Role of Co Clusters and Oxygen Vacancies in the Magnetic and

Article pubs.acs.org/JPCC

Role of Co Clusters and Oxygen Vacancies in the Magnetic and Transport Properties of Co-Doped In2O3 Films Yukai An,*,† Dongyan Yang,† Guanxiong Ma,*,‡ Yi Zhu,† Shiqi Wang,† Zhonghua Wu,§ and Jiwen Liu*,† †

Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Material Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ‡ College of Science, Shenyang Agricultural University, Shengyang, Liaoning 110866, China § Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: The (In1−xCox)2O3 films with x = (0.055, 0.08, 0.10, 0.15) have been prepared by a radio frequency magnetron sputtering technique and investigated by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure, Hall effect, and room-temperature magnetic measurements. The detailed structural analyses and full multiple-scattering ab initio calculations indicate that most Co2+ ions substitute for In3+ sites of In2O3 lattice and form CoIn2+ + VO complexes with the O vacancy in the nearest coordination shell, whereas a portion of the Co atoms form the precipitate of Co metal clusters for all the (In1−xCox)2O3 films. Despite the formation of Co clusters, magnetic characterizations show that the saturated magnetization Ms of films first increases and then decreases with the increase of Co concentration, suggesting that the small Co clusters are superparamagnetic. The electronic conducting mechanism is dominated by Mott variable range hopping behavior for all the films. The strong localization of carriers suggests the bound magnetic polarons scenario. It can be concluded that the observed roomtemperature ferromagnetism in the (In1−xCox)2O3 films is intrinsic and originates from electrons bound in defect states associated with oxygen vacancies. There exists an optimal localization radius ξ of variable range hopping for achieving the largest Ms in the (In1−xCox)2O3 films.

1. INTRODUCTION Diluted magnetic oxides (DMOs) have attracted substantial research interest because of their potential applications in spintronic devices.1,2 Among them, transition metal (TM)doped In2O3 has been intensively studied3,4 since Dietl et al. predicted that room-temperature (RT) ferromagnetism may be realized in oxide-based wide-band gap semiconductors.5 In2O3 is an n-type transparent, wide band gap (∼3.75 eV) semiconductor with a bixbyite cubic structure and has been extensively applied in electronics and optics, including flatpanel displays. So far, RT ferromagnetism has been observed for various TM elements (Co, Mn, Fe, Cr, Ni)-doped In2O3, but experimental results and conclusions obtained by different research groups are quite different and even contradictory.6−11 Yu et al. reported RT ferromagnetic Fe and Cu codoped In2O3 films and confirmed that the ferromagnetism is mediated by mobile carriers.8 However, Li et al. reported that the RT ferromagnetism in Fe-doped In2O3 films with Sn codoping is not due to charge carriers but is directly related to oxygen vacancies.9 Wu et al. reported that ferromagnetism could be achieved in Cr-doped In2O3 nanostructures,10 while Gaur et al. failed to find ferromagnetism in bulk ceramic Cr-doped In2O3.12 Moreover, the origin and mechanism of the observed ferromagnetism in In2O3 DMOs is still under intense debate. In many reports, the RT ferromagnetism is attributed to the © 2014 American Chemical Society

precipitation of magnetic clusters or the secondary magnetic phases,13 whereas others suggest that the ferromagnetism arises from double exchange mechanism, carrier-mediated indirect Ruderman−Kittel−Kasuya−Yoshida (RKKY) exchange interaction, and bound magnetic polarons (BMPs).8,14−16 Recently, Coey et al. proposed a defect-induced ferromagnetic exchange mechanism based on BMPs,17 and some experimental results as well as some theoretical calculations of the electronic structure and magnetic interaction18−21 also suggest that some kind of defects (such as O vacancy, In interstitial) induced during the film deposition process can play a crucial role in introducing ferromagnetism. Therefore, to clarify the mechanisms responsible for the ferromagnetic order in In2O3-based DMOs, further experimental investigations between the local structure and ferromagnetism are of great importance. We present a detailed investigation on the local Co structure and magnetic and transport properties in Co-doped In2O3 films. The XAFS technique was used to determine the occupation sites of the Co dopant and the possible presence of native defects. We aim to gain the dependence of magnetic and transport features on the local structure around the Co Received: February 13, 2014 Revised: April 23, 2014 Published: April 24, 2014 10448

dx.doi.org/10.1021/jp501560u | J. Phys. Chem. C 2014, 118, 10448−10454

The Journal of Physical Chemistry C

Article

atoms and the formation of the secondary phase. These results will be useful in advancing the studies on diluted magnetic semiconductors and allow us to explore the evidence of oxygen vacancy-induced ferromagnetism in Co-doped In2O3 films.

