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Structural and Magnetic Properties of Diluted Magnetic Oxide Based on Nanostructured Co-doped Anatase TiO (Ti CoO ) 2

1-x

x

2-#

Talita Evelyn Souza, Alexandre Mesquita, Angela Ortiz de Zevallos, Fanny Béron, Kleber R. Pirota, Person P Neves, Antonio Carlos Doriguetto, and Hugo Bonette de Carvalho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4017129 • Publication Date (Web): 29 Mar 2013 Downloaded from http://pubs.acs.org on April 6, 2013

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The Journal of Physical Chemistry

Structural and Magnetic Properties of Diluted Magnetic Oxide Based on Nanostructured Co-doped Anatase TiO2 (Ti1-xCoxO2-) Talita E. de Souza,† Alexandre Mesquita,† Angela O. de Zevallos,† Fanny Béron,‡ Kleber R. Pirota, ‡ Person P. Neves, † Antonio C. Doriguetto † and Hugo B. de Carvalho †, * †

Universidade Federal de Alfenas, 37130-000 Alfenas-MG, Brazil

‡

Instituto de Física GlebWataghin, Universidade Estadual de Campinas, 13083-859 Campinas-

SP, Brazil

ABSTRACT: Nanostructured Co-doped anatase TiO2 (Ti1xCoxO2) samples were prepared and studied with particular emphasis on their compositional, structural, and magnetic properties. A detailed microstructural analysis was carried out to investigate the nature of the Co incorporation into the anatase TiO2 matrix. Conjugating different techniques we confirmed the Ti4+ replacement by Co2+ ions in the anatase TiO2 structure. No segregated secondary phases neither Co-rich nanocrystals were detected. Co doping introduces oxygen vacancies in the system by means of a charge compensation process. Superconducting quantum interference device magnetometry demonstrates a paramagnetic Curie–Weiss behavior with antiferromagnetic interactions even in the presence of high density of oxygen vacancies. The fitting of the M(H) in

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the limit of low and high temperatures enable us to extract the fraction of isolated and antiferromagnetically coupled Co ions. We discuss the observed magnetic behavior of our samples considering the up to date main theories for the magnetic properties of diluted magnetic oxides.

KEYWORDS: diluted magnetic oxides, polymeric precursor method, nanostructured materials, defect vs. carrier related ferromagnetism.


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I. INTRODUCTION In the last decades diluted magnetic semiconductors (DMS) have raised in interest by their potential use in spintronic devices. In these kinds of semiconductors it would be possible to manipulate both charge and spin of the carriers by applying external electrical fields.1 Among the most studied DMS are the III-V and II-VI compounds doped with transition metals (TM). However, the highest Curie temperature that these semiconductors can achieve is lower than 190 K.2–4 Since Dietl et al.5 predicted theoretically ferromagnetism at room temperature in wide bandgap semiconductors, in special for the oxide materials doped with transition metals (Diluted Magnetic Oxides), several theoretical and experimental results have appeared. Matsumoto et al.6 were the first ones to report ferromagnetism at room temperature in these types of semiconductors. They grew anatase Ti1xCoxO2 thin films with cobalt concentration between 0 to 8% and obtained a magnetic moment of 0.32 µB per Co atom. Following this work, films and nanopowders with anatase and rutile structures have been synthesized by different physical and chemical techniques. In these studies magnetic moments between 0.16 µB to 1.7 µB per Co atom were reported.7 Such a large variety of magnetic moments has hindered the understanding of room temperature ferromagnetism (RTFM) in these materials. Matsumoto’s studies attribute the RTFM of Ti1xCoxO2 to local Co spins which substitute Ti atoms in the TiO2 matrix.6,8 On the other hand, other studies have found evidence of Co cluster formation instead of substitution doping of the matrix.9–11 For example, Kim et al.11 found evidence for cluster formation by using magnetic circular dichroism results. More recent theoretical and experimental results point out the importance of oxygen vacancies (VO). The interaction between Co ions and VO in Co-doped TiO2 was found to play an important role to achieve a magnetic ordering in such kind of



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systems.12-15 However, the influence of the VO and the electron density on the RTFM could not be clearly distinguished. Currently, the main theoretical models proposed to describe the origin and properties of ferromagnetism suppose that electrons are introduced by donor defects into the conduction band16 or forming bound magnetic polarons (BMP)17 that mediate ferromagnetic couplings between TM ions. VO was claimed to give rise to RTFM rather than the electron carriers,18 but very recent experimental results show, in the contrary, that it would be not the case.19 In the present work we report a study of the structural and the magnetic properties of nanostructured anatase Ti1xCoxO2 ( = oxygen vacancies) samples with Co molar concentrations up to 12 at.% prepared by the polymeric precursor method. This technique can be classified as a near-equilibrium process, which allows the study of nanostructures with control of size, shape, size distribution and crystallinity.20 The Co doping of the anatase TiO2 matrix introduces holes in the system, as Co2+ substitutes the Ti4+ in the structure. In this case, a charge compensation process takes place and VO are promoted to give the necessary compensating electrons.21 Therefore, in our samples, we would expect to introduce VO in a direct proportion to the Co concentration. It would also be reasonable to state that the Coulombic attraction energetically favors close proximity between VO and Co2+ sites, forming Co2+ + VO2 complexes.22,23 It has been shown that VO in anatase produce shallow donor levels that dope the material n type,24 for each , 2 electrons are introduced into valence band states consisting of Ti 3d and O 2p electrons.25 However, these VO have been found not to directly contribute to the electrical conductivity of the system.26,27 On the contrary, the resistivity increases by orders of magnitude upon doping.28,29 In this context, the aim of our work is to shine light over the


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ambiguous situation concerning the influence of the VO in the promoting and stabilization of the RTFM in TiO2 based diluted magnetic oxides.

II. EXPERIMENTAL The Ti1-xCoxO2- nanopowders were synthetized following the Pechini method illustrated on Scheme 1.30 Titanium isopropoxide, Ti[OCH(CH3)2]4 (Sigma-Aldrich, 97%) and cobalt nitrate hexahydrate, Co(NO3)2.6H2O (Dinâmica Química Comtemporânea LTDA, 98%) were the metal sources used in stoichiometric amounts in the synthesis. Citric acid and ethylene glycol were used to complex the metallic cations. In a first step, titanium isopropoxide was dissolved in an aqueous citric acid solution at 60º C by stirring. The amount of Ti in the solution (in mass) was determined by gravimetry. Another solution was obtained by the dissolution of cobalt nitrate hexahydrate in an aqueous citric acid solution. The solutions were mixed and after the formation of a completely homogenous solution ethylene glycol was then added to promote mixed citrate polymerization by polyesterification reaction. This solution was kept under stirring for approximately 1 h at 60º C. The citric acid/metal molar ratio and the citric acid/ethylene glycol mass ratio were 4:1 and 60:40, respectively. A viscous resin was formed by increasing the temperature to 120º C. The polymeric precursor resin was heat-treated at 300° C for 2 h at a heating rate of 5° C/min. The obtained solid was grounded in an agate mortar. Finally, the powder was heat-treated at 450ºC for 2 h in an electric furnace under air atmosphere in order to stabilize the crystalline phase of formed products.



