Solid State Communications 150 (2010) 1570–1574

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Solid State Communications

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Magnetic properties of Sn1−xNixO2-based diluted magnetic semiconductors

Sunita Mohanty, S. Ravi ∗

Department of Physics, Indian Institute of Technology Guwahati, Guwahati-781039, India

a r t i c l e i n f o

Article history:Received 16 March 2010Received in revised form8 May 2010Accepted 29 May 2010by E.V. SampathkumaranAvailable online 9 June 2010

Keywords:A. Diluted magnetic semiconductorD. Magnetic properties

a b s t r a c t

Polycrystalline Sn1−xNixO2 samples were prepared in single-phase form for x = 0.02 and 0.07 andwere characterized using several experimental techniques. Room-temperature ferromagnetism withtransition temperature, Tc , as high as 770Kwas observed from the temperature variation ofmagnetizationmeasurements. Nanoparticles of uniform crystalline phase and composition were identified from thetransmission electron microscope images. The initial magnetization curves recorded at 80 and 300 K forboth samples could be analyzed based on the boundmagnetic polaron (BMP)model, where the size of theBMP is found to increase with temperature. The magnitude of electrical resistivity is found to decreasewith doping, and its temperature dependence could be explained based on the variable range hoppingmechanism.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Semiconductor-based spintronics with room-temperature fer-romagnetism has turned out to be an active area of research.These materials are robust candidates for potential applications ofspin-controlled devices. Room-temperature ferromagnetism hasbeen reported by substituting transition elements in semiconduc-tor hosts such as ZnO and TiO2 [1–9]. However, a few authorshave not observed ferromagnetism in the same series [10–15].SnO2 is an attractive semiconductor with a wide band gap andoptical transparency in the visible region. In thin-film samples ofNi-doped SnO2, Hong et al. [16] reported room-temperature ferro-magnetism with transition temperature around 400 K; however,their measurements were limited up to 400 K. Moreover, theyhave reported a change in the magnitude of the magnetic mo-ments with different substrates. In order to determine the transi-tion temperature and to understand the role of Ni in promoting theroom-temperature ferromagnetism in the above serieswithout theinfluence of substrate-related ambiguity, we prepared bulkSn1−xNixO2 samples for x = 0.0, 0.02 and 0.07 and extended themagnetic measurement up to 1000 K. An effective magnetic mo-ment of the order of 1.4µB with transition temperature as high as770 K has been observed. The magnetization data could be ana-lyzed based on the bound magnetic polaron model (BMP) with atypical saturation magnetization of 466 A/m and a polaron radiusof 60 Å.

∗ Corresponding author. Tel.: +91 361 258 2707; fax: +91 361 2690762.E-mail addresses: sravi@iitg.ac.in, sravi@iitg.ernet.in (S. Ravi).

0038-1098/$ – see front matter© 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2010.05.045

2. Experimental details

Polycrystalline samples of Sn1−xNixO2 (x = 0.0, 0.02, 0.07)were prepared by following the standard solid-state route. Stoi-chiometric ratios of SnO2 andNiOwith 99.9% puritywereweighed,and theyweremixed thoroughly for several hours under acetone tocreate a homogeneous mixture; this was followed by presinteringat 400 °C for 24 h. The final sintering in pellet formwas carried outat 900 °C for 36 h in air with several intermediate grindings. X-raydiffraction (XRD) patterns were recorded at room temperature us-ing a Seifert 3003-TT XRDmachine employing Cu Kα radiation. Themicrostructure and the composition at crystallite level were stud-ied by recording images using a JEOL-JEM 2100 transmission elec-tron microscope (TEM) coupled with an energy-dispersive (EDS)analyzer. The Fourier transform infrared (FTIR) absorption spec-tra were recorded in the wavenumber range of 250–4000 cm−1using spectrum BX with a resolution of 2 cm−1. The isothermalmagnetization curves and the temperature variation ofmagnetiza-tion in the range of 80–1000 K were measured using a LakeShoremodel no. 7410 vibrating sample magnetometer (VSM). The tem-perature variation of electrical resistivity down to 100 K was mea-sured by using the standard two-probe technique and a Keithley2410 source meter.

