Antiferromagnetic Order in Disorder-Induced Insulating Phase of SrRu_{1-x}Mn_xO_3 (0.4x0.6)

AntiferromagneticOrderinDisorder-InducedInsulatingPhaseofSrRu1−xMnxO3(0.4≤x≤0.6)

arXiv:cond-mat/0503040v2 [cond-mat.str-el] 27 Jun 2005MakotoYokoyama,1∗ChimatoSatoh,1AkinoriSaitou,1HirofumiKawanaka,2

HiroshiBando,2KenjiOhoyama3andYoshikazuNishihara1

2National

ofScience,IbarakiUniversity,Mito310-8512

InstituteofAdvancedIndustrialScienceandTechnology,Tsukuba,Ibaraki305-85683InstituteforMaterialsResearch,TohokuUniversity,Sendai980-8577

(ReceivedFebruary2,2008)

1Faculty

WehaveperformedpowderneutrondiffractionmeasurementsonthesolidsolutionsofSrRu1−xMnxO3,andfoundthattheitinerantferromagneticorderobservedinpureSrRuO3changesintotheC-typeantiferromagnetic(AF)orderwithnearlylocalizeddelectronsintheintermediateMnconcentrationrangebetweenx=0.4and0.6.Withincreasingx,theAFmo-mentisstronglyenhancedfrom1.1µB(x=0.4)to2.6µB(x=0.6),whichisaccompaniedbytheelongationofthetetragonalc/aratio.TheseresultssuggestthatthesubstitutionofMnforRusuppressestheitinerantcharacteristicofthedelectrons,andinducesthesuperexchangeinteractionthroughthecompressioninthecplane.Wehavealsofoundthatthemagneticandtransportpropertiesobservedinourtetragonalsamplesaresimilartothoseofrecentlyreportedorthorhombicones.

KEYWORDS:metal-insulatortransition,SrRuO3,ferromagnetism,antiferromagnetism,neutronscattering

Strontiumruthenateshowsawidevarietyofphysi-calpropertiesattributedtotheitinerantfeatureofRu4delectronsandthestrongmixingbetweenRu4dandO2pelectrons.1–7)ThedistortedperovskitecompoundSrRuO3isknowntobeaferromagnetwiththeCurietemperatureof∼160Kandtheorderedmomentof∼1.1µB.5,6)Inthiscompound,itinerantRu4delec-tronsareconsideredtoberesponsibleforthemagneticandtransportproperties.7)Recentphotoemissionstud-ies,8,9)however,revealedthatthespectraldensityattheFermilevelissignificantlyreducedduetothestrongelec-troniccorrelation,comparedwiththatfromthebandcal-culation.Moreover,thiscompoundshowsananomaloustransportpropertycalledthe“badmetal”.7,10)Theelec-tricalresistivityincreaseslinearlywithincreasingtem-perature,andthenpassesthroughtheIoffe-Regellimitwithoutsaturationathightemperatures.Insuchahightemperatureregion,therelationkFl∼1(l:meanfreepath)holds,suggestingthebreakdownoftheitinerantquasi-particlepicture.Theseexperimentalresultsindi-catethatboththestrongcorrelationandtheitinerant-localizeddualityoftheRu4delectronsplayimportantrolesinthemagneticandtransportproperties.

Recently,Caoetal.11)revealedinthemixedcom-poundsSrRu1−xMnxO3(0≤x≤0.6)thatthesubstitu-tionofMnforRusuppressestheitinerantferromagneticphase,andtheninducesanewinsulatingphaseabovethecriticalpointxc=0.39.Forx≥xc,aclearcusplikeanomalyisobservedinthetemperaturevariationsofthemagneticsusceptibility,stronglysuggestingtheappearanceoftheantiferromagneticorderintheinsu-latingphase.InSrRuO3,thecarriermeanfreepathis

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MakotoYokoyamaetal.

