JOURNAL OF APPLIED PHYSICS 107, 093918 共2010兲
Structural, electronic, and magnetic properties of Co doped SnO2 nanoparticles Aditya Sharma,1,a兲 Abhinav Pratap Singh,2 P. Thakur,3 N. B. Brookes,3 Shalendra Kumar,4 Chan Gyu Lee,4 R. J. Choudhary,5 K. D. Verma,1 and Ravi Kumar2,6 1
Department of Physics, Material Science Research Laboratory, S.V. College, Aligarh 202001, Uttar Pradesh, India 2 Inter University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India 3 European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France 4 School of Nano and Advance Materials Engineering, Changwon National University, 9 Sarim dong, Changwon 641-773, Republic of Korea 5 UGC-DAE-CSR, Indore 452017, Madhya Pradesh, India 6 Centre for Materials Science and Engineering, National Institute of Technology, Hamirpur 177005, Himachal Pradesh, India
共Received 29 December 2009; accepted 31 March 2010; published online 7 May 2010兲 We present a detailed study on the structural, electronic, and magnetic properties of chemically synthesized Sn1−xCoxO2 共x = 0.00 to 0.05兲 nanoparticles. X-ray diffraction and transmission electron microscope measurements were performed to analyze the crystal structure and morphology of Sn1−xCoxO2 nanoparticles. The energy dispersive x-ray analysis measurements were performed to check the possible presence of any impurity elements in the nanocrystals. The near edge x-ray absorption fine structure 共NEXAFS兲 experiments at Sn M5,4-edge and Co L3,2-edge were performed to probe the local environment of Sn and Co ions in the SnO2 matrix. The NEXAFS at Co L3,2-edge, along with multiplet calculations, indicate that the Co is substituted at the Sn site in SnO2 matrix with +2 charge state and do not form metallic clusters and other oxide phases. The ferromagnetic nature of these materials was confirmed by x-ray magnetic circular dichroism and room temperature magnetization hysteresis loop measurements. © 2010 American Institute of Physics. 关doi:10.1063/1.3415541兴 I. INTRODUCTION
In recent years, diluted magnetic semiconductors 共DMS兲 have attracted intense interest owing to their potential applications in spintronics.1,2 Investigations on DMS were originally inspired by the discovery of low temperature ferromagnetism in Mn-doped GaAs exhibiting a curie temperature 共Tc兲 of about 110 K.3 Recent theoretical studies,4,5 on the basis of Zener’s mean field model, showed that transition metal 共TM兲 doped wide-band gap semiconductors are promising candidate for room temperature ferromagnetism 共RTFM兲. Among the other wide-band gap semiconductors, SnO2 is one of the important candidates because of its optical transparency, high carrier density, wide-band gap, remarkable chemical, and thermal stabilities. In previous studies, much attention was paid to the fabrication of SnO2 based DMS materials exhibiting RTFM.6–10 However, there is no consensus on the origin of the RTFM; whether it is intrinsic to the system or due to the secondary phases.11–15 The uncertainties are due to the difficulties in confirming the incorporation of doped ion into the host lattice and in establishing the absence of nanoscale secondary phases. Using conventional techniques, the presence of small amount of impurities cannot be ruled out completely. Recently, near edge x-ray absorption fine structure 共NEXAFS兲 spectroscopy technique has been employed to investigate the local structure of doped a兲
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ions in Co and Fe doped ZnO 共Refs. 16 and 17兲 thin films and nanocrystalline powders.18 The NEXAFS measurements provide information about the charge state, local environment, and hybridization of specific cations present in the material. The x-ray magnetic circular dichroism 共XMCD兲 technique can be used to obtain information regarding the contribution of a specific cation in any material toward the total magnetism of the system. It can also be used to separate out the spin and orbital contributions of the probed ion to the magnetism using simple sum rules.19,20 In this work, single phase, nanocrystalline powder samples of Sn1−xCoxO2 were synthesized by co-precipitation method. X-ray diffraction 共XRD兲, energy dispersive x-ray analysis 共EDAX兲, and transmission electron microscope 共TEM兲 measurements were performed to deduce the crystal structure, elemental composition, and morphology of the synthesized samples, respectively. The NEXAFS measurement at Sn M5,4-edge and Co L3,2-edge were performed to probe the local environment of Sn and Co ions in the SnO2 matrix and to investigate the effects of Co doping on the electronic structure of the host material. Multiplet calculations were performed at the Co L3,2-edge to determine its symmetry and hence the position in the host lattice. It also helps in confirming the charge state and the crystal field splitting of Co ions in SnO2 matrix. XMCD and room temperature hysteresis loop measurements are performed to confirm the ferromagnetic nature of the Sn1−xCoxO2 nanoparticles.