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Scheme 1. Scheme of the Pechini method reactions; n+ is the metallic cation oxidation state.

The structural properties were investigated by X-ray diffraction (XRD) recorded in the range of 2 = 20°80° with steps of 0.02° at 5 s/step by using Cu-Ka radiation (λ = 1.542 Å) of a Rigaku Ultima IV diffractometer. Structural analysis was performed using Rietveld method as implemented by the software General Structure Analysis System (GSAS) package with the graphical user interface EXPGUI.31,32 The morphology and structure were characterized by using a Tecnai G2-20 SuperTwin FEI high-resolution transmission electron microscopy (HRTEM), operating at 200 kV and a backscattered electron detector (BSE) of a SEM-LV JEOL JSM 6360LV that has a resolution down to 3 nm at 20 kV. Elemental analyses were performed by energy dispersive x-ray spectrometry (EDS). Unpolarized Raman scattering measurements was carried out at room temperature using a Jobin-Yvon-64000 micro-Raman system in the backscattering geometry using the 532 nm line of a solid state laser for excitation. We used an optical objective of 100X magnification, which gives an average laser spot size of 1 m. Co Kedge (7709 eV) X-ray absorption near-edge structure (XANES) and extended X-ray absorption


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fine structure (EXAFS) were used to determine the valence state and to evaluate the environment of Co in the TiO2 anatase lattice. The measurements were performed in the transmittance mode at the XAFS1 beamline at the Brazilian Synchrotron Light Laboratory (LNLS), Campinas, Brazil. Titanium L-edges and oxygen K-edge spectra were measured using the PGM beamline also at LNLS synchrotron facility. These XANES spectra (453 and 543 eV, respectively) were collected at room temperature using the electron yield mode. The extraction, normalization and fitting of the spectra were performed using the Multi-Platform Applications for X-Ray absorption (MAX) software package and theoretical spectra were calculated using FEFF9 code.33,34 The main magnetic measurements were performed in a Quantum Design superconducting quantum interference device (SQUID) while the room temperature ones were performed in a LakeShore vibrating sample magnetometer (VSM). Around 30 mg of each sample powder was encapsulated and measured following the same procedure. The magnetization function of applied field curve was first measured in the VSM at 300 K, from 20 to –20 kOe by 0.5 kOe increment in point by point mode. After, the sample was inserted in the SQUID at 300 K and zero field. A fixed field of 200 Oe was then applied and the magnetization recorded while sweeping the temperature between 300 and 2 K at a fixed rate of 2 K/min. The DC susceptibility was extracted from the ratio between the measured magnetization and the applied field (200 Oe). Finally, the magnetization curve was recorded at 2 K, from 50 to –50 kOe by 5 kOe increment.

III. RESULTS AND DISCUSSION Figure 1 shows the XRD spectra of the prepared Ti1-xCoxO2- samples. The peak positions and their relative intensities are consistent with the standard powder diffraction pattern of anatase



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TiO2. No Co or other foreign peaks were observed within the XRD detection limit. The Rietveld refinements were initiated with Ti+4 and O-2 atoms located at (0, 1/4, 3/8) and (0, 1/4, z), respectively. No occupancy was refined due to poor crystallite consequence of the nanometric particle size. The occupancy was fixed to the nominal composition. The final fitted patterns match quite well the experimental data. Table 1 presents the determined cell parameters (a) along the atomic positional parameters. The cell parameters increase with the Co concentration, revealing a presence of a tensile strain introduced in the system due the Co doping. This increase is expected considering that Co2+ ions (high spin state octahedral ionic radius = 0.885 Å) substitute the Ti4+ ions (octahedral ionic radius = 0.745 Å) in the anatase matrix.35 Therefore, the XRD results indicate that Co ions keep the 2+ oxidation state, since Co3+ ions has a high spin state octahedral ionic radius (0.75 Å) very similar to the Ti4+ one, not resulting in significant cell parameters variation. Keeping in mind the fact that the substitution of the 4+ by a 2+ cations in the anatase structure leads also to the introduction of VO in the system, we can conclude that a substitutional Co2+ doping of the TiO2 matrix, would lead to structure distortions in the sense as we, in fact, observe. The DRX results are an indication that Co in our Ti1-xCoxO2- nanopowders assumes a 2+ oxidation state taking the sites of the Ti4+ cations. The Co 2+ oxidation state will be further confirmed by the local-structural analyses.


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Figure 1. Refined XRD diffractograms of Ti1-xCoxO2- samples. It is shown the observed pattern (symbols), Rietveld calculated pattern (solid line), and the goodness of the fit or residual pattern (at the bottom).

Table 1. Structural data for the nanostructured Ti1-xCoxO2- samples obtained through the Rietveld refinement. xN is the Co nominal concentration, 2 is the square of goodness-of-fit indicator, and RWP is the refinement quality parameter. Volume (Å3)

Rwp (%)

χ2

xN = 0.03 3.7942(4)

136.34(4)

13.87

2.49

xN = 0.06 3.7954(4)

136.42(4)

14.30

2.37

xN = 0.09 3.7957(4)

136.49(4)

13.89

2.19

xN = 0.12 3.7972(3)

136.69(4)

14.59

2.45

Sample

a (Å)

In order to check the morphology of the samples and probe the possible presence of secondary phases or chemical phase separation, high-resolution transmission electron microscopy (HRTEM) studies were performed. Figure 2(a) presents a representative HRTEM micrograph of



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the sample with xN = 0.03. The particle size distribution was evaluated (Figure 2(b)), and the obtained results for the whole set of samples are listed in Table 2. The prepared Ti1-xCoxO2- samples are nanostructured with main average diameter size below 10 nm. To improve the statistical analysis of the HRTEM studies we have also taken a series of scanning electron microscope (SEM) images over large areas (not show). It is worth to point out that electron microscopy results do not reveal any evidence of crystallographic secondary phase or local aggregation of Co ions (Co-rich nanoclusters), in good agreement with XRD results. The effective Co concentrations of the Ti1-xCoxO2- samples (xE) are also presented in Table 2.

