Enhanced room temperature ferromagnetism in Ni doped SnO2 nanoparticles: A comprehensive study Ateeq Ahmed, T. Ali, M. Naseem Siddique, Abid Ahmad, and P. Tripathi
Citation: Journal of Applied Physics 122, 083906 (2017); doi: 10.1063/1.4999830 View online: http://dx.doi.org/10.1063/1.4999830 View Table of Contents: http://aip.scitation.org/toc/jap/122/8 Published by the American Institute of Physics
JOURNAL OF APPLIED PHYSICS 122, 083906 (2017)
Enhanced room temperature ferromagnetism in Ni doped SnO2 nanoparticles: A comprehensive study Ateeq Ahmed,1 T. Ali,1 M. Naseem Siddique,1 Abid Ahmad,2 and P. Tripathi1,a) 1
Department of Applied Physics, Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh 202002, India 2 School of Materials Science, Tsinghua University, Beijing, China
(Received 24 April 2017; accepted 10 August 2017; published online 25 August 2017) We emphasized on a detailed investigation of the structural, optical, and magnetic properties of pure and Ni-doped SnO2 nanoparticles (NPs) synthesized by a sol-gel process. An extensive structural study has been carried out using various characterization techniques. The X-ray Diffraction (XRD) spectra show the formation of the single phase tetragonal structure of pure and Ni-doped SnO2 NPs without any noticeable impurity phase such as NiO. XRD results indicate that the crystallite size of SnO2 is found to be decreased with Ni doping, which has also been confirmed by the Field Emission Scanning Electron Microscopy study. X-ray Photoelectron Spectroscopy (XPS) measurements displayed a clear sign for Ni2þ ions occupying the lattice sites of Sn4þ in the SnO2 host which also gives clear evidence for the formation of single phase Sn1-xNixO2 NPs. The optical analysis shows a significant decrease in the energy gap of SnO2, i.e., (from 3.71 eV to 3.28 eV) as Ni concentration increases which may be correlated with the core level valence band XPS analysis. Photoluminescence studies show that Ni doping creates oxygen vacancies due to dissimilar ionic radii of Ni2þ and Sn4þ. Superconducting quantum interference device measurements revealed that the Ni doped SnO2 NPs exhibit strong ferromagnetic behavior at room temperature and this analysis has been well fitted with a simple relationship to find out magnetic parameters proposed by Stearns and Cheng et al. Hence, our results demonstrate that Ni-doping has strong impact on the structural, optical, and magnetic properties. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4999830] I. INTRODUCTION
Tin oxide (SnO2) is one of the most important metal oxides with a wide band gap of 3.6 eV that has been widely used in flat panel displays, solar cells, and gas sensors. SnO2 evinces interest because it is a naturally non-stoichiometric prototype transparent conducting oxide (TCO). It has high transparency in the visible region and high reflectivity in the IR region. SnO2 also has low electrical resistance.1,2 To effectively use diluted magnetic semiconductors (DMSs) for real-world spintronic devices, they must show ferromagnetic storing far above room temperature. The nanostructures of DMSs of SnO2,3 TiO2,4 and ZnO5 could exhibit magnetic performance in the presence of magnetic and non-magnetic dopant ions. Some studies6,7 on transition metal (TM) doped semiconducting oxides have become one of the challenging topics to induce room temperature ferromagnetism (RTFM). In relation with this, many scholars reported Co/Fe/Ni/V/Crdoped TiO2 or ZnO,8,9 while few studies reported that doping of some elements such as Mn: SnO2 does not result in FM.10 However, Co:SnO2, Fe:SnO2, Cr:SnO2, and V:SnO2 films show RTFM.11,12 In continuation, there is still least work done on TM doped SnO2 nanoparticles (NPs). Among DMSs, SnO2 is also a good candidate material to trait longrange ferromagnetism with the doping of transition metal (TM) ions such as Fe, Ni, and Co, etc.13 In this context, the a)
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efforts and success are still relatively limited. Doping with transition-metals (TMs) has become the immense topic which was proposed by several authors to introduce magnetic functionality in conventional semiconductors14,15 in order to introduce magnetic ordering in conventional semiconductors. Recent reports on the observation of room-temperature ferromagnetism (RTFM) in thin films of TM-doped oxide semiconductors such as TiO216 and ZnO17 generated tremendous interest to investigate DMSs. Theoretical work explained RTFM by means of exchange interaction assisted by p-type free carriers in TM doped ZnO and GaN metal oxides.18 Adhikari et al.19 reported the synthesis of Fe doped tin dioxide (SnO2) nanoparticles by a chemical co-precipitation method and found that the antiferromagnetic interaction reduces with