The magnetic properties of the Fe oxides have been extensively studied; in particular, the enhancing magnetic recording properties of magnetic and maghemite for nanostructure materials, or the use of the latter in Fe2O3-SiO2 composite materials having magneto- optical properties. Most physico-chemical studies are centered in the alpha (corundum structure with a distorted hexagonal anion closed – packed) and gamma (cubic inverse spinel) phases. Size stability of the polymorphs has been studied but there is still a lack of consensus in a significant number of issues; particularly related to the existence of nano-particles with alpha structure.