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1MagneticPropertiesofNanocrystalline-SiCGopaMishra1,SankarMohapatra1,SasmitaPrusty1,ManojKumarSharma2,RatnamalaChatterjee2,SKSingh1andDKMishra1,*1AdvancedMaterialsTechnologyDepartment,InstituteofMineralsandMaterialsTechnology(CSIR),Bhubaneswar751013,Orissa,India2DepartmentofPhysics,IndianInstitueofTechnologyDelhi,NewDelhi110016,IndiaE.
mail:dilipiuac@gmail.
comAbstract:Four-hourball-milled-SiCproductsynthesizedbythethermalplasmatechniqueshowsroomtemperatureferromagnetism.
Thesemi-λsignatureofthefield-cooledmagnetization(FCM)andzerofield-cooledmagnetization(ZFCM)curvessuggestthepossiblesignatureofaglassyferromagnetismstateinthesample.
Theprominentfallinthemagnetizationvalueataround50KobservedinZFCMcurverevealstheexistenceofasharptransitionfromaferromagneticstatetoaglassyferromagneticstate.
ThepresenceofglassyferromagnetismatlowtemperatureisconfirmedfromtheM~Hcurverecordedat5K.
Keywords:Carbides;ThermalPlasma;Semiconductors;GlassyferromagnetismIntroduction:WidebandgapII-VIandIII-Vsemiconductorshaveattractedalotofattentionoftheresearcherstoproduceasystemlikedilutemagneticsemiconductor(DMS)fortheapplicationinthefieldofoptoelectronics,spintronicsandmagnetoelectronics[1-5].
ButtheintriguingphenomenonofferromagnetisminDMShasremainedunsolvedsofar.
Recentreportonferromagnetismincarbonandcarbonbasedcompoundshasopeneda2newavenueforthesearchofnewdilutedmagneticsemiconductors(DMSs)[6-12].
Developingamagneticsystemwithcarbonhasitsownadvantagesasitislightweight,stable,simpletoprocess,andlessexpensivetoproduce.
Investigationsperformedonvariousformsofcarbonandtheircompositeswithsemiconductorspointtowardsthefactthatitispossibletoproduceferromagneticcarbonsystems.
SomorefocushasbeenindicatedtowardstheIV-IVsemiconductorslikeSiliconcarbidematerials[6,7,13-17].
Recentlysiliconcarbideisunderinvestigationasanenablingmaterialforavarietyofnewsemiconductordevicesintheareaofspintronics[6,7,13,14].
Theseincludehigh-power,highvoltageswitchingapplications,hightemperatureelectronicsandhighpowermicrowaveapplicationsinthe1-10GHzregime.
Itisalsousedassubstratefordepositingseveralsemiconductormaterialslikegalliumnitride.
SiCistheonlycompoundsemiconductorwhichcanbethermallyoxidizedtoformahighqualitynativeoxide(SiO2).
ThismakesitpossibletofabricateMOSFET,insulatedgatebipolartransistorsandMOS-controlledthyristorinSiC[18].
Siliconcarbideexhibitsahighthermalconductivity,highresistancetowardsoxidation,highmechanicalstrength,lowspecificweight,andit'schemicallyinertnessmakeitacatalystsupportmaterial[19].
Duetothewidebandgapenergy,theleakagecurrentinSiCismanyorderslowerthaninsiliconandtheintrinsictemperatureiswellover800°C,whichmakesitanelectronicefficientmaterialforsemiconductorapplications[18].
Eventhoughitofferssubstantialadvantagesoversilicon,SiCisstillimmatureassemiconductormaterialsbecauseitexistsinmanypolytypicformsandalsothepresenceofminutemetallicimpuritieschangeitscrystalstructureandcreatelatticedefects.
Theselatticedefectsand3presenceofmagneticimpuritiesmaybeoneofthereasonsforobservingferromagnetisminthismaterial.
Inthismanuscript,-SiCproducthasbeensynthesizedbythermalplasmatechniqueandfurtherballmilledforthereductionofparticlesizes.
Roomtemperatureferromagnetismin-SiCandtheglassyferromagnetismatlowtemperaturehasbeendiscussed.
Experimental:Silica-richricehuskwasusedastherawmaterialforsynthesisofSiCpowder.
Therawmaterialwasplasmatreatedinanindigenouslydevelopedpottypeopenplasmareactor.
Thedetailoftheplasmareactorisdescribedelsewhere[20].