3. Results and discussions

Fig. 1a shows the typical plot of an XRD pattern recorded forthe x = 0.07 sample along with Rietveld refinement. All the ob-served peaks could be refined by using the P42/mnm space group,and this suggests the single-phase nature of the samples. The typ-ical lattice parameters are found to be a = b = 4.732 Å and

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Fig. 1a. XRD pattern along with Rietveld refinement for the x = 0.07 sample. Thecircles and solid line represent the experimental and refined data, respectively, andthe difference between them is shown as dashed lines.

Fig. 1b. XRD plots for the x = 0.0 and 0.07 samples with intensity in logarithmicscale.

c = 3.184 Å for x = 0.0 and a = b = 4.734 Å and c = 3.187 Åfor the x = 0.07 sample. Fig. 1b shows the intensity in the log-arithmic scale for the x = 0.0 and x = 0.07 samples, whereno additional peak due to Ni doping has been observed and thesamples are free from impurity. The average crystallite size wasestimated using Scherrer’s formula, S = λk/β cos θ , where theconstant k depends upon the shape of the grains and is taken as0.89 by assuming circular grains, λ = 1.5406 Å for Cu Kα radi-ation, β is the full width at half maximum (FWHM) of the diffrac-tion peaks and θ is the glancing angle. The experimental β valuewas corrected for instrumental broadening using the relationβ2 =β2m−β

2ins. Here βm is themeasured FWHMof the XRD peak and βins

is the instrumental broadening. The average crystallite sizewas es-timated to be 40 nm for x = 0.07 sample. Fig. 2 shows the TEMimages of the x = 0.07 sample taken using a carbon-coated cop-per grid. The average particle size is found to be around 50 nm.The inset of Fig. 2(a) shows the selected area electron diffractionpattern and it depicts the polycrystalline nature of the sample. Theabsence of a ring-like pattern suggests that the crystalline grainswere in some preferred orientation. The EDS measurement usingthe TEM facility was carried out and it showed the presence of Niwithin the crystallites. The high-resolution transmission electronmicroscope (HRTEM) image recorded at different locations showsa continuous (110) atomic plane, as can be seen in Fig. 2(b). The

Fig. 2. (a) TEM image of the x = 0.07 sample; the inset shows the selected areadiffraction pattern. (b) HRTEM image showing the (110) plane. The FFT image andinverse FFT image after filtering the background for the (110) plane are shown intop and bottom insets of the figure.

fast Fourier transform (FFT) image and inverse FFT image obtainedby filtering the noise are shown in the top and bottom insets ofFig. 2(b), where we can see the uniform (110) plane. To furthersubstantiate the doping of Ni ions, FTIR spectra were recorded.Fig. 3 shows the absorption spectra for different doped samplesalong with the parent SnO2 and NiO compounds. The absorptionbands observed at 3400 and 1640 cm−1 can be attributed to thehydroxyl group, which is in O–H mode. The weaker bands at 1370and 1130 cm−1 are basically from C–H vibrations as a result of thereaction of atmospheric CO2 with H2O, as reported in [17]. Thebands at around 530 and 680 cm−1 correspond to antisymmet-ric Sn–O–Sn vibrations of SnO2. So, it is evident from Fig. 3 thatthe prepared samples exhibit pure rutile structure without anysignature of contribution fromNiO clusters. Themagnetic hystere-sis loops recorded for the x = 0.02 and 0.07 samples at 85 and 300K are shown in Fig. 4, which demonstrates the presence of room-temperature ferromagnetism. For a comparison, we have also car-ried out M–H loop measurement on the parent compound SnO2,and it is found to exhibit weak diamagnetic behavior, as shownin Fig. 4a. The lack of saturation in M–H curves with considerablelinear contributions especially at low temperatures highlights thepresence of a considerable paramagnetic matrix or other compet-ing magnetic interaction. The increase in hysteresis loss with de-crease in temperature could bemainly due tomagnetic anisotropy.The observed saturation magnetization is found to be comparableto those reported in literature for ZnO-based and SnO2-based di-luted magnetic semiconductor materials [18–20].The resistivity of these samples is found to be in the order of

104 � cm at room temperature, and it enhances to 107 � cm ataround 100 K. The rather large value of resistivity indicates the

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Fig. 3. FTIR spectrum for x = 0.02 and 0.07 samples along with parent SnO2 andNiO compounds for comparison.