2. EXPERIMENTAL SECTION The (In1−xCox)2O3 films with a thickness of about 800 nm were grown on (001) Si substrates by an RF-magnetron sputtering technique. Targets were prepared by the sol−gel technique and sequent calcination treatments. Specifically, the starting reagents were chosen using indium nitrate hydrate (In(NO3)3·41/2H2O) and cobalt(II) acetate tetraphydrate (Co(Ac)2·4H2O) as precursors and absolute ethanol as solvent. After the solution was mixed at 60 °C for 3h, C4H13NO was injected into the solution dropwise. The reaction continued for 30 min; the sol was obtained and then dried at 150 °C to get the xerogel. The obtained xerogel was calcined at 1100 °C for 3h in air atmosphere to get the targets with stoichiometric amounts x = 0.055, 0.08, 0.10, and 0.15. The deposition chamber was evacuated to a base pressure of about 6 × 10−5 Pa. All films were deposited at an Ar (purity 99.999%) pressure of 0.8 Pa at a substrate temperature of 400 °C. The crystal structures of films were examined by θ/2θ X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.15406 nm). The valence state was confirmed by X-ray photoelectron spectroscopy (XPS), which was carried out on a PHI-1600 photoelectric spectrometer (Mg Kα, X-ray). The Co K-edge (7709 eV) XAFS spectra, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were measured in the total fluorescence mode at Beijing Synchrotron Radiation Facility (BSRF) on the 4B9A beamline of the X-ray diffraction station. As references, the spectra of Co, CoO, Co2O3, and Co3O4 foils were also measured for comparison and data analysis purposes. Resistivity and Hall effect were determined using physical property measurement system (PPMS, Quantum Design). The magnetic properties measurements were performed by the physical properties measurement system (PPMS-9 Quantum Design) at room temperature.

Figure 1. XRD patterns of (In1−xCox)2O3 (x = 0, 0.055, 0.08, 0.10, 0.15) films. The inset shows the enlarged view of (222) diffraction peaks.

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of (In1−xCox)2O3 films with x = 0, 0.055, 0.08, 0.10, 0.15. It is clear that all films are indexed as In2O3 cubic bixbyite structure with preferred (222) and (400) orientations. No detectable peaks corresponding to Co metal clusters or Co oxide secondary phases are observed with the XRD detection limit. As seen in the inset of Figure 1, a shift in the (222) peaks toward higher values of 2θ is observed as Co concentration increases, indicating the decrease of average lattice spacing. Considering the fact that the radii of the Co ions (RCo2+ = 0.74 Å and RCo3+ = 0.64 Å) are smaller than that of the In ion (RIn3+ = 0.94 Å), the reduction in the lattice constant with Co doping can be ascribed to the substitution of Co in the In3+ sites of the In2O3 lattice.22,23 In order to determine the composition and actual valence electron states of films, a detailed XPS analysis for the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15 was performed, as shown in Figure 2a−c. The XPS spectra have been charge-corrected by setting the C 1s binding energy at the energy location of 284.6 eV. The observed energies for In 3d5/2 and 3d3/2 of the (In1−xCox)2O3 films are located at 444.3 and 451.9 eV, respectively, corresponding to the binding energy of

Figure 2. (a) In 3d, (b) Co 2p , and (c) O 1s core-level XPS spectra for the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15. 10449



