Cationic-vacancy-induced room-temperature ferromagnetism in transparent, conducting anatase Ti _{1− x} Ta _x O ₂ ( x ∼0.05) thin films

Abstract

We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti 1−x Ta x O 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, * Authors for correspondence (phyandri@nus.edu.sg; eleds@nus.edu.sg; venky@nus.edu.sg). † These authors contributed equally to this work. 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.

Bandgap Control of the Oxygen‐Vacancy‐Induced Two‐Dimensional Electron Gas in SrTiO3

“…In addition, cationic vacancies in oxide films are expected to be more in high oxygen pressure depositions [33] and STO films deposited at various oxygen partial pressures (10 -2~1 0 -5 Torr) exhibit similar bandgap (see Figure S5 of the Supporting Information), which further supports that the bandgap enhancement is not due to cationic vacancies. Figure S6 of the Supporting Information).…”

Controlling the Electrical Properties of Undoped and Ta‐Doped TiO2 Polycrystalline Films via Ultra‐Fast‐Annealing Treatments

“…[ 12 ] Although such a result is expected from defect chemistry considerations, the exact mechanisms underlying this process and involving different phenomena (crystallization, oxygen incorporation) are still debated especially in the case of donor-doped TiO 2 . [20][21][22][23][24][25][26][27] In this contribution, we focus on how the post-deposition annealing treatments performed in different atmospheres and different heating/cooling rates affect the electrical properties of undoped as well as tantalum doped anatase TiO 2 thin fi lms. More importantly, we investigate the role of the change of the microstructure (from amorphous to crystalline) and of the oxygen exchange with the surroundings on the fi nal electrical conductivity of the material.…”

XPS study of cobalt doped TiO2 films prepared by pulsed laser deposition

“…The ferromagnetism has strong correlation with the defects induced by the deposition under low oxygen partial pressure, such as oxygen vacancies. In doped or modified TiO 2 materials, Ti 3+ state is supposed to play an important role in the room temperature ferromagnetism and also other physical properties …”