Therawmaterialwastakeninthegraphitecrucibleandthecruciblewascoveredwithagraphiteplatehavingaholeatthecentreinordertopreventthematerialtobeblownoutofthecrucibleduetoplasmapressure.
Therateofflowoftheplasmagengas(argon)wasregulatedto1.
5lit.
/min.
Thearcwasstruckbymovingtheupperelectrodeupordown.
Afterplasmatreatmentforaperiodof20minutestime,powerwasswitchedoffbuttheargongaswasallowedtopassforanotherhalfanhourinordertopreventoxidationoftheproduct.
TheplasmasynthesizedproductobtainedfromplasmareactorisinlooselyagglomeratedformandisamixtureofSiCandminutepercentageofcarbonandsilica.
Thisproductwasgroundinanagatemortartobreaktheagglomerationandwasthenheatedinafurnaceat700°Cfor2hoursforcompleteremovalofcarbon.
TheparticlesizeofthecarbonfreeSiCandSiO2mixtureisaround14.
69micron.
Forfurtherreductionofparticlesize,theproductwasgroundinaRetschPM-100planetaryballmillwith3mmstainlesssteelballs.
Thegrindingwascarriedoutina500mlstainlesssteel4jarinethylalcoholmediumatafixedrpmof350forfourhours.
Thenthisgroundsamplewasthoroughlywashedwith1:1HCl,1:2HNO3and40%Hfforthecompleteremovalofsilicaandothermetalimpuritiespresentinthesample.
Particlesizeanalyzer(ModelNanotracU2058I)wasusedtodeterminetheparticledistributionandaverageparticlesizeofthe-SiCpowder.
ATransmissionelectronmicroscope(TEM)(ModelJEOL,JEM–2010UHR)wasusedtodeterminetheshapeandsizeofthe-SiCnanocrystals.
EnergydispersiveX-rayandX-rayfluorescencespectrawererecordedtodeterminetheimpuritiesotherthanSi,CandOpresentinthesample.
PhaseandstructuralanalysisofthesampleswerecarriedoutusingX-raydiffractometer(XRD)(Model:X'PertPROPANalytical))usingMoKsource.
Fouriertransforminfraredspectroscopic(Model:PerkinElmerspectrumGx)andmicro-Ramanstudieswerecarriedouttoobtainthebondingandstructuralinformation.
Diffusedreflectivespectroscopy(DRS)studyhasbeendonetoestimatethebandgapof-SiCbyUV-Visiblespectrophotometer.
Fielddependentmagnetizationat300Kand5K,temperaturedependentzero-fieldcooled(ZFC)andfieldcooled(FC)magnetizationmeasurementswerecarriedoutusingQuantumDesignSuperconductingInterferometerDevice(SQUID)magnetometer.
ResultsandDiscussions:Fig.
1showsthedistributionofparticlesizeoffourhoursballmilled-SiCproduct.
Itisobservedfromthefigurethat78%ofparticleshavetheparticlesizewithin250nmwhereas22%ofparticleshavethesizewithin300nmto450nm.
TheGaussiancurvedistributionfittedtotheparticlesizedistributioncurveshowstheaverageparticlesizetobearound225nm.
Theresultobtainedfromparticlesizedistributioniswell5supportedbytheresultobtainedfromTEManalysis.
Transmissionelectronmicroscopyimagesof-SiCparticlesareshowninFig.
2.
Theparticlesarenonuniforminshapeandsize.
Theaveragesizeoftheparticlesisintheorderof250nm.
Theenergydispersivex-rayanalysispictureshowninFig.
3predictsthatthenanoparticlesarecomposedofSiandC.
NotransitionalmetalimpuritypeakisfoundotherthanCuwhichisnothingbutthesignatureofcarboncoatedCugridusedforTEManalysis.
ThesmalloxygenpeakobservedinthespectrumisduetothecontaminationofhydroxylgroupduringthesamplepreparationforTEManalysis.
ButXRFanalysispredictsthepresenceofmagneticimpurities(Fe,CoandNi)contentupto140ppmproductwithnon-magneticimpuritieslikeAl,Baetc.
ThespecificsurfaceareaoftheballmilledpowdersmeasuredbyBednortz-Edward-Tellertechniqueis12m2g-1.
Thex-raydiffractionpatternisshowninFig.
4.
Themajorreflectionpeaksof-SiCat(111),(200),(220),(311),(222)and(400)arefoundintheXRDpatternandmatchedwiththeJCPDSdata(#02-1050)havingspacegroup43Fm.