Article

In3+ in In2O3. The Co 2p XPS spectra are displayed in Figure 2b. The binding energy of Co 2p3/2 is located at 780.8 eV, and the energy difference between the binding energies of Co 2p3/2 and Co 2p1/2 is 15.5 eV, indicating that Co element exists in the ionic form with the high spin divalent state of Co2+ in In2O3 matrix.24 It is also noted that the Co 2p3/2 and 2p1/2 peaks have the satellite structure on the higher energy side separated by about 6 eV, which is also the typical character of Co2+ ions. Additionally, there exist additional peaks located at 778.3 and 793.1 eV for the (In1−xCox) 2O3 film with x = 0.15, corresponding to the 2p3/2 and 2p1/2 peak of metallic Co. These suggest that Co atoms start to segregate into metallic clusters in the films with high Co concentration. The O 1s XPS spectra for the (In1−xCox)2O3 films with x = 0.08, 0.10, 0.15 can be divided into two peaks utilized the Gaussian fitting, I and II, located at 529.9 and 531.5 eV, respectively. The I peak is assigned to In2O3 lattice oxygen, whereas the II peak is assigned to the oxygen ions in the oxygen vacancies (VO) region.25,26 However, the (In1−xCox)2O3 film with x = 0.055 shows a additional peak III (532.4 eV), which is assigned to the adsorbed oxygen and possibly comes from contaminations of the testing process.27 The peak ratio of II/I is 1/6.8 for the (In1−xCox)2O3 film with x = 0.055, implying that a large amount of oxygen vacancies are introduced in the film. The peak ratio of II/I increases to 1/4.6 when x goes from 0.055 to 0.15, suggesting that the amount of oxygen vacancies in the films increases with the increase of Co concentration. This may be due to local lattice distortion around the Co atoms or compensating for the charge nonequilibrium (Co2+ at In3+) leading to the creation of more oxygen vacancies in the films with high Co doping concentration. The XRD technique is not sensitive enough to detect nanoscale metal precipitates and exclude the existence of magnetic impurity phases. Thus, we employed the XAFS technique as a sensitive local structure probe to further investigate the structural characteristics of Co dopants in the (In1−xCox)2O3 films. Figure 3 shows experimental Fourier transform (FT) curves of Co K-edge EXAFS oscillation functions k3χ(k) for the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15. As references, the Co K-edge spectra of Co, CoO, Co2O3, and Co3O4 foils are also plotted. The two peaks at around 1.39 and 3.26 Å correspond to the Co−O and the Co−In neighbor coordination shell of Co. The existence of CoO, Co2O3, and Co3O4 can be excluded because their Co−O radial distances (CoO, 1.54 Å; Co2O3, 1.62 Å; Co3O4, 1.47 Å) are significantly different from those of films. The peak intensity of the Co−O nearest-neighbor coordination shell decreases with Co doping, implying that the disorder degree around the Co atoms becomes larger. In addition, it is obvious that there exist additional peaks located at 2.32, 2.23, 2.19, and 2.17 Å for the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15, respectively. The position of additional peaks gradually gets closer to the Co−Co radial distances of metallic Co, and the intensity also increases with the increasing of Co concentration. This phenomenon may be attributed to the composition segregation of metallic Co, indicating that not all doped Co atoms are incorporated into the lattice of In2O3 and a portion of them is separated to form the Co clusters. Therefore, it can be concluded that in the (In1−xCox)2O3 films, most Co atoms substitute the In sites of In2O3 lattices while a portion of the Co atoms form the precipitate of Co metal clusters. To obtain quantitative structural information, we fitted the main peaks including the Co−O and Co−Co nearest-neighbor

Figure 3. Experimental and fitting Fourier transform curves of Co Kedge EXAFS oscillation functions k3χ (k) for the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15 as well as Co, CoO, Co2O3, and Co3O4 foils. Circles, experimental; solid lines, fitting.

coordination shell. The solid lines in Figure 3 are the best fits to the experimental FT curves of the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15. Some structure models have been applied during fitting process: (1) structure model for CoIn1; (2) structure model for CoIn2; (3) structure model for Co metal. Attempts to fit the FT curves for the (In1−xCox)2O3 films using only model 1 or 2 failed (not shown here). The best fit to FT curves can be obtained by assuming the Co atoms substitution for In1 sites of the In2O3 lattice and coexistence with Co metal, namely a two-phase fitting using models 1 and 3. The fitted bond length R, coordination number N, and Debye−Waller factors σ2 of different atomic shells are given in Table 1. Within experimental error, the Co−O bond length of Table 1. Best Fit Structural Parameters around Co Atoms in the (In1−xCox)2O3 Films with x = 0.055, 0.08, 0.10, 0.15a sample

coordination

N

R (Å)

σ2 (Å2)

Co metal (In0.945Co0.055)2O3

Co−Co Co−O Co−Co Co−O Co−Co Co−O Co−Co Co−O Co−Co

5.4 10.2 5.0 10.7 4.7 11.2 4.7 11.4

2.495 2.171 2.556 2.168 2.531 2.166 2.502 2.158 2.492

0.005 28 0.008 77 0.020 00 0.012 82 0.016 00 0.020 00 0.011 0 0.024 18 0.009 2

(In0.92Co0.08)2O3 (In0.90Co0.10)2O3 (In0.85Co0.15)2O3

a N, R, and σ2 are the coordination number, bond length, and Debye− Waller factor, respectively. The uncertainties for N, R, and σ2 are 5%, 0.01 Å, and 5%, respectively.

