**3. Established methods for synthesis of nanocrystalline ferrites**

There are two methods for the preparation of magnetic nanoparticles: physical and chemical. The methods of generation of magnetic nanoparticles in the gas or solid phase using high-energy treatment of the material are usually called physical, while the nanoparticle syntheses, which are often carried out in solutions at moderate temperatures are chemical methods. Different routes have become an essential focus of the related research and development activities. Various fabrication methods to prepare spinel ferrites nanocrystals have been reported, e.g., sol-gel methods [18], the ball-milling technique [19], co-precipitation [20], polymeric assisted route [21] the hydrothermal method [22], the reverse micelles process [23], and the micro-emulsion method [24]. Various precipitation agents have been used to produce specific size and shape spinel ferrites nanocrystals, e.g., metal hydroxide in the co-precipitation method, surfactant and ammonia in the reverse micelles process and various micro-emulsion methods, and organic matrices in the sol-gel method. Most of these methods have achieved particles of the required sizes and shapes, but they are difficult to employ on a large scale because of their expensive and complicated procedures, high reaction temperatures, long reaction times, toxic reagents and by-products, and their potential harm to the environment.
