Electronic structure and optical properties of the Co-doped anatase<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">TiO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>studied from first principles

Weng, Hongming; Yang, Xiaoping; Dong, Jianji; Mizuseki, Hiroshi; Kawasaki, Masashi; Kawazoe, Yoshiyuki

doi:10.1103/physrevb.69.125219

Cited by 163 publications

(69 citation statements)

References 18 publications

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“…The partially occupied band is spin polarized, and this polarization may enhance or partially compensate the magnetization of transition metal ions depending on the net sign of effective exchange in this band. 8,28,30 To conclude, we have proposed in this paper a mechanism of ferromagnetic exchange in dielectric non-stoichiometric TiO 2 doped with transition metal impurities, which involves the interaction between magnetic ions and oxygen vacancies and proposed a qualitative explanation of the basic properties of these materials. A quantitative calculations of electronic structure of magnetically doped TiO 2 based on the model described in this paper will be presented in forthcoming publications.…”

Section: Discussionmentioning

confidence: 99%

Superexchange in dilute magnetic dielectrics: Application to(Ti,Co)O2

Kikoin

Fleurov

2006

Phys. Rev. B

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We extend the model of ferromagnetic superexchange in dilute magnetic semiconductors to the ferromagnetically ordered highly insulating compounds (dilute magnetic dielectrics). The intrinsic ferromagnetism without free carriers is observed in oxygen-deficient films of anatase TiO 2 doped with transition metal impurities in cation sublattice. We suppose that ferromagnetic order arises due to superexchange between complexes [oxygen vacancies + magnetic impurities], which are stabilized by charge transfer from vacancies to impurities. The Hund rule controls the superexchange via empty vacancy related levels so that it becomes possible only for the parallel orientation of impurity magnetic moments. The percolation threshold for magnetic ordering is determined by the radius of vacancy levels, but the exchange mechanism does not require free carriers. The crucial role of the non-stoichiometry in formation of the ferromagnetism makes the Curie temperatures extremely sensitive to the methods of sample preparation. * Email:fleurov@post.tau.ac.il. Among the most salient features of magnetism in dilute ferromagnetic oxides one should mention an extreme sensitivity of the magnetic order to the growth and annealing conditions. 5, 8 We believe that this is an integral feature of magnetism in these materials, and the ferromagnetic ordering with high T C is mediated by intrinsic or extrinsic defects, which form complexes with magnetic dopants. Such point of view is supported by recent experimental studies of Co-doped 9 and Cr-doped 10 TiO 2 . It was noticed, in particular, that in the most perfect (Ti, Cr)O 2 crystals the hysteresis loop at a given temperature is essentially less distinct than in 'bad quality' samples. 10 Besides, the enormous scatter of effective magnetic moment per cation is observed in all ferromagnetic oxides, and the concentration of magnetic dopants lies far below the percolation threshold x c associated with the nearest-neighbor cation coupling. 8 Basing on these facts, one concludes that non-magnetic defects are also involved in formation of the localized magnetic moments. They influence their magnitude and localization, and this influence is sensitive to the growth and annealing regimes.Recent theories of magnetism in n-type dilute magnetic oxides appeal to magnetic polarons as mediators of indirect exchange between magnetic dopants. 8,11 In this case the extrinsic defects are donor impurities, which donate free electrons occupying the bottom of conduction band. These electrons form shallow spin-polarized polaronic states due to strong exchange with the localized magnetic moments of transition metal dopants. A large radius of these states makes the polaronic percolation threshold essentially lower than x c .One should note, however, that the carrier concentration in these compounds is vanishingly small, and that is why they have been proposed to be called dilute magnetic dielectrics (DMD). 9 It means that we need now to find an exchange mechanism, which works in the case of really insulating ox...

