PropZnO

Structural, magnetic and electronic structure properties of Co dopedZnO nanoparticles

Shalendra Kumar a,b,*, T.K. Song b,*, Sanjeev Gautam c, K.H. Chae c, S.S. Kim a, K.W. Jang a

a Institute of Basic Sciences, Changwon National University, Changwon, Gyeongnam 641-773, Republic of Koreab School of Materials Science and Engineering, Changwon National University, Changwon, Gyeongnam 641-773, Republic of KoreacAdvanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

A R T I C L E I N F O

Article history:Received 5 October 2014Received in revised form 29 January 2015Accepted 6 February 2015Available online 9 February 2015

PACS:75.50.Pp18.70.Dm61.46. +w82.80.Ej

Keywords:Electronic materialNanostructuresChemical synthesisXANESMagnetic properties

A B S T R A C T

We reported structural, magnetic and electronic structure studies of Co doped ZnO nanoparticles. Dopingof Co ions in ZnO host matrix has been studied and conrmed using various methods; such as X-raydiffraction (XRD), eld emission scanning electron microscopy (FE-SEM), energy dispersed X-ray (EDX),high resolution transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy(FT-IR), near edge X-ray absorption ne structure (NEXAFS) spectroscopy, magnetic hysteresis loopmeasurements and X-ray magnetic circular dichroism (XMCD). From the XRD and HR-TEM results, it isobserved that Co doped ZnO nanoparticles have single phase nature with wurtzite structure and excludethe possibility of secondary phase formation. FE-SEMand TEMmicrographs show that pure and Co dopednanoparticles are nearly spherical in shape. O K edge NEXAFS spectra indicate that O vacancies increasewith Co doping. The Co L3,2 edge NEXAFS spectra revealed that Co ions are in 2+ valence state. DCmagnetization hysteresis loops and XMCD results clearly showed the intrinsic origin of temperatureferromagnetism in Co doped ZnO nanoparticles.

2015 Elsevier Ltd. All rights reserved.

1. Introduction

During the last decades dilute magnetic semiconductors(DMSs) have inspired a great deal of attention due to theirunderlying fundamental physics and their potential application insecond generation of spin electronic devices: such as spin lightemitting diodes, non-volatile memory, logic devices, spin valvetransistor and optical isolator etc. [14]. DMSs could pave the wayto exploit the charge and spin of the carrier in spintronics devicesand the combination of these two degree of freedom promises thenew functionality in the devices. The practical device needs thesuitable ferromagnetic semiconductors, which allow the simulta-neous control of the charge and spin state of the electron at roomtemperature. This phenomenon can be realized by introducing themagnetism in a semiconductingmaterials. Therefore, themagneticproperties of DMSs can be tailored by replacing a fraction of thecation of the host semiconducting materials by the transition

metal (TM) ions. Till now the magnetic properties of various TMions doped compound semiconductors such as ZnO, TiO2, SnO2,CeO2, etc. [519] doped with TM ions have been studied. Sharmaet al. reported the RT-FM in Co doped SnO2 nanoparticles preparedby the co-precipitation method [6]. Kumar et al. also reported theRT-FM in Mn doped TiO2 thin lms prepared by the pulsed laserdeposition method [12]. Moreover, Ahmed et al. fabricated the TMdoped ZnO nanostructures by the microwave assisted solutionroute and reported the detailed investigation of structural,magnetic, optical properties of these nanostructures [10]. Sharmaet al. also claimed the RT-FM in Fe doped CeO2 thin lms preparedby the pulsed laser deposition method. The experimentalobservation about the nature of the magnetic properties of DMSssynthesized by different methods are controversial [2025]. It isobserved that the DMSs prepared using different methods exhibitdistinctively different magnetic properties, ranging from para-magnetism to high temperature ferromagnetism [22,2628].Bouloudenine et al. reported the antiferromagnetism in Co dopedZnO bulk samples prepared by the co-precipitation route, whereasLawes et al. claimed the absence of the ferromagnetism in Co andMn substituted ZnO. These highly contradictory experimentalresults are probably associated with different defects or impurities

* Corresponding authors. Tel.: +82 55 213 3890; fax: +82 55 262 6486.E-mail addresses: [email protected] (S. Kumar), [email protected]

(T.K. Song).

http://dx.doi.org/10.1016/j.materresbull.2015.02.0200025-5408/ 2015 Elsevier Ltd. All rights reserved.