Figure 2. (a) Representative HRTEM micrograph and (b) particle size distribution histogram of Ti1-xCoxO2- sample with xN = 0.03. The line in (b) is lognormal fit.

Table 2. Particle size distribution analyses. d is the mean value of the particles diameter and s is the standard deviation obtained by the lognormal fit of particle size distribution histogram for each sample. N is the total number of the counted particles. xE is the effective Co concentration measured by EDS.


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Sample

d (nm) s

N

xE

xN = 0.03

8.5

0.4

139

0.030(3)

xN = 0.06

6.1

0.2

56

0.057(4)

xN = 0.09

5.4

0.3

120

0.087(2)

xN = 0.12

6.9

0.5

84

0.113(2)

Figure 3 shows typical Raman spectra acquired for Ti1-xCoxO2- samples. The main characteristic anatase Raman-active modes are observed at 147 (Eg(1)), 199 (Eg(2)), 399 (B1g(1)), 517 (overlap of A1g and B1g(2) modes), and 637 cm−1 (Eg(3)).36 A significant result from the Raman data is the complete absence of peaks related to rutile TiO2 phase and segregated secondary phases, as it has been observed for some Co-doped TiO2 samples.37 The inset of Figure 3 presents the position of the main vibrational mode Eg(1) as function of the nominal Co concentration in the samples. The Eg(1) mode shifts to lower energy, as Co concentration increases, due to the tensile strain introduced in the crystal lattice by Co incorporation, also confirming XRD results.



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Figure 3. Raman scattering spectra of nanostructured Ti1-xCoxO2- samples at room temperature. The inset presents the position of the Eg(1) main mode as a function of the Co effective content of the samples (xE). The error bars correspond to the statistical errors.

The structure of Ti1-xCoxO2- samples was also characterized by XAS measurements. XANES spectra give information on the coordination symmetry and the valence of ions incorporated in a solid. The energy of the absorption edge shifts according to the valence of the absorbing ion, since the binding energy of bound electrons rises as the valence increases. Also, the shape of the absorption edge depends on the unfilled local density of states and the coordination symmetry of the absorbing element. The spectrum at the Co K-edge is characteristic of the electron transition from the 1s state to empty 4p states.38 The pre-edge region (peak located around 7707 eV) is associated with transitions of electrons from 1s state to state 3d. Although this transition is originally forbidden, it occurs as a result of hybridization of the Co 3d states with the O 2p.38 Figure 4 shows the XANES spectra at Co K-edge are shown for the nanostructured Ti1-xCoxO2

samples and standards compounds. All samples exhibit similar K-edge white line shapes to

those previously reported for octahedral coordinated TM-doped TiO2 and XANES results undoubtedly indicate that Co on Ti1-xCoxO2- samples assumes predominantly the 2+ oxidation state. The upper-left inset of Figure 4 presents the evolution of the intensity of the peak relative to the white line maximum for the set of samples. The lower-right inset of this Figure shows the white line peak of calculated XANES spectra for Ti0.97Co0.03O2 (black line) compound using our XRD data obtained from Rietveld analysis and ab initio FEFF code.34 The input files for FEFF code with cluster radius of 6.0 Å were generated using CRYSTALFFREV software33 varying the occupation rate of Ti and O sites. In this inset, the spectra labeled as Ti0.50, Ti0.75, O0.83, O67 and


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O0.50 represent the calculated XANES spectra with occupation rate of 0.50, 0.67, 0.75 and 0.83 at Ti or O site. As can be seem in lower-right inset, the intensity of white line increases and the energy decreases as the occupation rate at Ti site decreases for calculated spectra. On the other hand, the diminution of the occupation rate at the O site in the XANES calculation results in a peak at white line with lower values of intensity and an increase in energy in concordance with the observed behavior for our set of samples. Thus, the calculated XANES spectra indicate that the decreasing in intensity and energy of the white line peak for the nanostructured Ti1-xCoxO2- samples with the increase of the Co content (upper-left inset) could be associated with the diminution of occupation rate at O site. In other words, the changes in the region of white line can be addressed to the formation of VO site as a result of the substitution of Ti4+ for Co2+ ions. Even more, these results can lead us to infer also that VO takes place close to the Co ions, as has been pointed out previously.27,39

Figure 4. Co K-edge XANES spectra for the nanostructured Ti1-xCoxO2-samples. Spectra of metallic Co, rocksalt CoO (valence 2+) and Co2O3(valence 3+) are also shown for comparison. The upper-left inset highlights the white line peak. The lower-right inset presents the calculated



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white line peak as a function of Ti and O occupation rate in respect to full structure (black dashed-dot line).

The short-range structural data provided by extended X-ray absorption fine structure (EXAFS) offers an element-specific insight, giving quantitative information about the number, position and identity of atoms surrounding the absorbing element as well as structural disorder within the coordination spheres. In order to obtain quantitative information of the local structure around Co atoms, Fourier transform curves were then back Fourier transformed between 1.0 and 2.0 Å to obtain the experimental EXAFS spectra to fit using a theoretical model calculated from FEEE9 code and crystallographic information according the XRD measurements. The fitted k3 weighted Fourier transforms of the nanostructured Ti1-xCoxO2- samples are shown in Figure 5(a) and fitted and experimental EXAFS spectra of back Fourier transformed are shown in Figure 5(b). In all fits, we considered single scattering paths corresponding to the two successive O shells around Co substitutionally placed at Ti-sites of the TiO2 host matrix according to the tetragonal anatase with I41/amd space group (inset of Figure 5(a)). The number of free parameters was kept smaller than the number of independent points, which is defined as Nind = 2ΔRΔk/π, where ΔR is the width of the R-space filter windows and Δk is the actual interval of the fit in the k-space.40 The reliability of the fit, determined by a quality factor (QF),40 coordination number (N), interatomic distances (R) and Debye-Waller factor (σ2) relative to the fits are shown in Table 3. According to the structural model, the more intense peak, between 1.0 and 2.0 Å in the Fourier transforms of nanostructured Ti1-xCoxO2- samples, corresponds to single scattering interaction between the first O atoms around absorber atom. The single scattering interactions relative to Co-Ti and CoO (beyond the first O neighbors) paths correspond to the peaks and shoulders observed between


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2.0 and 3.5 Å. This region also includes multiple scattering paths such as Co-O-O, Co-O-Ti-O, Co-O-Ti-O, Co-O-O-O and Co-Ti-O interactions. Due to the complexity of these components and the noise level, we could not performe the fitting of the peaks in this region. As can be viewed in Table 3, the obtained QF factors indicate the reliability of the fits, which is confirmed by the comparison of the fitted (lines) and experimental (symbols) spectra at Figure 5. No alterations within the uncertainty are observed in the average coordination number for Co-O interactions. The good agreement between experimental and fit data confirms the substitutional character of Co doping of the TiO2 matrix in our samples.