films is smaller than the In−O bond length (2.19 Å) in pure In2O3, which is in qualitative agreement with the smaller radius of Co ions, as compared to that of In ions. One can see that with the increase of Co concentration, the Co−O coordination number NCo−O and the Co−O bond length decrease and the Debye−Waller factor σ2Co−O in the first nearest neighbor around the Co atoms increases; these can be attributed to the 10450



Article

for the existence of oxygen vacancies around the Co dopant in our (In1−xCox)2O3 films. Figure 5 shows the RT M−H curves of the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15. The diamagnetic

relaxation of oxygen environment of Co ions upon substitution. The correlation between NCo−O and σ2Co−O implies that the disorder increases with the increase of Co concentration. Oxygen vacancies were considered to play a crucial role in introducing RT ferromagnetism for DMOs. The XANES spectrum is a fingerprint of the existence or absence of oxygen vacancy and In interstitial (Ini) in Co-doped In2O3 DMOs. The experimental Co K-edge XANES spectrum for the (In1−xCox)2O3 films with x = 0.055 is compared in Figure 4

Figure 5. RT M−H curves of the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15.

Figure 4. Experimental XANES spectrum for the (In1−xCox)2O3 films with x = 0.055 and XANES spectra calculated for different model structures. (M1: Co substitutes for In1 site with one Ini in the nearest coordination shell. M2: Co substitutes for In1 site with two VO in the nearest coordination shell. M3: Co substitutes for In1 site with one VO in the nearest coordination shell. M4: Co substitutes for In1 site. M5: Co substitutes for In2 site. M6: Co substitutes for In2 site with one VO in the nearest coordination shell. M7: Co substitutes for In2 site with one Ini in the nearest coordination shell.)

contribution from the substrate has been subtracted from the M−H curves. All the films show obvious RT ferromagnetism with a saturation magnetization (Ms) between 0.6 and 2 emu/ cm3. The Ms increases dramatically with a maximum value of 2 emu/cm3 when x goes from 0.055 to 0.10 and then decreases to 0.9 emu/cm3 with further increase of x to 0.15. The nonmonotonic Co concentration dependence of Ms is different from the previously reported cases of Fe-doped In2O3 and Modoped In2O3,28,29 where the Ms decreases or increases monotonously with the increase of TM concentration. Because the formation of Co clusters has been observed in all the (In1−xCox)2O3 films, to understand the ferromagnetic nature of these films, we should consider the ferromagnetic contributions from the substitutional Co atoms and the Co metal clusters for the (In1−xCox)2O3 films in detail. Shinde et al. showed that the Co metal clusters with the size of 10 nm observed by HRTEM were considered to be the origin of the ferromagnetism in the TiO2 films.30 However, Norton et al. found that the Co-doped ZnO films demonstrate superparamagnetism at 300 K, although the diffraction peaks corresponding to Co nanocrystals with the size of 3.5 nm appear in the XRD pattern.31 The theoretical studies by Vargas et al. indicated that small Co clusters may have no ferromagnetism at 300 K because of a pronounced temperature dependence on magnetization.32 According to the EXAFS results, a portion of the Co atoms form Co metal clusters, and the content of Co clusters increases with Co doping. If the observed RT ferromagnetism comes from Co clusters, it can be expected that the Ms has the same increasing tendency with the increase of Co concentration. However, the Ms increased first and then decreases. At the same time, no diffraction peaks of Co clusters can be detected by XRD; the size of Co clusters existing in the films should be very small (below 3.5 nm). As discussed above, it can be concluded that the small Co clusters in the grown (In1−xCox)2O3 films are superparamagnetic and exhibit no hysteresis curve at RT. Thus, the obvious RT ferromagnetism in the (In1−xCox)2O3 films is

