Magnetite and other iron-based materials are now expected for advanced medical applications. In a wet-chemical synthesis of magnetic particles, ultrasound is very useful to reduce the process time, to control the nucleation, and to control oxidation from Fe2+ to Fe3+. At a glance, the last term should be enhanced by ultrasonication. However, it is not always the case. If the initial precipitate of Fe(OH)2 is dissolved effectively by the ultrasound, more Fe2+ ions are supplied to the system, and consequent product of magnetite (Fe3O4) involves more amount of Fe2+. It should be indicated that chemical formula of Fe3O4 is somewhat misunderstanding because it has non-stoichiometry from Fe3O4 (= FeO-Fe2O3) to γ-Fe2O3, which are not distinguished by a usual XRD (x-ray diffraction) measurement.
