Improved resistance to oxidation of Ge-doped Fe3O4 thin films with cation excess composition using a sputtering target of FeO

The correlation between the composition of Ge-doped Fe3O4 thin films, including excess cation from stoichiometry, and resistance to oxidation during heat treatment in air was investigated. The films were prepared on glass substrates by radio-frequency sputtering using a target of Ge chips set on a ceramic FeO disk. Ge-doped Fe3O4 films with cation excess compositions were obtained by optimizing the deposition conditions, and five of these samples with Ge concentrations ranging from 0 to 4.3 at. % were heat treated in air at 673 K for up to 113 days. The magnetization after heat treatment was largest at 2.4 at. % Ge, which was larger than that of Fe3−δO4 (δ = 0.054) doped with Ge thin film. In other words, it is clear that the resistance to oxidation is better in the cation excess composition.