Fig. 4a. Magnetic hysteresis loops for the x = 0.02 sample at 80 and 300 K.The M–H curve of SnO2 exhibiting diamagnetic behavior at 300 K is shown forcomparison.

localized nature of the charge carriers. The localized charge carriersare expected to promote the bound magnetic polaron-mediatedferromagnetism. Here each trapped charge carrier polarizes thespin of the magnetic ions within its Bohr radius and this leads to aferromagnetic bubble or BMP embedded in a paramagneticmatrix.Ferromagnetism is observed when these ferromagnetic bubblesstart overlapping in such a way that all the magnetic spins arealigned in a particular orientation [21–24].In order to further understand themagnetic properties,we have

fitted the measured initialM–H curve in terms of the bound mag-netic polaron (BMP) model by following [21–23], as given below,

M = MsL(x)+ χmH. (1)

Here the first term is from the BMP contribution and the secondterm is due to the paramagnetic matrix. According to the aboveequation, the sample is visualized as a mixture of BMPs wherelocalized charge carriers strongly interact with the doped transi-tion element over the Bohr radius and the paramagnetic matrix.In the paramagnetic matrix, the doped Ni ions which do not formpart of the BMP are expected to behave like an independent para-magnetic entity. The spontaneous moment of the system Ms =Nms, where ms is the actual spontaneous moment of each BMPand N is the number of BMPs per unit volume. L(x) = coth x− 1/x is the Langevin function with x = meffH/(kBT ), where meffis the effective spontaneous magnetic moment of each BMP. Atrelatively high temperature, the interaction between BMPs can be

Fig. 4b. Magnetic hysteresis loops for the x = 0.07 sample at 80 and 300 K.

Fig. 5. Initial magnetization as a function of magnetic field for the x = 0.02 and0.07 samples at room temperature. The solid line represents the fit to the boundmagnetic polaron model.

Table 1List of parameters obtained from the analysis ofM–H data by using the BMPmodel.Ms is the spontaneous magnetization of the system, χm is the susceptibility of thematrix andmeff is the effective magnetic moment per bound magnetic polaron.

Sample/parameter x = 0.02 x = 0.07Temperature (K) 80 300 80 300

Ms (A/m) 142± 1.3 120± 1.4 466± 1.3 407± 1.1χm (10−4 SI) 0.5 0.02 1.47 0.06meff (10−20 J/T) 2.17 5.9 1.25 3.6

ignored and ms = meff can be taken [21]. However, at sufficientlyhigher temperature, where there is a considerable mobility ofcharge carriers, Eq. (1) cannot be used due to lack of BMPs.χm is thesusceptibility of the matrix. We analyzed the M–H curve in termsof BMP model, i.e. fitting to Eq. (1). The parameters Ms,meff , χmwere varied during the fit. Typical plots of themagnetization curvefor the x = 0.02 and 0.07 samples at 300 K along with fitted dataare shown in Fig. 5. The theoretical fit closely follows the experi-mental data; however, a minor deviationwas observed at low fieldregion. The fitted parametersMs, meff and χm are given in Table 1.For a given sample,Ms is found to increase but meff is found to re-duce with decrease in temperature. The increase in Ms could bedue to interaction between BMPs and the paramagnetic matrix;

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Fig. 6. Temperature variation of magnetization for the x = 0.07 sample. The insetshows the paramagnetic susceptibility along with Curie–Weiss law fit.