increasing Fe content. Very recently, Verma and Kotnala17 reported the ferromagnetic enhancement by doping of Ni, Cu, and Ce ions in ZnO nanoparticles. Some of the literature reported debatable outcomes in the existence of ferromagnetism and antiferromagnetism in TM materials.20 However, the perfect mechanism of induced magnetism occurring in magnetic and nonmagnetic doped SnO2 NPs is still under debate. Punnose et al.21 stated that the oxygen vacancies play a key role in RTFM. They observed enhanced ferromagnetism in Co-doped SnO2 NPs. Future spintronics devices based on DMSs need a better understanding of ferromagnetics at the microscopic level. Still, it is unclear, whether the ferromagnetism in DMS is inherent or due to dopant clustering, as it is an essential condition for spintronics. Few studies have been reported on RTFM in Ni-doped SnO2 NPs
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synthesized via a polyethylene glycol (PEG) assisted sol-gel process. Hence, in the light of the above understanding, we investigate systematically the influence of Ni ions on structural, optical, and magnetic properties of SnO2 NPs. Under the influence of Ni ions, we have found a remarkable change in the magnetic property of pure SnO2 NPs. Nevertheless, our both field and temperature dependent magnetic results have been well fitted with the derived and semiempirical relationship. We hope that the present study will give a good understanding for future spintronic devices. II. EXPERIMENTAL SECTION
The experimental work has been divided into two parts: The first one is synthesis of pure and Ni-doped SnO2 nanoparticles (NPs) via a polyethylene glycol (PEG) assisted solgel process and the second is characterization techniques which have been used in our present work to study the various properties such as structural, morphological, optical, and magnetic properties. A. Synthesis of Ni doped SnO2 NPs
SnCl2.2H2O (Fisher Scientific 98.7%) and NiCl2.6H2O (Fisher Scientific 98.7%) of AR grade without further purification were used as starting materials for the synthesis of Sn1-xNixO2 (x¼ 0, 0.02, 0.04, and 0.06) NPs by the simple chemical sol-gel method. In a typical synthesis process, the calculated required amounts of SnCl2.2H2O and NiCl2.6H2O were dissolved in 80 ml of double distilled water and the solution was stirred for 30 min. After which, 20 ml of polyethylene glycol (PEG, Mw ¼ 400) was added into the above solution with the constant stirring for about 15 min. Now, a sufficient amount of aqueous ammonia (15 mol/L) was added dropwise in a very precise manner to sustain the chemical homogeneity of the above solution under magnetic stirring to obtain pH values up to 8–10 at 80 C for 2 h. A dark milky gel was obtained which was then washed with double distilled water and ethanol several times to eliminate the
organic impurity. Then, the gel was dried at 100 C for 24 h. The dried material was crushed for half an hour using a pestle and mortar and further calcined for 4 h at 500 C in air, resulting in the formation of pure and Ni doped SnO2 NPs. B. Characterization techniques
The crystalline structure of the synthesized samples has been investigated using a Rigaku Miniflex-II X-ray diffractometer (XRD) with high intense Cu-Ka radiation ˚ ) operated at a voltage of 30 kV and current (k ¼ 1.5406 A 15 mA at a scan rate of 2 /min in 2h in the range of 20 –80 . The morphological analysis has been carried out by Field Emission Scanning Electron Microscopy (FESEM) (SU-8000, Hitachi, Japan). The functional vibrational bonds resolute were obtained by a Perkin-Elmer Fourier transform infrared (FTIR) spectrometer in the wavenumber 400–4000 cm1 using KBr as a background. To know the value of the optical band gap, UV–Visible spectroscopy has been performed using a Perkin Elmer Spectrophotometer in the wavelength region 200–800 nm. Photoluminescence (PL) spectra have been captured by a fluorescence spectrometer (Perkin Elmer LS-55). The field and temperature dependent magnetic properties have been obtained by a superconducting quantum interference device (SQUID). An X-ray photoelectron spectrometer (XPS) was used for the deep structural analysis of pure and Ni doped samples. III. RESULTS AND DISCUSSION A. Structural and morphological analyses
A Rigaku Miniflex-II X-ray diffractometer with high ˚ ) operated at a voltage intense Cu-Ka radiation (k ¼ 1.5406 A of 30 kV and current 15 mA at a scan rate of 2 /min in 2h in the range of 20 –80 has been used for the structural study of all samples. The typical XRD spectra of the pure and Ni doped SnO2 samples annealed at 500 C for 4 h are shown in Fig. 1(a). All indexed Bragg’s peaks corresponding to their
FIG. 1. (a) XRD spectra of pure and nickel doped SnO2 NPs and (b) Rutile structure of SnO2 NPs.