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II. EXPERIMENTAL DETAILS A. Synthesis
All the reagents used were of analytical grade without further purification. SnCl4 · 5H2O and Co共CH3COO兲2 · 5H2O were dissolved into deionized water with magnetic stirring until they form clear solutions. Ammonium hydroxide 共0.1 M兲 solution was added 共drop-wise兲 into the prepared clear solution of tin chloride and cobalt acetate with constant magnetic stirring and then aging for 30 min. The pH value of solution was leveled 共pH = 7兲 during the synthesis of all samples. The resultant white precipitates were washed several times with deionized water to remove Cl− ions and other ionic impurities. The precipitates were dried in air at 40 ° C and the light yellow colored powders were collected. Cobalt doping ratio was the molar ratio of Co to Co+ Sn, namely x. This procedure was employed for all the compositions of the cobalt doped tin oxide samples. From the nature of synthesis and experimental findings, the formation of Sn1−xCoxO2 nanoparticles in colloidal medium may occur via the following reaction: 共1 − x兲共SnCl4兲 + xCo共CH3COO兲2 + mH2O → 共1 − x兲Sn4+ + xCo2+ + mOH− + nHCl + 2xCH3COO− , 共1 − x兲Sn4+ + xCo2+ + mNH4OH → 共1 − x兲Sn共OH兲4 + xCo共OH兲2 + mNH+4 , 共1 − x兲Sn共OH兲4 + xCo共OH兲2 → Sn1−xCoxO2 + nH2O. In the reaction, tin chloride, cobalt acetate, and ammonium hydroxide underwent dissociation into ionic reaction and form Sn4+, Co2+, OH−, and NH+4 , etc., ions. The intermediate complexes of Sn共OH兲4 and Co共OH兲2 are expected to combine and form Sn1−xCoxO2 granules in the colloidal medium. Cerri et al.21 have reported the formation of Sn1−xCoxO2 nanocrystalline powders using a solid state reaction method. At high temperature synthesis, they reported the solubility limit up to x = 0.08, although at low temperature synthesis the solubility was only x = 0.02. The controlled wet chemical method, pH = 7, used in this work has helped us to prepare, x = 0.00 to 0.05, cobalt doped SnO2 nanoparticles, even at very low temperatures 共40 ° C兲. B. Characterizations
The phase and crystal structure of the samples were identified by powder XRD with Bruker D8 advanced diffractometer using Cu K␣ radiation 共 = 1.540 Å兲. The morphology and crystallite sizes were studied with FEI-Tecnai-20 TEM accompanied by EDAX attachment for compositional analysis, operated at 200 kV. The NEXAFS and XMCD measurements were performed at the ID08 beam line of the European Synchrotron Radiation Facility in Grenoble, which uses an APPLE II type undulator giving ⬃100% linear/ circular polarization. All the scans were collected simultaneously in both total electron yield 共TEY兲 and total fluorescence yield 共TFY兲 modes, ensuring both surface 共TEY兲 and bulk 共TFY兲 sensitivities. The spectra were normalized to incident photon flux and the base pressure of the experimental
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chamber was better than 3 ⫻ 10−10 Torr. The dc magnetization measurements were carried out at room temperature using commercial quantum design physical properties measurement system. III. RESULTS AND DISCUSSION
