# Magnetic Self-Healing Composites: Synthesis and Applications
## 2 Magnetic (Nano)Particles
### Synthesis Methods  
In the last two decades, several synthetic routes have been developed to prepare magnetic (nano)particles with good control over the particle size, polydispersity, shape, crystallinity and magnetic properties [1; 2]. Top-down and bottom-up approaches are the two main strategies for preparing magnetic (nano)particles. The former starts from macroscopic structures whose size is reduced in the nanoscale range by physical methods such as, among others, laser evaporation [15], ball milling [16; 17] and wire explosion [18]. The latter, generally based on biological or chemical routes, have been much more explored due to the lower cost of production, greater reaction yields and better spatial control over the system's characteristics. Among the chemical routes, coprecipitation and thermal decomposition were the first to be explored and became the most popular due to their improved performance compared to other strategies (see Figure 1) [2; 19; 20]. As the most conventional route, coprecipitation is based on mixing metal salts in an alkaline medium without surfactants [19; 20]. Temperature, precursor concentrations, reaction time and stirring are the key parameters that determine the final particle characteristics. The advantages of the coprecipitation route are its simplicity, low price and great efficiency, allowing for the production of magnetic (nano)particles in large amounts. Nevertheless, the systems synthesized by this method usually show poor control over the particle morphology, a broad size distribution, and the tendency to form particle agglomerates that hinder the control over their dispersion into the polymeric matrix. This methodology has been widely explored for the synthesis of ferrite magnetic (nano)particles in sizes ranging from a few to hundreds of nanometres. In particular, magnetic iron oxide nanoparticles (NPs) composed of magnetite (Fe\({}_{3}\)O\({}_{4}\)) or maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)) have been among the most commonly studied because of their ease of production, low toxicity and good magnetic properties [21; 22].  
Thermal decomposition employs, on the other hand, high temperatures to decompose organometallic precursors in the presence of organic surfactants, e.g., fatty acids, amines and alcohols, using organic solvents with high boiling points [23]. Generally, this route allows for the production of magnetic (nano)particles with an excellent control over the structural properties such as a narrow size distribution, a well-defined particle morphology and a good crystallinity. However, their main bottlenecks are the elevated price per synthesis, the high energy consumption and the difficult control over the complex reaction profiles that involve many reagents. In a similar way to coprecipitation, this synthetic route has proven to be ideal for the preparation of magnetic iron oxide (nano)particles in a wide range of sizes [24; 25; 26].  
Figure 1: (**a**) Scheme depicting the main routes to synthesize magnetic (nano)particles. Diagrams for the preparation of magnetic (nano)particles employing (**b**) coprecipitation and (**c**) thermal decomposition approaches [22].