Magnetic and electronic properties of Fe and Ni codoped SnO2 Jun Okabayashi, Shin Kono, Yasuhiro Yamada, and Kiyoshi Nomura Citation: J. Appl. Phys. 112, 073917 (2012); doi: 10.1063/1.4754454 View online: http://dx.doi.org/10.1063/1.4754454 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i7 Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 112, 073917 (2012)
Magnetic and electronic properties of Fe and Ni codoped SnO2 Jun Okabayashi,1,a) Shin Kono,2 Yasuhiro Yamada,2 and Kiyoshi Nomura3 1
Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, 113-0033 Tokyo, Japan Department of Chemistry, Tokyo University of Science, Shinjyuku-ku, Tokyo 162-8601, Japan 3 Department of Applied Chemistry, The University of Tokyo, Bunkyo-ku, 113-8656 Tokyo, Japan 2
(Received 25 July 2012; accepted 24 August 2012; published online 8 October 2012) We have investigated Fe and Ni codoping effect into SnO2. Room-temperature diluted ferromagnetic semiconductors with the enhancement of magnetization can be prepared in case of codoping. The saturation magnetization can be controlled by means of the Fe and Ni codoping ratios. The electronic structures were investigated by x-ray absorption spectroscopy (XAS) and M€ossbauer spectrometry. Fe3þ states were revealed by the isomer shift values. XAS revealed Ni2þ states, which suggests that the double-exchange-like ferromagnetic interaction between the diluted magnetic ions mediated by C 2012 American oxygen vacancies becomes a possible origin of room-temperature ferromagnetism. V Institute of Physics. [http://dx.doi.org/10.1063/1.4754454]
I. INTRODUCTION
Wide-band-gap oxide-semiconductor-based diluted magnetic semiconductors (DMS) are one of the promising candidates for future spintronic materials to be used for spin current controlling at room temperature. Transition-metal (TM) doped TiO2, ZnO, and SnO2 have attracted a great deal of attention because of their potential use as functionalities in optical and carrier controlling.1 SnO2, in particular, is widely utilized as a gas sensor, because of its chemical sensitivity and structural stability. TM doping into SnO2 can add further spin-related functionalities that extend beyond current semiconductor application technology. Although theoretical calculations predict that TM-doped oxide DMS would represent the roomtemperature ferromagnetism mediated by charge carriers,2 the origin of ferromagnetism is still debated in regard to DMS materials. The main challenge is to fabricate a DMS with a high Curie temperature (TC) and to control its magnetic properties via chemical doping. Magnetic interaction between doped TM ions through carriers is also essential to the understanding of the mechanism of ferromagnetism in DMS, which opens up a new research field of oxygen-vacancy-induced ferromagnetism. To develop new DMS, the magnetic ion distributions, carrier densities, and electronic structures must be explicitly clarified. Even as its fundamental properties have not yet been established, there are numerous reports on TM-doped SnO2, which is derived from preparation conditions that are extremely sensitive to magnetic properties. Fe-doped SnO2 has been prepared by means of pulsed laser deposition3,4 and sol-gel methods.5–7 However, its maximum saturation magnetization (Ms) is limited to 0.03 emu/g.5,6 Like other candidates for TM doping, Ni-doped SnO2 became a new DMS material8–15 for the following reasons: (1) NiO exhibits antiferromagnetism and ferromagnetic behavior in Ni-doped SnO2 does not originate from the secondary phase formations. (2) Because the ionic radius of Ni2þ is close to that of a)
Author to whom correspondence should be addressed. Electronic mail:
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0021-8979/2012/112(7)/073917/5/$30.00
