Room-temperature ferromagnetism in highly conducting, transparent anatase Ti1−xTaxO2 (x∼0.05) thin films grown by pulsed laser deposition onto LaAlO3 substrates was studied. Rutherford back-scattering spectrometry, X-ray diffraction, proton-induced X-ray emission, X-ray absorption spectroscopy and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of ferromagnetism with concomitant large carrier densities was determined by a combination of superconducting quantum interference device magnetometry, electrical transport measurements, soft X-ray magnetic circular dichroism, X-ray absorption spectroscopy and optical magnetic circular dichroism, and was supported by first-principles calculations. The soft X-ray magnetic circular dichroism and X-ray absorption spectroscopy measurements revealed a 90% contribution to ferromagnetism from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements revealed complete Ta substitution at the Ti sites, though carrier activation was only 50% at a 5%Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (VTi) and Ti3+ was studied via X-ray absorption spectroscopy and X-ray photo-emission spectroscopy, respectively. It was found that, in films with strong ferromagnetism, the VTi signal was strong while the Ti3+ signal was absent. It was proposed, in the absence of any obvious exchange mechanisms, that the localized magnetic moments, VTi sites, were ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism was an alternative route to ferromagnetism in wide-band-gap semiconducting oxides without any magnetic elements.

Cationic-Vacancy-Induced Room-Temperature Ferromagnetism in Transparent, Conducting Anatase Ti1-xTaxO2 (x∼0.05) Thin Films. Rusydi, A., Dhar, S., Barman, A.R., Ariando, Qi, D.C., Motapothula, M., Yi, J.B., Santoso, I., Feng, Y.P., Yang, K., Dai, Y., Yakovlev, N.L., Ding, J., Wee, A.T.S., Neuber, G., Breese, M.B.H., Ruebhausen, M., Hilgenkamp, H., Venkatesan, T.: Philosophical Transactions of the Royal Society A, 2012, 37[1977], 4927-43