## 2 Magnetic Ferrite Nanoparticles  
Magnetic ferrite nanoparticles are magnetic, metallic oxides containing iron oxide with the formula AB\({}_{2}\)O\({}_{4}\), where A and B refer to metal atoms at two different sites in the lattice, typically called tetrahedral and octahedral sites, respectively, and O refers to the corner oxygen position (Figure 2). In particular, these polycrystalline magnetic nanoparticles are new materials for treating wastewater [66]. Figure 1: A graph showing the number of documents with magnetic ferrite nanoparticles [55]. Spinal structured magnetic ferrite nanoparticles doped with transition metal atoms such as MnFe\({}_{2}\)O\({}_{4}\), CoFe\({}_{2}\)O\({}_{4}\), CuFe\({}_{2}\)O\({}_{4}\), NiFe\({}_{2}\)O\({}_{4}\), and ZnFe\({}_{2}\)O\({}_{4}\) have attracted increasing attention due to their unique structural and optical properties [67,68]. Moreover, There magnetic nature makes them fast to collect from reactive mediums [29,69]. These distinctive properties make them suitable for a wide range of high-tech applications, such as photocatalysis, drug delivery, biosensing, sensing applications, wastewater treatment, etc. The significance of magnetic ferrite nanoparticles for a broad range of applications instigates researchers to develop customize ferrites nanoparticles with specific structural, morphological, electrical, and magnetic properties. The easiest approach to tune such properties is controlling nanoparticles' microcrystalline structure [70]  
Magnetic ferrite nanoparticles have two main sublattice structures, tetrahedral and octahedral, depending on the ions present in the lattice. These sublattices determine many of their electrical and magnetic properties. Normally, ferromagnetic behavior originates from the antiparallel alignment of magnetic moments of unpaired electrons of atoms [70]. It is evident that the physical properties of ferrite materials depend on cationic distribution, which is why different kinds of cations exhibit different types of magnetizations in the structure, as well as different cations having different coordination numbers and sizes, affecting the structure of sublattices. Introducing different cations in the ferrite structure can control ferrite nanoparticles' structural and magnetic properties. Many researchers have studied these cation distributions in magnetic ferrites. A variety of metal cations, e.g., Mn\({}^{2+}\), Co\({}^{2+}\), Cu\({}^{2+}\), Ni\({}^{2+}\), and Zn\({}^{2+}\), have been used for doping in magnetic ferrite nanoparticles to influence their functionality. Recent research has already explored the co-doping of various atoms typically having the same valence state. However, differences in ionic radii cause changes in the microlattice structure that led to significant changes in certain aspects of functionality [71]. [PERSON] et al. studied the effect of Cu and Ni doping on the ferrite crystal structure and, eventually, their electrical and magnetic properties. They introduced Cu ions due to the bigger ionic radius of Cu\({}^{2+}\) (0.73 A) compared to the Ni\({}^{2+}\) (0.69 A) atom in the octahedral site, which results in an increase in lattice constant and, eventually, in the bigger size of the crystal. The doping with Cu\({}^{2+}\) ions significantly changes the crystal composition, microstructure, particle size, and antibacterial activity, and the Ni\({}^{2+}\) ions doped into ferrite nanoparticles suppress the grain formation, resulting in low surface roughness [72]. [PERSON] et al. studied the doping of Ti\({}^{4+}\) and Zn\({}^{2+}\) ions in nickel ferrites and changes in various properties of magnetic ferrite nanoparticles. Due to Ti\({}^{4+}\) ions' larger ionic radius and a different oxidation state compared to Fe\({}^{3+}\), they observed lattice distortions due  
Figure 2: Crystal structure of magnetic ferrites having AB\({}_{2}\)O\({}_{4}\) structure. Reproduced with permission from Ref. [65]. to Ti\({}^{4+}\) ions, which modifies grain boundaries and enhances electrical resistivity.