Materials Research Bulletin 66 (2015) 7682

Contents lists available at ScienceDirect

Materials Research Bulletin

journa l homepage: www.elsevier .com/ locate /matresbu

created by different preparation methods. Venkatesan et al. [29]have studied a correlation between the magnitude of magneticmoments with the oxygen partial pressure during annealing, andthey observed that annealing in higher oxygen partial pressurereduces the amount of magnetization. Kumar et al. [21] alsoclaimed the RT-FM in pure ZnO nanorods due to the oxygenvacancies. Therefore, from the reported literature it clear thatdefects play a crucial role in ferromagnetic ordering in DMSs.Herein, we have chosen ZnO as a host semiconductor. ZnO is aninteresting direct band gap (3.27 eV) semiconductor, which isexplored for various applications in optoelectronics. Therefore, theincorporation of ferromagnetism in ZnO can lead to a diversity ofnewmultifunctional devices. However, in spite of being one of themost representative and widely investigated material, Co dopedZnO still lacks clear explanation about the ferromagnetism at roomtemperature. Despite of extensive efforts in this area, still there hasbeen a great deal of controversy, particularly on fundamentalissues such as the origins and characteristics of the observedferromagnetism (FM).

In thiswork, we report the ferromagnetism in the Co doped ZnOnanoparticles at room temperature. We have performed asystematic investigation of the structure and magnetic propertiesusing various methods. X-ray diffraction (XRD), high resolutiontransmission electronmicroscopy (HR-TEM) and Fourier transforminfrared spectroscopy (FT-IR) measurements demonstrate that Codoped ZnO nanoparticles exhibit a single phase nature withwurtzite lattice and ruled out the formation of any impurity phase.NEXAFS measurements performed at Co L3,2-edge clearly showsthe absence of Co clusters and also reveals that Co ions are in 2+valance state. DCmagnetization and XMCDmeasurements showedthat Co doped ZnO nanoparticles exhibits RT-FM.

2. Experimental

Zn1xCoxO (x = 0.00, 0.01, and 0.03) nanoparticles wereprepared by co-precipitation technique using Zn(NO3)26H2Oand Co(NO3)26H2O. The precursor of zinc and cobalt nitrates(purity 99.99% from Aldrich) was employed tomake Co doped ZnOnanoparticles. These metal nitrates were dissolved in de-ionizedwater to get a 0.06M solution and then kept for stirring for 1h. Inthis solution, NH4OH solutionwas added drop wise until the pH ofsolution reached to 8. This mixture was stirred for 3h at roomtemperature and after that washed 5 times with de-ionized waterand ethanol. The precipitate obtained after wash was dried at100 C for 12h. Finally, the dried samples were ground and kept forcalcination at 500 C for 4h. Thermal gravimetric (TG) anddifferential thermal (DT) measurements were performed using2960 SDT TA instrument. Philips X-pert X-ray diffractometer withCu Ka (l =1.5405) was used to study single phase nature of theZn1xCoxO (x =0.00, 0.01, and 0.03) nanoparticles at roomtemperature. The surface morphology of pure and Co dopedZnO nanoparticleswas studied by using FE-SEM (Mira II LHM) at anaccelerating voltage 200kV with an attached energy dispersedX-ray (EDX) spectrometer. Before the FE-SEM measurements, asmall quantity of powder was dissolved in acetone and put for10min in ultrasonic bath and then few drops of this solution weredisperse on glass slide. The glass slide was then dried at 80 C forhalf an hour. Finally, the glass slide dispersed with nanoparticleswas coated with platinum to avoid the charging effect during themeasurements. TEM image and HR-TEM measurements wereperformed using FE-TEM (JEOL/JEM 2100F). For TEM measure-ments the nanoparticles of Co doped ZnO were dispersedhomogeneously in ethanol using ultrasonic treatment. A minutedrop of this solution was cast on to a carbon-coated copper gridfollowed by subsequently drying in air before transfer it in tomicroscope. FT-IR measurements were performed using Nicolet

