Solid State Communications 150 (2010) 739–742

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Solid State Communications

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Ferromagnetism and bound magnetic polaron behavior in bulk Sn1−xCoxO2Sunita 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 8 September 2009Received in revised form2 December 2009Accepted 18 January 2010by D.D. SarmaAvailable online 25 January 2010

Keywords:A. Diluted magnetic semiconductorD. Magnetic properties

a b s t r a c t

Co doped bulk SnO2 based diluted magnetic semiconductors were prepared in single-phase form up to10% of doping. The magnetic properties were studied from magnetization and electron spin resonancemeasurements. Themagnetization of the 2% doped sample annealed under N2 gas atmosphere exhibited asaturationmagnetization of 0.34µB/Co ion and it could be analysed based on the Brillouin functionmodelby taking into account the ferromagnetic contribution. The magnetization versus field (M–H) curvesfor the other samples recorded at three different temperatures could be analyzed based on the boundmagnetic polaron model. The variation of the spontaneous moment of the bound magnetic polaron withtemperature and doping concentration is explained in detail.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Diluted magnetic semiconductors have evolved as a fasci-nating area of research for spintronics applications at practicaltemperatures. The generic problem of resistance mismatch atmetal–semiconductor interfaces hinders effective spin injection;therefore, much attention is being focused on the development ofroom-temperature ferromagnetic semiconductors. Transition el-ement doped oxide semiconductors such as TiO2, ZnO and SnO2have potential applications in spintronics devices with room-temperature ferromagnetism [1–13]. They are generally called di-lutedmagnetic semiconductors (DMSs). However, a few studies onthin film and bulk samples report other types of behavior, suchas simple paramagnetism, antiferromagnetism (AFM), spin-glass,etc. [14–20]. SnO2is an attractive semiconductor with wide bandgap, Eg = 3.6 eV, and optical transparency in the visible region.Ogale et al. [9] reported a giant magnetic moment of 7.55 µB/ionat room temperature in thin film samples of Co doped SnO2. Sub-sequent to the above report, room-temperature ferromagnetism(FM) with saturation magnetic momentms = 0.133 µB/Co ion forx = 0.01 was obtained by Punnoose et al. [10] on polycrystallineSn1−xCoxO2. However, Liu et al. [20] reported AFM behavior in theabove series with TN ∼ 60 K. Recently, Gopinadhan et al. reportedferromagnetism with Tc > 573 K and ms = 0.26 µB/ion on thinfilm sample of 10 at.% Co doped SnO2 [13]. In order to understandthe magnetic behavior of Co doped SnO2 without the influence ofsubstrate for consistent results, we have prepared bulk Sn1−xCoxO2for x = 0.02, 0.05, 0.07 and 0.10. We have found that these mate-rials exhibit ferromagnetism at room temperature with magnetic

∗ 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.2010.01.029

moment up to 0.34µB/Co ion, especially on material prepared un-der N2 gas atmosphere. The magnetization data could be analyzedin terms of the bound magnetic polaron model, and for one of thesamples the data could be fitted to the Brillouin function modelwith ferromagnetic interaction.

2. Experimental details

Sn1−xCoxO2 for (x = 0.02, 0.05, 0.07 and 0.10) were preparedby following the standard solid-state route. Stoichiometric ratiosof SnO2 and Co3O4 with 99.9% purity were weighed, mixed underacetone and presintered at 400 °C. The final sintering in pellet formwas carried out in air at 900 °C for 24h (air annealed). For a compar-ison, one of the pellets with x = 0.02 was annealed under N2 gasin atmosphere pressure at 500 °C for 12 h. X-ray diffraction (XRD)patterns were recorded at room temperature using a Seifert 3003-TT XRD machine by employing Cu Kα radiation. The microstruc-ture and compositional analysis were studied with a LEO scanningelectronmicroscope (SEM)with energy-dispersive X-ray spectrum(EDX) facilities. The samples were also characterized by record-ing the Raman spectra by using a LabRam HR 8000 spectrometer.The field variation of magnetization at different temperatures wasmeasured by using a Lakeshore model no. 7410 vibrating samplemagnetometer (VSM). The electron spin resonance (ESR) spectrawere recorded in the temperature range 295–445 K by using anX-band JEOL ESR spectrometer model no. JES-FA200.