Sn4þ, Ni atoms are most likely to be substituted in Sn sites. To enhance the ferromagnetic exchange interaction, a codoping technique that employs different d electron numbers becomes essential for the stabilization of ferromagnetism, with the control of both Ms and the coercive field (Hc) at room temperature. In our previous paper, we reported the synthesis of Fe and Co codoped SnO2, and the enhancement of ferromagnetic behavior was observed at room temperature.16 TM d5 systems, such as Fe3þ and Mn2þ configurations, are efficient for the enhancement of the Ms. Fe and Ni codoping is suitable for superexchange ferromagnetism in d5-d8 systems.17 We then employed Fe and Ni codoping into SnO2 for the development of a new DMS. The electronic structure of the codoping system has to be determined, because the codoping effects cannot be explained by the simple summation of single-ion doping. In addition, the defectinduced model of ferromagnetism proposed by Coey et al.1 cannot be applied to TM codoped DMS. This suggests that Fe and Ni codoped SnO2 would be the most suitable material system for the room-temperature DMS. In this paper, we report upon the TM concentration dependence of the magnetic properties of Fe and Ni codoped SnO2. We also systematically discuss its magnetic and electronic properties. II. EXPERIMENTAL
The Fe and Ni codoped SnO2 samples were prepared using a sol-gel method, which is a suitable way to achieve uniform distribution of doped TM ions in the host oxides. SnCl22H2O, metallic Fe, and Ni were dissolved using citric acid, HCl, and ethylene glycol to make a 0.01 M solution. Each solution was mixed according to the appropriate amounts of compounds. These solutions were then condensed at 80 C and calcinated at 250 C for 2 h. The black shiny gels were annealed at 550 C in air for 1 h and then again annealed at 550 C for 3 h after milling. The obtained nanoparticles became a white-brown color with increasing TM concentration. The structure of the particles was determined through x-ray diffraction (XRD) using Cu Ka x-ray and transmission
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electron microscopy (TEM). Local distribution of TM ions was observed by energy-dispersive x-ray analysis (EDS). The magnetic properties at room temperature were measured using a vibrating sample magnetometer (VSM). To investigate the valence states of the Ni ions, x-ray absorption spectroscopy (XAS) was used in the total-electron-yield mode at BL-7A of the Photon Factory, High Energy Accelerator Research Organization (KEK). M€ossbauer spectrometry in transmission geometry with 57Co sources was also employed to investigate the magnetic states of the Fe ions. III. RESULTS AND DISCUSSION
XRD patterns for Fe and Ni codoped SnO2 are shown in Fig. 1. The vertical axis is displayed on a logarithmic scale to enhance the small peaks. The observed peaks correspond to the rutile-type crystalline structure belonging to the P42/ mnm space group. No secondary phases were observed within the detection limits of XRD in 2% Fe and Ni codoped case. With increasing Fe and Ni concentrations, lattice expansion in the range of 0.01 nm was observed from the peak shifts of XRD. With increasing the Ni concentration, extra peaks at 43 appear which correspond to NiO. On the other hand, with increasing the Fe concentration, extra peaks at 36 appear which correspond to a-Fe2O3 (hematite)-like compounds. The size of each particle was estimated to be approximately 40 nm, based on an analysis using Sherrer’s formula. The TEM images in Fig. 1(b) represent the sizes of the nanoparticles, which are almost consistent with those estimated by XRD. Clear lattice images in a rutile-type structure can be observed. EDS measurements reveal the uniform distribution of Fe and Ni ions in 2% Fe and Ni codoped case. Figure 2 shows the Fe and Ni concentration dependencies in the magnetization, as measured at room temperature by VSM. For the Fe or Ni single-ion doped SnO2, no clear