Impact 410DSP spectrometer. The XMCD experiment for 3% Codoped ZnO was performed at 2A MS beam line of PohangAccelerator Laboratory (PAL) operating at 3GeV with a maximumstorage current of 300mA. This beam line has elliptically polarizedundulator with greater than 90% degree of circular polarization.The XMCD spectrum was taken for a xed helicity of the light byreversing the applied magnetic eld (0.8T) for each hn. Thespectrumwas normalized to the incident photon ux. The NEXAFSmeasurements of pure and Co doped ZnO along with the referencecompounds of Co3O4 and CoO at O K and Co L3,2-edge wereperformed at the soft X-ray beam line 10D KIST of PAL. The spectrawere simultaneously collected in the total electron yield (TEY)mode and the uorescence yield (FY)mode at room temperature ina vacuum 1.5108 Torr. The spectra in the two modes turnedout to be nearly identical indicating that the systems are so stablethat the surface contamination effects are negligible even in theTEY mode. The spectra were normalized to incident photon uxand the energy resolution was 0.2 eV. Magnetic hysteresis loopmeasurements were performed at room temperature using acommercial Quantum Design physical properties measurementsystem (PPMS).

3. Results and discussion

Thermal gravimetric (TG) and differential thermal (DT) analysisof the as synthesized samples of Co doped ZnO nanoparticles werecarried out to determine the weight loss process and approximatecalcinations temperature. The precipitate obtained after washedby the distilled water and ethanol was dried at 100 C. The driedsamples again need the thermal treatment to form themetal oxidepowder. DT-TGA curve for 3% Co doped ZnO nanoparticles is shownin Fig. 1. The as synthesized Co doped ZnO nanoparticles wereheated up to 1000 C in the nitrogen atmosphere at a rate of 10 C/min. It is clear from Fig. 1 that DTA peak closely corresponds to theweight changes observed in the TGA curve. It is observed that thethermal decomposition of the dried sample occurs in three stepsbelow 400 C. The rst stage of the decompositionwas observed inthe temperature range 40108 C with an endothermic peak at96.9 C, that indicates a weight loss of 8.7%, which may be due tothe evaporation of ethanol. The second and third stages of thedecomposition were found in the temperature range 122154 Cand 200400 C, respectively. The weight losses in the second andthird stages were 3.4% and 28.7%, respectively. The weight lossobserved in the second stage may be due to the evaporation of thewater content. However, the weight loss noticed in the third stage

[(Fig._1)TD$FIG]

Fig. 1. TGA and DTA curve of Zn0.97Co0.03O nanoparticles.

S. Kumar et al. /Materials Research Bulletin 66 (2015) 7682 77

is due to the decomposition of the ZnCo precursor to theformation of Zn0.97Co0.03O. Since, there is no signicant change inthe weight loss above the 400 C, the calcination temperature ofthe powders was decided to be over 400 C.

Powder XRD pattern obtained from Zn1xCoxO (0.00 x0.03)nanoparticles are shown in Fig. 2. The XRD datawere analyzed andindexed using Powder-X [30] software. It can be seen from the XRDpattern that all the samples have a single phase nature withhexagonal wurtzite structure. No Co cluster or Co related complexwas detected, at least within the sensitivity of X-ray diffarac-tometer, which ruled out the possibility of any secondary phase.The full width at half maximum calculated using (0 02) peaks hasbeen found to increase as increasing the Co content. Moreover, theposition of theXRDpeak corresponding to the (0 02) plane is foundto shift towards lower 2u value with increasing the Co contents.The d (lattice spacing) values from (0 02) peak have beenplotted asa function of Co contents is shown in the inset of Fig. 2. It isobserved that the d (0 0 2) value increases linearly with the Cocontent which is consistent with the results reported by the othergroups on this system [31,32]. A linear increase of the latticespacing thus reects, in accordance with Vegards law.

FT-IR measurements of Co doped ZnO nanoparticles have beencarried out to examine the change in ZnO bonding due to the Cosubstitution. FT-IR measurements performed in the wavenumberrange 4004000 cm1 using KBr method at RT are shown in Fig. 3.It is observed that undoped and Co doped ZnO nanoparticlesexhibited the absorption band at 3450, 1620, 1080, 867,700, 500 and 425 cm1 and which are well matched withearlier reported results [3335]. The absorption peak appeared at3450 cm1 is attributed to normal polymeric OH stretchingvibration of H2O in CoZnO lattice [34], whichmight be due to the

adsorption of moisture, when FT-IR sample disks were prepared inan open air atmosphere. Anotherweak intense peak at1620 cm1is assigned to HOH bending vibration mode, which is due to asmall amount of H2O in the ZnO nanoparticles [35]. The absorptionbands 1080 cm1 may be originated due to asymmetric stretch-ing of resonance interaction betweenvibrationmodes of oxide ionsin nanomaterials [33]. The absorption bands at 500 and425 cm1 are attributed to the ZnO stretching in the ZnOlattice. The absorption bands in the range of 425410 cm1