3. Results and discussion

Fig. 1(a) shows the XRD patterns recorded for Sn1−xCoxO2samples for x = 0 to 0.10; they are found to be in single-phase form. The patterns could be refined by using the P42/mnmspace group with the aid of the Rietveld refinement technique and

740 S. Mohanty, S. Ravi / Solid State Communications 150 (2010) 739–742

a

b

Fig. 1. (a) XRD patterns of Sn1−xCoxO2 samples for different doping concentrations.(b) Expanded view of the (110) peak for different concentrations.

Fig. 2. Room-temperature Raman spectra for x = 0.0, 0.02, 0.10 samples.

Fullprof programme. The lattice parameters are found to decreasemarginally with increase in doping, and this can be understood interms of Co2+ or Co3+ replacing Sn4+ ions. The lattice parametersfor pure SnO2 were found to be a = b = 4.732 Å and c =3.184 Å, and they reduce to a = b = 4.712 Å and c =3.142 Å for x = 0.1. The lattice parameters are comparableto those reported by Duan et al. [21]. Expanded view of the(110) peak are shown in Fig. 1(b), where one can see the shift inpeak position towards higher 2θ value with increase in dopingconcentration. The above observation substantiates the argumentof lattice parameter variation due to Co ions replacing Sn ions. TheSEM images show uniform surface morphology. The cation ratioobtained from EDX analysis is found to be comparable to that ofnominal starting composition. The typical Raman spectra recordedfor x = 0.0, 0.02 and 0.10 samples are shown in Fig. 2. The mainpeak at 633 cm−1 corresponds to theA1g mode, i.e. symmetric Sn–Ostretching.We have also observed aminor peak at∼693 cm−1 andit is comparable to that reported in the literature for SnO2 [11,12].It may be noted that in all the Co doped samples no additionalpeak has been observed. The doping gives rise to only reductionin intensity of peaks. So, the samples are basically free from Co3O4and other related impurity phases. Typical magnetization loopsrecorded at two different temperatures, namely 85 K and 400 K,

a

b

Fig. 3. M–H loops recorded for Sn1−xCoxO2 samples at (a) 85 K and (b) 400 K forx = 0.02, 0.05, 0.07, 0.10.

are shown in Fig. 3 for all the samples. The magnetization dataare expressed in the unit of µB/Co ion, calculated from the valuein emu/mole by taking into account the actual Co concentrationin the material. They all exhibit ferromagnetic behavior, evenat 400 K. We have not observed any magnetic transition fromthe temperature variation of magnetization measurement carriedout up to 400 K, and this suggests that the Tc is beyond 400 K.The 2% Co doped sample exhibits magnetic saturation with itsmagnitude considerably larger than that of other higher dopedsamples. In addition to the FM behavior, a considerable linearcontribution can be seen, especially for x ≥ 0.05 at lowtemperature, and this suggests the presence of a considerableparamagnetic matrix in the system. The saturation moment of0.036 µB/Co ion (0.19 emu/cm3) has been observed for x = 0.02at 400 K and it enhances to 0.055 µB/Co ion (0.28 emu/cm3)when the temperature is reduced to 85 K. As the temperature isreduced, there is a considerable increase in hysteresis loss due topossible enhancement in magnetic anisotropy or other competingmagnetic interaction. The M–H curve of the N2 annealed x =0.02 sample is shown in Fig. 4, and we can see a large increasein magnitude of magnetization compared to that of air annealedmaterial. The N2 annealing is expected to reduce the oxygencontent in the sample and that leads to electron doping in thematerial. In other words, there would be an increase in electronconcentration with N2 annealing. This would enhance the chargecarriermediated ferromagnetic interaction, thereby enhancing thesaturation magnetization.In order to further understand themagnetic properties,we have

fitted themeasured initialM–H curves of the air annealed samplesin terms of the boundmagnetic polaron (BMP) model by followingRefs. [22–24], where

