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Open accessJournal ArticleDOI: 10.1063/1.3690113

Interplay between carrier and cationic defect concentration in ferromagnetism of anatase Ti1-xTaxO2 thin films

16 Feb 2012-AIP Advances (American Institute of Physics)-Vol. 2, Iss: 1, pp 012148
Abstract: Thin films of Ta incorporated TiO2 grown by pulsed laser deposition under specific growth conditions show room temperature ferromagnetism. Ta introduces carriers and concomitantly cationic defects, the combination of which leads to ferromagnetism. In this paper, we report on the dependence of the carrier and cationic defect density (compensation) on various parameters such as oxygen growth pressure, temperature and Ta concentration. Most likely, the Ti vacancies act as magnetic centers and the free electrons help with the exchange leading to ferromagnetism via Ruderman-Kittel-Kasuya-Yosida mechanism.

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Topics: Ferromagnetism (56%), Pulsed laser deposition (53%), Thin film (51%)
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Journal ArticleDOI: 10.1063/1.4905150
Xianjie Wang1, Yongli Song1, L. L. Tao2, Jiafeng Feng2  +7 moreInstitutions (3)
Abstract: In this paper, we combine first-principles calculations and experiments to investigate the magnetic properties of aluminum-doped TiO2 films of rutile structure. Density-functional theory with generalized gradient approximation based calculations were carried out for three cases, where the TiO2 lattice contains oxygen vacancies VO only, an oxygen is substituted by a fluorine atom, or a Ti is substituted by an aluminum. Magnetic moments associated with the formation of Ti3+ ions are found in all cases but they couple differently resulting in different magnetic states. Al-doped samples prepared in our labs exhibit ferromagnetism at room temperature with a TC near 340 K. The experimental results are consistent with the first principles calculations, and the magnetism is associated with the VO defect electrons induced by the Al doping. The defect electron occupies nearby Ti sites giving rise to the Ti3+ moments and, at the same time, has spatially extended wavefunctions assuring overlapping between neighbors.

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Topics: Magnetism (58%), Ferromagnetism (57%), Magnetic moment (54%)

36 Citations


Journal ArticleDOI: 10.1039/C2TC00172A
Vishal Thakare1, Vishal Thakare2, Onkar Game1, Onkar Game2  +1 moreInstitutions (2)
Abstract: Ferromagnetism in metal oxide systems has always attracted scientific attention in view of the intriguing and interesting interplay of spin, charge, orbital and lattice degrees of freedom that such systems display. This trend appears to be continuing with enhanced focus on interface systems, multiferroics, diluted magnetic semiconducting oxides (DMSOs) and nanomaterial magnetism. Newer techniques are being applied to bring out materials issues that are critical to the precise understanding of the origin of magnetism in certain cases. Interest is also beginning to grow in exploring application potential of some such systems. In this article we review the developments in this field by late 2011 and 2012.

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Topics: Ferromagnetism (51%)

23 Citations


Journal ArticleDOI: 10.1063/1.4756799
Abstract: We report a Ni impurity induced reversible ferromagnetism and surface conduction in rutile TiO2 crystals subjected to specific thermal annealing. For annealing in vacuum at 800 °C, a growing ferromagnetic signal is seen with time while for a similar annealing in air, the magnetism vanishes. The magnetism is concomitant with a surface conductivity which at low temperatures shows tunneling characteristics. Here, we show that Ni magnetic impurity (in TiO2 crystals at <100 ppm) under vacuum annealing segregates to the surface over a 50 nm layer where the Ni concentration exceeds 10%–20% and drops with subsequent air annealing.

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Topics: Ferromagnetism (55%), Magnetic impurity (54%), Annealing (metallurgy) (53%) ...read more

18 Citations


Open accessJournal ArticleDOI: 10.1038/SREP13011
12 Aug 2015-Scientific Reports
Abstract: We report the observation of spatially separated Kondo scattering and ferromagnetism in anatase Ta0.06Ti0.94O2 thin films as a function of thickness (10–200 nm). The Kondo behavior observed in thicker films is suppressed on decreasing thickness and vanishes below ~25 nm. In 200 nm film, transport data could be fitted to a renormalization group theory for Kondo scattering though the carrier density in this system is lower by two orders of magnitude, the magnetic entity concentration is larger by a similar magnitude and there is strong electronic correlation compared to a conventional system such as Cu with magnetic impurities. However, ferromagnetism is observed at all thicknesses with magnetic moment per unit thickness decreasing beyond 10 nm film thickness. The simultaneous presence of Kondo and ferromagnetism is explained by the spatial variation of defects from the interface to surface which results in a dominantly ferromagnetic region closer to substrate-film interface while the Kondo scattering is dominant near the surface and decreasing towards the interface. This material system enables us to study the effect of neighboring presence of two competing magnetic phenomena and the possibility for tuning them.