Figure 5. (a) Fitted k3 weighted Fourier transforms of the nanostructured

Ti1-xCoxO2- samples

and (b) fitted and experimental EXAFS spectra of back Fourier transformed. Open symbols are the experimental data and the solid lines represent the fittings using the parameters shown in Table 3. The inset in (a) presents the anatase TiO2 structure with the central Ti replaced by a Co atom used in the EXAFS fits. Table 3. Results of fits to the EXAFS spectra of Co K-edge EXAFS simulation results obtained by assuming Co substitutionally placed at Zn sites in the ZnO matrix. R is the distance



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from the central atom, N is the average coordination number, σ2 the Debye-Waller factor and QF the quality factor.40 Sample xN = 0.03 xN = 0.06 xN = 0.09 xN = 0.12

Shell Co-OI Co-OII Co-OI Co-OII Co-OI Co-OII Co-OI Co-OII

R (Å) 2.02(3) 2.16(6) 2.02(2) 1.88(5) 2.01(2) 1.88(4) 1.94(3) 2.06(6)

N 3.8(6) 1.5(7) 4.1(8) 2.1(6) 4.1(7) 2.1(6) 4.0(7) 2.0(8)

σ2 (Å2)

QF

0.0001(45) 0.46 0.0032(44) 0.35 0.0035(39) 0.46 0.0046(31) 0.18

Figure 6 presents XANES spectra at Ti L2,3,-edges of the nanostructured Ti1-xCoxO2- samples. The features labeled as A and B are related to forbidden transitions in L-S coupling (spin-orbit coupling) but become allowed due to pd multipolar interactions.41 Features C and D occur because in Ti L3-edge the 3d band splits in two subbands: t2g (peak C) e eg.42 In turn, Ti 3d eg subband also splits in 3dx2-y2 (peak D) and 3dz2 (peak E) orbitals.42 The crystal field at L2-edge also splits the 3d band into t2g (peak F) and eg (peak G) subbands and a similar splitting of the eg states occurs at the L2-edge. However, it is not well resolved due to the lifetime-related broadening of the L2-edge.42


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Figure 6. Ti L2,3-edges XANES spectra for the Ti1-xCoxO2-samples. The spectra are offsetted for clarity. At the bottom a spectrum for an pure TiO2 sample prepared under the same conditions as the doped ones is also presented. As it can be seen in Figure 6, the splitting between eg orbitals (peaks D and E) becomes less discernible as Co2+ ions are added to the TiO2 structure (highlighted area in yellow in Figure 6). To stress this point, an obtained spectrum for a pure TiO2 sample prepared at the same condition as the doped ones is also presented in Figure 6. The origin of this splitting in the eg states is a controversial subject in the literature.43–45 Recently, Kruger studied the Ti L2,3-edge spectra of SrTiO3 and TiO2 compounds.46 According to this author, the distortion of TiO6 octahedra is not a sufficient condition for the L3-eg peak splitting, since there is no splitting for an isolated (distorted) TiO6 octahedron. Moreover, Kruger’s studies show that in an ideal rutile made from undistorted octahedral, a splitting on the L3-eg peak can be observed and it is comparable to the width of the splitting in the real rutile.46 It was concluded that characteristic L3-eg peak splitting in TiO2 is a long-range band-structure effect, which reflects the crystal structure of TiO2 on a length scale of about 1 nm and distortion has, however, a non-negligible influence on the fine



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structure of the L3-eg peak.46 Thus, the changes observed in peak D for nanostructured Ti1xCoxO2-

samples can be considered an indication of the effect of Co substitution in the TiO2

structure. The O K-edge XANES spectra for the nanostructured Ti1-xCoxO2- samples are shown in Figure 7. The features labeled as A, B and C are due to the transition of O 1s electron to the hybridized orbitals of O 2p and Ti 3d.47 The peaks at higher energy and denoted as D, E and F are attributed to delocalized states derived from Ti 4sp-O 2p band.47 The upper-left inset of Figure 7 shows the evolution of the intensity for A, B and C features as a function of the Co content. As can be seen in it, the intensity of peaks A and C remarkably decreases for samples with higher Co concentration. In order to settle these changes, we calculated theoretical XANES spectra at O K-edge using FEFF input files generated from the same atom cluster taken in account for Co K-edge XANES calculations (inset of Figure 5(a)). Calculated XANES spectra reproduce satisfactorily the experimental spectra and the theoretical features A, B and C are shown in upper-right inset in Figure 7. In these calculations, we used a cluster with 6 Å of radius and varying the O occupation from 1.00 to 0.67 (O1.00, O0.83 and O0.67). The intensity of features A and C decreases when decreasing the O occupation. A similar behavior is observed for the experimental spectra as the Co content in the samples increases. Thus, the decrease in the intensity of these peaks can be associated to the reduction of the transition probability of O 1s to the hybridized Ti 3d-O 2p bands due the hybridization of Co 3d with Ti 3d states and the formation of VO proportionally to the Co concentration present in the samples.47 Again, the good comparison agreement between our calculations and the experimental data enables us to state that these results indicate that the VO is located close to the Co ions in the anatase TiO2 structure,


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in accordance with the conclusions obtained from the analyses of the Co k-edge XANES results (Figure 4).

Figure 7. O K-edge XANES spectra for the Ti1-xCoxO2-samples. The spectra are offsetted for clarity. The left inset highlights the changes in white line peak in respect to the Co content. The right inset presents the calculated white line peak as a function of O occupation rate in respect to full structure (O1.00).