Apartfrom-SiC,smallshoulderpeakof-SiCisfoundatthe2valueof17.
31degree.
WithinthelimitationofXRD,thesmaller%ofmetallicimpuritiespresentinthesamplecannotbedetected.
ThecrystallitesizecalculatedusingScherer'sformulat=0.
89λ/1/2Cosisaround97nm.
Latticeparameterof-SiCproduct,a=4.
312calculatedusingtheformulad=a/(h2+k2+l2)1/2whichiswellmatchedwiththetheoreticalandexperimentalvalueofa=4.
349[21].
Fig.
5showstheFTIRspectrumoffourhourballmilled-SiCproduct.
Thesharpreflectancepeakat800.
73cm-1correspondstothevibrationalmodeofSiC.
Apartfromthis,peaksataround400to600cm-1areattributedtotheSi-O-Sistretchingmodesof6vibration.
1072.
4cm-1peakisattributedtotheSi-Omodeand2350.
72cm-1peakcorrespondtotheC-Cmodeofvibration.
1488.
7cm-1peakiscorrespondingtothehydroxylgroup(OH)whichisduetocontaminationduringhandlingofthesampleforexperimentinopenatmosphere.
TheXRDandFTIRspectraclearlypredictthatthefourhourballmilledproductisin-SiCform.
Fig.
6showstheRamanspectrumoffourhourballmilled-SiCproduct.
ItisreportedthatSiCgivesRamanscatteringfromatransverseoptic(TO)phononatapproximately790cm-1andalongitudinalopticphonon(LO)at973cm-1[3].
Inourspectrum,twoprominentpeaksareobservedat783and982cm-1representedastransverseoptics(TO)andlongitudinaloptics(LO)peaksrespectively.
TheTOandLOpeakpositionsoftheSiCcrystallitesindicatethatthepredominantSiCpolytypeis-SiC[6,22].
TheDRSstudyhasbeenundertakentoevaluatethebandgapof-SiCandisshowninFig.
7.
Itisverywellknownthat-SiCisanindirectbandgapsemiconductor.
ThebandgapcalculatedusingtheTaucsplot[23](i.
e.
hvs(h)1/2)isaround2.
17eVwhichiswellmatchedtothebandgapof2.
19eVreportedinliterature[24,25].
Presenceofminutepercentageofmetalimpuritiesdoesnotplayanyroleinvaryingbandgapof-SiC.
TheM~HcurveatroomtemperatureshowninFig.
8for-SiCproductisferromagneticinnature.
Saturationmagnetizationof0.
004emu/ghasbeenobservedwitharemnantmagnetization1.
1x10-3emu/gandcoercivityof106Oe.
ThehysteresisloopisshownintheinsetofFig.
8.
TheoriginofFMorderinsuchIV-IVsemiconductorslike-SiCislessstudied.
Alsoitisverydifficulttogetthe-SiCinitscompletepureform.
Sotheexactmechanismforexhibitingferromagnetisminthesematerialsisnotclear.
The7saturationmagnetizationarisingfrommagneticimpuritiesareestimatedtobeintheorderof0.
00254emu/g.
Ourexperimentallyobservedvalueis1.
5timesgreaterthanthevalueofsaturationmagneticmomentarisingfrommagneticimpurities.
Hence,itconfirmsthatthemagnetismcominginthismaterialisnotfromthemagneticimpurities.
Thepossiblereasonfortheobservationofferromagnetismmaybetheformationoflatticedefectsinducedduringthesynthesisprocesses.
Duetopresenceofmagneticandnonmagneticimpuritiesandthecontaminationofoxygen,sp3configurationofSiCisconvertedtoamixtureofsp3/sp2hybridizationtoinduceferromagneticorderinginthismaterial[6,7,26,27].
Infact,thepresenceofimpuritiesinSiCmayintroducelargescaledefectsintothelattice,suchasvacanciesandinterstitials.
ThesurfacecontaminationofoxygeninSiCisalsoascribedtoafactorofcreatinglatticedefects.
SuchdefectsatlowtemperaturebecomesisolatedfromeachotherandcreateshortrangeFMordering,thusreducingthemagnetizationvalue.
Furtherinvestigationisrequiredtoestablishtheoriginofferromagnetism.
TheM~Hcurverecordedatlowtemperature(i.
e.
5K)isshowninFig.
9.
Aclearhysteresisloopisobservedwitharemnantmagnetizationof3.