SrRu)1-xMnxO(b) st(a) 3x = 0.4 (I4/mcm)in4ux = 0.15 (Pbnm)2 .42b2ra2004(0 04ytisne0t0n4I66.56767.5686767.56868.52θ (deg)Fig.1.X-raydiffractionprofilesaroundthe(400)and(040)peaksforSrRu1−xMnxO3with(a)x=0.15and(b)x=0.4,wherethepeaksareindexedbyassumingtheorthorhombicPbnmsymmetryfor(a)andthetetragonalI4/mcmsymmetryfor(b).Notethatthe(040)peakdoesnotexistindependentlyin(b).

thetetragonalstructure.Ourpreliminaryinvestigationsforthesamplesbetweenx=0and0.6indicatethattheboundarybetweentheorthorhombicphaseandtetrago-nalphaseliesbetweenx=0.2and0.3.Thereasonforthediscrepancyisnotclearinthepresentstage,probablycausedbytheunexpecteddifferencesinthesynthesizedconditionsofthesamples.

InFig.2,thetemperaturevariationsofthemagneticsusceptibilityχ(T)forx=0.4,0.5and0.6,measuredus-ingaSQUIDmagnetometerinthetemperaturerangebe-tween2Kand350K,areshown.Thehystereticbehaviorisobservedinχ(T)betweenthedataunderthezero-field-cooling(ZFC)andfield-cooling(FC)conditions,indicat-ingtheevolutionoftheferromagneticorder.Themagni-tudeofthelow-temperaturemagnetizationis,however,estimatedtobeonlylessthan1%ofthatforx=0.Wehavefoundthatthemagnitudeofthehysteresisinχ(T)ismuchlargerinthesamplessinteredonlyonce,andreducedbyiteratingtheheatingprocess.Inaddi-tion,theonsetofthehysteresisisnearlyindependentofx.Thesepropertiesstronglysuggestthatthehystereticbehaviorisduetothemixtureoftheextrinsicferromag-neticcomponentwithlessthan1%volumefraction.Ontheotherhand,cusplikeanomaliesareobservedinχ(T)at180Kand220K(≡TN)forx=0.5and0.6re-spectively,suggestingtheantiferromagnetic(AF)tran-sition.Forx=0.4,theanomalyappearsat∼70K,butitisobscuredbythehystereticbehavior.Athightemperatures,χ(T)roughlyfollowstheCurie-Weisslawχ=Nµ2eff/3kB(T−θ)+χ0.Thebestfitfortheχ(T)databetween280Kand350Kyieldsµeffvaluesof3.70(18)µBforx=0.4,3.70(29)µBforx=0.5and4.12(34)µBforx=0.6.Theθvaluesforthesesamplescannotbedeterminedforafairlylargeuncertaintyduetonarrowfittingranges.Wehavealsomeasuredthetemperaturevariationsoftheelectricalresistivityρ(T)forx=0.4,0.5and0.6,andfoundthatρ(T)showstheinsulatingbehavior(theinsetofFig.2).Thepropertiesofχ(T)andρ(T)arequitesimilartothosereportedbyCaoetal.,11)suggestingthattheelectronicstatesinthesam-pleswiththetetragonalstructurearenearlyidenticaltothosewiththeorthorhombicones.Thedetailedexperi-

810)SrRu1-xMnxO3)ml1x = 0.6ocm6H = 1 kOe Ω( 0.1ρ/u0.01m0200T (K)400600800e 4x = 0.62-01x = 0.5x( 2χ0x = 0.40100T (K)200300Fig.2.Temperaturevariationsofthemagneticsusceptibilityforx=0.4,0.5and0.6.Theopenandsolidsymbolsindicatethedataobtainedunderthezero-field-cooling(ZFC)andfield-cooling(FC)conditions,respectively.Theinsetshowsthetem-peraturevariationsoftheelectricalresistivityforx=0.6.

mentalresultsandanalysesonoursamplesintheentirexrangewillbedescribedelsewhere.14)