with the calculated spectra with different model structures. The simulations of Co K-edge XANES spectra were calculated by the real-space multiple-scattering approach using FEFF 9.0 code for seven model structures which contain 108 atoms within a sphere of radius 6.7 Å from the central Co atom. It is obvious that the intensity in the “valley” between the pre-edge 1s−3d feature (peak D) and the white line peak (peak A) for the (In0.94Co0.055)2O3 film is significantly increased compared to those for the calculated XANES spectra with different model structures. This is due to the presence of metallic Co clusters in the film. From Figure 4, it can be clearly seen that for the calculated XANES spectra of M5 (CoIn2), M6 (CoIn2 + VO), and M7 (CoIn2 + Ini) there exist three main features marked by A, B, and C that are not consistent with the experimental spectrum features. However, the Co atom substituting In1 atom by the introduction of VO in the first coordination can greatly improve the quality of the fit. The XANES spectra calculated for model structures M2 (CoIn1 + 2 VO), M3 (CoIn1 + VO) and M4 (CoIn1) in Figure 4 change obviously with the introduction of VO defects. The intensity of peak A increases dramatically, while the intensity of peak B decreases with the decrease of VO number. One can see that the XANES spectra calculated for model structure M2 (CoIn1 + VO) can reproduce well the experimental spectrum features. These results further suggest that the doped Co atom substitutes for In1 site with one VO in the nearest coordination shell. This is also a powerful witness 10451



Article

Table 2. Parameters of the (In1−xCox)2O3 Films with x = 0.055, 0.08, 0.10, 0.15 Obtained from Hall Effect and RT Measurements (Electron Concentration n, kFl, and Mean Free Path l, Characteristic Hopping Temperature T0, and Localization Radius ξ) x

n (cm−3)

0.055 0.08 0.1 0.15

× × × ×

8.76 6.91 4.97 9.26

19

10 1019 1019 1018

kFl 3.72 9.15 2.48 7.01

× × × ×

−2

10 10−2 10−1 10−1

l (Å)

Tv (K)

T0 (K)

ξ (nm)

0.72 1.14 3.11 15.7

14 34 36 39

18 739 2 541 371 1.31

1.296 2.87 5.21 38.4

intrinsic and is related to the substitutional Co atoms at the In1 sites of In2O3 lattices. To further investigate the possible magnetic mechanism responsible for the observed RT ferromagnetism in the (In1−xCox)2O3 films, the Hall effect was measured at 300 K. The type of carriers is confirmed to be n-type for all the films, as expected for films grown in an oxygen-deficient environment. The carrier concentration nc decreases monotonically with the increase of Co concentration (Table 2). This can be explained by electrons arising from the oxygen vacancies that may be partly compensated by the holes produced by the substitution of Co2+ for In3+, resulting in a decrease in the carrier concentration nc. The electric parameters of the (In1−xCox)2O3 films are characterized by kFl, which was estimated using the formula kFl = ℏ(3π2)2/3/(e2ρn1/3), where kF is the Fermi wave vector, l the mean free path, ℏ the Planck constant, e the electron charge, ρ the resistivity, and n the electron concentration. From Table 2, one can see that all the (In1−xCox)2O3 films fulfill the Anderson−Mott localization criterion with kFl < 1, suggesting that the carriers in the films are strongly localized and carrier-mediated ferromagnetism is ruled out.33,34 Therefore, the BMP mechanism involving oxygen vacancies can account for the ferromagnetism order in the (In1−xCox)2O3 films. Figure 6a shows the ρ−T curves of the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15. It can be clearly seen that semiconducting behaviors characterized by the negative temperature coefficient can be observed for all the films. Figure 6b show data plotted as ln ρ(T) versus T−1/4 for the (In1−xCox)2O3 films with x = 0.055, 0.08, 0.10, 0.15. The linear relationship between ln ρ and T−1/4 suggests the Mott variable range hopping (VRH) behavior as the main conduction mechanism. The hopping resistivity can be described as ρ = ρ0 exp[(T0/T)1/4], where ρ0 is the resistivity coefficient and T0 is the characteristic hopping temperature.35 The deviation from linear relation at high temperature can be attributed to thermally activated conduction. To further investigate the VRH model, we calculate the localization radius of VRH, which is expressed as ζ = (2β/Nd)1/3(Tv/T0)1/4, where β is a constant, Nd the donor concentration, and Tv the onset temperature of VRH.36 Behan et al. reported that a high T0 corresponds to a low carrier hopping probability and fewer disturbances to the exchange field of BMPs.37 From Table 2, one can see that with the increase of Co concentration, the localization radius ξ increases and the characteristic hopping temperature T0 decreases, suggesting that the Co doping can to some extent avert the localization radius ξ and hopping probability of carriers. The variation of localization effect could also strongly modify the ferromagnetism in the films with the BMPs scenario. When the localization radius ξ of carriers is obviously smaller than the radius of magnetic polaron rBMP in low Co concentration films, actually this effective rBMP should be decreased, resulting in some Co ions being outside the effective