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TABLE I. Variation of structural parameters for pure and Ni doped SnO2 NPs. Lattice parameters Ni conc. (%) 0 2 4 6
Crystallite size (nm)
˚ ) ¼ b (A ˚) a (A
˚) c (A
˚ 3) Unit cell volume (A
Lattice distortion
10.4 6 1.436 9.5 6 1.436 8.3 6 1.436 7.1 6 1.436
4.739 6 0.0047 4.735 6 0.0047 4.732 6 0.0047 4.728 6 0.0047
3.206 6 0.0034 3.203 6 0.0034 3.201 6 0.0034 3.198 6 0.0034
72.00 6 0.2209 71.82 6 0.2209 71.67 6 0.2209 71.48 6 0.2209
1.473 1.477 1.475 1.478
planes obtained at different diffraction angles display the rutile structure (P42/mnm) of SnO2 (JCPDS file no. 411445). The diffraction peaks in each sample show that they are polycrystalline in nature. Further, no characteristic peaks of impurities, like nickel oxide (NiO) or another mixed tin oxide (Sn2O4) phase were observed in the XRD pattern, showing the single phase sample formation. From the inset of Fig. 1(a), it is quite clear that there is no presence of any significant Bragg’s reflection in the XRD spectra corresponding to NiO. It is marked that nickel doping does not disturb the tetragonal unit cell of SnO2, signifying that Ni ions successfully inhabit lattice sites rather than interstitial sites of SnO2. However, the diffraction peaks shift to higher angles in the case of the doped samples as clearly shown in the inset of Fig. 1(a), which is due to the change in lattice parameters22 as well as which can also be attributed to the small ionic radii of Ni2þ as compared to Sn4þ. This result supports the substitution of Sn4þ ions by Ni2þ ions in the assynthesized Ni doped SnO2 nanoparticles. The average crystallite size thkl has been calculated using the Debye-Scherer formula23 as given by the following equation: thkl ¼
0:98k ; b cos h
(1)
where k is the wavelength of the X-Ray radiation and b is the full width at half maximum (FWHM) at diffraction angle h. The most intense peak (110) has been used to estimate the average crystallite size of each sample. It can be clearly seen from the inset of Fig. 1(a) that doping of Ni ions increases the FWHM; this broadening determines the decrease of the crystallite size. This decrease in the crystallite size is due to shrinkage of the crystal structure as the slightly smaller ˚ ) replaced that of Sn4þ (0.71 A ˚ ); ionic radius of Ni2þ (0.69 A 2þ hence, the presence of Ni ions in host SnO2 prevented the growth of crystal grains. Reduction in the crystallite size may also be due to the reduction in the diffusion rate which makes the NPs intact and due to the distortion in the host lattice by the doping.24 Furthermore, the creation of oxygen vacancies may also decrease the crystallite size.25 The unit cell volume and lattice distortion of the tetragonal structure are given by the following relationship: V ¼ a2 c and U ¼ ac, respectively. Where a and c are the lattice parameters. The variation in the crystallite size, lattice parameters, and unit cell volume can be observed from Table I. Further, a continuous reduction of the lattice parameters (a and c) and the unit cell volume was observed when the Ni content has grown as shown in Fig. 2. Such lattice narrowing
can be credited to the ion size and local coordination effects linked to the substitution of Sn4þ ions by Ni2þ ions. Furthermore, the replacement of the Sn4þ by Ni2þ ions of lesser oxidation state may lead to the probable exclusion of some oxygen-ions in order to keep the charge neutrality of the host system.26 ˚ ) and Sn4þ is Since the ionic radius of the O2 (1.32 A 2þ larger than that of Ni , the lattice contraction suggests a substitutional doping of the Sn4þ ions by the Ni2þ ions. Therefore, the continuous shrinkage in the unit cell volume confirms the uniform incorporation of the Ni dopant into the SnO2 lattice. Hence, Ni doping in the SnO2 lattice produces crystal defects around the dopant and the charge imbalance that resulted from these defects modifies the stoichiometry of the materials. In addition to structural analysis, the rutile structure of SnO2 with two formula units per unit cell has also been shown in Fig. 1(b) which is designed by “Diamond” software. To study the morphology and shape of the Pure and Nidoped SnO2 NPs, we use field emission scanning electron microscopy (FESEM JSM-6700, JEOL, Japan). The FESEM micrographs with magnification (i.e., x75000) at the same 16 kV were taken at different parts of the sample. Figs. 3(a) and 3(b) show FESEM micrographs of Pure and 6% Ni doped SnO2 NPs, while Figs. 3(c) and 3(d) represent the average particle size distribution histogram curve with the Gaussian fitting of pure and Ni doped SnO2 NPs, respectively. It is well observed that PEG (as a capping agent) assisted synthesized pure and Ni doped SnO2 NPs are
FIG. 2. Variation in the Crystallite size & lattice parameter with Ni concentration.