Theneutrondiffractionmeasurementsforthepow-deredsampleswithx=0.4,0.5and0.6wereperformedinthetemperaturerangebetween20Kand290K,us-ingtheHERMESspectrometerlocatedattheresearchreactorJRR-3MoftheJapanAtomicEnergyResearchInstitute.Wechosethewavelengthoftheincidentneu-tron1.82035˚Aproducedbythe(331)reflectionofaGemonochromator.Figure3(a)showsthepowderneutrondiffractionprofileforx=0.6at290K.Thediffractionpatternsforx=0.4,0.5and0.6wereanalyzedusingtheRietveldmethodwiththeRIETAN2000program.15)WeassumedthatthecrystalstructurehasatetragonalI4/mcmsymmetry,whereionicpositionsareassignedtoSr(0,1/2,1/4),Ru/Mn(0,0,0),O(1)(0,0,1/4),andO(2)(1/4+u,3/4+u,0).Theparametersandreliabil-ityfactorsobtainedbybestfitsarelistedinTableI.Thequalitiesoftherefinementareslightlyworse,probablyduetothesmallmixtureoftheferromagneticphaseob-servedinχ(T).Wehavefoundthatboththeaandcaxislatticeparametersshrinkwithincreasingx,reflect-ingthesmallradiusofMnioncomparedtothatofRuion.Ontheotherhand,thedisplacementuoftheO(2)ionfromthesymmetricpositiondecreasesbyincreasingx.WenotethatsomenuclearBraggpeakssuchasthe(211)and(213)reflectionsarisepurelyduetofiniteuvalues.Thesepeaksareclearlyobservedinthepresentmeasurements,eliminatingthepossibilitythatthesam-plesformthetetragonalstructurewiththeP4/mmmsymmetry,13)wheretheO(2)ionoccupiesthepositionwithu=0.

Figure3(b)showsthepowderneutrondiffractionpro-fileforx=0.6at20K.WehavefoundnewBraggreflec-tionsatpositionscorrespondingto(hkl)withh+k=oddandl=even.Thepeakintensitiesinthehighscatter-inganglearesmallerthanthoseinthelowscatteringangle,stronglysuggestingthatthesepeaksoriginateinthemagneticorder.Wehavenotobservedtheotherad-ditionalreflectionduetoaferromagneticorAForder

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Intensity (arb. units)(a) T = 290 K112200002110211004220213202SrRu0.4Mn0.6O3224400(b) T = 20 KcM100M210M212aa1020304050607080901001102θ (deg)Fig.3.NeutronpowderdiffractionprofilesofSrRu0.4Mn0.6O3at(a)T=290Kand(b)T=20K.ThemirrorindicesofseveralBraggpeaksarealsoshown.In(a),thecalculatedprofile,thecalculatedBragg-peakpositions,andthedifferencebetweentheobservedandthecalculatedprofilesarealsoplottedwiththesolidline,verticalmarkers,andthelowertrace,respectively.Theinsetof(b)showsthespinarrangementontheRu/MnionsintheC-typeAFstructure,depictedintheunitcellofthetetragonalI4/mcmsymmetry.

a(˚A)c(˚A)u3

V(˚A)c/aRwp(%)Re(%)S(=Rwp/Re)

5.470(11)7.94696(23)0.03200(13)237.835(9)1.4526(1)3.952.371.67

M1025.45744(17)7.916(40)0.02921(20)235.793(15)1.4506(2)4.932.631.885.44732(18)7.86858(51)0.02348(25)233.486(19)1.4445(2)4.2.471.98

4224044MakotoYokoyamaetal.

3SrRu1-xMnxO3).2uT = 20 K.f/Bµ13).( u.of/2µ0Bµ( o1µ0x = 0.61.48a/c1.462381.491.440100T (K)200300a236/cV (Å3)1.482342321.470.40.50.6230Mn concentration xFig.5.xvariationsoftheAFmomentµo,theunit-cellvolumeVandthec/aratioatT=20KforSrRu1−xMnxO3.Thetemperaturevariationsofµoandc/aratioforx=0.6arealsoplottedintheinsets.

