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I NSTITUTE OF P HYSICS P UBLISHING N ANOTECHNOLOGY Nanotechnology17(2006)1520–1526doi:10.1088/0957-4484/17/5/059Effects of temperature on the ferromagnetism of Mn-doped ZnO nanoparticles and Mn-related Raman vibrationC J Cong1,L Liao2,Q Y Liu1,J C Li2and K L Zhang1,3,41College of Chemistry and Molecular Sciences,Wuhan University,Wuhan430072,People’s Republic of China2Department of Physics,Wuhan University,Wuhan430072,People’s Republic of China3Centre of Nanoscience and Nanotechnology Research,Wuhan University,Wuhan430072,People’s Republic of ChinaE-mail:klzhang@Received15September2005,infinal form28December2005Published16February2006Online at /Nano/17/1520AbstractMn-doped ZnO nanoparticles were synthesized by a rheologicalphase-reaction–precursor method and the thermal decomposition of theoxalate precursors was studied by thermogravimetry and differential thermalanalysis in air.The Mn-doped ZnO obtained at lower temperature crystallizesin a wurtzite structure and a new phase appears at higher temperature.Thelattice parameters of Zn1−x Mn x O(0.09 x 0.1)increase gradually withincreasing temperature up to650◦C and then decrease.X-ray photoelectronspectroscopy indicates that there are different Mn valence bonds inMn-doped ZnO nanoparticles.Furthermore,two additional Raman peakswere observed.One peak is considered to have an origin related to theincorporation of Mn ions into the Zn site of the ZnO lattice;the other may beattributed to the ZnMnO3phase.Magnetization measurements underfieldcooling conditions reveal that the Zn1−x Mn x O nanoparticles exhibitferromagnetic behaviour,and the Curie temperatures of some samples areabove room temperature.The difference of the ferromagnetic properties ofthe Mn-doped ZnO nanoparticles is primarily attributed to the Mn2+content.(Somefigures in this article are in colour only in the electronic version)1.IntroductionDiluted magnetic semiconductors(DMSs)obtained by doping a small amount of magnetic impurities in semiconductors have recently attracted broad interest for their possible applications in generating and manipulating spin-polarized currents[1]. While a DMS based on III–V semiconductors shows ferromagnetism only at very low temperatures,an oxide-based DMS may be ferromagnetic at higher temperatures.Stable ferromagnetic configurations arising from carrier-mediated 4Address for correspondence:College of Chemistry and Molecular Sciences, Wuhan University,Wuhan430072,People’s Republic of China.exchange interactions have been predicted for several transition-metal-doped ZnO DMSs[2,3].Among them, Mn-doped ZnO would show ferromagnetic behaviour with a Curie temperature T c above room temperature[2],and may be applied to short-wave magneto-optical devices[4]. Although room-temperature ferromagnetism(FM)has been observed in several ZnO-based materials,many controversial results have also been reported.For Mn-doped ZnO,several groups have obtained various properties.Paramagnetism was observed in Zn0.93Mn0.07Ofilms prepared by magnetron sputtering[5]and in Mn-doped bulk ZnO prepared by solid-state reaction[6].Fukumaura et al found a spin-glass behaviour[7];Tiwari et al observed paramagnetism[8];0957-4484/06/051520+07$30.00©2006IOP Publishing Ltd Printed in the UK1520Effects of temperature on the ferromagnetism of Mn-doped ZnO nanoparticles and Mn-related Raman vibrationJung et al observed FM with T c in the range30–45K[9]. Han et al reported a ferromagnetic phase transition of Zn0.95Mn0.05O at1170K,which was attributed to the(Mn, Zn)Mn2O4spinel impurity[10].Shmarma et al found that low-temperature synthesis could favour FM in Mn-doped ZnO,and room-temperature ferromagnetism(T c> 420◦C)was obtained,but the samples prepared at highertemperatures(>700◦C)did not exhibit ferromagnetism[11].Luo et al found an antiferromagnetic ordering in Mn-doped ZnO nanoparticles by a combustion method[12].Chen et al prepared Mn–ZnO