# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 1 Introduction  
Magnetic nanoparticles are recognized for their distinct nanoscale properties, opening fascinating possibilities for understanding how structure affects their physical properties. They play a vital role across various domains, including medicine, technology, and environmental science, driving progress in areas like targeted drug delivery, hyperthermiatreatments for cancer, data storage innovations, and environmental cleanup. Their effectiveness stems from the arrangement of magnetic domains when subjected to external magnetic fields, revealing intriguing quantum effects due to their tiny size. Additionally, the interaction between their surface and core is crucial to their magnetic characteristics. Initially, researchers focused on magnetic nanoparticles in the context of ferrofluids, which are mixtures of magnetic nanoparticles in a liquid. These were some of the earliest practical applications of these materials. As techniques for synthesizing nanoparticles improved and our understanding of their properties deepened, they became integral to more advanced applications such as magnetically guided drug delivery in the field of biomedicine [1; 2; 3; 4; 5; 6; 7; 8]. Ferrites are gaining prominence thanks to their diverse properties and applications. These materials mainly consist of iron oxide mixed with metal cations like cobalt, nickel, manganese, and zinc. They are ferrimagnetic ordered and can form various crystal structures, including spinel, garnet, and hexagonal varieties. Spinel ferrites are recognized for their high saturation magnetization and relatively high Curie temperatures, allowing them to retain magnetic properties over a broad temperature range. The arrangement of atoms in these ferrites, with metal cations occupying tetrahedral and octahedral positions, critically influences their magnetic characteristics. For instance, the way these cations are distributed greatly affects coercivity, with hexagonal ferrites exhibiting higher coercivity due to their uniaxial anisotropy, making them ideal for use in permanent magnets. Recent developments showcase the impressive physical, optical, magnetic, and electrical properties of spinel ferrite nanomaterials, which are becoming increasingly appealing for a wide range of uses. These include high-density data storage solutions [9], gas sensors [10; 11], rechargeable lithium batteries [12], tools for medical diagnostics [13; 14; 15], magnetic bulk cores [16], catalysts [17], adsorbents [18; 19], microwave adsorbers [20], and water treatment [21; 22; 23]. Cobalt ferrite stands out as one of the most researched magnetic nanoparticles, forming a key foundation for many studies. Its versatile applications span across various sectors, including biomedicine--where it serves as an anti-cancer agent and supports cellular therapy--magnetic refrigeration, energy storage, sensor technology, and high-frequency devices. Moreover, cobalt ferrite is essential in areas like catalysis, magnetic cell separation, hyperthermia, targeted drug delivery, magneto-electric systems, magneto-plasmonics, and medical imaging [24; 25; 26; 27; 28]. Cobalt ferrite is considered a unique class of material owing to its remarkable properties as a hard ferrite. It exhibits moderate saturation magnetization, high coercivity, significant magnetocrystalline anisotropy, mechanical hardness, and excellent chemical stability [29; 30]. Structurally, spinel cobalt ferrite has a cubic close-packed arrangement comprising 96 voids created by large O\({}^{2-}\) anions--64 at tetrahedral (A) sites and 32 at octahedral (B) sites. Among these, 8 A-site and 16 B-site voids are typically occupied by metal cations, forming an open structure that allows smaller cations to migrate freely between tetrahedral and octahedral sites [31; 32; 33]. While bulk cobalt ferrite generally adopts an inverse spinel structure, reducing its particle size to the nanometer scale results in a mixed spinel configuration.