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FIG. 3. FESEM images of [(a) and (b)] Pure SnO2 & Ni-SnO2 and (c, d) Gaussian fitting curve of Particle size distribution of Pure SnO2 & Ni-SnO2.
uniform and spherical/round in shape with very low agglomeration as shown in Figs. 3(a) and 3(b). FESEM images of pure and doped samples also confirmed that particles are in the nano regime with an average particle size of 14.3 nm and 10.5 nm. The particle size estimated from SEM images is slightly larger than the crystallite size calculated by Scherrer’s formula because an individual grain may usually contain several crystallites.27 Zhao et al.28 studied the morphological deviation from SnO2 nanospheres to nanosheets due to Zn doping. However, in our case, any remarkable variation in morphology of SnO2 NPs has not been observed with Ni doping into the host SnO2 lattice as clearly displayed in Figs. 3(a) and 3(b). The chemical compositional analysis of pure and Ni doped SnO2 NPs has been performed by energy dispersive X-ray spectroscopy (EDX). The EDX spectra of pure and 6% Ni doped SnO2 NPs are shown in Figs. 4(a) and 4(b), which show the Sn, Ni, O, and C signal peaks. The peak for carbon appears from the carbon tape substrate which is used for holding the sample during analysis. The EDX analysis of pure SnO2 NPs shows that the prepared nanoparticles consist of Sn and O elements only, which indicates the high purity of SnO2 NPs. It is also clear from Fig. 4(b) that Ni ions have been successfully incorporated into the host SnO2 lattice and validate the results found by XRD analysis.
B. FTIR analysis
FTIR is plausibly the utmost dominant tool for identifying the functional groups or the types of chemical bonds. FTIR spectra of all the Sn1-xNixO2 (x¼ 0, 0.02, 0.04, and 0.06) NPs have been recorded in solid phase using the KBr pellet technique in the region 4000–400 cm1 as presented in Fig. 5. There are several bands present in the wavenumber range 4000–400 cm1. The band in the range 3213–3532 cm1 and that at 1621 cm1 have been assigned to the vibration of hydroxyl groups due to the absorbed water and show a stretching vibrational mode of O–H groups, respectively.29 The band located in the region 490–710 cm1 has been assigned to SnO2 (anti-symmetric O–Sn–O stretching).30 In addition to main peaks, few peaks are also observed at 1092 cm1 (C–O) and 1408 cm1 (C ¼ C) due to adsorption of CO.31,32 Figure 5 clearly shows that there is small shifting in the positions of IR peaks specifying that Ni ions have been incorporated into pure SnO2 NPs. It is also clear from Fig. 5 that all the vibrations broaden with the increase in dopant concentration. This can be ascribed to the diminution of the particle size with doping. C. XPS measurements
In order to examine the oxidation states of the constituent elements, substitution and the elemental surface composition
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FIG. 4. (a) EDX spectra of Pure SnO2 NPs and (b) Ni-SnO2 NPs.