The structural analysis confirms that Co ions are occupying Ti sites of the anatase TiO2 structure in our samples. Clearly, the results excludes the presence of magnetic extrinsic sources as Co-rich nanocrystals and segregated secondary magnetic phases. The Co2+ doping of TiO2 matrix also leads to an introduction of VO in the system located close to the Co2+ ions due the process of charge compensation. As the Co concentration in the samples increases, the VO also increases. With these conclusions we proceed to the magnetic characterization. The measurements for the magnetic moment (M) as a function of magnetic field (H) are presented in Figure 8, which reveal a typical paramagnetic behavior for all the samples. The



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inverse of the DC magnetic susceptibility as a function of the temperature is presented in Figure 9 for our set of samples. Previous measurements confirmed that neither the magnetization curve, neither the magnetization function of temperature measurement present hysteretic behavior. The diamagnetic background of the TiO2 matrix was determined by measuring an undoped TiO2 sample used as a reference and then subtracting this value from the raw data. The results show a clear linear behavior for high temperatures. In this range, the magnetic susceptibility can be described using the paramagnetic Curie–Weiss equation48



C ( x) (1) T  ( x)

where both the Curie–Weiss temperature, , and the Curie constant per gram, C, should present a linear dependence on the Co concentration x:  =0 · x and C = C0 · x. The constants C0 and 0 are defined as

C0 

Ng 2 B2 S ( S  1) (2) 3kB

and

0 

2 zS ( S  1) J1 (3) 3kB

where N is the number of cations per gram, g is the effective gyromagnetic factor of the Co ion, we assumed J = S = 3/2 (high spin state), B is the Bohr magneton, z is the number of nearest neighbors (z = 8 in the anatase structure), J1 is the effective exchange integral constant, and kB is the Boltzmann constant. The values of  and C obtained by fitting the results from Figure 9 using Eq. (1) are presented in the inset of Figure 9 as a function of the effective Co concentration (xE). All samples present negatives , indicating a majority antiferromagnetic (AF) interaction between Co ions in the limit of high temperatures. We can also determine the parameters C0 and


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0 by fitting the results presented in the inset of Figure 9. Finally, using the known values for the other constants involved in C0 and 0 in the Eq. (2) and (3), we can estimate the two main magnetic parameters for Co2+: g = 2.2 ± 0.2 and J1/kB = −10.3 ± 6.8 K. These values are in good agreement with other results obtained for Co2+ in TiO2 matrix49 and for other Co-doped II-VI semiconductors50–53 and lead to an average magnetic moment per Co atom of 4.3 ± 0.3 B, a value quite close to the one reported in literature for isolated Co2+.54 The good agreement between the reported magnetic parameters and our data confirms the high spin state adopted for the Co ions.

Figure 8. M(H) curves of the nanostructured Ti1-xCoxO2- samples at (a) 300 K and (b) 2 K. Symbols are the experimental data and lines are fits using Brillouin function (Equation 4).



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Figure 9. Inverse DC susceptibility  vs. temperature for the nanostructured Ti1-xCoxO2- samples. The diamagnetic contribution of the TiO2 matrix was subtracted. The best fit of the high temperature data to Curie–Weiss law is shown as solid lines. The inset shows  (red squares) and C (black circules) as a function of percentage of Co content, solid lines are linear fit to the data. We estimated the fraction of the total number of Co ions (NCo) AF coupled (NAF) at room temperature by assuming that the number of isolated Co ions (NPR) are those that remain paramagnetic at low temperature (NAF = NCo − NPR). The M(H) curves were fitted by48

M  NgS B tanh( gS B H / kBT ) (4) In the limit of high/ low temperatures N = NCo/NPR. Here we have used again J = S = 3/2 and g = 2.2, as obtained from the inverse of the susceptibility analyses presented before. The fittings are shown with the experimental data in Figure 8. Table 4 presents the obtained numbers and their relative values. NCo determined by the M(H) fitting is in quite good agreement with the effective number of Co ions measured by chemical analyses, corroborating the magnetic results. The fraction of AF coupled Co (NAF/Co) is larger than 50% for all the samples, and its value


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increases as Co concentration is increased. To present an AF coupling, the Co concentration has to be larger than the dopant percolation threshold (xP). For the anatase TiO2 structure xP = 0.25.17 These data suggest that Co distribution over the entire volume of our nanostructured samples is actually heterogeneous in fact. The local Co concentration in some parts of the nanoparticle volume, even at the lower Co doping level (xN = 0.03), should reaches values higher than 0.25, in order to leading to the observed AF coupling. The increase of the AF coupling with Co concentration can be easily understood by remembering that the particle diameter of our samples does not change significantly with Co doping (Table 2); therefore, while increasing the Co doping, for the same nanoparticle volume, the average distance between the Co ions decreases, leading to the increase of the number of AF coupled Co ions. The tendency for Co atoms to cluster near each other, forming Co-rich nanodomains have already been theoretically predicted.22 Nevertheless, to experimentally probe the spatial distributions of Co dopants in the nanocrystals is a very difficult task that requires state-of-the-art nanocharacterization tools.55,56 These results point out that non-equilibrium process would be a better choice for the synthesis route in order to achieve a truly Co diluted condition.

Table 4. Number of total Co ions (NCo) obtained by the fitting of M(H) curves at 300 K, the number of isolated Co ions (NPR) obtained by the fitting at 5 K. For comparison, N is the relative value of the NCo in respect to the effective numbers of Co ions measured for each sample by EDS (NE), and NAF/Co is the relative value of NAF in respect to the NCo.

Sample xN = 0.03

NCo

N

NPR

NAF/Co

(ൈ 1020)

(%)

(ൈ 1020)

(%)

1.00(1)

54(1)

2.190(1)

97(9)



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xN = 0.06

4.280(2)

100(7)

1.33(2)

69(1)

xN = 0.09

6.417(2)

99(2)

1.65(3)

74(1)

xN = 0.12

8.466(2)

101(2)

2.08(4)

75(1)

However a significative fraction of the Co ions remains isolated over the nanoparticle volume (46% for sample with xN = 0.03). In principle, under the scope of the BMP model, they would present a ferromagnetic behavior in the case of the presence of the right defects in the samples that could form enough magnetic polarons to couple the isolated Co ions. In the context of the BMP model, a long range magnetic order occurs when two conditions are satisfied: i) the dopant concentration (x) has to be lower than the dopant percolation threshold (xP) and ii) the percentage of defects () has to be larger than the polaron percolation threshold (P). For the anatase TiO2 system P = 0.0059 and xP = 0.25,17 as presented before. In our samples the concentration of isolated Co ions is far below the xP value for the system in case, while the concentration of defects, associated to the VO, is at least of the same order of the Co doping level, the lowest value for our samples is 0.03, which is one order of magnitude larger then P. The VO are also located in the vicinity of the Co2+ ions, assuring the necessary condition to forming Co2+ + VO2 complexes.12,13,22 As a conclusion, contrary to some reports,12,13,18,57-59 we cannot associate directly the main defect responsible for magnetic coupling between the Co ions to VO, since, from our structural and magnetic results, no evidence of RTFM could be observed. Besides, very recent results have pointed out that high electron densities can be the responsible for the observed RTFM of Co-doped TiO2 systems.19,60,61 Remembering that the Co2+ substitutional doping of our anatase TiO2 samples would not lead to significative changes in the electron carrier densities; we can infer that the lack of observation of a RTFM in our samples might be associated to a low electron density carrier.


