Anomalous itinerant magnetism in single crystal Sr

AnomalousitinerantmagnetisminsinglecrystalSr4Ru3O10:AthermodynamicandtransportinvestigationG.Cao,S.ChikaraandJ.W.BrillDepartmentofPhysicsandAstronomy,UniversityofKentucky,Lexington,KY40506P.SchlottmannDepartmentofPhysics,FloridaStateUniversity,Tallahassee,FL32306(Dated:November27,2006)AbstractAthermodynamicandtransportstudyofSr4Ru3O10asafunctionoftemperatureandmagneticfieldispresented.ThecentralresultsincludeagrowingspecificheatCwithincreasingfieldB,amagneticcontributiontoC/Tatlowtemperaturesproportionalto–log(T),anabruptjumpandapeakinC/Tat2.90Tand7TforB||ab-planeandB||c-axis,respectively,andcorrespondingchangesinthelowTpowerlawsoftheresistivity.Thenoveltyofthisworkliesinthefactthatthissystemisstronglyanisotropicdisplayingspontaneousferromagnetismalongthec-axisandanintralayermetamagnetictransitionwithapossibilityofanearbyquantumcriticalpoint.Theexoticbehaviorreflectsnewphysicsthatisyettobeunderstood.PACSnumbers:75.30.Gw,75.40.-sI.IntroductionAquantumcriticalpoint(QCP)canberealizedbytuningthecriticaltemperatureofaphasetransitiontoabsolutezerobyvaryingexternalparameterssuchasthemagneticfield,pressureorthedopinglevel.WhileaphasetransitionatfiniteTisgovernedbythermalfluctuations,therelevantfluctuationsclosetoaQCPareofquantummechanicalnature.AQCPusuallyimpactsthephysicalpropertiesoverawiderangeoftemperatures.Quantumcriticalityiswellillustratedinheavyfermions[1]andhighTccuprates,and,recently,inmaterialsintimatelyassociatedwithitinerantmetamagnetism.Forinstance,UPt3[2],CeRu2Si2[3]andSr3Ru2O7[4]areenhancedparamagnetsshowingafield-inducedmetamagnetictransition.Whenthecriticalendpointofthemetamagnetictransitionistunedtozerotemperature,thecriticalfluctuationscausethebreakdownofFermiliquidbehaviorwithprofoundconsequences,suchasdivergentspecificheat[C/T~-log(T)]andunusualpowerlawsintheresistivityatlowT[1-6].However,unusualpowerlawsarealsoobservedwhenthecriticaltemperatureremainsfinite.Inthispaper,wereportthetemperatureandfielddependenceofthespecificheat,resistivityandmagnetizationoftriple-layeredSr4Ru3O10.Criticallydrivenbyboth,intralayerfield-inducedmetamagnetismandinterlayerspontaneousferromagnetism[7-12],unusualpowerlawsinTarefound.ThesefeaturesaredistinctfromthoseobservedinothersystemsinvolvingaQCP.OurresultsforSr4Ru3O10revealahostofanomalousproperties,namely,agrowingspecificheatCwithincreasingmagneticfieldB,a–log(T)contributiontoC/TatlowT,anabruptjumpandanomalyinC/TatBc=2.90TandBc=7TforB||ab-planeandB||c-axis,respectively,andcorrespondingchangesinthepowerlawoftheresistivity2betweenT2andT3/2.Thenoveltyofthisworkliesinthefactthatthequantumfluctuationsoccurinasystemwherebothinterlayerspontaneousferromagnetismandintralayerfield-inducedmetamagnetismcoexist,afeaturedistinctivelydifferentfromallothersystemsinvolvingaQCP.(Theterm“spontaneousferromagnetism”istodistinguishitfromfield-inducedferromagneticorderormetamagnetismbecausethissystemexhibitsboth.)Sr4Ru3O10belongstothelayeredRuddlesden-Popperseries,Srn+1RunO3n+1(n=numberofRu-Olayers/unitcell).Thecomparableandcompetingenergiesofcrystallinefields(CEF),Hund’sruleinteractions,spin-orbitcouplingandelectron-latticecouplingcruciallydeterminetheCEFlevelsplittingandthebandstructure,hencethegroundstate.Thephysicalpropertiesarethushighlydimensionality(orn)dependent.Asaresult,themagneticstateofSrn+1RunO3n+1systematicallyevolvesfromparamagnetism(n=1),enhancedparamagnetism(n=2)tospontaneousferromagnetism(n=∞)withincreasingn.Situatedbetweenn=2andn=∞,Sr4Ru3O10(n=3)displaysinterestingphenomenarangingfromquantumoscillations[8],tunnelingmagnetoresistance[8],unusuallowtemperaturespecificheat[9],strongspin-latticecoupling[10,11],switchingbehavior[12]andotherproperties[13,14].Theuniquefeature,however,isborderlinemagnetism:Forafieldalongthec-axis(perpendiculartothelayers),Sr4Ru3O10displaysspontaneousferromagnetismataCurietemperature,whileforafieldwithintheab-planeitfeaturesapronouncedpeakinmagnetizationandafirst-ordermetamagnetictransition[7-11].Thec-axisferromagnetismindicatesthattheStonercriterionissatisfied,Ug(EF)≥1,whereUisanexchangeinteractionandg(EF)thedensityofstatesattheFermisurface.BelowTCthespin-upandspin-downbandsarethusspontaneouslysplitbythe3exchangesplittingΔintheabsenceofanappliedfield.Theresponsetoafieldintheplane,however,isstrikinglysimilartoStonerenhancedparamagnetismwithUg(EF)1[4].ThecoexistenceoftheinterlayerferromagnetismandtheintralayermetamagnetismmakesSr4Ru3O10significantlydifferentfromSr3Ru2O7wherespontaneousferromagnetismisabsent.ThecrucialpropertiesofSr4Ru3O10discoveredfrompreviousstudies[7-11]arethefollowing:(1)Thec-axismagnetization,MC,isferromagneticwithTCat105KfollowedbyanincreasedspinpolarizationbelowTM=60Kwithlargeirreversibilityuponin-fieldandzero-fieldcooling[7-8].Incontrast,theab-planemagnetization,Mab,ismuchsmallerandexhibitsaweakcuspatTCandabroadpeakatTM[7-8].(2)TheisothermalmagnetizationMCillustratesthatthespinsarereadilypolarizedandsaturatedalongthec-axisatB=0.2T,yieldingasaturationmomentof1.2μB/Ru.Onthecontrary,Mabdisplaysafirst-ordermetamagnetictran