Figure 6. (a) Temperature dependence of the resistivity of (In1−xCox)2O3 (x = 0.055, 0.08, 0.10, 0.15) films. (b) Plot of ln ρ versus T−1/4 for the (In1−xCox)2O3 (x = 0.055, 0.08, 0.10, 0.15) films.

rBMP and small Ms. When the ξ is much larger than the rBMP in high Co concentration films, especially for the (In1−xCox)2O3 film with x = 0.15, the hopping probability of carriers remarkably increases (lowest T0), leading to weak stabilization of BMPs and decrease in the Ms. Therefore, it can be considered that there exists an optimal localization radius ξ of carriers for achieving the largest Ms in the (In1−xCox)2O3 films. This is also consistent with the nonmonotonic Co concentration dependence of Ms in the films.

4. CONCLUSIONS The (In1−xCox)2O3 films have been synthesized by RFmagnetron sputtering technique. According to the structure analysis, all films have cubic bixbyite structure without forming Co oxide-related secondary phases. Most Co ions substitute for In3+ sites in the valence of +2 states and form CoIn2+ + VO complex with the O vacancy in the nearest coordination shell, while a portion of the Co atoms form the precipitate of Co metal clusters in all the (In1−xCox)2O3 films. All films display a clear RT ferromagnetic behavior; the Ms increases first and then decreases with the increase of Co concentration, suggesting that 10452



Article

(11) Peleckis, G.; Wang, X. L.; Dou, S. X. High Temperature Ferromagnetism in Ni-Doped In2O3 and Indium-Tin Oxide. Appl. Phys. Lett. 2006, 89, 022501. (12) Gaur, A. P. S.; Sahoo, S.; Katiyar, R. K.; Rinaldi, C.; Scott, J. F.; Katiyar, R. S. Absence of Magnetism in Cr-Doped In2O3: A Case Study of Phase Separation versus Phase Formation. J. Phys. D: Appl. Phys. 2011, 44, 495002. (13) Ohno, T.; Kawahara, T.; Tanaka, H.; Kawai, T.; Oku, M.; Okada, K.; Kohiki, S. Ferromagnetism in Transparent Thin Films of Fe-Doped Indium Tin Oxide. Jpn. J. Appl. Phys. 2006, 45, L957−L959. (14) Park, C. Y.; You, C. Y.; Jeon, K. R.; Shin, S. C. Charge-Carrier Mediated Ferromagnetism in Mo-Doped In2O3 Films. Appl. Phys. Lett. 2012, 100, 222409. (15) Ma, R. R.; Jiang, F. X.; Qin, X. F.; Xu, X. H. Effects of Oxygen Vacancy and Local Spin on the Ferromagnetic Properties of Ni-Doped In2O3 Powders. Mater. Chem. Phys. 2012, 132, 796−799. (16) Hakimi, A. M. H. R.; Blamire, M. G.; Heald, S. M.; Alshammari, M. S.; Alqahtani, M. S.; Score, D. S.; Blythe, H. J.; Fox, A. M.; Gehring, G. A. Donor-Band Ferromagnetism in Cobalt-Doped Indium Oxide. Phys. Rev. B 2011, 84, 085201. (17) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Donor Impurity Band Exchange in Dilute Ferromagnetic Oxides. Nat. Mater. 