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IV. CONCLUSIONS In summary, we have presented a complete structure and magnetic analysis of nanostructured anatase Ti1xCoxO2 synthetized via the Pechini method. Structural results confirm that Co2+ ions substitute Ti4+ ions in the anatase TiO2 matrix. There is no indication of Co-rich nanoclusters or segregated secondary phases in the samples. The Co doping leads, due to the difference between the cations valences, to the introduction of VO in the system close to the Co ions and in number proportional to the Co content. Magnetization measurements reveal a Curie– Weiss behavior of the susceptibility at high temperatures characterized by an AF interaction between Co ions. By the fitting of the M(H) curves at high (300 K) and low temperature (5 K) we could estimate the proportion of isolated and the AF coupled Co ions. These results leads to two major conclusions, the Co distribution over the nanoparticle volume is not homogeneous and that, under the scope of BMP model, the VO cannot be attributed to defects associated to the formation of magnetic polarons that could ferromagnetically couple the isolated Co ions, giving support to the carrier mediated theory for the ferromagnetism in diluted magnetic oxides.

AUTHOR INFORMATION *E-mail: [email protected]

ACNOWLEDGEMENTS Support from agencies FAPEMIG (APQ-02600-12 and PPM-00524-12), CNPq (306867/20095 and 476870/2011-9), CAPES (PNPD-2007 and PNPD-2011), FINEP (Ref. 134/08) and



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FAPESP are gratefully acknowledged. We also thank CNPq and CAPES for research fellowships (ACD, TES, AOZ and AM). The authors also acknowledge LNLS for the XAS measurements, the Rede Mineira de Química (RQ-MG) and Centro de Microscopia da UFMG for the HRTEM microscopy facilities and Prof. Dr. F. Iikawa and Profa. Dra. M. J. S. Brasil of the Universidade de Campinas (UNICAMP) for the Raman measurements.

REFERENCES (1) Ohno, H. Semiconductors – Toward Functional Spintronics. Science 2001, 291, 840-841. (2) Wang, M.; Campion, R. P.; Rushforth, A. W.; Edmonds, K. W.; Foxon, C. T.; Gallagher, B. L. Achieving High Curie Temperature in (Ga, Mn)As. Appl. Phys. Lett. 2008, 93, 132103-1-3. (3) Olejník, K.; Owen, M. H. S.; Novák, V.; Mašek, J.; Irvine, A. C.; Wunderlich, J.; Jungwirth, T. Enhanced Annealing, High Curie Temperature, and Low-Voltage Gating in (Ga, Mn)As: A Surface Oxide Study. Phys. Rev. B 2008, 78, 054403 1-4. (4) Chen, L.; Yan, S.; Xu, P. F.; Lu, J.; Wang, W. Z.; Deng, J. J.; Qian, X.; Ji, Y.; Zhao, J. H. Low-Temperature Magnetotransport Behaviours of Heavily Mn-Doped (Ga,Mn)As Films with High Ferromagnetic Transition Temperature. Appl. Phys. Lett. 2009, 95, 182505 1-3. (5) Dietl, T; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Zener Model Description of Ferromagnetism in Zinc-Blend Magnetic Semiconductors. Science 2000, 287, 1019-1022. (6) Matsumoto, Y. Room-Temperatura Ferromagnetism in Transparent Transition MetalDoped Titanium Dioxide. Science 2001, 291, 854-856.


Page 27 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Janisch, R.; Gopal, P.; Spaldin, N. A. Transition Metal-Doped TiO2 and ZnO – Present Status of the Field. J. Phys. Condens. Matter 2005, 17, R657-R689. (8) Matsumoto, Y.; Takahashi, R.; Murakami, M.; Koida, T.; Fan, X.-J.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Koshihara, S.-Y.; Koinuma, H. Ferromagnetism in Co-Doped TiO2 Rutile Thin Films Grown by Laser Molecular Beam Epitaxy. Jpn. J. Appl. Phys. Part 2 2001, 40, L1204-L1206. (9) Chambers, S. A.; Thevuthasan, S.; Farrow, R. F. C.; Marks, R. F.; Thiele, J. U.; Folks, L.; Samant, M. G.; Kellock, A. J.; Ruzycki, N.; Ederer, D. L.; et al. Epitaxial Growth and Properties of Ferromagnetic Co-Doped TiO2 Anatase. Appl. Phys. Lett. 2001, 79, 3467-3469. (10) Chambers, S. A.; Droubay, T.; Wang, C. M.; Lea, A. S.; Farrow, R. F. C.; Folks, L.; Deline, V.; Anders, S. Clusters and Magnetism in Epitaxial Co-Doped TiO2 Anatase. Appl. Phys. Lett. 2003, 82, 1257-1259. (11) Kim, J.-Y.; Park, J.-H.; Park, B.-G.; Noh, H.-J.; Oh, S.-J.; Yang, J.; Kim, D.-H.; Bu, S.; Noh, T.-W.; Lin, H.-J.; et al. Ferromagnetism Induced by Clustered Co in Co-Doped Anatase TiO2 Thin Films. Phys. Rev. Lett. 2003, 90, 017401 1-4. (12) Yan, W.; Sun, Z.; Pan, Z.; Liu, Q.; Yao, T.; Wu, Z.; Song, C.; Zeng, F.; Xie, Y.; Hu, T.; et al. Oxygen Vacancy Effect on Room-Temperature Ferromagnetism of Rutile Co:TiO2 Thin Films. Appl. Phys. Lett. 2009, 94, 042508 1-3. (13) Singhal, R. K.; Kumar, S.; Kumari, P.; Xing, Y. T.; Saitovitch, E. Evidence of DefectInduced Ferromagnetism and its “Switch” Action in Pristine Bulk TiO2. Appl. Phys. Lett. 2011, 98, 092510 1-3.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