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Topics: Kondo effect (70%), Kondo insulator (65%), Anderson impurity model (57%) ...read more

12 Citations


Journal ArticleDOI: 10.1016/J.JPCS.2016.09.011
Zhaorui Zou1, Zhongpo Zhou1, Zhongpo Zhou2, Haiying Wang1  +1 moreInstitutions (2)
Abstract: In this paper, we investigated the physical properties especially the magnetic properties of the TiO 2 films and Au cluster doped TiO 2 films fabricated by sol-gel and sputtering methods combined experiments and first-principles calculations. All the samples annealed under air and N 2 atmosphere respectively exhibit room temperature ferromagnetism with the crystal phase of anatase. The values of the saturation magnetizations are in the order of Au δ-doped TiO 2 (annealed in N 2 )>undoped TiO 2 (annealed in air)>Au δ-doped TiO 2 (annealed in air). The first principles calculation results show that the formation energy of Au cluster doped TiO 2 films is lower than that of the oxygen vacancy and Au cluster codoped TiO 2 films. The effects of the Au cluster dopant are the retard of the formation of surface oxygen vacancy and the electrons transfer from 3d states of Ti atoms to Au 5d states in Au cluster doped TiO 2 films. The codoping of surface oxygen vacancies, bulk oxygen vacancies and Au clusters led to the spin-split of Ti 3d and O 2p in Au cluster doped TiO 2 films (annealed in N 2 ) which yield the highest saturation magnetization.

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Topics: Vacancy defect (51%), Doping (50%)

8 Citations


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Journal ArticleDOI: 10.1126/SCIENCE.1065389
16 Nov 2001-Science
Abstract: This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

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Topics: Spin transistor (64%), Spin polarization (64%), Spin pumping (63%) ...read more

9,209 Citations


Open accessJournal ArticleDOI: 10.1103/REVMODPHYS.76.323
Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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  • FIG. 4. (Color in online edition) Pedagogical illustration of the concept of electrical spin injection from a ferromagnet (F) into a normal metal (N). Electrons flow from F to N: (a) schematic device geometry; (b) magnetization M as a function of position—nonequilibrium magnetization dM (spin accumulation) is injected into a normal metal; (c) contribution of different spin-resolved densities of states to both charge and spin transport across the F/N interface. Unequal filled levels in the density of states depict spin-resolved electrochemical potentials different from the equilibrium value m0 .
    FIG. 4. (Color in online edition) Pedagogical illustration of the concept of electrical spin injection from a ferromagnet (F) into a normal metal (N). Electrons flow from F to N: (a) schematic device geometry; (b) magnetization M as a function of position—nonequilibrium magnetization dM (spin accumulation) is injected into a normal metal; (c) contribution of different spin-resolved densities of states to both charge and spin transport across the F/N interface. Unequal filled levels in the density of states depict spin-resolved electrochemical potentials different from the equilibrium value m0 .
  • FIG. 7. (Color in online edition) Spatial variation of the electrochemical potentials near a spin-selective resistive interface at an F/N junction. At the interface x50 both the spinresolved electrochemical potentials (ml , l5↑ ,↓ , denoted with solid lines) and the average electrochemical potential (mF , mN , dashed lines) are discontinuous. The spin diffusion lengths LsF and LsN characterize the decay of ms5m↑2m↓ (or equivalently the decay of spin accumulation and the nonequilibrium magnetization) away from the interface and into the bulk F and N regions, respectively.
    FIG. 7. (Color in online edition) Spatial variation of the electrochemical potentials near a spin-selective resistive interface at an F/N junction. At the interface x50 both the spinresolved electrochemical potentials (ml , l5↑ ,↓ , denoted with solid lines) and the average electrochemical potential (mF , mN , dashed lines) are discontinuous. The spin diffusion lengths LsF and LsN characterize the decay of ms5m↑2m↓ (or equivalently the decay of spin accumulation and the nonequilibrium magnetization) away from the interface and into the bulk F and N regions, respectively.
  • FIG. 31. (Color in online edition) Giant magnetoresistance (GMR) effect in magnetic diodes. Current/spin-splitting characteristics (I2z) are calculated self-consistently at V50.8 V for the diode from Fig. 30. Spin splitting 2qz on the p side is normalized to kBT . The solid curve corresponds to a switchedoff spin source. The current is symmetric in z. With spin source on (the extreme case of 100% spin polarization injected into the n region is shown), the current is a strongly asymmetric function of z, displaying large GMR, shown by the dashed curve. Materials parameters of GaAs were applied. Adapted from Žutić et al., 2002.
    FIG. 31. (Color in online edition) Giant magnetoresistance (GMR) effect in magnetic diodes. Current/spin-splitting characteristics (I2z) are calculated self-consistently at V50.8 V for the diode from Fig. 30. Spin splitting 2qz on the p side is normalized to kBT . The solid curve corresponds to a switchedoff spin source. The current is symmetric in z. With spin source on (the extreme case of 100% spin polarization injected into the n region is shown), the current is a strongly asymmetric function of z, displaying large GMR, shown by the dashed curve. Materials parameters of GaAs were applied. Adapted from Žutić et al., 2002.
  • FIG. 16. Measured and calculated ts in Al. The low-T measurements are the conduction-electron spin resonance (Lubzens and Schultz, 1976b) and spin injection (Johnson and Silsbee, 1985). Only the phonon contribution is shown, as adapted from Johnson and Silsbee (1985). The solid line is the first-principles calculation, not a fit to the data (Fabian and Das Sarma, 1998). The data at T5293 K are results from room-temperature spin injection experiments of Jedema et al. (2002a, 2003). Adapted from Fabian and Das Sarma, 1999b.
    FIG. 16. Measured and calculated ts in Al. The low-T measurements are the conduction-electron spin resonance (Lubzens and Schultz, 1976b) and spin injection (Johnson and Silsbee, 1985). Only the phonon contribution is shown, as adapted from Johnson and Silsbee (1985). The solid line is the first-principles calculation, not a fit to the data (Fabian and Das Sarma, 1998). The data at T5293 K are results from room-temperature spin injection experiments of Jedema et al. (2002a, 2003). Adapted from Fabian and Das Sarma, 1999b.
  • FIG. 27. (Color in online edition) Electric-field control of ferromagnetism. RHall vs field curves under three different gate biases. Application of VG50, 1125, and 2125 V results in a qualitatively different field dependence of RHall measured at 22.5 K (sample B): almost horizontal dash-dotted line, paramagnetic response when holes are partially depleted from the channel (VG51125 V); dashed lines, clear hysteresis at low fields (,0.7 mT) as holes are accumulated in the channel (VG52125 V); solid line, RHall curve measured at VG50 V before application of 6125 V; dotted line, RHall curve after application of 6125 V. Inset, the same curves shown at higher magnetic fields. From Ohno, Chiba, et al., 2000.
    FIG. 27. (Color in online edition) Electric-field control of ferromagnetism. RHall vs field curves under three different gate biases. Application of VG50, 1125, and 2125 V results in a qualitatively different field dependence of RHall measured at 22.5 K (sample B): almost horizontal dash-dotted line, paramagnetic response when holes are partially depleted from the channel (VG51125 V); dashed lines, clear hysteresis at low fields (,0.7 mT) as holes are accumulated in the channel (VG52125 V); solid line, RHall curve measured at VG50 V before application of 6125 V; dotted line, RHall curve after application of 6125 V. Inset, the same curves shown at higher magnetic fields. From Ohno, Chiba, et al., 2000.
  • + 32