9x10-3emu/gandcoercivityof290Oerespectively.
Theremnantmagnetizationvalueis3.
5timesgreaterthanthevalueobtainedat300K.
Themagnetizationvalueincreasessteeplywiththeincreaseofmagneticfieldupto10000Oe.
Thereisnoobservationofsaturationmagnetizationfromthecurvewithinthelimitedappliedfieldof10000Oe.
ThetemperaturedependentZFCmagnetization(ZFCM)andFCmagnetization(FCM)measurementswerecarriedoutatamagneticfieldof100Oewithinatemperaturerangeof300K-5KandisshowninFig.
10.
AsshowninthispicturetheZFCandFC8startsbranchingfromeachotheratorabove300K.
ThedifferencebetweenFCMandZFCMincreasessignificantlywiththedecreaseoftemperatureandexhibitsapromptcuspespeciallyinZFCMcurveat50K.
Itisverymuchinterestingtonotethattheobservedthermo-magneticirreversibilityandsemi-λnatureoftheFCMandZFCMcurvessuggestthepossibilityofspinglasssignatureinthesample[6].
Atthesametimeferromagnetismisobservedatlowtemperaturei.
e.
at5K(showninFig.
9).
Thecombiningfeatureofspinglassandferromagneticbehaviorexhibitsaglassyferromagneticbehavior.
MostoftheglassyFMbehaviorisexhibitedduetothecompetitionbetweenlongrangeferromagneticorderingandshortrangeantiferromagneticinteractionandthusreducingthemagnetizationvalueatlowtemperature.
Conclusion:Inconclusion,itisconfirmedfromtheXRDandFTIRstudiesthattheproductobtainedfromplasmareactorisin-SiCformhavinglatticeparameterof4.
31,whichisincloseagreementwiththelatticeparameterofbulk-SiC.
Roomtemperatureferromagnetismandglassyferromagneticbehavioratlowtemperaturehasbeenpredicted.
Observedthermo-magneticirreversibilityandsemiλ-shapenatureoftheFCMandZFCMcurvesatvaryingtemperaturesuggestthepossibilityofglassyferromagneticstateinthesample.
Acknowledgement:AuthorsaregratefultoDirector,IMMT(CSIR),Bhubaneswarforprovidingresearchsupport.
AuthorsarethankfultoDSTforprovidingSQUIDfacilitytoIIT-DelhiunderprojectRP01993.
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11FigureCaptions:Fig.
1:SchematicfigureofParticlesizedistributionof4hoursballmilled-SiCproduct.
Fig.
2:Transmissionelectronmicroscopepictureof4hoursballmilled-SiCproduct.
Fig.
3:EnergydispersiveX-rayanalysisspectrumof4hoursballmilled-SiCproductFig.
4:XRDpatternof4hoursballmilled-SiCproduct.
Fig.
5:FTIRspectrumof4hoursballmilled-SiCproduct.
Fig.
6:Ramanspectrumof4hoursballmilled-SiCproduct.
Fig.
7:Diffusedreflectivespectrumof4hoursballmilled-SiCproduct.
Fig.
8:Roomtemperaturehysteresiscurveof4hoursballmilled-SiCproduct.
Fig.
9:Hysteresiscurveof4hoursballmilled-SiCproductatlowtemperature(5K).
Fig.
10:Temperaturedependentzerofieldcooledandfieldcooledmagnetizationcurveof4hoursballmilled-SiCproduct.
12Fig.
1:SchematicfigureofParticlesizedistributionof4hoursballmilled-SiCproduct.
13Fig.
2:Transmissionelectronmicroscopepictureof4hoursballmilled-SiCproduct.
14Fig.
3:EnergydispersiveX-rayanalysisspectrumof4hoursballmilled-SiCproduct.
15Fig.
4:XRDpatternof4hoursballmilled-SiCproduct.
16Fig.
5:FTIRspectrumof4hoursballmilled-SiCproduct.
17Fig.
6:Ramanspectrumof4hoursballmilled-SiCproduct.
18Fig.
7:Diffusedreflectivespectrumof4hoursballmilled-SiCproduct.
19Fig.
8:Roomtemperaturehysteresiscurveof4hoursballmilled-SiCproduct.
20Fig.
9:Hysteresiscurveof4hoursballmilled-SiCproductatlowtemperature(5K).
21Fig.
10:Temperaturedependentzerofieldcooledandfieldcooledmagnetizationcurveof4hoursballmilled-SiCproduct.