correspond to the E2 mode of the hexagonal ZnO (Raman active).However, the absorption bands in 506493 cm1 range may beassociatedwith oxygen deciency and/or oxygen vacancy defect inZnO [36]. Although, in case of undoped ZnO nanoparticles, weobserved these two strong absorption bands at 425 and 506 cm1,but in case of 1% Co doped ZnO nanoparticles, the value ofabsorption bands was found at 419 and 498 cm1, whereas for 3%Co doped ZnO nanoparticles, these values were at 410 and496 cm1, respectively. It can be clearly seen from the FT-IRmeasurements that these absorption bands have blue shift with Codoping. The shift in the peak position of ZnO absorption bands issuggestive of the fact that ZnOZn network is perturbed by thepresence of Co in ZnO host matrix. Thus, from FT-IR results, it maybe concluded that Co ions are occupying Zn position in ZnOmatrixwhich are consistent with the results as observed in XRDmeasurements.

The morphology and chemical composition of pure and Codoped ZnO nanoparticles were investigated by FE-SEM and EDXmeasurements. From the FE-SEMmicrograph (not shown here dueto the brevity of article), it is observed that all the particles arenearly spherical in shape. Fig. 4(a) and (b) highlights the typicalEDX spectra taken from pure and 3% Co-doped ZnO samples. Thechemical analysis of Zn1xCoxO nanoparticles for x =0.00 and 0.03measured by EDX analysis shows the presence of Zn, O and Co only.The EDX results indicate that the nanoparticles of pure ZnO aremade up of zinc, oxygen ions only whereas Co doped ZnO have Zn,O and Co ions only.

In order to get more insight of the structural properties, we hadalso performed high resolution transmission electron microscopy(HR-TEM) measurements. Fig. 5(a) and (c) represents TEMmicrograph of pure and 3% Co doped ZnO. It can be clearly seenfrom TEM micrographs that particles are agglomerated. TEMmicrograph indicates the nano-crystalline behavior of the sampleswith almost spherical shapewhich are in good agreement with theFE-SEM results. In order see the particle size distribution, the

[(Fig._2)TD$FIG]

Fig. 2. XRD pattern of Zn1xCoxO (x = 0.00, 0.01 and 0.03) nanoparticles. Inset-1shows the d (0 0 2) value as a function of Co content and inset-2 represents theFWHM as function of Co content.

[(Fig._3)TD$FIG]

Fig. 3. FTIR spectra of Zn1xCoxO (x = 0.0, 0.01 and 0.03) nanoparticles.

78 S. Kumar et al. /Materials Research Bulletin 66 (2015) 7682

particle size was measured by using Image-J software. The meanparticle size calculated using the Lorentzian tting (see Fig. 5(b)and (d)) of the histogramswas found 16.5 and 14.0nm for pure and3% Co doped ZnO, respectively. The particle size obtained from theTEM micrographs infer that Co doping hindered the particlegrowth of Co-doped ZnO which are in good agreement with theXRD results. Furthermore, to see any impurity phase in doped

samples, we have performed HR-TEM measurements of pure andCo doped ZnO nanoparticles. HR-TEM images were taken atdifferent part of the samples, to investigate the presence of anysecondary phase related to Co in doped ZnO nanoparticles. Inset inFig. 5(a) and (c) shows the HR-TEM image of pure and 3% Co dopedZnO nanoparticles. A careful analysis of the inter-planer distancecalculated from HR-TEM image shows the (10 0) planes of

[(Fig._4)TD$FIG]

Fig. 4. EDX spectrum of (a) ZnO and (b) Zn0.99Co0.01O nanoparticles.

[(Fig._5)TD$FIG]

Fig. 5. TEM micrograph of (a) ZnO and (c) Zn0.97Co0.03O nanoparticles. Insets in (a) and (c) show high resolution TEM image. Insets in (b) and (d) show the particle sizedistribution histogram.

S. Kumar et al. /Materials Research Bulletin 66 (2015) 7682 79

hexagonal structure of ZnO. HR-TEM image taken on the sampleswith different Co concentration also indicates that individualnanoparticle has single phase nature. HR-TEM observations oflarge number of randomly selected particles showed the absence ofimpurity phase and suggesting their good crystallinity.