bulk by a solid-state reaction method and observed ferromagnetic behaviour at room temperature after it was calcined in Ar gas;however,the ferromagnetic behaviour disappears in samples calcined in air[13].Recently,we synthesized Mn-doped ZnO nanoparticles by a rheological phase-reaction–precursor method and observed ferromagnetic behaviour at room temperature[14].The different magnetic properties of Mn-doped ZnO are most likely due to the different preparation methods,since the properties of ZnO are sensitive to the preparation conditions.In this paper we report the effects of calcining temperature on the ferromagnetism of Mn-doped ZnO nanoparticles and discuss the Mn-related Raman spectra.We found that the calcining temperature affects the ferromagnetism of Mn-doped ZnO nanoparticles and that the optimal calcining temperature was in the range550–650◦C.In addition, the Mn-related Raman spectra and the dependence of the ferromagnetic properties on the Mn valence bond in Zn1−x Mn x O nanoparticles were studied by x-ray photoelectron spectroscopy(XPS).2.Experimental detailsNanoparticles of Zn1−x Mn x O(x=0.1)were synthesized by a rheological phase-reaction–precursor method[14–19].Stoi-chiometric quantities of Zn(CH3COO)2·2H2O,Mn(CH3COO)2·4H2O and H2C2O4·2H2O were mixed well,and a proper quan-tity of water was added into the mixture to prepare the rheo-logical bodies.A precursor was prepared from the rheological bodies in a closed container at70–90◦C for8h and a series of nanoparticles of Zn1−x Mn x O was obtained by thermal decom-position of the precursors at450,550,650,750and850◦C, respectively,for2h in air.The processes of their thermal decomposition were stud-ied by differential thermal analysis(DTA)and thermogravime-try(TG)with a NETZSCH STA449C instrument in air at a heating rate of15◦C min−1in alumina sample holders with alumina as a reference sample.The powder samples were characterized by powder x-ray diffraction(XRD)using a SHI-MADZU XRD-6000diffractometer(Cu Kαradiation,λ= 1.54056˚A).The mean crystallite sizes were estimated us-ing the Scherrer equation,and Rietveld refinement analysis was then performed on the XRD data to obtain the lattice con-stants.The EPR spectra were recorded using a Bruker EMX-10/12spectrometer at X-band frequency.The morphology and microstructure were characterized by a JEOL JEM2010FEF high-resolution transmission electron microscope(HRTEM) with a point resolution of0.19nm.Samples for the TEM were prepared by making a clear dispersion of the nanoparticles in ethanol and putting a drop of the solution on acarbon-coatedFigure1.The TG/DTG/DTA curves of the precursors. copper grid;the solution was allowed to evaporate,leaving the nanocrystals behind on the carbon grid.The Raman-scattering experiments were carried out using a laser confocal Raman mi-crospectroscope RM1000at room temperature.The514.5nm line of an Ar+laser was used for excitation.The Mn content was determined using x-rayfluorescence(XRF)spectrometer S4PIONEER-4KW.The micro-composition and the Mn va-lence bond of samples were determined by x-ray photoelectron spectroscopy(XPS)using Mg Kαradiation(1256.5eV)after 3keV Ar+ion etching for10min.All binding energies are referenced to the C1s hydrocarbon peak at285eV.Magnetic behaviours were measured using a superconducting quantum interference device MPMS-7(SQUID)magnetometer in the temperature range10–300K.3.Results and discussionTG-DTG/DTA curves of the oxalate precursor are shown in figure1.The DTA peaks closely corresponding to the weight changes are observed on the TG curves.The curves show that the thermal decomposition of the oxalate precursor below 420◦C occurs in two well-defined steps.Thefirst step is from 147.0to172.1◦C,which is characterized by an endothermic peak at169.7◦C in accordance with calculated weight loss of18.19%(theoretical weight loss19.11%),attributed to the dehydration of the oxalate precursor and formation of anhydrous oxalate.The