Figure 1共a兲 shows the XRD patterns of Sn1−xCoxO2 共x = 0.00– 0.05兲 nanoparticles. The diffraction peaks in the figure are indexed to the rutile-type phase of SnO2 共JCPDS, Card No. 77–0450兲 with a tetragonal unit cell. No trace of cobalt metal, oxides, or other binary tin-cobalt phases could be detected, within the detection limit of XRD. These results indicate that highly crystalline and single phase Sn1−xCoxO2 nanoparticles have been synthesized with x up to 0.05. The average size of the nanoparticles was calculated using the Scherrer relation; D = 0.9 / 共 cos 兲, where  is the full width at half maximum of the peaks, expressed in radian. Thus calculated particle size fall in the range of ⬃3.9– 4.3 nm for all the samples. The calculated lattice parameters of nanocrystalline SnO2 was found to be a = 4.767共5兲 Å which is higher than the corresponding bulk value of 4.745 共0兲 Å. This is in agreement with earlier reports that lattice expands in oxide nanoparticles.22 Besides, the peak positions and their intensities show systematic change with cobalt doping. The 共110兲 and 共101兲 peaks shift toward the higher 2 values as x increases 关see Fig. 1共b兲兴. Evolution of cell parameters, a and c, with cobalt content, x, is shown in Fig. 1共c兲. It can be seen that the lattice parameters decrease, and hence, the volume of the unit cell decrease, with increase in the cobalt concentration. Such a contraction of lattice parameters can be understood, qualitatively, by considering the size of the ions. Here, substitution of bigger Sn4+ ions 共ionic radius= 0.69 Å兲 by the smaller Co2+ ions 关ionic radius= 0.58 Å 共Ref. 8兲兴 reduces the interatomic spacing significantly and therefore contraction in lattice parameters is observed with Co doping. Hays et al.8 have reported such a contraction in the lattice parameters for x = 0.01 cobalt doped Sn1−xCoxO2 nanoparticles. Figure 1共d兲 shows the continuous decrease in the intensity ratio of 共110兲 plane to 共101兲 plane 共I共110兲 / I共101兲兲 with respect to cobalt concentration. The observed decrease in the intensity ratio, 共I共110兲 / I共101兲兲 up to x = 0.05, may be ascribed to the substitution of Sn by Co in the tin oxide lattice. It may be noted here that the atomic scattering factor for Co 共Z = 27兲 is almost half the value for Sn 共Z = 50兲, therefore, any substitution of Sn by Co may lead to change in the structure factor of SnO2 lattice, which will reflect in the change in XRD peak intensity ratios on Co doping. Gopinadhan et al.23 have also reported similar changes in the XRD peak intensities in their Co doped SnO2 thin films. From the XRD measurements, we conclude that the doped Co ions are substituting for Sn in the host matrix and there are no Co metallic clusters or other impurity phases in the Sn1−xCoxO2 nanoparticles. In order to reveal the shape and size of pure and doped tin oxide nanoparticles, TEM measurements were performed on pure SnO2 and Sn0.95Co0.05O2 samples, which are shown in Figs. 2共a兲 and 2共b兲, respectively. The figures show that only spherical shaped nanoparticles are obtained for undoped
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FIG. 1. 共a兲 XRD patterns of Sn1−xCoxO2 nanoparticles, 共b兲 change in 共110兲 and 共101兲 peak position with cobalt doping, 共c兲 change in lattice parameters “a” and “c” with cobalt doping, and 共d兲 change in peak intensity ratio 共I共110兲 / I共101兲兲 with cobalt doping.