of pure and Ni doped SnO2 NPs, X-ray photoelectron spectroscopy (XPS) measurements have been performed. The XPS survey scan spectrum of the Ni–doped SnO2 is shown in Fig. 6 and core-level spectra of Sn3d, Ni2p, and O1s peaks are displayed in Figs. 7–9, respectively. The XPS survey spectrum of Ni doped SnO2 NPs contains only Sn, Ni, and O signal peaks which confirm the high chemical purity of the Ni–doped SnO2 NPs. The doublet spectral lines of the Sn-3d state have been detected at binding energies of 486.4 eV and 494.8 eV corresponding to Sn 3d5/2 and Sn 3d3/2 lines, respectively. The spin-orbit splitting of these two peaks is found to be 8.4 eV and this splitting revealed the þ4 oxidation state of Sn in pure SnO2 NPs.33 It can also be clearly seen from Fig. 7, the Sn 3d5/2 peak for the Ni doped sample is shifted towards lower binding energy at 485.7 eV which is associated with the Sn2þ state.34 This shifting of Sn 3d5/2 confirms that the oxidation state of Sn changes from Sn4þ to Sn2þ as Ni ions are doped into the host SnO2 system. The estimated percentage change in the oxidation state of Sn from þ4 to þ2 is found to be approximately (10%–12%). The core level Ni 2p peaks positioned at around 855.2 and 872.6 eV with splitting 17.4 eV have been assigned to the Ni 2p3/2 and Ni 2p1/2 levels, respectively, as shown in Fig. 8. The position of Ni 2p3/2 is found to be reasonably different from those of
metallic Ni (852.3 eV), NiO (853.4 eV), and Ni2O3 (856.7 eV).35,36 The presence of NiO may be ruled out further by another characteristic of the energy splitting between Ni 2p3/2 and Ni 2p1/2 peaks and this value lies in the range of 17.31–17.85 eV which is less than that of NiO (18.4 eV).37 This finding confirms that Ni has þ2 oxidation state in our synthesized samples. Therefore, the above consequences strongly recommend that Ni2þ ions successfully incorporated into the SnO2 lattice without creating any detectable impurity phase such as Ni, Ni2O3, or NiO. Further, in order to explore the local environment around the Ni dopant and rule out the existence of secondary phases with more precision, XANES and EXAFS techniques are considered to be powerful analytical and characterizing tools for the study of the actual local environment of Ni. The broad O1s peak for pure and Ni doped NPs has been fitted using the Gaussian function as shown in Figs. 9(a) and 9(b). The structural oxygen peaks located at 530.5 and 529.7 eV correspond to Sn4þ (SnO2) and Sn2þ (SnO), respectively,38 while one Gaussian peak centered at higher binding energy (531.3 eV) was also observed for Ni doped NPs. This peak possibly is due to the creation of oxygen vacancies at the cost of removal of oxygen atoms from the SnO2 lattice due to charge neutrality requirements as Sn4þ is replaced by Ni2þ.39,40 Hence, the formation of oxygen vacancies in Ni doped samples will have a good impact on the enhancement of ferromagnetic ordering. However, it
FIG. 5. FTIR spectra of pure and Ni doped SnO2 NPs.
FIG. 6. XPS survey spectra of Ni doped SnO2 NPs.
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FIG. 7. High resolution Sn 3d XPS spectra of pure and 6% Ni doped SnO2 NPs.
has been clearly seen from Fig. 9(b). that the dominance of the Sn4þ ions at the particle’s surface is decreased as Ni is doped into SnO2, as specified by the increasing the relative spectral area ratio OSn2þ/OSn4þ which is a clear indication of a progressive change of the oxidation state from Sn4þ to Sn2þ at the nanoparticle’s surface. This result is in good agreement with the analysis of the valence state of tin ions resolute from the Sn (3d) core level region. The ratio O:Sn for pure and 6% Ni doped NPs is found to be nearly 1.83 and 1.74, respectively. This reduction in O:Sn for 6% Ni doped SnO2 is due to the formation of oxygen vacancies. In support of our study, similar results have also been obtained earlier for Pr-doped SnO2 nanoparticles.41 Furthermore, XPS valence band region spectra of pure and Ni doped SnO2 NPs have been plotted in Fig. 10. Valence band maximum (VBM) of both samples has been obtained by extrapolating the leading edges of valence band spectra to the base lines. Observed VBM values are found to
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FIG. 8. Ni 2p core level XPS spectrum of Ni doped SnO2 NPs.
be 3.08 and 2.61 eV for pure and doped NPs, respectively. To determine the actual VBM values, calibration with reference to Fermi level has been done. After calibration, the true VBM values are found to be 3.65 and 3.18 eV for pure and Ni doped samples, respectively. D. Optical properties
The room temperature UV-visible absorption spectra were recorded from stable transparent suspensions of pure and Ni doped SnO2 NPs in dimethyl sulfoxide (DMSO) as shown in Fig. 11(a). The absorbance spectra show a UV cutoff wavelength around 340–420 nm accredited to the photo excitation of electrons originating from the valence band to the conduction band.42 The band gap values of the Sn1-x NixO2 (x¼ 0, 0.02, 0.04, and 0.06) NPs are estimated Using the Tauc’s realtion43 as given by Eq. (2), ah ¼ Aðh EgÞn ;
(2)
FIG. 9. XPS spectra of O (1s): (a) Pure SnO2 and (b) Ni doped SnO2, the black symbols are the experimental data whereas the red, blue, and brown solid lines are the best Gaussian fit.
