Magnetite (Fe3O4) has been synthesized by several methods. Among these methods, I thing that "coprecipitation" is a rapid, facile and good method for obtaining nanoparticles of magnetite. In this case, aqueous solutios of FeCl2 and FeCl3 should be mixed, and then the pH of this solution is elevated using diuluted ammonia. Reaction could be carried out under nitrogen. A temperature about 40°C is needed.
Other successful method is using microwave irradiation. for these synthesis.