(14) Quilty, J. W.; Shibata, A.; Son, J.-Y.; Takubo, K.; Mizokawa, T.; Toyosaki, H.; Fukumura, T.; Kawasaki, M. Signature of Carrier-Induced Ferromagnetism in Ti1-xCoxO2-: Exchange Interaction Between High-Spin Co2+ and the Ti 3d Conduction Band. Phys. Rev. Lett. 2006, 96, 027202 1-4. (15) Yamada, Y.; Fukumura, T.; Ueno, K.; Kawasaki, M. Control of Ferromagnetism at Room Temperature in (Ti,Co)O2- via Chemical Doping of Electron Carries. Appl. Phys. Lett. 2011, 99, 242502 1-3. (16) Calderón M. J.; Das Sarma, S. Theory of Carrier Mediated Ferromagnetism in Diluted Magnetic Oxides. Annals of Physics 2007, 322, 2618-2634. (17) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Donor Impurity Band Exchange in Diluted Ferromagnetic Oxides. Nat. Mater. 2005, 4, 173-179. (18) Griffin, K. A.; Pakhomov, A. B.; Wang, C. M.; Heald, S. M.; Krishnan, K. M. Intrinsic Ferromagnetism in Insulating Cobalt Doped Anatase TiO2. Phys. Rev. Lett. 2005, 94, 157204 14. (19) Yamada, Y.; Ueno, K.; Fukumura, T.; Yuan, H. T.; Shimotani, H.; Iwasa, Y.; Gu, L.; Tsukimoto, S.; Ikuhara, Y.; Kawasaki, M. Electrically Induced Ferromagnetism at Room Temperature in Cobalt-Doped Titanium Dioxide. Science 2011, 332, 1065-1067. (20) Chen, Y.; Lin, A.; Gan, F. Preparation of Nano-TiO2 from TiCl4 by Dialysis Hydrolysis. Powder Technology 2006, 167, 109-116. (21) Barsoum, M. W. Fundamentals of Ceramics, Series in Materials Science and Engineering, IoP: London, 2003.


Page 29 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Errico, L. A.; Rentería, M.; Weissmann, M. Theorical Study of Magnetism in TransitionMetal-Doped TiO2 and TiO2-delta. Phys. Rev. B 2005, 72, 184425 1-8. (23) Jaffe, J. E.; Droubay, T. C.; Chambers, S. A. Oxygen Vacancies and Ferromagnetism in CoxTi1-xO2-x-y. J. Appl. Phys. 2005, 97, 073908 1-6. (24) Forro, L.; Chauvet, O.; Emin, D.; Zuppiroli, L.; Berger, H.; Levy, F. High-Mobility NType Charge-Carriers in Large Single-Crystals of Anatase (TiO2). J. Appl. Phys. 1994, 75, 633635. (25) Woicik, J. C.; Nelson, E. J.; Kronik, L.; Jain, M.; Chelikowsky, J. R.; Heskett, D.; Berman, L. E.; Herman, G. S. Hybridization and Bond-Orbital Components in Site-Specific XRay Photoelectron Spectra of Rutile TiO2. Phys. Rev. Lett. 2002, 89, 077401 1-4. (26) Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.; Koinuma, H. Room-Temeprature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide. Science 2001, 291, 854-856. (27) Chambers, S. A.; Heald, S. M.; Droubay, T. Local Co Structure in Epitaxial CoxTi1-xO2-x Anatase. Phys. Rev. B 2003, 67, 100401 1-4. (28) Ogale, S. B.; Choudhary, R .J.; Buban, J. P.; Lofland, S. E.; Shinde, S. R.; Kale, S. N.; Kulkarni, V. N.; Higgins, J.; Lanci, C.; Simpson, J. R.; et al. High Temperature Ferromagnetism With a Giant Magnetic Moment in Transparent Co-Doped SnO2-. Phys. Rev. Lett. 2003, 91, 077205 1-4.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

(29) Shinde, S. R.; Ogale, S. B.; Das Sarma, S.; Simpson, J. R.; Drew, H. D.; Lofland, S. E.; Lanci, C.; Kulkarni, V. N.; Higgins, J.; Sharma, R. P.; et al. Ferromagnetism in Laser Deposited Anatase Ti1-xCoxO2- Films. Phys. Rev. B 2003, 67, 115211 1-6. (30) Pechini, M. P. Method of Preparing Lead and Alkaline Earth Titanates and Niobates and Coating Method Using the Same to Form a Capacitor. US Patent 3,330,697, July 11, 1967. (31) Larson, A. C.; Von Dreele, R. B. In General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 2000, 86-748. (32) Toby. B. H. EXPGUI, a Graphical User Interface for GSAS. J. Appl. Cryst. 2001, 34, 210213. (33) Michalowicz, A.; Moscovici, J.; Muller-Bouvet, D.; Provost, K. MAX: Multiplataform Applications for XAFS. J. Phys.: Conference Series 2009, 190, 012034 1-4. (34) Ankudinov, A. L.; Ravel, B.; Conradson S. D.; Rehr, J. J. Real-space Multiple-Scattering Calculation and Interpretation of X-Ray-Absorption Near-Edge Structure. Phys. Rev. B 1998, 58, 7565-7576. (35) Shannon, R. D. Revised Effective Ionic-Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallographica Section A 1976, 32, 751-767. (36) Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D. G.; Muddle, B. C. Finite-Size and Pressure Effects on the Raman Spectrum Nanocrystalline Anatase TiO2. Phys. Rev. B 2005, 71, 184302 1-11.


Page 31 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Lin, Y. B.; Yang, Y. M.; Zhuang, B.; Huang, S. L.; Wu, L. P.; Huang, Z. G.; Zhang, F. M.; Du, Y. W. Ferromagnetism of Co-Doped TiO2 Films Prepared by Plasma Enhanced Chemical Vapour Deposition (PECVD) Method. J. Phys. D: Appl. Phys. 2008, 41, 195007 1-5. (38) Orikasa, Y.; Ina, T.; Nakao, T.; Mineshige, A.; Amezawa, K.; Oishi, M.; Arai, H.; Ogumi, Z.; Uchimoto, Y. X-Ray Absorption Spectroscopic Study on La0.6Sr0.4CoO3- Cathode Materials Related with Oxygen Vacancy Formation. J. Phys. Chem. C 2011, 115, 16433-16438. (39) Roberts, K. G.; Varela, M.; Rashkeev, S.; Pantelides, S. T.; Pennycook, S. J.; Krishnan, K. M. Defect-Mediated Ferromagnetism in Insulating Co-doped Anatase TiO2 Thin Films. Phys. Rev. B 2008, 78, 014409 1-6. (40) Hasnain S. S. In X-ray Absorption Fine Structure: Proceedings of the VI International Conference on X-ray Absorption Fine Structures, 05-11 Aug 1990, (Ellis Horwood, New York, 1991). (41) De Groot, F. M. F.; Fuggle, J. C.; Thole B. T.; Sawatzky, G. A. L2,3 X-Ray Absorption Edges of Compounds: K+, Ca2+, Sc3+, and Ti4+ in Oh (Octahedral) Symmetry. Phys. Rev. B 1990, 41, 928-938. (42) Nachimuthu, P.; Thevuthasan, S.; Adams, E. M.; Weber, W. J.; Begg, B. D.; Mun, B. S.; Shuh, D. K.; Lindle, D. W.; Gullikson E. M.; Perera, R. C. C. Near-Edge X-ray Absorption Fine Structure Study of Disordering in Gd-2(Ti1-yZry)(2)O7 Pyrochlores. J. Phys. Chem. B 2005, 109, 1337-1339. (43) Jan, J. C.; Tsai, H. M.; Pao, C. W.; Chiou, J. W.; Asokan, C. K.; Kumar, K. P. K.; Pong, W. F.; Tang, Y. H.; Tsai, M. H.; Kuo S. Y.; et al. Direct Experimental Evidence of Hybridization