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FIG. 10. VB-XPS spectra of pure and Ni doped SnO2 NPs.
where Eg is the optical band gap, h is incident photon energy, A is the absorbance, and a is the absorption coefficient which is given by Beer Lambert’s law as follows: a ¼ 2.303 A/t where A is the absorbance and t is the thickness of the sample container. The value of exponent (n) is 1=2 for allowed direct transition as that occurring in the case of SnO2 NPs. Figure 11(b) shows the plots of (aht)2 versus photon energy (ht) for pure and Ni doped SnO2 NPs. The band gap of the pure SnO2 is found to be 3.72 eV, which is higher than that of bulk SnO2 (3.59 eV) as reported value.44 Doping of Ni ions into SnO2 can introduce defective energy levels into the band gap of tin oxide, and produce new energy states located in the gap of tin oxide nanoparticle for electron-hole pair recombination with lower energy. This indicates a band gap narrowing of the SnO2 which may be credited to the creation of a huge amount of defects/oxygen vacancies as Ni is doped into the host lattice. Furthermore, this type of red shift occurring in the energy gap may also be credited to the charge-transfer between the confined Ni2þ d electrons and the band electrons.45 This type of narrowing in the band gap well agrees with the actual VBM values determined by VB-XPS analysis as presented in Fig. 10.
Photoluminescence spectroscopy is a very important tool to explore the surface defects, impurities, energy bands, and exciton structure. The synthesis process can also affect the emission properties of SnO2 NPs. Figure 12(a) displays the room temperature PL spectra of pure and Ni doped SnO2 NPs with an excitation wavelength of 320 nm in the range of 200–800 nm. A broad emission band is found between 385 and 405 nm in all samples. This spectral emission band corresponds to the direct transition from the conduction band edge (CBE) to valence band edge (VBE) which is assigned to direct recombination of free excitons, i.e., electrons in the Sn 4p conduction band and holes in the O 2p valence band.46 It has been observed that the emission band is shifted to higher wavelength as Ni is doped into the host SnO2 that may be due to a reduction in the optical band gap as shown in Fig. 11(b). Two emission bands of small intensity are also detected at around 460 and 510 nm in Ni doped NPs. The blue emission band at nearly 460 nm may be due to the formation of doubly charged oxygen vacancies (VOþþ).47 The green emission band at about 510 nm can originate from radiative recombination of singly charged oxygen vacancies (Voþ).48 Moreover, as the Ni doping level increases the intensity of emission band increases which may be due to the increase in density of oxygen vacancies.49 For a better understanding of emission bands, a proposed band structure model of the Ni doped SnO2 nanostructure has also been presented in Fig. 12(b). Therefore, the oxygen vacancies play a key role in PL emission through a recombination of Voþ electrons with photo excited holes in the valence band as well as in enhancement of RTFM in Ni doped SnO2 NPs which will be discussed in Sec. III E. E. Magnetic properties
The room temperature magnetic properties of pure and nickel doped SnO2 nanoparticles have been investigated using a superconducting quantum interference device (SQUID). The magnetization as a function of magnetic field strength, i.e., (M-H) curves have been plotted at room temperature for pure and Ni-doped SnO2 NPs as displayed in Fig.13. M-H
FIG. 11. (a) Absorption spectra and (b) Tauc’s plots of pure and Ni doped SnO2 NPs.
