Cationic-vacancy-induced room-temperature ferromagnetism in transparent, conducting anatase Ti _(1-x)Ta _xO _2 (x ～0.05) thin films

摘要

We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti _(1-x)TaxO _2 (x ～0.05) thin films grown by pulsed laser deposition on LaAlO _3 substrates. Rutherford backscattering spectrometry (RBS), X-ray diffraction, protoninduced X-ray emission, X-ray absorption spectroscopy (XAS) and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of FM 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 (SXMCD), XAS and optical magnetic circular dichroism, and was supported by first-principles calculations. SXMCD and XAS measurements revealed a 90 per cent contribution to FM from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites, though carrier activation was only 50 per cent at 5 per cent Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (V _(Ti)) and Ti ~(3+) was studied via XAS and X-ray photoemission spectroscopy, respectively. It was found that, in films with strong FM, the V _(Ti) signal was strong while the Ti ~(3+) signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localized magnetic moments, V _(Ti) sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to FM in wide-band-gap semiconducting oxides without any magnetic elements.