Iron oxides such as Fe
3O
4 and γFe
2O
3 have been used as magnetic materials. It is difficult to synthesize these oxides directly from iron by sputtering. F
3O
4 is synthesized by transformation from αFe
2O
3 using reduction reaction in H
2 gas. Thermodynamics states that O
2 gas pressure should be below 10
-30Pa in order for this transformation to take place at 250°C without H
2 gas. When αFe
2O
3 film was heated to 250°C by a tantalum filament, the transformation to Fe
3O
4 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+O
2→2CO, and the reaction of 2CO+O
2→2CO
2 occurs near a sample of αFe
2O
3 at 250°C. These reactions reduce O
2 gas pressure to 10
-42Pa so that the O
2 gas pressure is low enough to transform αFe
2O
3 to Fe
3O
4.
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