The NEXAFS techniques have been established as a powerfultool to understand the local structure of the transitionmetal dopedvarious oxide systems. The spectral features in NEXAFS spectra arevery sensitive to the symmetry of the probed ions as well as localenvironment. The peak position and spectral features of theNEXAFS spectra are affected not only by the oxidation state ofprobed ion, but also by its structural symmetry and covalent/ioniccharacter of the bonds between cations and neighboring atoms. InNEXAFS experiment, the photons of characteristic energies areabsorbed to produce the transition of a core electron to an emptystate above the Fermi level and are governed by the dipoleselection rules. Fig. 6 illustrates O K-edge NEXAFS spectra ofZn1xCoxO nanoparticles. Basically the O K-edge spectra originatedue to a transition from O 1s core state to the unoccupied O 2pderived states, which are hybridized states with the moderatelynarrow 3d band and broader 4sp bands of the 3d transition metalions. The spectral feature in the energy range of 530539eV can beascribed to the hybridization between O 2p and Zn and Co 4s statesfollowed by the region between 540 and 550 eV, which is due tothe hybridization between O 2p and Zn and Co 4p states. Thespectral feature above 550 eV is due to the hybridization betweenO 2p and Zn and Co 4d states. By comparing the NEXAFS spectra ofCo doped ZnO nanoparticles with undoped ZnO nanoparticles, it isobserved that the some extra spectral feature evolve at 528(marked by a) and 532 eV (marked by b) and its intensity increaseswith Co doping. The new feature appeared at 532 eV, nearby theconduction-bandminimum, can be assigned to the cobalt 3d and O

2p hybridized states. Furthermore, the broadening of the spectralfeatures observed in doped samples at 535eV (b) is thought to bedue to the presence of the oxygen vacancies. Thus, increase inbroadening of spectral features at 535eV infer that Co atoms areoccupying the interstitial site of the ZnOmatrix surrounded by theoxygen. Therefore, we may say that Co ions are coordinated withoxygen site tetrahedrally in ZnO. The broadening of the spectralfeatures observed for 3% Co doped ZnO nanoparticles is higher incomparison to other composition at 535eV. The broadening isassigned due to the presence of the oxygen vacancies indicatingthat the oxygen-related defect concentration is higher in 3% cobaltdoped ZnO nanoparticles. This may be the reason that 3% dopedsample shows the highermagneticmoment in comparison to othercomposition.

Lots of controversies have been reported in the literature aboutthe origin of ferromagnetism in Co doped ZnO. So in order to checkthe role of Co doping and origin of ferromagnetism, we haveperformed NEXAFS and XMCDmeasurements at Co L3,2-edge of Codoped ZnOnanoparticles. NEXAFS is a very effective tool to conrmthe secondary phase such as Co clusters, Co oxide phase orwhether

[(Fig._6)TD$FIG]

Fig. 6. NEXAFS spectra measured at the O K-edge for Zn1xCoxO (x =0.00, 0.01, and0.03) nanoparticles. Inset-1 shows the NEXAFS spectra measured at Co L3,2-edge ofZn1xCoxO (x = 0.01, and 0.03) nanoparticles. Inset-2 shows the XMCD spectrum ofZn0.97Co0.03O nanoparticles measured at Co L3,2-edge.

[(Fig._7)TD$FIG]

Fig. 7. MH hysteresis loop of Zn0.99Co0.01O nanoparticles at different temperature.Inset showing the expanded view of low eld region.

[(Fig._8)TD$FIG]

Fig. 8. MH hysteresis loop of Zn0.97Co0.03O nanoparticles at different temperature.Inset showing the expanded view of low eld region.

80 S. Kumar et al. /Materials Research Bulletin 66 (2015) 7682

it is substituting Co at Zn position in the ZnO nanoparticles or not.Inset-1 in Fig. 6 shows the Co L3,2-edgeNEXAFS spectra of Co dopedZnO nanoparticles at room temperature. It is clear from the insetthat the spectral features of Co L3,2-edge are neither similar toCo3O4 nor Co metal [3739]. The Co L3,2-edge spectral featuresresult from the Co 2p!3d dipole transitions and are stronglyinuenced by the large 2p core-hole spin coupling energy whichdivide the spectra into L3 and L2 regions at low and high photonenergies, respectively. In comparison with the spectrum of Cometal, Co3O4 and CoO, one can see that the spectra of Co doped ZnOnanoparticles are very similar to the CoO, which show that the Coions are in 2+ valance state in Co doped ZnO nanoparticles.