however, further analysis is required to verify such interactions.The enhanced hysteresis loop at low temperature can be explainedon the basis of magnetic anisotropy in the above interaction. Thepossible reason for low meff at low temperature could be due tothe reduced size of the BMPs. The origin of such reduced BMP sizecan be due to the presence of competing magnetic interaction. Theconsiderable increase in χm value at low temperature is consistentwith the smaller size of the BMPs. The above variation ofχm cannotbe simply explained based on the Curie law for the paramagneticmatrix, and this supports the above argument of BMP size. Withincrease in doping concentration, the Ms and χm values are foundto increase as a result of the higher concentration of magnetic ions.In the present series of samples, the parameterMs/meff is found tovary with temperature, so in such conditions, one cannot assumethatms = meff . In view of the above restriction, we could not esti-mateN , the number of BMPs per unit volume. The average radius ofthe BMPswas estimated from the fitted value ofmeff and by assum-ing a spherical shape of the BMPs, and it was found to be 60 Å forthe x = 0.07 sample at 80 K. The BMP radius is found to be in thesame order of magnitude as reported in other magnetic systems,such as in CdMnSe by Dietl et al. [24]. The main uncertainty in theabove calculation is from the estimation of the number of transi-tion element ions, Ni, within each BMP, especially in weakly dopedmaterials. On the other hand, in other magnetic systems follow-ing the BMP model such as Cu2Mn0.9Zn0.1SnS4 and Y0.9Ce0.1MnO3[21,25], the number of magnetic ions per BMP is large by an orderof magnitude. Themeff values are found to increase with tempera-ture, and this could be mainly due to change in size of the BMPs. Inorder to study the ferromagnetic transition, the temperature varia-tion ofmagnetizationwasmeasured for awide temperature range,i.e. 80–1000 K. A typical M–T curve for the x = 0.07 sample isshown in Fig. 6, where a clear paramagnetic to ferromagnetic tran-sition can be seen. Itmaybenoted that the observedmagnetic tran-sition is different from superparamagnetic behavior, where onewould expect a thermal blocking transition, etc. Themagnetizationis found to increase gradually with decrease in temperature. Theparamagnetic susceptibility was fitted to the Curie–Weiss law,

χ = χ0 +C(x)T − θc

, (2)

where χ0 is temperature-independent susceptibility. C(x) = xC0= xNµ2eff /3kB is the Curie constant, where x is the concentrationof doped Ni, and θc is Curie temperature. The Curie temperature

Fig. 7. ln ρ versus (1/T )1/4 for the x = 0.02 and 0.07 samples (symbols) alongwithVRHmodel fit (solid line). The inset shows the temperature dependence of electricalresistivity, ρ, for the x = 0.02 sample.

is found to be 770 K and 730 K respectively for the x = 0.02 andx = 0.07 samples. The typical value of effective magnetic momentµeff estimated from the Curie constant is found to be 1.4µB/Niion for the x = 0.07 sample, and this suggests that the doped Niions are expected to be mostly in the Ni2+ state. If the doped Niions are in the Ni2+ state, one would expect the µeff value to beof the order of 2.8µB/Ni ion; the observed difference between theexperimental and theoretical values could be due to only a frac-tion of Ni ions entering the Sn site. χ0 is mainly to account for thetemperature-independent susceptibility originating from the hostmaterial, etc., and its typical value for the x = 0.07 sample is foundto be−7.0× 10−5 SI units.The temperature variation of electrical resistivitywasmeasured

down to 100 K for the x = 0.02 and 0.07 samples. The resistivityof the parent SnO2 is very high, and it could not be measured.The resistivity data could be analyzed based on the variable rangehopping (VRH) model [26]

ρ = ρo exp[To/T ]1/4, (3)where ρ is the resistivity of the sample; ρo and To are constants.The plots of ln ρ as a function of (1/T )1/4 for both samples alongwith VRH model fit are shown in Fig. 7. The density of states inthe vicinity of the Fermi level N(EF ), hopping distance Rhop(T ) andhopping energy Ehop(T ) were determined by following [26], andthey were found to be of the order of 6.1 × 1024 eV−1 m−3, 59 Åand 215 meV, respectively, for x = 0.02. The above parameters forthe x = 0.07 sample were found to be 1.3 × 1025 eV−1 m−3, 49 Åand 178 meV, respectively.

4. Conclusions

Ni-doped SnO2-based diluted magnetic semiconductors wereprepared in single-phase form. They exhibit room-temperatureferromagnetism with transition temperature Tc as high as∼770 K. The initial M–H loops measured for both samples couldbe explained based on the bound magnetic polaron model. Theincrease in matrix susceptibility and decrease in effective spon-taneous magnetic moment of the BMPs at low temperature is ex-plained on the basis of the reduction in size of the BMPs.

Acknowledgement

The authors are thankful to DST, New Delhi, for financialsupport towards a magnetometer.

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