Selective growth of α-Fe2O3, γ-Fe2O3 and Fe3O4 at low temperatures and under ambient pressure

Abstract

We fabricated thin films of α- and γ-Fe2O3 and Fe3O4 at 400 °C under ambient (non-vacuum) conditions by using a mist chemical vapor deposition (mist CVD) method and investigated their structural and physical properties. We found that the structural phases of the films with preferred orientations depend on surface structures of substrate crystals and that the valence states of Fe in the films depend on solvents in which the Fe precursor (Iron (III) acetylacetonate) was dissolved. When films are grown by atomizing aqueous solutions of the precursor, (001) α-Fe2O3 and (001) γ- Fe2O3, both of which have only Fe3+, are grown on (001) Al2O3 and (001) SrTiO3 substrates, respectively. On the other hand, when methanol-rich solutions (98% methanol and 2% water in volume) are atomized, (111) and (001)-oriented Fe3O4 films which accommodate Fe3+ and Fe2+ were grown on the Al2O3 and SrTiO3 substrates, respectively. Magnetization measurements show that the γ-Fe2O3 and Fe3O4 films have room temperature saturated magnetizations as large as 200 and 500 emu/cc. Furthermore, the Fe3O4 films exhibit drastic decreases in the magnetization at 110 K, which are attributed to the Verwey transition. These results indicate that mist CVD is a nice technique that can enable one to control both crystal structures and Fe valence states in iron oxide films.