Origin of magnetic moments in defectiveTiO2single crystals

Abstract

In this paper we show that ferromagnetism can be induced in pure TiO2 single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman-scattering, and electron spin resonance spectroscopy, a defect complex, i.e. Ti 3+ ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated TiO2. This kind of defect complex results in a local (TiO6−x) stretching Raman mode. We elucidate that Ti 3+ ions with one unpaired 3d electron provide the local magnetic moments.Recently, ferromagnetism has been observed in nonmagnetically doped, but defective oxides, including TiO 2 1,2,3,4 . This kind of observation challenges the conventional understanding of ferromagnetism, which is rather due to spin-split states or bands. Thus, one fundamental question must be answered: where are the moments located? Intensive theoretical work has been performed to understand the ferromagnetism in defective oxides 5,6,7 . In these papers, the triplet states of p-like electrons, located at cation or oxygen vacancies, yield the local moments, leading to a kind of ferromagnetism without the involvement of 3d electrons. Experimentally the ferromagnetism in undoped TiO 2 has been found to relate with oxygen vacancies (O V ) 2,3 , however, its mechanism remains unclear. It is worth to note that Ti 3+ ions with one 3d electron are usually generated in slightly reduced TiO 2 . When O is removed, the excess electrons are unpaired 8 . They can occupy the nearby localized Ti 3d orbit and therefore convert Ti 4+ ions to Ti 3+ ions. In a reduced rutile TiO 2 (110) surface, such a defect complex, Ti 3+ -O V , has been well studied by first-principles calculations 9,10 and experimentally by resonant photoelectron diffraction 11 . Therefore, experimental work is needed to clarify whether the magnetic moments in defective TiO 2 is due to unpaired 3d electrons localized on Ti 3+ ions.Ion irradiation is a non-equilibrium and reproducible method of inducing defects. Energetic ions displace atoms from their equilibrium lattice sites, thus creating mainly vacancies and interstitials. The amount of defects can be controlled by the ion fluence and energy. In this paper, we irradiated rutile TiO 2 single crystals with 2-MeV O ions, resulting in a projected range of 1.52 µm and a longitudinal straggling of 0.16 µm as calculated by SRIM code (The Stopping and Range of Ions in Matter) 12 . As a result of this irradiation, the formation of Ti/O vacancies/interstitials is expected 12 . We selected high-energy oxygen ions as projectiles to avoid the introduction of foreign elements. Moreover, from a ballistic point of view, the creation of oxygen vacancies is more efficient, e.g., by a factor of 1.5 larger than the Ti-vacancy creation. From SRIM calculations it is also evident that, at the given energy, the maximum atomic concentration of the implanted oxygen ions is by a factor of 500 smaller than the concentration of oxygen recoils. For the region of maximum defect creation, i.e., at the end of the ion range, those project...

Magnetic and nonlinear optical properties of BaTiO3 nanoparticles

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Room temperature ferromagnetism in liquid-phase pulsed laser ablation synthesized nanoparticles of nonmagnetic oxides

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“…Saturation magnetization value for TiO 2 NPs in the present work is almost 4.5 fold larger than the only one report available until now for particles. 17 Three dimensional AFM and MFM images of the as synthesized titanium dioxide and zinc oxide NPs are shown in Fig. 4.…”

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