G-typemagneticstructureinpureSrMnO319)aswellastheitinerantferromagneticphaseinpureSrRuO3.5,6)Furthermore,wehaveobservedthattheratioµo/µeffisenhancedwithincreasingxfromx=0to0.6.Thisenhancementmaybeascribedtotheitineranttolocal-izedelectrontransitionduetotheelectroncorrelationeffect,thatis,thesystemisinthecrossoverregionfromW/U>1toW/U≪1,whereWandUarethed-bandwidthandtheon-siteCoulombpotential,respectively.Underthelattercondition,weexpecttheoccurrenceofAForderduetothesuperexchangeinteraction,asisar-guedintheMott-typeinsulators.Inthesuperexchangemechanism,thepositionsoftheRu/MnandOionsplayakeyroleinthestabilityoftheAForder.WehavefoundinthepresentneutrondiffractionmeasurementsthattheintensitiesofsomeBragg-peaks,suchasthe(211),(213)and(215)reflections,significantlyincreasewithdecreas-ingtemperature.Theenhancementofthesepeakscanbesimplyexplainedbytheincreaseinparameteru,in-dicatingthatthedisplacementoftheO(2)ionfromthesymmetricpositionincreasesintheAFphase.WehaveestimatedtheRu/Mn-O(1)bondlengthd1,theRu/Mn-O(2)bondlengthd2andtheRu/Mn-O(2)-Ru/Mnbondangleαfromthelatticeparametersandtheuvalues.Astemperatureislowered,d2forx=0.6isreducedfrom1.934˚A(290K)to1.921˚A(20K),whiled1increasesfrom1.967˚Ato1.996˚A.Inaddition,thedifferencesind1andd2between20Kand290Kincreasewithincreas-ingx.Thesevariationsareconsistentwiththeincreaseinc/aratio.Fromtheseresults,weexpectthattheMndopingmakesthehybridizationbetweenthedelectronsinRu/MnionsandthepelectronsinO(2)ionsstrongthroughthecompressioninthecplane.Sincethecon-ditionW/U≪1holdsintheintermediatexrange,thiseffectenhancesthesuperexchangeinteractionbetweennearestneighborRu/Mnions,bringingtheC-typeAF

order.Ontheotherhand,αforx=0.6slightlyde-creasesfrom169.27(11)◦at290Kto166.26(9)◦at20K.Thismayalsoaffectthemagneticstructureandthemagneticexcitationspectrum,althoughtheycannotbedetectedinthepresentmeasurements.

ItisnaturaltothinkthatthevalenceandspinstatesinRuandMnionsarealsoconnectedwiththemagneticstructure.InpureSrRuO35)andSrMnO3,19)therealiza-tionofthelow-spinRu4+(S=1)andhigh-spinMn4+(S=3/2)stateswasreported.Byassumingasimplecombinationofthesestates,theeffectivemomentsforx=0.4,0.5and0.6areestimatedtobe3.25µB,3.35µBand3.46µB,respectively.Thesevaluesare,however,smallerthantheµeffvaluesobtainedfromthepresentexperiments,indicatingthattheothervalencestatesorthehigh-spinRu4+(S=2)statearemixedwiththelow-spinRu4+(S=1)andhigh-spinMn4+(S=3/2)states.Weexpectthatsuchamixingmightreducethedissim-ilarityofthespinstatesbetweenRuandMnions,andstabilizethesimplecolinearmagneticstructureobservedinSrRu1−xMnxO3forintermediateMnconcentrations.Oneofus(M.Y.)isgratefultoH.Amitsukaforhelpfuldiscussionsandcomments.ThisworkwassupportedbyaGrant-in-AidforScientificResearchfromtheMinistryofEducation,Culture,Sports,ScienceandTechnology.