2005, 4, 173−179. (18) He, J.; Xu, S.; Yoo, Y. K.; Xue, Q.; Lee, H. C.; Cheng, S.; Xiang, X. D.; Dionne, G. F.; Takeuchi, I. Room Temperature Ferromagnetic n-Type Semiconductor in (In1−xFex)2O3−σ. Appl. Phys. Lett. 2005, 86, 052503. (19) Yoo, Y. K.; Xue, Q.; Lee, H. C.; Cheng, S.; Xiang, X. D.; Dionne, G. F.; Xu, S.; He, J.; Chu, Y. S.; Preite, S. D.; et al. Bulk Synthesis and High-Temperature Ferromagnetism of (In1−xFex)2O3−σ with Cu coDoping. Appl. Phys. Lett. 2005, 86, 042506. (20) Errico, L. A.; Rentería, M.; Weissmann, M. Theoretical Study of Magnetism in Transition-Metal-Doped TiO2 and TiO2−δ. Phys. Rev. B 2005, 72, 184425. (21) Rodriguez-Torres, C. E.; Cabrera, A. F.; Errico, L. A.; Adan, 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. (22) Punnoose, A.; Hays, J.; Gopal, V.; Shutthanandah, V. RoomTemperature Ferromagnetism in Chemically Synthesized Sn1−xCoxO2 Powders. Appl. Phys. Lett. 2004, 85, 1559. (23) Hays, J.; Punnoose, A.; Baldner, R.; Engelhard, M. H.; Peloquin, J.; Reddy, K. M. Relationship between the Structural and Magnetic Properties of Co-Doped SnO2 Nanoparticles. Phys. Rev. B 2003, 72, 075203. (24) Subias, G.; Stankiewicz, J.; Villuendas, F.; Lozano, M. P.; Garcia, J. Local Structure Study of Co-Doped Indium Oxide and Indium-Tin Oxide Thin Films using X-ray Absorption Spectroscopy. Phys. Rev. B 2009, 79, 094118. (25) Yan, S. M.; Ge, S. H.; Qiao, W.; Zuo, Y. L.; Xu, F.; Xi, L. Control of Ferromagnetism in Fe-Doped In2O3 by Carbothermal Annealing. J. Magn. Magn. Mater. 2010, 323, 264−267. (26) Samariya, A.; Singhal, R. K.; Kumar, S.; Xing, Y. T.; Sharma, S. C.; Kumari, P.; Jain, D. C.; Dolia, S. N.; Deshpande, U. P.; Shripathi, T. RETRACTED: “Effect of Hydrogenation vs. Re-heating on Intrinsic Magnetization of Co Doped In2O3”. Appl. Surf. Sci. 2010, 257, 585−590. (27) Yan, S.; Ge, S.; Zuo, Y. L.; Qiao, W.; Zhang, L. Effects of Carbothermal Annealing on Structure Defects, Electrical and Magnetic Properties in Fe-Doped In2O3. Scr. Mater. 2009, 61, 387−390. (28) Xu, X. H.; Jiang, F. X.; Zhang, J.; Fan, X. C.; Wu, H. S.; Gehring, G. A. Magnetic and Transport Properties of n-Type Fe-Doped In2O3 Ferromagnetic Thin Films. Appl. Phys. Lett. 2009, 94, 212510. (29) Park, C. Y.; Yoon, S. G.; Jo, Y. H.; Shin, S. C. RoomTemperature Ferromagnetism Observed in Mo-Doped Indium Oxide Films. Appl. Phys. Lett. 2009, 95, 122502. (30) Shinde, S. R.; Ogale, S. B.; Higgins, J. S.; Zheng, H.; Millis, A. J.; Kulkarni, V. N.; Ramesh, R.; Greene, R. L.; Venkatesan, T. Co-