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hysteresis loop was observed. On the other hand, for the Fe and Ni codoped SnO2, hysteresis with the 500 Oe of Hc was observed. Figure 2(a) deduces that the increase in the Ni concentration did not contribute to the enhancement of the Ms. With increasing Fe concentration, the Ms was enhanced as shown in Fig. 2(b). To confirm the codoping effects, we prepared 5% Fe and 1% Ni codoping and 1% Fe and 5% Ni codoped SnO2. In cases of higher Fe doping, the Ms could be enhanced. On the other hand, a higher Ni concentration was not effective for enhancing the ferromagnetism. This suggests that the spinel-type NiFe2O4 (Ni-ferrite)-like secondary phase formation is not dominant for ferromagnetism. In order to investigate the local electronic structures around Fe ions, M€ossbauer spectrometry was performed. M€ossbauer spectra for the Fe and Ni codoped SnO2 are shown in Fig. 3, along with the fitting results. The spectra are decomposed into subcomponents of the sextets (S1 and S2), a broad magnetic relaxation component (R1), and the doublets (D1 and D2) due to different quadrupole splitting (QS). The parameters used for the fitting are listed in Table I. The values of the isomer shift, i.e., the shift of the center of gravity in subcomponents, were almost 0.3 mm/s, which suggest Fe3þ states in Fe and Ni codoped SnO2. For the singleion doping of Fe into SnO2, similar isomer shift values have been observed.5 D1 originates from the substitution of Fe in the Sn site. D2 is assigned as the presence of oxygen vacancies around the Fe ions, which is brought about by the large QS by the electric field gradient. Sextet peaks are derived from the magnetic ordering. With increasing Fe concentrations, the peak intensity of S1 increases. An internal hyperfine field of about 49-50 T is estimated, which is almost consistent with the results obtained for Fe-doped SnO2.5 S1 is regarded as the ferromagnetic contribution of Fe ions. For the fitting of M€ossbauer spectrum of Ni-ferrite, different parameters are required.18–21 Therefore, Ni-ferrite segregation is not
FIG. 1. (a) XRD patterns of Fe and Ni codoped SnO2 in the logarithmic scale. Arrows show the secondary phase formations of Fe2O3 (36 ) and NiO (43 ). (b) TEM image of 2% Fe and 2% Ni codoped SnO2.
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FIG. 2. Magnetization vs. magnetic field for various Fe and Ni concentrations in Fe and Ni codoped SnO2. (a) Ni concentration dependence. (b) Fe concentration dependence.
dominant in cases of dilution with Fe and Ni ions into SnO2. With increasing Fe concentrations, S2 component becomes dominant. Small line widths in S2 originate from the Fe oxide compounds. It suggests that the segregation of hematite (Fe2O3) or Ni-ferrite-like compounds starts in high Fe concentration. The broad R1 components represent the spin relaxation within the detection time of 107 s in M€ossbauer spectrometry. The presence of R1 is related to the superparamagnetism in the nanoparticles.22 With an increase in the Fe concentration, S1 is enhanced, which is related to the enhancement of Ms in VSM. R1 is suppressed in high Fe concentrations. Therefore, R1 can become a monitor for the particle size which Fe ions contribute to the ferromagnetism. To ascertain the electronic structure in Fe and Ni codoped SnO2, we performed XAS for the Ni L-edges. Figure 4 shows the Ni L-edge spectra of Fe and Ni codoped SnO2, Ni-ferrite, and NiO. The spectral line shapes between
TABLE I. M€ ossbauer parameters of Fe and Ni codoped SnO2: IS denotes isomer shift; QS, quadrupole splitting; and Bhf, hyperfine field. Fe1Ni1 denotes the 1% Fe and 1% Ni codoped SnO2. The error bars are 60.02 mm/s for an isomer shift, 60.04 mm/s for QS, and 60.1 T for a hyperfine field.
FIG. 3. M€ossbauer spectra of various Fe and Ni concentrations in Fe and Ni codoped SnO2.