M = M0L(x)+ χmH. (1)

Here the first term is from the BMP contribution and the secondterm is due to the paramagnetic matrix contribution. Here, M0 =Nms, N is the number of BMPs involved and ms is the effectivespontaneous moment per BMP. L(x) = coth x − 1/x is theLangevin function with x = meffH/(kBT ), where meff is the true

S. Mohanty, S. Ravi / Solid State Communications 150 (2010) 739–742 741

Table 1List of parameters obtained from the bound magnetic polaron model fit.M0 is the spontaneous magnetization, χm is the susceptibility of the matrix andmeff is the effectivespontaneous moment per bound magnetic polaron.

Sample/ parameter x = 0.05 x = 0.07 x = 0.10

Temperature (K) 85 295 400 85 295 85 295 400M0 (10−3 emu/cm3) 430± 1.2 289± 1.3 195± 1.01 411± 1.4 282± 1.2 342± 1.2 270± 1.4 173± 1.01χm (10−4 cgs) 0.03 0.03 0.002 0.09 0.03 0.12 0.11 0.003meff (10−17 emu) 0.42 3.8 5.2 0.41 3.3 0.22 2.7 4.5

Fig. 4. Magnetic hysteresis loop of nitrogen annealed Sn0.98Co0.02O2 samplerecorded at 400 K.

Fig. 5. Magnetization versus field at 295 K, for x = 0.05 and x = 0.10 samples ofSn1−xCoxO2 . The solid lines represent the fit to the bound magnetic polaron model(Eq. (1)).

spontaneous moment per BMP. At relatively high temperature,where the interaction betweenBMPs can be ignored,ms = meff canbe taken [22]. However, at sufficiently higher temperature, wherethere is a considerable mobility of charge carriers, Eq. (1) cannotbe used due to lack of BMPs. χm is the susceptibility of the matrix.TheM–H curves recorded at three different temperatures, namely85 K, 295 K and 400 K, could be fitted to Eq. (1) for samples withx ≥ 0.05. The typical plots of the BMP model fit along with theexperimental data are shown in Fig. 5 for x = 0.05 and 0.10 at T =295K. The fitted parametersM0,χm andmeff are given in Table 1 fordifferent samples. For a given doping concentration, theM0 valuesare found to decreasewith increase in temperature, and this can beunderstood as a result of the decrease in ferromagnetic interactionwith increase in temperature. On the other hand, the M0 valueis found to decrease with increase in doping concentration,and such a decrease in ferromagnetism with increase in dopingconcentration has been reported in the Mn doped SnO2 baseddiluted magnetic semiconductor [21]. It is mainly due to reductionin average Co–Co interatomic distance, which might contributeto nearest-neighbour antiferromagnetic interaction at the expenseof ferromagnetism. The paramagnetic susceptibility χm is foundto decrease with increase in temperature, as expected for anyparamagnetic matrix, and its value marginally increases withdoping. The spontaneous moment per BMP, meff , is found toincrease with temperature, and such a variation of meff in

Fig. 6. The initial magnetization curve along with the Brillouin function model fitfor the N2 annealed x = 0.02 sample.