Topics: Spin engineering (75%), Spin polarization (70%), Spin Hall effect (67%) ...read more

8,325 Citations


Journal ArticleDOI: 10.1126/SCIENCE.281.5379.951
Hideo Ohno1Institutions (1)
14 Aug 1998-Science
Abstract: REVIEW Semiconductor devices generally take advantage of the charge of electrons, whereas magnetic materials are used for recording information involving electron spin. To make use of both charge and spin of electrons in semiconductors, a high concentration of magnetic elements can be introduced in nonmagnetic III-V semiconductors currently in use for devices. Low solubility of magnetic elements was overcome by low-temperature nonequilibrium molecular beam epitaxial growth, and ferromagnetic (Ga,Mn)As was realized. Magnetotransport measurements revealed that the magnetic transition temperature can be as high as 110 kelvin. The origin of the ferromagnetic interaction is discussed. Multilayer heterostructures including resonant tunneling diodes (RTDs) have also successfully been fabricated. The magnetic coupling between two ferromagnetic (Ga,Mn)As films separated by a nonmagnetic layer indicated the critical role of the holes in the magnetic coupling. The magnetic coupling in all semiconductor ferromagnetic/nonmagnetic layered structures, together with the possibility of spin filtering in RTDs, shows the potential of the present material system for exploring new physics and for developing new functionality toward future electronics.

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Topics: Spin polarization (63%), Ferromagnetic resonance (62%), Magnetic domain (62%) ...read more

4,189 Citations


Journal ArticleDOI: 10.1063/1.341700
Abstract: We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++‐Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susc...

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2,789 Citations


Journal ArticleDOI: 10.1126/SCIENCE.1056186
Yuji Matsumoto1, Makoto Murakami1, T. Shono1, Tetsuya Hasegawa1  +6 moreInstitutions (1)
02 Feb 2001-Science
Abstract: Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin.

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Topics: Magnetic semiconductor (63%), Magnetic domain (61%), Ferromagnetism (58%) ...read more

2,238 Citations


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