Fe1Ni1
Fe1Ni2
Fe1Ni5
Fe2Ni2
Fe5Ni1
D1; area (%) IS (mm/s) QS (mm/s) Line width (mm/s)
40.3 0.34 0.75 0.58
20.8 0.34 0.79 0.54
19.8 0.34 0.76 0.52
25.2 0.35 0.80 0.59
25.1 0.36 0.81 0.52
D2; area (%) IS (mm/s) QS (mm/s) Line width (mm/s)
11.7 0.26 1.66 0.58
6.1 0.27 1.73 0.66
7.9 0.31 1.49 0.71
5.6 0.28 1.82 0.67
8.9 0.29 1.76 0.62
S1; area (%) IS (mm/s) QS (mm/s) Bhf [T] Line width (mm/s)
16.3 0.38 0.13 50.82 1.09
36.3 0.39 0.13 49.10 1.42
35.3 0.39 0.03 49.2 1.44
35.5 0.39 0.09 49.44 1.32
35.3 0.39 0.15 49.84 1.26
S2; area (%) IS (mm/s) QS (mm/s) Bhf [T] Line width (mm/s)
… … … … …
9.0 0.38 0.09 51.56 0.35
7.4 0.36 0.16 51.40 0.35
10.3 0.36 0.17 51.26 0.35
17.0 0.37 0.19 51.11 0.35
R1; area (%) IS (mm/s) Line width (mm/s)
31.6 0.54 7.47
27.8 0.47 5.21
29.5 0.42 5.26
23.5 0.47 5.26
13.8 0.45 5.59
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Ms not by the segregation of secondary phases but by the exchange interaction of the oxygen vacancies. Next, we address the electronic structures in codoped cases. If the secondary phase formations are excluded, the defect-induced model becomes the most applicable scenario. TM doping induces a spin-polarized defect level, which stabilizes room-temperature ferromagnetism. We would suggest that codoping with different d-electron numbers facilitates the further stabilization of spin-splitting in the defect levels, which can be tuned by codoping in the low carrier density regions.25 Further studies, including the observation of spinpolarized defect levels and quantitative estimation of the oxygen vacancies are needed. IV. CONCLUSIONS
FIG. 4. X-ray absorption spectra of Fe and Ni codoped SnO2, NiFe2O4, and NiO.
We have investigated Fe and Ni codoped SnO2. Roomtemperature diluted ferromagnetic semiconductors with the enhancement of magnetization can be prepared in cases of codoping. The Ms can be controlled by the Fe and Ni codoping ratios. Fe3þ states were revealed by the isomer shift values, and XAS represents the Ni2þ states, which suggests that the double-exchange-like ferromagnetic interaction between the diluted magnetic ions mediated by oxygen vacancies becomes a possible origin for room-temperature ferromagnetism. ACKNOWLEDGMENTS
them are quite similar. The XAS line shape of NiO was reported in Ref. 23. XAS of NiFe2O4 also reveals a line shape similar to that of Ni2þ states.24 Because Ni2þ states in NiFe2O4 and NiO are assumed, Ni2þ states with d8 electrons are formed in codoped cases. Because Ni2þ states have also been reported in Ni-doped SnO2, the electronic structure is not drastically modified by the codoping effect. Unfortunately, Fe L-edge XAS spectra could not be obtained, because the Fe L-edge region overlaps the M4.5-edge absorption peaks. Considering the above results, we were able to elucidate the following possibilities for the origins of room-temperature ferromagnetism. (1) Fe and Ni codoping induce a ferromagnetic exchange interaction between the diluted magnetic ions through the carriers generated by the oxygen vacancies. It is well known that the d5-d8 system promotes ferromagnetic superexchange pathways. (2) Carriers polarized by oxygen vacancies stabilize the magnetic polarons that mediate the ferromagnetic interaction. (3) Secondary phases such as Fe or Ni oxides promote the ferromagnetism. We note that stable Fe oxides (a-Fe2O3) and Ni mono-oxide (NiO) behave like antiferromagnetism. (4) The formation of Fe and Ni complex oxides might become one of the candidates for ferromagnetic ordering, since Ni-based spinel-type ferrite is well known as a ferromagnetic material. However, the observed TM doping dependencies in Ms of Fe and Ni codoped SnO2 cannot be explained by the simple formation of secondary phases. The enhancement of Ms in higher Fe concentrations cannot be explained by the simple secondary phase formation. We can therefore deduce that the TM codoping effect enhances the
A part of this work was supported by a research granted from The Murata Science Foundation. A part of this work that involved TEM was conducted at the Center for Nano Lithography and Analysis at the University of Tokyo and was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. XAS was performed under Project No. 2010G120 and 2011G657 at the Institute of Materials Structure Science in KEK. 1
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