Nonstoichiometry of epitaxial FeTiO_3+δ films

Epitaxial thin films of (001)-oriented FeTiO_3+δ were prepared on α-Al₂O₃(001) single crystalline substrates by helicon plasma sputtering technique. The FeTiO_3+δ films had large oxygen nonstoichiometry, which seriously depended on both substrate temperature and oxygen pressure during the sputtering deposition. The valence states of Fe ions in FeTiO_3+δ changed monotonically from Fe²⁺ to Fe³⁺ with decreasing the substrate temperature from 900 to 400°C or with increasing the oxygen pressure from 0.9 to 1.8×10^-6 Pa. The change of Fe valence states from Fe²⁺ to Fe³⁺ induced the magnetic phase transition only for the films prepared at 900°C. The films containing Fe²⁺ were paramagnetic while those with Fe³⁺ were antiferromagnetic at room temperature. The oxygen nonstoichiometry of the FeTiO_3+δ films was probably produced by cation vacancies and disarrangement of Fe³⁺ and Ti⁴⁺ ions, which randomly occupied both interstitial and substitutional sites of the FeTiO₃ related structure.