A Raman study of multiferroic LuMnO3

SolidStateSciences11(2009)1639–1642

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SolidStateSciences

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ARamanstudyofmultiferroicLuMnO3

AnirbanGhosha,b,JyotiRanjanSahua,S.VenkataprasadBhata,C.N.R.Raoa,b,*

a

ChemistryandPhysicsofMaterialsUnit,NewChemistryUnitandCSIRCentreofExcellenceinChemistry,JawaharlalNehruCentreforAdvancedScienti cResearch,JakkurP.O.,Bangalore560064,Indiab

MaterialsResearchCentre,IndianInstituteofScience,Bangalore560012,India

articleinfo

Articlehistory:

Received9December2008Receivedinrevisedform26May2009

Accepted7June2009

Availableonline13June2009Keywords:Multiferroics

RamanspectroscopyLuMnO3Phonons

Hexagonalmanganites

abstract

Magneticanddielectricmeasurementscon rmthemultiferroicnatureofLuMnO3.RamanspectraofLuMnO3havebeenrecordedinthe77–800Krangecoveringboththeantiferromagnetictransitionat90Kandtheferroelectric–paraelectrictransitionat750K.Thechangesinthephononmodesfrequenciesandband-widthsindicatethepresenceofphonon–spincouplingintheantiferromagneticallyorderedphase.Theferroelectric–paraelectrictransitionisaccompaniedbythebroadeninganddisappearanceofmanyofthephononmodes.Someofthephononmodesalsoshowanomaliesattheferroelectrictransition.

Ó2009ElsevierMassonSAS.Allrightsreserved.

1.Introduction

Amongthevariousmultiferroicmaterials,oneoftheearliestexamplesisthatofhexagonal(spacegroup:P63cm)YMnO3whichhasbeenofconsiderableinterestfrombothacademicandtechno-logicalpointsofview.TheferroelectricnatureofthismaterialarisesfromthetiltingofMnO5trigonalbipyramidsbeloww910Kgivingrisetotheappearanceofferroelectricpolarizationalongthec-axisofthecrystal[1–3].Itexhibitscantedantiferromagneticbehavioratlowtemperatures(TN¼70K)duetogeometricalfrustrationofthetriangularlatticeofMnions.Thus,ferroelectricbehaviorcoexists

´eltemperaturewherethewithmagneticorderingsbelowtheNe

ferroelectricdomainwallsactasantiferromagneticdomainwallsaswell[4].Adielectricconstantanomalyisobservednearthemagnetictransitiontemperatureindicatingacouplingbetweenferroelectricandmagneticordering[5].Evidenceforastrongcouplingbetweenmagnonsandphononsisalsoindicatedbyinelasticneutronscat-teringmeasurements.[6].Heavyrare-earthmanganitesLnMnO3(Ln¼Ho–Lu)alsocrystallizeinthehexagonalstructure(spacegroup:P63cm)atambientpressureandshowpropertiessimilartothoseofYMnO3[7].MagneticpropertiesofLuMnO3anditsdielectric

behavioratTNhavebeenreportedinsomedetail,buttheferro-electrictransitionhasnotbeenfullydocumented[8–10].Wewereinterestedinexaminingthecoexistenceofmagnetismandferro-electricityandtheirinterplayinhexagonalLuMnO3.Forthispurpose,wehavecharacterizedthemagneticanddielectricprop-ertiesofLuMnO3andcarriedoutaRamanspectroscopicstudythroughitsmagneticandferroelectrictransitionsinordertoobtainanunderstandingofthemultiferroicphenomenon.SinceRamanspectroscopyisparticularlyusefulinexaminingthelatticedynamicsassociatedwiththephasetransitionsofferroicmaterials,itshouldthrowlightonthedynamicsassociatedwiththephasetransitionsofmultiferroicssuchasYMnO3andLuMnO3wherepolyhedraltiltingisexpectedtooccur[11].2.Experimental

PolycrystallineLuMnO3wassynthesizedbytheconventionalsolidstatereactionprocedurestartingwithstoichiometricquanti-tiesofLu2O3(99.99%)andMnO2(99.99%).Themixturewasgroundthoroughlyinanagatemortarwithpropanolandpelletized.The rstsinteringwascarriedoutat900 Cfor24h.Afterintermediategrindingsatdifferenttemperatures(withintervalsof100 C),thepelletswere nallysinteredat1400 Cfor48h.Thepelletswereannealedat800 Cfor24hinoxygeninordertoassuretheoxygenstoichiometry.

EnergydispersiveX-rayanalysis(EDAX)wasusedtochecktheoverallchemicalhomogeneityandcompositionofthesample.

*Correspondingauthor.ChemistryandPhysicsofMaterialsUnit,NewChemistryUnitandCSIRCentreofExcellenceinChemistry,JawaharlalNehruCentreforAdvancedScienti cResearch,JakkurP.O.,Bangalore560064,India.Fax:þ91802208276.

E-mailaddress:cnrrao@jncasr.ac.in(C.N.R.

Rao).1293-2558/$–seefrontmatterÓ2009ElsevierMassonSAS.Allrightsreserved.doi:10.1016/j.solidstatesciences.2009.06.002

1640A.Ghoshetal./SolidStateSciences11(2009)1639–1642

TheX-raydiffractionpatternofthepowdersamplewasrecordedwithaSeiferts3000TTdiffractometerusingCuKaradiation(l¼1.54056Å).MagneticpropertiesofthepolycrystallinesampleweremeasuredbyusingtheVSMoptioninthePPMS(QuantumDesign)instrument.Inordertostudytheferroelectricproperties,pelletsofthematerialwerepolishedproperly,coatedwithgoldinanargonatmosphere.Thesewereheatedfor2hat300 Cataheatingrateof1 C/mintoensureproperelectricalcontacts.DielectricpropertiesoftheresultingpelletsweremeasuredbyusingaPrecisionImpedanceAnalyzer(Agilent4294A).Ferroelec-trichysteresisloopswereobtainedbyemployingRadiantTech-nologiesInc.Precision4kVHVIinstrument.Ramanspectrawererecordedonasmallpellet(3mmÂ3mm)withaLabRAMHR800high-resolutionRamanspectrometer(HORIBA-JobinYvon)usinganHe–Nelaser(l¼632.8nm).Themeasurementswerecarriedoutinthe77–800KrangeemployingtheLinkamTHMS600stageusingliquidN2asthecoolant.3.Resultsanddiscussion

TheXRDpatternofthesamplecon rmedtheformationofsingle-phaseLuMnO3inthehexagonalstructure(P63cm)inagreementwiththeliteraturereport(a¼6.039Å,c¼11.367Å)[12].Fig.1(a)showsthetemperaturevariationofthezero- eld-cooled(ZFC)andthe eld-cooled(FC)magnetization(at100Oe).

ThematerialisacantedantiferromagnetbelowtheNe

´eltemper-atureof90K.Previousreportsofmagneticmeasurements[2]onsinglecrystalsofLuMnO3andthermalconductivitymeasurementsonapolycrystallinesample[13]areinagreementwiththepresentresults.WenoticeapergencebetweentheFCandZFCdatabelow

76

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rε( tna22.2tsnoC21.7 cirtcel21.2

eiD20.720.2

T (K)

Fig.1.Temperaturevariationof(a)themagnetizationand(b)thedielectricconstantofLu …… 此处隐藏：10810字，全部文档内容请下载后查看。喜欢就下载吧 ……