Systematic control of size, shape, structure, and magnetic properties of uniform magnetite and maghemite particles

As an application of the gel–sol method especially developed for the synthesis of general monodisperse particles in large quantities, uniform hematite ( α-Fe 2O 3), magnetite (Fe 3O 4), and maghemite ( γ-Fe 2O 3) particles, precisely controlled in size, aspect ratio, and internal structure, have been prepared. For the synthesis of uniform ellipsoidal single-crystal particles of α-Fe 2O 3, a highly condensed suspension of fine β-FeOOH particles doped with a prescribed amount of PO 4 3− ion in their interiors was aged at 140 °C for 24 h with seed particles of α-Fe 2O 3 in an acidic medium containing optimum concentrations of HCl and NaNO 3. Systematic control of the aspect ratio and mean size was achieved by regulating the concentration of PO 4 3− ion incorporated into the β-FeOOH particles and the number of seeds added. The resulting hematite particles were converted into magnetite by reduction in a H 2 stream at 330 °C for 6 h; the magnetite was then oxidized to maghemite in an air stream at 240 °C for 2 h. Magnetite and maghemite thus prepared retained the original shape of the hematite. On the other hand, polycrystalline hematite particles of different sizes and aspect ratios were also prepared by aging a condensed Fe(OH) 3 gel in the presence of different concentrations of SO 4 2− ion and seeds. The polycrystalline hematite particles were similarly converted into magnetite and then maghemite. The magnetic properties of these magnetite and maghemite particles were analyzed as a function of their mean particle volume, aspect ratio, and internal structure.