the small Co clusters are superparamagnetic. Transport properties show that only semiconducting behavior in the whole temperature range is observed for all the films and the carrier concentration nc decreases monotonically with Co doping. The electronic conducting mechanism is dominated by Mott variable range hopping behavior, suggesting that the carriers are strongly localized. It can be concluded that the RT ferromagnetism of the (In1−xCox)2O3 films can be ascribed to rely on the percolation of bound magnetic polarons and that there exists an optimal localization radius ξ of variable range hopping for achieving the largest Ms in the films.

■

AUTHOR INFORMATION

Corresponding Authors

*Y.A.: e-mail, [email protected]. *G.M.: e-mail, [email protected]. *J.L.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by National Natural Science Foundation of China (Grant 10904110, 11174217), Tianjin Natural Science Foundation of China (Grant 10JCYBJC01600), and the Beijing Synchrotron Radiation Laboratory.

(1) Pan, Z. Y.; Hu, F. C.; He, S.; Liu, Q. H.; Sun, Z. H.; Yao, T.; Xie, Y.; Oyanagi, H.; Xie, Z.; Jiang, Y.; et al. Co Cluster Formation Induced by Cu Codoping in Co:ZnO Semiconductor Thin Films. J. Phys. Chem. C 2012, 116, 4855−4861. (2) Gao, D. Q.; Zhang, J.; Yang, G. J.; Qi, J.; Si, M. S.; Xue, D. S. Ferromagnetism Induced by Oxygen Vacancies in Zinc Peroxide Nanoparticles. J. Phys. Chem. C 2011, 115, 16405−16410. (3) Xu, T. S.; Qiao, R. M.; Tian, Y. F.; Yan, S. S.; Zhang, K.; Cao, Y. L.; Kang, S. S.; Chen, Y. X.; Liu, G. L.; Mei, L. M. Magnetoresistance and Electron-Hole Exchange Interaction in (Co1−xInx)2O3−v Concentrated Ferromagnetic Semiconductors. Appl. Phys. Lett. 2013, 103, 202411. (4) Hsua, C. Y. Origin of Ferromagnetism Enhancement in Bi-Layer Chromium-Doped Indium Zinc Oxides. Appl. Phys. Lett. 2012, 101, 062416. (5) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors. Science 2000, 287 (11), 1019−1022. (6) Meng, X. Q.; Tang, L. M.; Li, J. B. Room-Temperature Ferromagnetism in Co-Doped In2O3 Nanocrystals. J. Phys. Chem. C 2010, 114, 17569−17563. (7) Jiang, F. X.; Xu, X. H.; Zhang, J.; Fan, X. C.; Wu, H. S.; Gehring, G. A. Role of Carrier and Spin in Tuning Ferromagnetism in Mn and Cr-Doped In2O3 Thin Films. Appl. Phys. Lett. 2010, 96, 052503. (8) Yu, Z. G.; He, J.; Xu, S.; Xue, Q.; van’t Erve, O. M. J.; Jonker, B. T.; Marcus, M. A.; Yoo, Y. K.; Cheng, S.; Xiang, X.-d. Origin of Ferromagnetism in Semiconducting (In1−x−yFexCuy)2O3−σ. Phys. Rev. B 2006, 74, 165321. (9) Li, S. C.; Ren, P.; Zhao, B. C.; Xia, B.; Wang, L. Room T e m p e r a t u r e F e r r o m a g n e t is m o f B u l k P o l y c r y s t a l l i n e (In0.85−xSnxFe0.15)2O3: Charge Carrier Mediated or Oxygen Vacancy Mediated? Appl. Phys. Lett. 2009, 95, 102101. (10) Xing, G. Z.; Yi, J. B.; Wang, D. D.; Liao, L.; Yu, T.; Shen, Z. X.; Huan, C. H. A. T.; Sum, C.; Ding, J.; Wu, T. Strong Correlation between Ferromagnetism and Oxygen Deficiency in Cr-Doped In2O3−δ Nanostructures. Phys. Rev. B 2009, 79, 174406. 10453



Article

occurrence of Superparamagnetism and Anomalous Hall Effect in Highly Reduced Cobalt-Doped Rutile TiO2−δ Films. Phys. Rev. Lett. 2004, 92, 166601. (31) Norton, D. P.; Overberg, M. E.; Pearton, S. J.; Pruessner, K.; Budai, J. D.; Boatner, L. A.; Chisholm, M. F.; Lee, J. S.; Khim, Z. G.; Park, Y. D.; et al. Ferromagnetism in Cobalt-Implanted ZnO. Appl. Phys. Lett. 2003, 83, 5488. (32) Vargas, P.; Castro, J. D.; Altbir, D. Magnetism of Nanosized Metallic Particles. Phys. Rev. B 1999, 60, 6541. (33) Andrearczyk, T.; Jaroszynski, J.; Grabecki, G.; Dietl, T.; Fukumura, T.; Kawasaki, M. Spin-Related Magnetoresistance of nType ZnO:Al and Zn1−xMnxO:Al Thin Films. Phys. Rev. B 2005, 72, 121309. (34) Xu, Q.; Hartmann, L.; Schmidt, H.; Hochmuth, H.; Lorenz, M.; Spemann, D.; Grundmann, M. s-d Exchange Interaction Induced Magnetoresistance in Magnetic ZnO. Phys. Rev. B 2007, 76, 134417. (35) Mott, N. F.; and Davies, E. A. Electronic Processes in Noncrystalline Materials, 2nd ed.; Oxford University: New York, 1979. (36) Lisunov, K. G.; Arushanov, E.; Thomas, G. A.; Bucher, E.; Schon, J. H. Variable-Range Hopping Conductivity and Magnetoresistance in n-CuGaSe2. J. Appl. Phys. 2000, 88, 4128. (37) Behan, A. J.; Mokhtari, A.; Blythe, H. J.; Score, D.; Xu, X. H.; Neal, J. R.; Fox, A. M.; Gehring, G. A. Two Magnetic Regimes in Doped ZnO Corresponding to a Dilute Magnetic Semiconductor and a Dilute Magnetic Insulator. Phys. Rev. Lett. 2008, 100, 047206.

10454


Role of Co Clusters and Oxygen Vacancies in the Magnetic and

Recommend Documents