anhydrous oxalate is stable up to 368◦C,and then it decomposes in the second step.This step shows an exothermic process with DTA peak at388.9◦C, indicating a weight loss of38.57%(theoretical weight loss 38.22%)due to the decomposition of the anhydrous oxalate precursor and the formation of Zn1−x Mn x O.Figures2(1)(a)–(e)show the powder x-ray diffraction patterns of Zn0.9Mn0.1O obtained at T=450,550,650,750 and850◦C,respectively.Figures2(1)(a)and(b)indicate that the as-synthesized samples can be indexed to a hexagonal wurtzite structure(space group P63mc,JCPDF#36-1451)as ZnO.No trace of manganese metal,oxides,or any binary zinc manganese phase is clearly observed infigures2(1)(a)and(b). However,a second phase may be amorphous or the peaks too weak,although a secondary phase was not detectable by XRD in the powders produced at lower temperatures(450,550◦C). Besides the diffraction peaks from the ZnO wurtzite structure, weak additional peaks from ZnMnO3have been observed (figures2(1)(c)–(e)),and their intensity increases with the1521C J Cong et alTable 1.The lattice constants calculated from the XRD data of Zn 0.9Mn 0.1O nanoparticles obtained at different temperatures.Samplesa (˚A)c (˚A)V (˚A3)ZnO [14] 3.2404(0.0010) 5.2066(0.0009)47.3312(0.0021)T =450◦C 3.2538(0.0009) 5.2157(0.0024)47.8225(0.0026)T =550◦C 3.2593(0.0001) 5.2206(0.0003)48.0292(0.0039)Zn 0.9Mn 0.1OT =650◦C 3.2606(0.0005) 5.2189(0.0014)48.0498(0.0025)T =750◦C 3.2509(0.0002) 5.2107(0.0004)47.6891(0.0041)T =850◦C3.2490(0.0001)5.2054(0.0003)47.5858(0.0038)E P R S i g n a l (a .u .)Figure 2.Powder x-ray diffraction patterns of Zn 0.9Mn 0.1O (1)and room-temperature EPR spectra of Zn 0.9Mn 0.1O obtained at T =450,550,650,750and 850◦C,respectively (2).increasing of the calcining temperature (figures 2(1)(c)–(e)).A slight shift of XRD peaks to higher angles for the samples (b)–(e)was also clearly observed.The particle sizes for all five products are about 11,15,20,38,47nm,corresponding to the calcining temperature T =450,550,650,750and 850◦C,respectively;these were calculated from x-ray line broadening using the Scherrer formula D hkl =κλ/βhkl cos θ,where κis a coefficient (equal to 0.89here),βis the half-maximum line width,and λis the wavelength of the x-rays.Table 1shows that the lattice parameters of Zn 1−x Mn x O obtained at T =450,550,650,750and 850◦C were slightly larger than that of pureZnO,because the ionic radius of Mn 2+(0.66˚A)is larger than that of Zn 2+(0.60˚A).The expansion of the lattice constants of Zn 1−x Mn x O and the slight shift of XRD peaks indicates that manganese ions were really incorporated into the ZnO structure.However,comparing the lattices of Zn 0.9Mn 0.1O obtained at T =450,550,650,750and 850◦C,it can be found that the lattice parameters of Zn 1−x Mn x O increase gradually with increasing temperature up to 650◦C,and then decrease.This may be attributed to the incorporation of manganese ions into the ZnO structure and the formation of ZnMnO 3phase when the temperature is over 650◦C.Figure 2(2)shows the typical EPR spectra of the as-prepared Mn-doped ZnO nanoparticles obtained at different temperatures.The spectra show a broad EPR signal without clear six-line splitting in figures 2(2)(a)–(c),which indicates the incorporation of Mn 2+ions into Zn sites in the interior of Mn-doped ZnO nanostructure [20].However,a weak six-line spectrum can be seen for both figures 2(2)(d)and (e),which indicates that some of the Mn is at the surface of the ZnO [21].These results also indicate that Mn clusters were not clearly observed [22].The TEM micrograph of Zn 1−x Mn x O nanoparticles obtained at 450and 650◦C,shown in figures 3(a)and (b),indicates that most of the individual particles are of size 10–13and 20–25nm,respectively,which is consistent with the results calculated from the XRD data.The insets show the corresponding high-resolution image of a single particle.The selected area electron diffraction (SAED)patterns (figures 3(b)and (d)),taken from the same nanoparticles shown in figures 3(a)and (c),respectively,can be indexed to the wurtzite ZnO structure,which is consistent with the above XRD results.Also,in the selected area electron diffraction (SAED)pattern (figure 3(d)),besides the diffraction rings from the ZnO wurtzite structure,no additional ring was clearly observed.These results are also consistent with the above XRD result.In addition,from the SAED patterns (figures 3(b)and (d)),the nanocrystals are found to have preferential instead of random orientations,which is consistent with the report in [23].The Mn content after calcining was examined by XRF.Mn contents were 9.4,9.5,9.7,9.4and 9.3mol%,respectively,corresponding to the calculated concentration in preparations for 10.0mol%.The valence bond of Mn was investigated using XPS.Figures 4(a)–(e)show XPS Mn 2p spectra of the Zn 1−x Mn x O nanoparticles obtained at different temperatures.Overlapping bands were deconvoluted into separated peaks by Gaussian fitting using XPS PEAK41software.From the fitting,it has been found that there are three peaks at 641.0,641.60and 642.65eV attributed to Mn 2+,Mn 3+and Mn 4+,1522Effects of temperature on the ferromagnetism of Mn-doped ZnO nanoparticles and Mn-related Raman vibrationZnO(112)ZnO(110)ZnO(002)ZnO(100)ZnO(102)ZnO(103)ZnO(112)ZnO(110)ZnO(002)ZnO(100)ZnO(002)ZnO(103)10 nm10 nm(a)(b)(c)(d)Figure 3.TEM micrograph of Zn 0.9Mn 0.1O nanoparticles obtained at (a)450◦C,(c)650◦C.The insets show the corresponding high-resolution image of a single particle.SAED pattern of Zn 0.9Mn 0.1O nanoparticles obtained (b)450◦C,(d)650◦C,respectively.Table 2.The fitting results of Mn 2p XPS spectra.Area of peakQuantity of Mn 2+in all 641.0(eV)641.6(eV)642.65(eV)644.6(eV)Mn valence SampleMn 2+Mn 3+Mn 4+bonds/%T =450◦C 142.3184.9193.637.825.4T =550◦C 198.3451.44470.4528.4427.4Zn 0.9Mn 0.1OT =650◦C 373.7286.67579.0231.2929.4T =750◦C 274.51336.35483.8725.08T=850◦C138.05106.48299.7325.36respectively [24].However,a high-energy feature between 644and 647eV (644.6eV)gradually weakened with the increasing temperature,therefore we only added the peak about 644.6eV in the figures 4(a)–(c).The extra high-energy feature at about 644.6eV in the Mn 2p XPS spectra can be attributed to ligand to metal charge transfer shake-up transitions [25,26].In addition,the peak areas of the different valence bonds of Mn and the percentages of Mn 2+in each sample were calculated,and the results are summarized in table 2.It has been found that the largest Mn 2+atomic concentration appeared in Zn 1−x Mn x O nanoparticles when the samples werecalcined at 650◦C,which is consistent with the XRD results of the lattice of Zn 1−x Mn x O obtained at 650◦C.Raman scattering experiments were performed at room temperature.The output power and the power on the sample are 25and 4.8mW,respectively.The Raman spectra of both ZnO and Zn 1−x Mn x O nanoparticles in the range 200–1400cm −1are shown in figures 5(1)and (2).The assignments of the Raman peaks of ZnO and Zn 1−x Mn x O nanoparticles obtained at different temperature are summarized in table 3.The Raman spectrum of the fine ZnO powder is shown in figure 5(1),where the peaks at 332,383and 437cm −1are1523C J Cong etalI n t e n s i t y (a .u )Figure 4.The XPS Mn 2p spectra for Zn 0.9Mn 0.1O nanoparticles:(a)–(e)correspond to Zn 0.9Mn 0.1O samples obtained at T =450,550,650,750and 850◦C,respectively;the fitting curves numbered 1–4correspond to Mn 2+,Mn 3+,Mn 4+,and the shake-up transitions,respectively.clearly observed in the low wavenumber region,and there is a very weak peak at 538cm −1.On the other hand,a broad peak (1050–1185cm −1)was also found in the large wavenumber pared with the vibration modes of ZnOpowder,Figure 5.Room-temperature micro-Raman spectra of (1)ZnO and (2)(a)–(e)Zn 0.9Mn 0.1O nanoparticles obtained at T =450,550,650,750and 850◦C,respectively.two additional vibration modes can be observed at