as well as cobalt doped tin oxide. In some previous reports,7,8 elongated morphology has been reported while the samples were doped for x ⱕ 0.05 cobalt concentrations. In this study, the nanocrystals do not have elongated morphology and are only spherical in shape. This could be achieved by controlling the synthesis parameter such as, pH value of the solution and low temperature heat treatment of all the samples. However, some aggregation of nanocrystals has been observed in both samples. This aggregation in wet chemically synthesized nanoparticles is expected due to the presence of substantial OH− ions in samples.24 The size of individual nanoparticles, calculated from the TEM images, is about 4 nm, which is in good agreement with the results obtained from XRD measurements. To check the elemental presence of Sn, O, Co, and other possible impurity elements in the samples, EDAX measurements were also performed. Figures 2共c兲 and 2共d兲 show the EDAX spectra of Sn1−xCoxO2 nanoparticles for x = 0.00 and 0.05, respectively. It is clearly seen from the spectra that for pure SnO2 powders, only the peaks corresponding to Sn and O have been detected while, additionally, cobalt peaks were observed in cobalt doped samples. These measurements exclude the presence of any impurity elements 共Cl, C, etc.兲 in the samples. The NEXAFS measurements involve the excitation of electrons from a core level to the partially filled and empty
states. The peak position and spectral features of the NEXAFS spectra are affected not only by the oxidation state of probed ion but also by its structural symmetry and covalent/ ionic character of the bonds between cations and neighboring atoms.25,26 The Sn M5,4-edge spectra reflect electronic transitions from Sn 3d core level 共spin-orbit split 3d5/2 and 3d3/2 levels, giving rise to M5 and M4-edges, respectively兲 into unoccupied electronic states having p and f character 共according to the dipole selection rules兲 above the Fermi level. Figure 3 shows the Sn M5,4-edge NEXAFS spectra of Sn1−xCoxO2 共x = 0.00, 0.01, 0.04, and 0.05兲 nanoparticles, recorded in TEY mode. The main spectral features of the Sn1−xCoxO2 samples are similar to those of the reported SnO2 nanoparticles,27 which further strengthen the XRD and TEM results that the powders are in nanocrystalline form. Interestingly, a small intensity pre-edge peak ⬃487 eV was observed in all of the Sn1−xCoxO2 samples. This kind of preedge peak was previously observed in SnO2 nanoparticles, but not in bulk SnO2 and is proposed to have an origin from the surface Sn atoms and surface reconstructions in nanomaterials.28 This conclusion is supported by the theoretical studies on the SnO2 nanoparticles, which predict that energetically favorable reconstruction of SnO2 共110兲 and 共101兲 surfaces results into Sn/O interstitials/deficiencies at the surface.29 The NEXAFS spectra at Sn M5,4-edge, col-
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FIG. 2. TEM images of 共a兲 Sn0.99Co0.01O2 nanoparticles, 共b兲 Sn0.95Co0.05O2 nanoparticles, Figs. 2共c兲 and 2共d兲 show the EDAX spectra of Sn0.99Co0.01O2 and Sn0.95Co0.05O2 nanoparticles, respectively.
lected for the Co doped SnO2 samples, do not show any appreciable changes in their spectral features with increasing Co concentrations. These results suggest that the incorporation of Co ions do not substantially change the valence state of Sn and its lattice symmetry in tin oxide. The L-edge of TM is known to be highly sensitive to the symmetry and the charge state of probed ion. Thus to investigate the position of Co ions in SnO2 lattice, NEXAFS spectra at Co L3,2-edge were collected in TFY mode, which are shown in Fig. 4. It can be seen from Fig. 4 that spectral features of all the samples are very similar to each other and do not match with the spectral features of reported cobalt metal, Co2O3, and CoO.17,30 This is in accordance with the XRD results, in which no peaks corresponding to any of the cobalt oxide or metallic phases were detected. The reported spectral features of the Co L3,2-edge, for the Co doped SnO2
and TiO2, by Luisser et al.31 are similar to each other and also similar to the Co L3,2-edge reported for Co doped ZnO.17 Since the symmetry of Co in these different materials will be different, so different spectral features of the Co L3,2-edge are expected. In the present work, the observed features at the Co L3,2-edge are different from those reported by Luisser et al.31 To clarify these doubts, we have performed multiplet calculations to simulate the Co L3,2-edge. Since, in the SnO2 lattice, each tin atom is situated amidst of six oxygen atoms, which approximately form the corners of a regular octahedron, therefore, any substitution of tin by cobalt should provide an octahedral coordination for cobalt ions with neighboring oxygen atoms. To confirm the charge state and local environment of doped Co ions in the SnO2 lattice, we have performed the multiplet calculations in various symmetries 共results only for octahedral symmetry are
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FIG. 3. 共Color online兲 NEXAFS spectra at the Sn M5,4-edge collected at 300 K for Sn1−xCoxO2 共x = 0.00, 0.01, 0.04, and 0.05兲 nanoparticles.