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FIG. 12. (a) Room temperature PL spectra of pure and Ni-doped SnO2 NPs and (b) proposed band structure model for Ni doped SnO2 with oxygen vacancies.
curves clearly show hysteresis behavior for all Ni doped SnO2 NPs, indicating the well-defined ferromagnetism behavior at room temperature, while pure SnO2 NPs exhibited nearly diamagnetic behavior with negative susceptibility at room temperature which may be due to the valence state of tin (Sn4þ) which favors the 4d10 electronic configuration of Sn in SnO2 and, hence, there are no unpaired d electrons in the host system for any kind of ferromagnetic ordering.45,50 The synthesis process can also affect the intrinsic magnetic properties of pure SnO2. In this context, many researchers reported intrinsic ferromagnetic behavior in pure SnO2.51,52 In the present work, when Ni is doped into the SnO2 lattice, a remarkable change in magnetic behavior has been observed, i.e., the transition from the diamagnetic state to ferromagnetic state. It can also be clearly seen from Fig. 13 that as the Ni ion concentration increases into the host SnO2 lattice, saturation magnetization (Ms) as well as remnant magnetization (Mr) increases. The inset of Fig. 13 shows a small FM loop in the low field region which may be due the presence of a small amount of oxygen vacancies located on the surface53 of pure SnO2 NPs. To know the actual mechanism of intrinsic ferromagnetism in transition metal (TM) doped DMSs, various models such as free-carrier-mediated exchange sp–d exchange mechanism, super-exchange, double-exchange between the d states of TMs, etc.,54 have been proposed. However, still all are under debate. Therefore, to confirm it we have gone through with the various structural analyses such as XRD, FESEM, and XPS techniques and found that there is no impurity phase like Ni, NiO, and Sn2O4 phase, which would be accountable for the ferromagnetism. All structural examination confirmed the uniform incorporation of Ni2þ ions into SnO2 and suggested that the Ni2þ ion substitution for Sn4þ ions results in the creation of oxygen vacancies in order to maintain charge neutrality. In this context, first principles density functional study also intensely establishes that the oxygen vacancies play a vital role in the enrichment of
ferromagnetism in Ni-doped SnO2.55,56 In our case, enhancement in ferromagnetic ordering can also be explained on the basis of creation of free charge carriers and oxygen vacancies due to the substitution of Ni2þ for Sn4þ into the host SnO2 lattice. To continue the charge neutrality, an amount of O2– ions need to escape from the lattice forming oxygen vacancies. Moreover, doping of Ni ions decreases the crystallite size as observed by XRD spectra; therefore, the surface to volume ratio increases; hence, the density of oxygen-vacancy increases as Ni concentration increases as confirmed by XPS and PL study. These oxygen vacancies tend to locate near the Ni ions, which play a key role in trapping of unpaired electrons. Therefore, the localized spins of the Ni ions interact with the charge carriers which are bound to oxygen vacancies resulting in a magnetic polarization and forming the bound magnetic polaron (BMP)57 and lead to improvement in
FIG. 13. Room temperature M–H curves for Pure and Ni doped SnO2 NPs with fit to Eq. (3).
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ferromagnetism. Thus, the defect/oxygen vacancy-assisted ferromagnetism is the satisfactory description in our case. In order to extract the magnetic parameters from the M-H curves, a well-known fitting has been carried out which was proposed by Stearns and Cheng58 as given by Eq. (3), 2Ms H6Hc pS tan1 tan þ vH: (3) Mð H Þ ¼ p Hc 2 The first term on the right side stands for FM contribution where Ms, Hc, H, and S (¼Mr/Ms) are the saturation magnetization, coercivity, applied field, and Squareness of the FM loop, respectively, and the second term defines the PM component with v as the magnetic susceptibility. Equation (3) holds good to explain the magnetic behavior in the present study and well-defined fitted curves are in good agreement with M-H plots which are plotted using experimental data as shown in Fig. 13. The obtained fit parameters such as Ms, Hc, S, and v are depicted in Table II. The magnetic parameters obtained from experimental data well agree with those parameters that are extracted from a simple relationship as established by Stearn et al. Table II clearly indicates that total magnetization increases, while lB/Ni decreases with Ni content. This may be due to the fact that as the Ni ion content on the surface is enhanced with the Ni doping,59 it may be supposed that these additional ions are responsible for the paramagnetic contribution. Moreover, the presence of antiferromagnetic interactions which coexist with the paramagnetism must have deviation from the linear relationship between Ms and the Ni dopant (as shown in Table II). It can also be seen from Table II that the magnetic moment decreases from 0.682 to 0.298 lB/Ni as the Ni content increases, which is smaller than theoretical spin value 2 lB/ Ni for Ni, indicating that only a fraction of Ni ions participate in ferromagnetism. The temperature-dependent magnetization (M-T) plot for Ni doped SnO2 NPs has been recorded at under field cooling (FC) in the temperature range from 5 K to 300 K at constant field 500 Oe. The best fit for the observed M-T curve is shown by the red color curve in Fig. 14. It is clearly seen from Fig. 14 that the magnetization shows a rapid decay of magnetization in the low temperature region with the increase of the measured temperature, and then the magnetization drops gradually and does not attain zero value until 300 K. After 300 K, a sudden drop in magnetization is observed which is clearly shown in the inset of Fig. 14. This drop in magnetization as a function of temperature is due to the dominance of thermal energy over the exchange interaction energy among the magnetic spins. The TABLE II. Parameters extracted from fitting Eq. (3) of M–H data for pure and Ni doped SnO2 NPs. Ni conc.(%) 0 2 4 6
Ms (emu/g)
Hc (Oe)
S (Mr/Ms)
v (emu/g-Oe)
lB/Ni
0.02 0.51 0.57 0.68
563 160 186 213
0.31 0.23 0.25 0.28
9.19 106 1.06 106 4.73 106 7.21 106
… 0.682 0.376 0.298
FIG. 14. Normalized magnetization vs. temperature plots of Ni doped SnO2 NPs.
