## 3 Application Fields of Ferrite  
The spinel ferrites have a broad range of very different applications [17]. They are used as inductors (as low-noise amplifiers, filters, voltage-controlled oscillators, and impedance-matching networks) [18], high-frequency materials (circulators, isolators, phase shifters, and antennas) [9], power supplies (computers, peripherals, TV, and video systems), [19] and Electromagnetic Interference (EMI) Suppression [20] and in biosciences. Many biotechnological applications are based on magnetic nanoparticles. For example, nanoparticles have been used to guide radionuclides to specific tissues as well as in magnetic resonance imaging (MRI). Magnetite superparamagnetic particles are selectively associated with healthy regions of some tissues, liver for instance [6; 7; 21] (Figure 3). Thermal energy from hysteresis loss of ferrites can be used in hyperthermia (Figure 3), a technique that makes use of heating of specific tissues or organs to treat cancer: The temperature in tumour tissues rises and more sensitivity to radio- or chemotherapy can be attained [21]. Figure 2: Schematic representation of the normal (**left**) and inverse (**right**) AB\({}_{2}\)O\({}_{4}\) spinel structure. A more recent field of application of spinels is electrochemistry. In these last several years, many spinels have been proposed as electrodes for lithium-ion batteries. We can cite ZnCoO\({}_{4}\), LiMn\({}_{2}\)O\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\), NiFe\({}_{2}\)O\({}_{4}\), Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\), and LiV\({}_{2}\)O\({}_{4}\) applied as cathodes or anodes in LIBs, but ZnFe\({}_{2}\)O\({}_{4}\) stands out from other analogous compounds as the most competitive [22; 23]. The main reasons are found in the low cost, abundance, and environmental friendliness of both Zn and Fe precursors, the high surface-to-volume ratio, the relatively short path for Li-ion diffusion, the low working voltage of about 1.5 V for lithium extraction and, most importantly, the high theoretical specific capacity (1072 mAh g\({}^{-1}\)) [24; 25]. The first report about ZnFe\({}_{2}\)O\({}_{4}\) as anode is quite recent and dates to 2004 [26]. Up to now, about 200 papers have been published on this topic [27; 28; 29; 30]. However, a large part of works are only academic researches because its practical application is still seriously hampered by many factors: fast capacity fading and poor rate capability, resulting from an inherent low electronic conductivity, severe agglomeration, and large volume change during the lithiation/delithiation processes [12; 22; 23]. To overcome these limits, different strategies have been proposed, even though the most satisfying and practicable is represented by the coating/embedding of ferrite particles in various architectures with carbon sources (Figure 4) [31; 32; 33; 34; 35; 36]. Figure 4: TEM (**a**,**b**) and HR-TEM (**c**,**d**) images of the Acetylene black/ZnFe\({}_{2}\)O\({}_{4}\)-NPs@C; the inset in (**d**) is the SAED pattern [31]. Reproduced with permission, Copyright Elsevier 2016. Figure 3: An example of the use of manganese ferrite, properly coated with citrate, in MRI and Hyperthermia [21]. Reproduced with permission, Copyright Elsevier 2015. The doping route, which is still however scarcely exploited, is also proposed as a successful strategy for improving the electrochemical performances of ferrites, particularly to increase their poor electronic conductivity. The most encouraging dopings of ZnFe\({}_{2}\)O\({}_{4}\) were performed with cations, such as Mn, Ni, Co, Mg, and Cu ions [37; 38; 39; 40], to cite the most used. Anionic doping is not common for the ferrite phases, while more diffused is the insertion of N and/or S species on the carbon sources used to obtain composites [41; 42; 43].