about 524–533and 650–665cm −1in the Raman spectra of the Zn 1−x Mn x O nanoparticles obtained at different temperatures;the intensities and positions of the two additional vibration peaks change with temperature.The detailed changes can be seen from table 3:there are peaks at 524,527,528and 533cm −1in the range 524–533cm −1,and in the range 650–665cm −1,there are peaks at 648,659,665,652and 657cm −1at different pared with these ‘classical’Raman modes (321–324and 430–436cm −1)of Zn 0.9Mn 0.1O nanoparticles,the Raman modes of ZnO show a red shift.Furthermore,it should be noticed that the intensities of the peak at 524–527cm −1became weak and at 650–665cm −1they increased with increasing temperature.Yang et al discussed the origin of the vibration mode at 522cm −1;they considered that it could be associated with Mn doping.Because the ionic radius of Mn 2+(0.66˚A)is larger than that of Zn 2+(0.60˚A),lattice defects are introduced or intrinsic host-lattice defects are activated when Mn 2+ions occupy the Zn sites [20].Therefore additional vibration modes could appear after doping.In our experiments,the additional vibration peak (about 524cm −1)disappeared gradually and merged into a broad peak (538–540cm −1);at the same time,the1524Effects of temperature on the ferromagnetism of Mn-doped ZnO nanoparticles and Mn-related RamanvibrationFigure 6.Temperature variation of magnetization of Zn 0.9Mn 0.1O nanoparticles obtained at different temperatures under a field of 1000Oe.peak became weaker and weaker with increasing temperature,which may be attributed to the decrease of Mn 2+occupation at the Zn sites and the formation of ZnMnO 3impurities when the calcining temperature increased.This is consistent with the XRD results that the lattice of Zn 1−x Mn x O obtained at 650◦C is the largest.The second set of additional vibration peaks (650–665cm −1)becomes stronger and stronger with increasing temperature.Wang et al reported the vibration mode at 663cm −1in Mn-doped ZnO nanoparticles and assigned it to Zn 2MnO 4partly [27].Cheng et al reported the vibration mode at 667cm −1in ZnO:Ce nanostructure and assigned it to an additional mode,which is connected with the metastable Ce-rich solid solution layer [28].In our experiments,the vibration mode at 650–665cm −1is observed in all Zn 1−x Mn x O samples and becomes stronger and stronger with increasing calcining temperature.It may be attributed to the Mn-based compounds,such as ZnMnO 3precipitates.These results also are consistent with the XRD results,in which a new phase is too weak to be detected by XRD when the nanoparticles were calcined at lower temperature.The temperature dependence of magnetization of the Zn 1−x Mn x O nanoparticles obtained at different temperatures under the field-cooled (FC)mode with a magnetic field of 1000Oe is shown in figure 6.Ferromagnetic ordering of the Zn 0.9Mn 0.1O nanoparticles obtained at different temperatures was clearly observed in the temperature range from 10to 300K.The Curie temperatures of samples obtained at 450and 850◦C are 250and 230K,respectively,and the Curie temperatures of other samples are above room paring the ferromagnetic properties of all samples,it can be found that the magnetic susceptibility of Zn 1−x Mn x O increases gradually with increasing temperature before 650◦C and then decreases.For these materials to have application in spintronics devices,it is essential that they are homogeneous,and that the magnetic properties do not arise from phase aggregation.If the ferromagnetic properties arise from a segregated phase within the semiconducting matrix,this material would not have the desired properties for spintronics devices such as spin injection,etc.Therefore,considering all kinds of factors,we think that the optimal calciningtemperature for Zn 0.9Mn 0.1O nanoparticles is in the range 550–650◦C.In order to elucidate the origin of the difference displayed in the magnetic properties,we turn to the