Origin of Magnetism in undoped and Ta doped anatase TiO₂ free-standing films

Thin films of anatase titanium dioxide (TiO₂) are semiconducting and exhibit signs of room temperature ferromagnetism. However, the role of the surface, substrate and interface in the observed magnetism are controversial. Here, the magnetic properties of two systems, Ta-doped anatase and undoped-anatase have been studied in detail. To avoid the effects from the substrate as well as interface we have fabricated free-standing films using an epitaxial, water-soluble Sr₃Al₂O₆ sacrificial buffer layer. We have found a strong linear correlation between the observed magnetism and Ta-doping, in addition to an intrinsic contribution. The magnetization exhibits a minimum as a function of the oxygen pressure present during film deposition and the result are consistent with a model where Ti interstitials and oxygen vacancies may be responsible for the magnetism at low oxygen pressure while at the high oxygen pressures, Ti vacancies dominate. This study proves that magnetism in anatase TiO₂ system is intrinsic and cannot be arising from either the interface or the substrate.