XMCD measurements were also performed to know about thesource of room temperature ferromagnetism in Co doped ZnOnanoparticles. The magnetization of the sample is proportional tothe XMCD signal, which results from the difference between XASspectrum recorded for the parallel (m+) and antiparallel (m)alignment of the photon helicity with applied led. Inset in Fig. 6shows the XMCD spectrum of 3% Co doped ZnO nanoparticles. Itcan be clearly seen from the XMCD (m+m+) signal with a negativesign at energy (hv) 777.5 eV conrm the intrinsic origin offerromagnetism in the Co doped ZnO nanoparticles. The XMCDsignal observed at 777.5 eV indicated that ferromagnetism is due toCo2+ ion, which also conrm that Co ions are in 2+ valence state inZnO matrix.

Magnetic hysteresis loop measurements of Zn1xCoxO (x =0.01and 0.03) with different Co doping have been performed atdifferent temperature. Figs. 7 and 8 show themagnetization versuseld (MH) curves for 1% and 3% Co doped ZnO nanoparticles atdifferent temperature (temperature range (100300K)), whichinfers that all the samples exhibit ferromagnetic behavior with TCabove the room temperature. The deduced value of coercive eldand remanence magnetization is shown in Table 1. It is observedthat the value of coercive eld and remenance magnetizationdecreases with increase in the temperature for both the sample. Itcan be seen from the hysteresis loops measurements measured at100, 200 and 300K that the value of the saturation magnetizationis almost same, except the slight higher value of HC and Mr at lowtemperatures (see Table 1). Moreover, it is observed that thesaturationmagnetization value of 3% doped samples is higher than1% Co doped ZnO. However, the value of the magnetic moment isfound to be very small in the present case. Some groups havereported that small value of the moments in DMSs may originatedue to the presence of the additional antiferromagnetic typecoupling between some neighboring ions [23,40]. As a result, thisantiferromagnetic coupling may lead to the canting of the spin.Hence, the observed weak FM in Co doped ZnO nanoparticles is anintrinsic property and is not due to the ferromagnetism of Coclusters as supported by our XRD, FTIR, HRTEM and NEXAFSspectroscopy results. Moreover, in addition to themagnetic dopingeffect, the oxygen vacancies have been found to play an importantrole in the RTFM for Co doped ZnO nanoparticles. Some theoreticalgroups have reported that magnetic ordering in DMSs can betailored by creating the oxygen vacancies, because oxygen

vacancies can cause a noticeable change in the band structure ofthe host matrix [41,42]. From the O K-edge spectra, it is observedthat an oxygen vacancy increases with Co doping, which mayincrease the magnetic moment of the system. The boundmagneticpolarons (BMPs) model may be applied to explain the observedferromagnetism in the present case. According to BMP model, theoxygen vacancies locally exist around magnetic ions i.e., Co ionssubstitute for Zn in ZnOmatrix. An electron trapped by the oxygenvacancies undergo orbital couplingwith the d shells of the adjacentCo2+ ions and form the BMPs. In each BMP, the neighboring Co2+

cations create a spin-alignment of Co2+/Co2+ magnetic exchangecoupling. When a large number of BMPs overlap, a continuouschain of Co2+/Co2+ magnetic exchange coupling is shaped, andferromagnetic ordering in Co doped ZnO is established. A higherdensity of oxygen vacancies results in a greater overall volumebeing occupied by BMPs, thereby, increasing the probability ofoverlapping more Co ions into the ferromagnetic domains, whichwill enhance the ferromagnetism [43].

4. Conclusions

We have successfully synthesized single phase Co doped ZnOnanoparticles. From XRD pattern, HR-TEM and FTIR measure-ments, it is observed that Co ions are occupying the Zn position inZnOmatrix. NEXAFS measurements infer the absence of Co metalsclusters and the observed spectral features are similar to the CoO,which indicates that Co ions are in 2+ valence state. The DCmagnetization and XMCD measurements clearly reect Co dopedZnO nanoparticles exhibits intrinsic ferromagnetism at roomtemperature.

Acknowledgements

This work was supported by the Priority Research CentersProgram through the National Research Foundation of Korea (NRF)funded by the MOE (2012-045424) and by the National ResearchFoundation of Korea [KRF] grant funded by the Korea government[MSIP] (2012-0009457).