presented here兲 with different valence states of Co ions at the Co L3,2-edge. From the multiplet calculations, it was found that the spectral features were better reproduced with the cobalt in the 2+ state surrounded octahedrally with six O ions. Figure 5 shows the results of these calculations. The crystal field splitting values 共10 Dq兲, used in the calculations, are mentioned in the each plot of the figure. To compare the discrete lines of the atomic multiplets with the experiment and to simulate the resolution, they have been broadened with the Lorentzian of 0.08 eV and Gaussian of 0.2 eV. The results were better reproduced when 3d spin-orbit coupling was reduced to zero. It is clear from the Figs. 5共a兲–5共d兲 that the calculations for Co2+ in the octahedral symmetry are in better agreement with the experimentally observed NEXAFS spectra with 10 Dq= 0.6 eV. This value of 10 Dq is consistent with the values reported for the Co doped ZnO thin films.17 The other common valence state of Co is 3+, therefore, calculations were also performed at Co3+ L3,2-edge 关see Figs. 5共e兲–5共h兲兴. These calculations show large spread and do not match well with the spectral features of the experimental spectra, and hence, exclude the presence of Co in 3 + state. The conclusions derived from these calculations are that the Co is substituted at the Sn site in SnO2 lattice with 2+ charge state; it do not form metallic clusters or other oxide phases in the host matrix, and is octahedrally coordinated with six oxygen atoms with a crystal field splitting of 0.6 eV. The XMCD signals, shown in Fig. 6, are the difference between NEXAFS spectra recorded for parallel 共+兲 and antiparallel 共−兲 alignments of the photon helicity with the applied field at 10 K. As is evident, a clear XMCD 共+ − −兲
FIG. 4. 共Color online兲 NEXAFS spectra at the Co L3,2-edge collected at 300 K for Sn1−xCoxO2 共x = 0.01, 0.02, 0.04, and 0.05兲 nanoparticles.
with a negative sign at h = 779.8 eV confirms that Co2+ ions are responsible for ferromagnetic ordering. The relative intensity of the Co L3-XMCD signal decreases with increasing cobalt concentration, indicating that the ferromagnetic contribution of Co2+ ions depends on the cobalt concentration. The following sum rules were used to separate the spin and the orbital part of the magnetic contribution of cobalt. 具Lz典 =
2共A + B兲 nh , 3C
具Sz典 =
共A − 2B兲 nh , 2C
where A represents the area under the L3-XMCD curve, B represents the area under the L2-XMCD curve, C represents the area of the total NEXAFS absorption curve, and nh is the number of holes in 3d level. The orbital and spin magnetic moments, L and S, are calculated as follows:
L = − B具Lz典, S = − 2B具SZ典, where, B is the Bohr magneton. Our experimental and calculated NEXAFS spectra at Co L3,2-edge have confirmed the +2 charge state of Co in SnO2; therefore, the value of nh was
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FIG. 5. Multiplet calculation plots of Co L3,2-edge, 共a兲–共d兲 for +2 and 共e兲–共f兲 for +3 charge states of Co, respectively with different 10 Dq values.
taken to be three. The values of L and S so calculated, for different samples, are listed in Table I. These calculated values of L and S are very close to the previously reported value of L and S for cobalt doped ZnO.17 Thus the cobalt contribution to the magnetism in our samples is ferromagnetic. The orbital and spin moments of the cobalt metal are reported to be 0.153 B and 1.55 B, respectively,32 which
are an order of magnitude larger than the calculated values in the present study. This further confirms that the ferromagnetism in our samples is not due to cobalt metal clusters and is due to the Co+2 ions substituting at the Sn sites in the SnO2 matrix. Room temperature magnetization measurements were performed for the samples with x = 0.01 and 0.05, which are shown in Fig. 7. It can be seen that the magnetization values increase rapidly at lower fields which indicate their ferromagnetic behavior, with coercive field 共Hc兲 of 262 Oe and 152 Oe for x = 0.01 and 0.05 samples, respectively. The magnetic moments per cobalt ion, calculated from the magnetization data, was found to decrease from 0.155 to 0.0223 B / Co when cobalt concentration were increased from x = 0.01 to 0.05, which is in agreement with the XMCD results. The decrease in magnetic moment of cobalt, with increasing doping concentrations, may be related to the structural property of Sn1−xCoxO2 system. The evolution of cell parameters show that the lattice parameters, and hence unit cell volume, decrease with increasing cobalt concentration. The unit cell volume contraction may leads to reduction in distances between nearby cobalt ions in the SnO2 matrix, which may lead to the antiferromagnetic type superexchange interaction among neighboring cobalt ions, leading to the observed decrease in magnetic moment with increasing Co concentration. TABLE I. Orbital and spin magnetic moments 共L and S兲, calculated from XMCD measurements, of Sn1−xCoxO2 共x = 0.01, 0.04, and 0.05兲 nanoparticles.
