Heating Effect of a Filament on Transformation from [alpha]Fe2O3 to Fe3O4 Thin Film

Iron oxides such as Fe3O4 and γFe2O3 have been used as magnetic materials. It is difficult to synthesize these oxides directly from iron by sputtering. F3O4 is synthesized by transformation from αFe2O3 using reduction reaction in H2 gas. Thermodynamics states that O2 gas pressure should be below 10-30Pa in order for this transformation to take place at 250°C without H2 gas. When αFe2O3 film was heated to 250°C by a tantalum filament, the transformation to Fe3O4 occurred in a total pressure of 10-3Pa. This contradiction is explained thermodynamically by the formation of carbon oxides. The heated filament at 2000-3000°C enhances the reaction of 2C+O2->2CO, and the reaction of 2CO+O2->2CO2 occurs near a sample of αFe2O3 at 250°C. These reactions reduce O2 gas pressure to 10-42Pa so that the O2 gas pressure is low enough to transform αFe2O3 to Fe3O4.