupts the equilibrium distribution of cations, promoting the migration of Co\({}^{2+}\) ions to the tetrahedral sites. To decrease the lattice strain caused by RE\({}^{3+}\) incorporation, some Co\({}^{2+}\) ions may shift from octahedral to tetrahedral positions, while a compensatory migration of Fe\({}^{3+}\) ions from tetrahedral to octahedral sites may also occur. On the other hand, [PERSON] and co-workers reported an increase in the saturation magnetization. The authors have explained this behavior to the higher magnetic moment of Gd\({}^{3+}\), which prefers octahedral sites, being higher than that of Fe\({}^{3+}\)[49]. At the same time, literature reports that the rare-earth substitution on cobalt sites in cobalt ferrites is limited. [PERSON] et al. [50] have reported that the substitution of rare-earth ions on the Co site increases the lattice constant, decreases the grain size, and increases the coercivity. However, the substantial mismatch in ionic radii between RE ions and the host cations (Fe\({}^{3+}\) and Co\({}^{2+}\)) presents a significant challenge for their successful integration into the spinel framework. We previously explored the structural and magnetic properties of CoFe\({}_{2}\)O\({}_{4}\) nanoparticles doped with Zn or Mn at Co sites [51; 52].