References

[1] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75(1999) 3366.

[2] S.W. Jung, S.-J. An, G.-C. Yi, C.U. Jung, S.-I. Lee, S. Cho, Appl. Phys. Lett. 80 (2002)4561.

[3] H. Ohno, Science 281 (1998) 951.[4] S. Gayathri, O. Sivaraman, N. Ghosh, S. Sathishkumar, J. Jayaramudu, S.S. Ray, A.

K. Viswanath, Appl. Sci. Lett. 1 (2015) 8.[5] S. Kumar, B.H. Koo, S.K. Sharma, M. Knobel, C.G. Lee, Nano 05 (2010) 349.[6] A. Sharma, A.P. Singh, P. Thakur, N.B. Brookes, S. Kumar, C.G. Lee, R.J.

Choudhary, K.D. Verma, R. Kumar, J. Appl. Phys. 107 (2010) .[7] S. Kumar, B.H. Koo, C.G. Lee, S. Gautam, K.H. Chae, S.K. Sharma, M. Knobel,

Funct. Mater. Lett. 04 (2011) 17.[8] F. Ahmed, S. Kumar, N. Arshi, M.S. Anwar, B.H. Koo, C.G. Lee, Funct. Mater. Lett.

04 (2011) 1.[9] S. Kumar, F. Ahmed, M.S. Anwar, H.K. Choi, H. Chung, B.H. Koo, Mater. Res. Bull.

47 (2012) 2980.

Table 1Calculated value of coercive eld (HC), remanent magnetization (Mr) and saturation magnetization (MS) for Zn1xCoxO2 (x = 0.01 and 0.03) nanoparticles at differenttemperature.

Sample Temperature (K) Coercive eld (HC) (Oe) Remanence magnetization (Mr)104 (emu/g) Saturation magnetization (MS)103 (emu/g)Zn0.99Co0.01O 100 50 4.41 6.4

200 40.6 3.9 6.4300 35.8 3.3 6.3

Zn0.97Co0.03O 100 60.4 7.2 10.7200 32.2 4.43 9.7300 28.8 3.72 9.2

S. Kumar et al. /Materials Research Bulletin 66 (2015) 7682 81

[10] F. Ahmed, S. Kumar, N. Arshi, M.S. Anwar, S.N. Heo, B.H. Koo, Acta Mater. 60(2012) 5190.

[11] M.S. Anwar, S. Kumar, F. Ahmed, N. Arshi, G.-S. Kil, D.-W. Park, J. Chang, B.H.Koo, Mater. Lett. 65 (2011) 3098.

[12] S. Kumar, S. Gautam, G.W. Kim, F. Ahmed, M.S. Anwar, K.H. Chae, H.K. Choi, H.Chung, B.H. Koo, Appl. Surf. Sci. 257 (2011) 10557.

[13] R. Prakash, S. Kumar, F. Ahmed, C.G. Lee, J. Il Song, Thin Solid Films 519 (2011)8243.

[14] F. Ahmed, S. Kumar, N. Arshi, M.S. Anwar, B.H. Koo, C.G. Lee, Thin Solid Films519 (2011) 8199.

[15] S.K. Sharma, P. Thakur, S. Kumar, D.K. Shukla, N.B. Brookes, C.G. Lee, K.R. Pirota,B.H. Koo, M. Knobel, Thin Solid Films 519 (2010) 410.

[16] F. Ahmed, S. Kumar, N. Arshi,M.S. Anwar, B.H. Koo, C.G. Lee,Microelectron. Eng.89 (2012) 129.

[17] S. Kumar, Y.J. Kim, B.H. Koo, C.G. Lee, J. Nanosci. Nanotechnol. 10 (2010) 7204.[18] F. Ahmed, S. Kumar, N. Arshi, M.S. Anwar, G.W. Kim, S.N. Heo, E.S. Byon, S.H.N.J.

Lee Lyu, B.H. Koo, J. Nanosci. Nanotechnol. 12 (n.d (2015) 5464.[19] M. Varshney, A. Sharma, K.H. Chae, S. Gautam, H.J. Shin, Appl. Sci. Lett. 1 (2015)

19.[20] M.H. Kane, K. Shalini, C.J. Summers, R. Varatharajan, J. Nause, C.R. Vestal, Z.J.

Zhang, I.T. Ferguson, J. Appl. Phys. 97 (2005) 23906.[21] S. Kumar, Y.J. Kim, B.H. Koo, S. Gautam, K.H. Chae, R. Kumar, C.G. Lee, Mater.