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Section: Discussionmentioning

confidence: 99%

Superexchange in dilute magnetic dielectrics: Application to(Ti,Co)O2

Kikoin

Fleurov

2006

Phys. Rev. B

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“…28 A recent theoretical model for high-T C ferromagnetism in oxide DMS is also based on the oxygen vacancy, which causes impurity band exchange. 29 Elucidation of the microscopic structure around Co and its relation with the mechanism of the ferromagnetism is strongly required.…”

Section: Discussionmentioning

confidence: 99%

X-ray anomalous scattering of diluted magnetic oxide semiconductors: Possible evidence of lattice deformation for high temperature ferromagnetism

et al. 2007

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We have examined whether the Co ions crystallographically substitute on the Ti sites in rutile and anatase Ti1−xCoxO 2−δ thin films that exhibit room-temperature ferromagnetism. Intensities of the x-ray Bragg reflection from the films were measured around the K-absorption-edge of Co. If the Co ions randomly substitute on the Ti sites, the intensity should exhibit an anomaly due to the anomalous dispersion of the atomic scattering factor of Co. However, none of the anatase and rutile samples did exhibit an anomaly, unambiguously showing that the Co ions in Ti1−xCoxO 2−δ are not exactly located at the Ti sites of TiO2. The absence of the anomaly is probably caused by a significant deformation of the local structure around Co due to the oxygen vacancy. We have applied the same method to paramagnetic Zn1−xCoxO thin films and obtained direct evidence that the Co ions are indeed substituted on the Zn sites.

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“…al. [14], a ferromagnetic superexchange coupling was instead proposed by Janish et. al., who did not find oxygen vacancies to play any role in the observed ferromagnetism in Co-doped TiO 2 [17].…”

Section: Introductionmentioning

confidence: 99%

“…Liu et al [13], on the other hand, proposed the combination of Al-donor and oxygen vacancy defects to be responsible for a strong carrier mediated ferromagnetic interaction in Co-doped ZnO. Transition metal-doped rutile and anatase TiO 2 are another class of oxide semiconductors that have been extensively studied for room temperature ferromagnetism [14][15][16][17]. Although, here too, defects have been proposed to enhance ferromagnetism, their exact role and the mechanism, by which they induce magnetic interactions, remain highly debatable.…”

Section: Introductionmentioning

confidence: 99%

Role of oxygen vacancies in room-temperature ferromagnetism in cobalt-substitutedSrTiO3

et al. 2014

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Ferromagnetic insulating behaviour has recently been demonstrated in cobalt-substituted SrTiO 3 at room temperature [1]. Experimentally, it was found that a well-defined hysteresis loop only occurs at high Co concentrations of 30-40%. X-ray photoelectron spectroscopy also indicated that Co substitutes for Ti with Co being in high-spin +2 oxidation state. In this work, we employ density-functional theory to explain the experimentally observed properties of cobalt-substituted SrTiO 3 . We examine in detail the role of oxygen vacancy (OV) defects and their formation of defect complexes with the Co ions as the origin of the ferromagnetic insulating behaviour. Our first-principles thermodynamic calculations indicate that OV defects are much more likely to occur next to Co atoms where their formation energies could be reduced by as much as 1.28 eV compared to that in bulk SrTiO 3 . We also find that Co in these Co-OV complexes occurs in the high-spin state in agreement with core level spectroscopy, and identify a linear arrangement of the Co-OV defect complexes to be the most energetically favourable structure. These defect complexes are also shown to interact ferromagnetically and that their magnetic interaction is found to be short-ranged, consistent with the relatively high Co concentrations needed experimentally for ferromagnetism to be observed in cobalt-substituted SrTiO 3 . I.Introduction The discovery of room-temperature ferromagnetism in doped transition metal oxides by Matsumoto et al. [2] has initiated many theoretical and experimental studies, owing to potential applications of these materials in spintronics. For example, , SnO 2 [5-6], In 2 O 3 [7][8] and TiO 2 [9][10], have all been reported to show room-temperature ferromagnetism when doped with magnetic ions. Increasing the Curie temperature and being able to control the spin degrees of freedom remain major challenges in this field. In this respect, defects in these materials, both intrinsic and extrinsic, play an important role. Defects provide additional carriers to the system (electrons or holes) which could significantly alter magnetic interactions and the Curie temperature in these materials.

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Electronic structure and optical properties of the Co-doped anataseTiO2studied from first principles

Cited by 163 publications

References 18 publications

Superexchange in dilute magnetic dielectrics: Application to(Ti,Co)O2

Superexchange in dilute magnetic dielectrics: Application to(Ti,Co)O2

X-ray anomalous scattering of diluted magnetic oxide semiconductors: Possible evidence of lattice deformation for high temperature ferromagnetism

Role of oxygen vacancies in room-temperature ferromagnetism in cobalt-substitutedSrTiO3

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