thermal energy randomizes the aligned spins; therefore, the degree of disorder or randomness in the system increases due to which the entropy of the system increases. Thus, net magnetization decreases as temperature increases. Qualitatively, it can be clearly seen from the M-T curve that magnetization is found to be non-zero at 300 K. This fact indicates that the Curie point (Tc) is above room temperature which is reliable with our M-H analysis. In order to find Curie temperature in a quantitative way, the M-T plot has been fitted using a semi-empirical equation60 [Eq. (4)] which explains the temperature dependence of magnetization and the expected Tc value can be obtained by extrapolation of the fitted M-T curve as shown in Fig. 14, h b 3 MðT Þ ¼ Mð0Þ 1 sðT=Tc Þ2 ð1 sÞ ðT=Tc ÞÞp ; (4) where M (0) is magnetization at the initial temperature (5 K), s is the shape factor, and the value of s is found to be 0.37. A smaller value of s 0.4 is the clear sign of long-range ferromagnetic ordering. The exponent p (>3/2) is a semiempirical fit parameter and the value of p comes out to be 1.9. b is the critical exponent of the order parameter and the best fit value of b in our case is found to be 0.43 which is nearly equal to a theoretical mean value (1/2) that indicates for the long-range interacting ferromagnetic system.61 The FC magnetic moment shows a slight increase from 300 K to 40 K, followed by a steep increase below 25 K, which may be due to a paramagnetic signal as observed in the fitting of the M-H loop. In the low temperature region (T Tc), the M-T curve follows the Bloch’s 3/2 power law while at a higher temperature it deviates from the 3/2 law and tracks Dyson’s 5/2 power law.62 After extrapolation, the expected value of Tc of synthesized Ni doped SnO2 NPs is found to be around 322 K. IV. CONCLUSION
To summarize, pure and Ni doped SnO2 NPs were successfully synthesized by a sol-gel method. XRD, FESEM,
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and XPS studies confirm the uniform incorporation of Ni2þ ions into the host SnO2 lattice without creating any secondary phases. XRD spectra and FE-SEM images reveal that the particle size in Ni doped SnO2 decreases as the Ni content increases. The UV-Visible absorption edge is shifted towards the visible region on doping of Ni. A remarkable reduction in the band gap of SnO2 NPs with Ni doping has been observed which is due to the formation of extra energy states in the band gap of SnO2. PL studies demonstrate that Ni doping creates a large number of defects/oxygen vacancies in Sn1-xNixO2 NPs. SQUIDS results revealed that the Ni doping introduces the room temperature ferromagnetism in the SnO2 nanoparticles. The relationship between the induced VO defects and the improved ferromagnetic ordering implies that VO defects are accountable for the RTFM in Ni doped SnO2 NPs. Thus, all results show a strong correlation among structural, optical, and magnetic properties. Meanwhile, these outcomes may open a path to make SnO2 NPs as valuable candidates for spintronics just by monitoring VO defects with the suitable dopant. Finally, our experimental results may be useful for tuning the magnetic and optical properties of Ni doped SnO2 based DMS materials. ACKNOWLEDGMENTS
The authors are thankful to the Department of Applied Physics, A.M.U, Aligarh, India for providing experimental facilities. Special thanks is given to the School of Materials Science, Tsinghua University, Beijing, China for experimental support. One of the authors Ateeq Ahmed is also thankful to UGC India for availing Maulana Azad National Fellowship. 1
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