Lett. 63 (2009) 194.[22] G. Lawes, A. Risbud, A. Ramirez, R. Seshadri, Phys. Rev. B 71 (2005) 45201.[23] M. Bouloudenine, N. Viart, S. Colis, J. Kortus, A. Dinia, Appl. Phys. Lett. 87 (2005)

525501.[24] S. Kumar, Y.J. Kim, B.H. Koo, S.K. Sharma, J.M. Vargas,M. Knobel, S. Gautam, K.H.

Chae, D.K. Kim, Y.K. Kim, C.G. Lee, J. Appl. Phys. 105 (2009) .[25] F. Ahmed, S. Kkumar, N. Arshi, M.S. Anwar, B.H. Koo, C.G. Lee, Int. J. Nanosci. 10

(2011) 1025.[26] Y.P. Zhang, S.-S. Yan, Y.-H. Liu, M.-J. Ren, Y. Fang, Y.X. Chen, G.L. Liu, L.M. Mei, J.P.

Liu, J.H. Qiu, S.Y. Wang, L.Y. Chen, Appl. Phys. Lett. 89 (2006) 42501.

[27] J.H. Yang, Y. Cheng, Y. Liu, X. Ding, Y.X. Wang, Y.J. Zhang, H.L. Liu, Solid StateCommun. 149 (2009) 1164.

[28] L.Q. Liu, B. Xiang, X.Z. Zhang, Y. Zhang, D.P. Yu, Appl. Phys. Lett. 88 (2006)63104.

[29] M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93(2004) 177206.

[30] C. Dong, J. Appl. Crystallogr. 32 (1999) 838.[31] S. Kumar, S. Gautam, Y.J. Kim, B.H. Koo, K.H. Chae, C.G. Lee, J. Ceram. Soc. Japan

117 (2009) 616.[32] M. Naeem, S.K. Hasanain, M. Kobayashi, Y. Ishida, S. a Fujimori, Buzby, S.I. Shah,

Nanotechnology 17 (2006) 2675.[33] V. Gandhi, R. Ganesan, H. Hameed, A. Syedahamed,M. Thaiyan, J. Phys. Chem. C

118 (2014) 9715.[34] A.J. Reddy, M.K. Kokila, H. Nagabhushana, R.P.S. Chakradhar, C. Shivakumara, J.

L. Rao, B.M. Nagabhushana, J. Alloys Compd. 509 (2011) 5349.[35] S. Muthukumaran, R. Gopalakrishnan, Opt. Mater. (Amst.) 34 (2012) 1946.[36] A. Kaschner, U. Haboeck, M. Strassburg, M. Strassburg, G. Kaczmarczyk, A.

Hoffmann, C. Thomsen, A. Zeuner, H.R. Alves, D.M. Hofmann, B.K. Meyer, Appl.Phys. Lett. 80 (2002) 1909.

[37] A.P. Singh, R. Kumar, P. Thakur, N.B. Brookes, K.H. Chae, W.K. Choi, J. Phys.Condens. Matter 21 (2009) 185005.

[38] S. Krishnamurthy, C. McGuinness, L.S. Dorneles, M. Venkatesan, J.M.D. Coey, J.G. Lunney, C.H. Patterson, K.E. Smith, T. Learmonth, P.-A. Glans, T. Schmitt, J.-H.Guo, J. Appl. Phys. 99 (2006) 08M111.

[39] S. Gautam, P. Thakur, P. Bazylewski, R. Bauer, A.P. Singh, J.Y. Kim, M.Subramanian, R. Jayavel, K. Asokan, K.H. Chae, G.S. Chang, Mater. Chem. Phys.140 (2013) 130.

[40] H. Ohno, J. Magn. Magn. Mater. 200 (1999) 110.[41] J.E. Jaffe, T.C. Droubay, S.A. Chambers, J. Appl. Phys. 97 (2005) 73908.[42] H.Weng, X. Yang, J. Dong, H.Mizuseki,M. Kawasaki, Y. Kawazoe, Phys. Rev. B 69

(2004) 125219.[43] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173.

82 S. Kumar et al. /Materials Research Bulletin 66 (2015) 7682

Structural, magnetic and electronic structure properties of Co doped ZnO nanoparticles1 Introduction2 Experimental3 Results and discussion4 ConclusionsAcknowledgementsReferences