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

of Pb States with O 2p States in Ferroelectric Perovskite Oxides. App. Phys. Lett. 2005, 87, 012103 1-3. (44) Jan, J. C.; Kumar, K. P. K.; Chiou, J. W.; Tsai, H. M.; Shih, H. L.; Hsueh, H. C.; Ray, S. C.; Asokan, K.; Pong, W. F.; Tsai, M. H.;.et al. Effect of the Ca Content on the Electronic Structure of Pb1-xCaxTiO3 Perovskites. App. Phys. Lett. 2003, 83, 3311-3313. (45) Kucheyev, S. O.; Buuren, T. V.; Baumann, T. F.; Satcher, J. J. H.; Willey, T. M.; Meulenberg, R. W.; Felter, T. E.; Poco, J. F.; Gammon S. A.; Terminello, L. J. Eletronic Structure of Titania Aerogels From Soft X-Ray Absorption Spectroscopy. Phys. Rev. B 2004, 69, 245102 1-7. (46) Krüger, P. Multichannel Multiple Scattering Calculation of L-2,L-3-edge Spectra of TiO2 and SrTiO3: Importance of Multiplet Coupling and Band Structure. Phys. Rev. B 2010, 81, 125121 1-6. (47) Yan, W.; Sun, Z.; Pan, Z.; Liu, Q.; Yao, T.; Wu, Z.; Song, C.; Zeng, F.; Xie, Y.; Hu T.; et al. Oxygen Vacancy Effect on Room-Temeprature Ferromagnetism of Rutile Co:TiO2 Thin Films. App. Phys. Lett. 2009, 94, 042508 1-3. (48) Cullity B. D.; Graham, C. D. Introduction to Magnetic Materials; Addison-Wesley: Reading, 1972. (49) Rodríguez-Torres, C. E.; Cabrera, A. F.; Errico, L. A.; Adán, C.; Requejo, F. G.; Weissmann M.; Stewart, S. J. Local Structure and Magnetic Behaviour of Fe-Doped TiO2 Anatase Nanoparticles: Experiments and Calculations. J. Phys. Condens. Matter 2008, 20, 135210 1-9.


Page 33 of 35

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(50) Twardowski, A.; Swagten, H. J. M.; de Jonge W. J. M.; Demianiuk, M. MagneticBehavior of the Diluted Magnetic Semiconductor Zn1-xMnxSe. Phys. Rev. B 1987, 36, 70137023. (51) Lewicki, A.; Schindler, A. I.; Furdyna J. K.; Giriat, W. Magnetic-Susceptibility of Zn1xCoxS

and Zn1-xCoxSe Alloys. Phys. Rev. B 1989, 40, 2379-2382.

(52) Alawadhi, H.; Miotkowski, I.; Lewicki, A.; Ramdas, A. K.; Miotkowska S.; McElfresh, M. Magnetic Susceptibility and Compositional Dependence of the Energy Gap in Cd1-xCoxTe. J. Phys: Condens. Matter 2002, 14, 4611-4620. (53) de Carvalho, H. B.; de Godoy, M. P. F.; Pais, R. W. D.; Mir, M.; Ortiz de Zevallos, A.; Iikawa, F.; Brasil, M. J. S. P.; Chitta, V. A.; Ferraz, W. B.; Boselli, M. A.; et al.. Absence of Ferromagnetic Order in High Quality Bulk Co-Doped ZnO Samples. J. Appl. Phys. 2010, 108, 033914 1-5. (54) Kittel, C. Introduction to Solid State Physics; Wiley: New York,1996. (55) Bonanni, A.; Navarro-Quezada, A.; Li, Tian; Wegscheider, M.; Matej, Z.; Holy, V.; Lechner, R. T.; Bauer, G.; Rovezzi, M.; D’Acapito, F.; et al. Controlled Aggregation of Magnetic Ions in a Semiconductor: An Experimental Demonstration. Phys. Rev. Lett. 2008, 101, 135502 1-4. (56) Rossell, M. D.; Ramasse, Q. M.; Findlay, S. D.; Rechberger, F.; Erni, R.; Niederberger, M. Direct Imaging of Dopant Clustering in Metal-Oxide Nanoparticles.ACS Nano 2012, 6, 70777083.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

(57) Singhal, R. K.; Samariya, A.; Kumar, S.; Xing, Y. T.; Jain, D. C.; Dolia, S. N.; Deshpande, U. P.; Shripathi, T.; Saitovitch, E. B. Study of Defect-Induced Ferromagnetism in Hydrogenated Anatase TiO2:Co. J. Appl. Phys. 2010, 107, 113916 1-7. 58 Choudhury, B.; Choudhury, A.; Maidul Islam, A. K. M.; Alagarsamy, P.; Mukherjee, M. Effect of Oxygen Vacancy and Dopant Concentration on the Magnetic Properties of High Spin Co2+ Doped TiO2 Nanoparticles. J. Magn. Magn. Mater. 2011, 323, 440-446. 59 Roberts, K. G.; Varela, M.; Rashkeev, S.; Pantelides, S. T.; Pennycook, S. J.; Krishnan, K. M. Deffect-Mediated Ferromagnetism in Insulating Co-Doped Anatase TiO2 Thin Films. Phys. Rev. B 2008, 78, 014409 1-6. 60 Ohtsuki, T.; Chainani, A.; Eguchi, R.; Matsunami, M.; Takata, Y.; Taguchi, M.; Nishino, Y.; Tamasaku, K.; Yabashi, M.; Ishikawa, T.; et al. Role of Ti 3d Carriers in Mediating the Ferromagnetism of Co:TiO2 Anatase Thin Films. Phys. Rev. Lett. 2011, 106, 047602 1-4. 61 Singh, V. R.; Ishigami, K.; Verma, V. K.; Shibata, G.; Yamazaki, Y.; Kataoka, T.; Fujimori, A.; Chang, F.-H.; Huang, D.-J.; Lin, H.-J.; et al. Ferromagnetism of Cobalt-Doped Anatase TiO2 Studied by Bulk- and Surface-Sensitive Soft X-Ray Magnetic Circular Dichroism. Appl. Phys. Lett. 2012, 100, 242404 1-5.


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