FIG. 6. 共Color online兲 The XMCD spectra at the Co L3,2-edge collected at 300 K for Sn1−xCoxO2 共x = 0.01, 0.04, and 0.05兲 nanoparticles.
L S
x = 0.01
x = 0.04
x = 0.05
0.182 B 0.516 B
0.105 B 0.243 B
0.061 B 0.210 B
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Project No. SR/S2/CMP/0051/2007. A.P.S. is also thankful to Council for Scientific and Industrial Research 共CSIR兲 for providing financial support through fellowship. G. A. Prinz, Science 282, 1660 共1998兲. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. V. Molnar, M. L. Roukes, A. Y. Chtcheljanova, and D. M. Treger, Science 294, 1488 共2001兲. 3 H. Ohno, Science 281, 951 共1998兲. 4 T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 共2000兲. 5 T. Dietl, H. Ohno, and F. Matsukura, Phys. Rev. B 63, 195205 共2001兲. 6 S. B. Ogale, R. J. Choudhary, J. P. Bhuban, S. E. Lofland, S. R. Shinde, S. N. Kale, V. N. Kulkarni, J. Higgins, C. Lanci, J. R. Simpson, N. D. Browning, S. D. Sarma, H. D. Drew, R. L. Greene, and T. Venkatesan, Phys. Rev. Lett. 91, 077205 共2003兲. 7 A. Punnoose, J. Hays, V. Gopal, and V. Shutthanandan, Appl. Phys. Lett. 85, 1559 共2004兲. 8 J. Hays, A. Punnoose, R. Baldher, M. H. Engelhard, J. Pelequin, and R. M. Reddy, Phys. Rev. B 72, 075203 共2005兲. 9 D. Menzel, A. Awada, H. Dierke, J. Schaenes, F. Ludwig, and M. Schilling, J. Appl. Phys. 103, 07D106 共2008兲. 10 X. L. Wang, Z. Zeng, X. H. Zheng and Q. Lin, J. Appl. Phys. 101, 09H104 共2007兲. 11 X. L. Wang, Z. X. Dai, and Z. Zheng, J. Phys.: Condens. Matter 20, 045214 共2008兲. 12 J. M. D. Coey, A. P. Douvalis, C. B. Fitzgerald, and M. Venkatesan, Appl. Phys. Lett. 84, 1332 共2004兲. 13 A. Punnoose, M. S. Sechra, W. K. Park, and J. S. Moudera, J. Appl. Phys. 93, 7867 共2003兲. 14 L. B. Duan, G. H. Rao J, Yu, Y. C. Wang, G. Y. Liu, and J. K. Liang, J. Appl. Phys. 101, 063917 共2007兲. 15 Y. Xiao, S. Ge, Li Xi Y. Zuo, X. Zhou, B. M. Zhang, Li Zhang, C. Li, X. Han, and Z. C. Wen, Appl. Surf. Sci. 254, 7459 共2008兲. 16 R. Kumar, A. P. Singh, P. Thakur, N. B. Brookes, K. H. Chae, W. K. Choi, B. Angadi, S. D. Kaushik, and S. Patnaik, J. Phys. D: Appl. Phys. 41, 155002 共2008兲. 17 A. P. Singh, R. Kumar, P. Thakur, N. B. Brookes, K. H. Chae, and W. K. Choi, J. Phys. Condens. Matter 21, 185005 共2009兲. 18 S. Kumar, Y. J. Kim, B. H. Koo, S. K. Sharma, J. M. Vargas, M. Knobel, S. Gautam, K. H. Chae, D. K. Kim, Y. K. Kim and C. G. Lee, J. Appl. Phys. 105, 07C520 共2009兲. 19 B. T. Thole, P. Carra, F. Sette, and G. Van der Lann, Phys. Rev. Lett. 68, 1943 共1992兲. 20 P. Carra, B. T. Thole, M. Altarelli, and X. Wang, Phys. Rev. B 70, 000694 共1993兲. 21 J. A. Cerri, E. R. Leite, D. Gouvea, E. Longo, and J. A. Varela, J. Am. Ceram. Soc. 79, 799 共1996兲. 22 S. Tsunekawa, K. Ishikawa, Z.