Rounding of a first-order magnetic phase transitio

1Roundingofafirst-ordermagneticphasetransitioninGadopedLa0.67Ca0.33MnO3S.Röβler1,U.K.Röβler1,K.Nenkov1,D.Eckert1,S.M.Yusuf2,K.Dörr1,K.–H.Müller11Leibniz-InstitutfürFestkörper-undWerkstoffforschungIFWDresden,Postfach270116,D-01171Dresden,Germany.2SolidStatePhysicsDivision,BhabhaAtomicResearchCenter,Mumbai400085,India.TheeffectofdisorderonthecriticalpropertiesoftheferromagneticphasetransitionincolossalmagnetoresistivemanganiteLa0.67Ca0.33MnO3hasbeenstudiedbysubstitutingGaforMn.Itisfoundthat,upon10%Gasubstitution,thepeakinthespecificheatattheCuriepointCTchangesdrasticallyandappearsasasmallanomaly.StaticmagnetizationdataanalyzedintheasymptoticcriticalregionusingmodifiedArrottplotsandtheKouvel-Fishermethodgivevaluesforthecriticalexponents0.387(6)β=,1.362(2)γ=,and4.60(3)δ=.Theresultsshowthatthefirst-ordertransitioninLa0.67Ca0.33MnO3becomescontinuousbyGasubstitution.ThecriticalpropertiesoftheroundedtransitioninLa0.67Ca0.33Mn0.9Ga0.1O3suggestthatthemagneticsubsysteminthismixed-valentperovskiteisclosetothatofaconventionalisotropicferromagnetbelongingtotheHeisenberguniversalityclasswithshort-rangeinteractions.Itisconcludedthatthefirst-ordermagnetictransitioninpureLa0.67Ca0.33MnO3isinducedbyfluctuationsfromacompetingmode,whichcouplestothemagneticsubsystem.PACSnumbers:75.47.Lx,64.60.Fr,75.40.Cx,75.50.Lk2IntroductionColossalmagnetoresistive(CMR)manganitesofthetypeR1-xAxMnO3,whereRisatrivalentrare-earthionandAisadivalentalkalineearthion,hasbeenasubjectofintensiveresearchinrecenttimesduetotheirintricatemagneticandelectricalproperties.1Forthesecompoundsvariouselectronic,magneticandstructuralorderscompeteleadingtoveryrichphasediagrams.Recently,theimportanceofmulticriticalpointsfortheirunconventional“colossal”behaviorhasbeenemphasized.2-4Foracertainrangeofcompositions(0.20.4x≤≤),theR1-xAxMnO3mayshowinterdependentparamagnetic-ferromagnetic(pm-fm)andinsulator-metaltransitions.Anexplanationofthisphenomenonhasbeengivenbyadouble-exchangemodel,5whereelectronhoppingbetweenMn-eglevelsaligntheMn-t2gspinsduetoastrongintra-atomicHund’scoupling.ForaquantitativeexplanationofCMR,additionalmechanismssuchasmagneto-elasticcouplingsduetoJahn-Teller(JT)effectshavebeeninvoked.6SeveralexperimentalandtheoreticalworkshavedemonstratedacloseconnectionbetweenJTdistortionsandlocalizationofchargecarriersorinotherwords,formationofpolarons,intheinsulatingphase.7,8Asaresult,someofthetheoriesassociateCMRwithafirst-orderpm-fmtransitiontriggeredbyadiscontinuoustransitionbetweenpolaronicandextendeditinerantstatesofchargecarriers.9ForthearchetypicalCMR-materialLa0.67Ca0.33MnO3clearevidenceofafirst-orderpm-fmtransitionhasbeenfound,suchasstrongvolumeanomalyattheCuriepoint3(CT),8thermalhysteresis,10andnegativeslopesobservedinmagneticisothermplots2/vs.HMM.11Theexistenceofafirst-orderpm-fmtransitionintheLa1-xCaxMnO3systemisalsocorroboratedbyatricriticalpointattheCurie-temperatureTCforx=0.4.12Ontheotherhand,acontinuousphasetransitionhasbeenobservedinLa1-xSrxMnO3(0.125x=,0.3),13,14La0.67Mg0.33MnO3,15andNd0.6Pb0.4MnO3.16Valuesofthecriticalexponentscharacterizingthepm-fmtransitioninthesecompoundsareclosetothosepredictedfora3DHeisenbergmodelwithshort-rangeinteractions.Thiscanbeexpectedalthoughthedoubleexchangeisdrivenbythemotionofconductionelectrons(Mn-eg),theeffectivemagneticinteractionnearthetransitionisrenormalizedtoisotropicshortrangeinteractionswhichcouplethecore-likeMn-t2g-spins.Thus,thefirst-orderandthecontinuouspm-fmtransitionsobservedindifferentmanganitesindicatethatthenatureofthepm-fmtransitionseemstodependontheA-sitedoping.Theexistenceofafirst-orderpm-fmtransitioninLa0.67Ca0.33MnO3isanintriguingprobleminitself.Itisknownthatthermallydrivenfirst-ordertransitionsmayberoundedbyquencheddisorder.17-23Thismeansthatafirst-ordertransitioninapuresystembecomescontinuousupondoping.Then,eitheraconventionalcriticalbehaviorornoveltypesofcontinuousorderingtransitionsareobservedinthedisorderedsystem.Thus,theexistenceofafirst-orderpm-fmtransitionintheLa1-xCaxMnO3-systemindicatesthattheinevitablemixedoccupationoftheA-siteoftheperovskitelatticewithLaandCadoesnotaffectthemagneticordering-transitionsufficientlytocauseacontinuouspm-fmtransition.Interestingly,forasystemwithacontinuousorderparametersymmetry,asthemagnetizationinaHeisenberg-likemagnet,thereisarigorousproofthat,ifthesystemundergoesathermallydrivenfirst-4ordertransitionasapuresystem,anyamountofquencheddisorderleadstoaroundedcontinuoustransitioninspatialdimensions4d≤.19,24Ingeneral,thecouplingofthemagneticsubsystemtootherdegreesoffreedommaycauseafluctuation-inducedfirst-ordertransition.SuchascenariohasbeenanalyzedintermsofphenomenologicalLandau-theorybyusingrenormalization-groupcalculationsforanisotropicmagneticsystemcoupledtoanIsing-likeorderparameter,whichdescribese.g.achargeorder.3Thisanalysis,togetherwiththeabsenceoffirst-ordertransitionsindisorderedisotropicmagnets,suggeststhatthefirst-orderpm-fmtransitioninLa0.67Ca0.33MnO3ismostprobablyclosetoamulticriticalpointwhereanadditionalfluctuatingorderparametertransformsthe