structure analysis.The XRD results indicate that the intensities of additional peaks from ZnMnO 3increase (figures 2(d)–(e))with increasing calcining temperature and the lattice of Zn 1−x Mn x O obtained at 650◦C is the largest.At the same time,the XPS results show that the Mn 2+concentration reaches a maximum when the Zn 1−x Mn x O nanoparticles were calcined at 650◦C.Regarding the secondary phase,MnO,MnO 2and Mn 3O 4are the possible phases.Of these manganese oxides,MnO and MnO 2may be excluded because they are antiferromagnetic.The Curie temperature of ferromagnetic Mn 3O 4is 43K,far below that of all the samples [29].ZnMnO 3cannot be responsible for the ferromagnetic properties in our experiment,either [30].According to the results of XRD,XPS,Raman spectra and the magnetic properties,we may conclude that the Mn 2+plays the most important role in the ferromagnetic properties of Mn-doped ZnO [14].According to RKKY theory [31,32],the magnetism is due to the exchange interaction between local spin-polarized electrons (such as the electrons of Mn 2+ions)and conductive electrons.This interaction leads to the spin polarization of conductive electrons.Subsequently,the spin-polarized conductive electrons perform an exchange interaction with local spin-polarized electrons of other Mn 2+ions.Thus after the long-range exchange interaction,almost all Mn 2+ions exhibit the same spin direction.The conductive electrons are regarded as a medium to contact all Mn 2+ions.As a result,the material exhibits ferromagnetism.However,direct Mn–Mn interactions have been used to explain antiferromagnetism by some researchers [33].In our experiment,the XRD,EPR,XPS and Raman results imply that most Mn ions locate at Zn sites,and the occupation rate of Mn 2+ions at the Zn sites increases with increasing calcining temperature up to 650◦C,and then decrease;at the same time,the amount of ZnMnO 3impurity increases with increasing temperature.This may explain the difference in the ferromagnetic properties in all samples,although MnO and MnO 2may be induced to the difference of the magnetic properties to some extent because there are antiferromagnetic.4.ConclusionIn conclusion,nanoparticles of Zn 1−x Mn x O (0.09 x 0.1)were synthesized by a rheological phase-reaction–precursor method at different temperatures,and the microstructure,morphologic and magnetic properties were investigated.Ferromagnetic ordering for the Zn 0.9Mn 0.1O nanoparticles obtained at different temperatures was clearly observed in the temperature range from 10to 300K.The Curie temperatures of samples calcined at 450and 850◦C are 250and 230K,respectively,and the Curie temperature of other samples calcined at 550,650,and 750◦C is above room temperature.The optimal calcining temperature for Zn 0.9Mn 0.1O nanoparticles was in the range 550–650◦C.Preparations conditions affect the magnetic properties of Mn-doped ZnO.The difference in the ferromagnetic properties of the Mn-doped ZnO nanoparticles may be primarily attributed to the Mn 2+-content.1525C J Cong et alTable3.The observed Raman peak positions of the Zn0.9Mn0.1O nanoparticles and their assignments.Vibration frequency(cm−1)Zn0.9Mn0.1OZnO450◦C550◦C650◦C750◦C850◦C Symmetry Process3323223243213213242-LA(M)Second order383360377367357363A TO1First order437436435433430433E high2First order524527528533Dopant vibration538541550554556A LO1First order648659665652657Dopant vibration 110110851088110111111102A1,E2AcknowledgmentsThis work was supported by the National Nature Science Foundation of China(No.20071026).The authors thank Dr Zuotao Zeng(in America),Dr Lei Zhang and Dr Jianhe Hong(Wuhan University)for helpful discussions on the ferromagnetic properties,improvement of the language and discussions on how to plot thefigures.References[1]Wolf S A,Awschalom D D and Buhrman R A2001Science2941488–95[2]Dietl T,Ohno H and Matsukura F2000Science2871019–22[3]Sato K and 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