-Q. Li, Y. Kawazoe, and A. Kasuya, Phys. Rev. Lett. 85, 3440 共2000兲. 23 K. Gopinadhan, D. K. Pandya, S. C. Kashyap, and S. Chaudhary, J. Appl. Phys. 99, 126106 共2006兲. 24 S. Das, S. Kar, and S. Choudhary, J. Appl. Phys. 99, 114303 共2006兲. 25 F. D. Groot, Coord. Chem. Rev. 249, 31 共2005兲. 26 T. Kroll, M. Knupfer, J. Geck, C. Hess, T. Schwieger, G. Krabbes, C. Sekar, D. R. Batchelor, H. Berger, and B. Buchner, Phys. Rev. B 74, 115123 共2006兲. 27 H. Ahn, H. Choi, K. Park, S. Kim, and Y. Sung, J. Phys. Chem. B 108, 9815 共2004兲. 28 T. F. Baumann, S. O. Kucheyev, A. E. Gash, and J. H. Satcher, Jr., Adv. Mater. 共Weinheim, Ger.兲 17, 1546 共2005兲. 29 M. Batzill and U. Diebold, Prog. Surf. Sci. 79, 47 共2005兲. 30 X. F. Liu, J. Iqbal, W. M. Gong, S. L. Yang, R. S. Gao, F. Zeng, R. H. Yu, B. He, Y. P. Hao, and X. P. Hao, J. Appl. Phys. 105, 093931 共2009兲. 31 A. Lussier, J. Dvorak, Y. U. Idzerda, S. B. Ogle, S. R. Shinde, R. J. Choudhary, and T. Venkatesan, J. Appl. Phys. 95, 7190 共2004兲. 32 C. T. Chen, Y. U. Idzerada, H.-J. Lin, N. V. Smith, G. Meigs, E. Chaban, G. H. Ho, E. Pelegrin, and F. Sette, Phys. Rev. Lett. 75, 152 共1995兲. 1 2
FIG. 7. 共Color online兲 Hysteresis loop 共M-H curve兲 of Sn1−xCoxO2 共x = 0.01 and 0.05兲 nanoparticles at room temperature.
IV. CONCLUSIONS
In summary, we have investigated structural and magnetic properties along with electronic structure of chemically synthesized Sn1−xCoxO2 共x = 0.00 to 0.05兲 nanoparticles. The XRD studies reveal single phase rutile-type crystal structure without any secondary phases. TEM images along with EDAX measurements confirm that Sn1−xCoxO2 nanocrystals have spherical morphology with a narrow size distribution and are free from the impurity elements. NEXAFS measurements performed at Sn M5,4-edge have also strengthened the XRD and TEM results of the formation of nanocrystalline SnO2. The observed NEXAFS spectra at Co L3,2-edge, show entirely different features than that of metallic cobalt clusters and/or other cobalt oxide phases. The multiplet calculations show that the cobalt is in +2 charge state and substitutes at the Sn site in the octahedral symmetry with 10 Dq = 0.6 eV. The ferromagnetic character of these materials was further confirmed by XMCD and room temperature magnetization hysteresis loop measurements. All the findings suggest that the prepared samples are in nanocrystalline form and strengthened the view of substitution of cobalt at the Sn site in the SnO2 host matrix, which is responsible for the RTFM. ACKNOWLEDGMENTS
Authors 共A.S. and K.D.V.兲 acknowledge the financial support from Inter University Accelerator Center 共IUAC兲, New Delhi, India, for this research work, under the UFUP research project scheme 共Code-41304兲. One of the authors 共R.K.兲 acknowledges the financial support from Department of Science and Technology 共DST兲, India under the research