contradiction to the variation of M0 can be understood as a resultof the increase in size of the BMPs with temperature. At highertemperature, even though the magnetic moments are alignedparallel within each BMP, the overall alignment of BMPs along theapplied field may not be complete due to the increase in thermalenergy. The value of meff is found to be in the order of 10−17 emu(10−20 J/T), and it is comparable to that reported by Quinteroet al. [25]. However, the present meff value is found to be oneorder ofmagnitude larger than that reported in Cu2Mn0.9Zn0.1SnS4and Y0.9Ce0.1MnO3 [22,26]. In the present series of samples, theparameter M0/meff is found to vary with temperature, so in suchconditions, one cannot assume thatms = meff . In view of the aboverestriction, we could not estimate N , the number of BMPs per unitvolume. The average radius of the BMPs was estimated from thefitted value ofmeff and by assuming a spherical shape of the BMPs.The typical radius of BMPs for the x = 0.05 sample is found to be38 Å. Our result is comparable to that reported by Dietl et al. [27]in the CdMnSe based DMS, where the radius was determined to be40 Å. However, themagnetization data for the N2 annealed samplecould not be fitted to the BMP model; on the other hand, it couldbe fitted to the Brillouin function model by taking into account theferromagnetic contribution.

M = MSBJ(x) (2)MS = ng µBS and

BJ(x) =1S

[(S +

12

)coth x

(S +

12

)−12coth

x2

].

Here n is the number of magnetic atoms per unit volume, S is themagnetic spin quantum number and x = g µBB

kT . The fitted dataare shown as a solid line in Fig. 6; the line closely follows theexperimental data. The values of S and Ms are found to be 1.38and 1.05 × 103 emu/cm3, respectively. The S value suggests thepresence of Co2+ ions in high spin states; however, we cannot ruleout the presence of Co3+ ions. The magnetization data of the airannealed x = 0.02 sample could not be fitted to the Brillouinfunction model due to the weak signal. To further investigatethe magnetic properties of Sn1−xCoxO2 samples, ESR spectra wererecorded. Typical ESR spectra for x = 0.02, 0.07, 0.10 samplesare shown in Fig. 7(a), where we can see a shift in resonancefield towards higher field with increase in doping concentration,and this supports the magnetization results of the decrease in FMinteraction. In order to study the variation of ESR signal intensity

742 S. Mohanty, S. Ravi / Solid State Communications 150 (2010) 739–742

a

b

c

d

e

Fig. 7. (a) ESR spectra at room temperature for different doped samples andintegrated ESR spectra for the x = 0.02 (air annealed) sample recorded at (b) 295K, (c) 345 K, (d) 395 K, (e) 445 K, respectively. The simulated data obtained by usingtwo overlapping Gaussian curves are also shown.

with temperature, we have plotted the integrated ESR signalfor the x = 0.02 sample (air annealed) recorded at differenttemperatures, as shown in Fig. 7(b). The observed increase inintensity with temperature can be explained as a result of theincrease in paramagnetic signal as the sample approaches the Curietemperature. The low-field shoulder in the above curves signifiesthe presence of ferromagnetism at room temperature (295 K). Theobserved ESR signal could be simulated by using two Gaussiancurves, as shown in Fig. 7(b). As the temperature approaches 445 K,where the material is almost in the paramagnetic state, the signalcould be reproduced by using a single Gaussian function.

4. Conclusion

Single-phase samples of Co doped SnO2 based dilutedmagneticsemiconductorswere prepared for x = 0.02 to 0.10. These samplesare found to exhibit room-temperature ferromagnetism, and thehighest magnetic moment of 0.34 µB/Co ion was observed forthe x = 0.02 sample prepared under N2 gas atmosphere. Themagnetization of the x = 0.02 sample could be fitted to theBrillouin function model by taking into account the ferromagneticcontribution. The magnetization data measured at three differenttemperatures for x ≥ 0.05 samples could be fitted to the BMPmodel. The variation of total spontaneous magnetization, M0, andspontaneous moment per BMP, meff , with temperature has beenexplained in terms of the increase in size of the BMPs withtemperature. The decrease in M0 and meff values with increase indoping concentration has been explained on the basis of possibleantiferromagnetism due to the reduction in average interatomicdistance of doped Co ions.

Acknowledgements

The authors are grateful to DST, NewDelhi, for financial supportfor a magnetometer.

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