Electron tunneling measurements on ferromagnetically doped lanthanum manganite films

99 Pages·2000·3.4 MB·English

by  Hudspeth, Heather D. 1972-

ELECTRONTUNNELINGMEASUREMENTSONFERROMAGNETICALLYDOPED
LANTHANUMMANGANITEFILMS
By
HEATFIERD.HUDSPETH
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHE
UNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHEREQUIREMENTS
FORTHEDEGREEOFDOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2000
Iwouldliketodedicatethisdissertationtomyhusband,Quentin.
WithouthislovingsupportIwouldhavenevergottenthisfar. Thanks,Q.
ACKNOWLEDGMENTS
Iwouldliketothankallthepeoplewhohavehelpedmeachievemygoalof
becomingadoctor. Inparticular,Iwishtothankmyadvisor,FredSharifi. Iamsincerely
gratefulforallhisencouragement,advice,andsupport. SpecialthanksgotoPengXiong
andStephenvonMolnarforthethinfilmsusedinthestudyaswellasforallthehelpful
discussions. ManythanksgotoSelmanHershfield,ArthurHebard,StephenPearton,
ChristopherStanton,andMarkMeiselforservingonmysupervisorycommitteeatone
timeoranother.
Iamalwaysgratefulfortheencouragementofmyhusband,parents,family,and
friends. Iwouldneverhaveachievedmysuccesseswithoutalltheirloveandsupport. I
wouldespeciallyliketothankmyDad,JeffLynn,forthecriticalreadingofthis
dissertation.
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3
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
iii
TABLEOFCONTENTS iv
LISTOFFIGURES vi
ABSTRACT viii
CHAPTERS
1 INTRODUCTION I
2 TUNNELINGSPECTROSCOPY 4
2.12.1.M1etalT-I=ns0ulSatteoard-yM-eSttaalteTCuanlnceullaJtuinocntions 48
2.1.2 T 0Steady-StateCalculation 9
2.2 Metal-Insulator-SuperconductorTunneling 10
2.3 SchottkyBarrierEffects 1
3 REVIEWOFLai-x(Ca,Sr)xMn03 18
3.1 CrystalStructure 18
3.2 PhaseDiagram 21
3.3 ColossalMagnetoresistance 23
3.4 DoubleExchange 24
3.5 Jahn-TellerDistortion 28
3.6 Electron-LatticeCoupling 30
3.7 BandStructure 31
4 REVIEWOFEXPERIMENTALEVIDENCE 33
4.1 TransportandThermodynamicstudies 33
4.1.1 LowTemperatureResistivity 34
4.1.2 HighTemperatureResistivityandThermopower 34
4.1.3 SpecificHeat 35
IV
4.1.4 ThermalConductivity 35
4.1.5 IsotopeEffect 36
4.1.6 Optical/RamanSpectroscopy 36
4.2 StructuralExperiments 37
4.2.1 EXAFS 37
4.2.2 NeutronScattering 38
4.2.3 NeutronScatteringPairDensity-FunctionAnalysis 39
4.3 PreviousTunnelingExperiments 39
4.3.1 ScanningTunnelingSpectroscopy 41
4.3.2 TrilayerTunnelingSpectroscopy 41
5 FABRICATION 43
5.1 FilmCharacterization 43
5.2 TunnelJunctionPreparation 46
5.2.1 Ion-milledBarrier 47
5.2.2 ChemicallyFormedBarrier 52
6 DATAACQUISITION 54
6.1 Measurements 54
6.2 JunctionQuality 58
6.3 Lao.67Cao33Mn03TunnelingResults 64
6.3.1 Zero-BiasConductanceMeasurements 64
6.3.2 LowTemperatureConductanceMeasurements 69
6.3.3 MiddleTemperatureConductanceMeasurements 72
6.3.4 HighTemperatureConductanceMeasurements 75
6.4 PreliminaryLao67Sro.33Mn03TunnelingResults 77
6.4.1 Zero-BiasConductanceMeasurements 77
6.4.2 LowTemperatureMeasurements 80
7 SUMMARY 82
REFERENCES 84
BIOGRAPHICALSKETCH 87
V
LISTOFFIGURES
Figure page
2.1 Sketchofacross-stripetunneljunction 5
2.2 T=0EnergydiagramofaNINtunneljunction 7
2.3 T=0EnergydiagramofaSINtunneljunction 11
2.4 T=01vs.Vsketchofasuperconductor 12
2.5 T=0EnergydiagramofaSESljunctionwithaSchottkybarrier 14
2.6 Schottkybarriersketchwithdifferentappliedvoltages 15
2.7 SketchofaSchottkybarrierprofilefordifferentresistancejunctions 16
2.8 SketchofaGvs.Vplotfordifferentresistancejunctions 17
3.1 Sketchofanundistortedperovskitestructure 19
3.2 Lai-xSrxMnOsresistancevs.temperaturegraph 25
3.3 Lai-xSrxMn03magnetizationvs.temperaturegraph 26
3.4 EnergydiagramoftheeffectsofJahn-Tellerdistortions 29
5.1 Lai-xCaxMnOsRvs.Tgraph 44
5.2 Lai-xCaxMnOsMvs.Tgraph 45
5.3 Sketchofsamplebeforebarrierformation 47
5.4 Ivs.VgraphcomparingArandXebarriers,T=2K 50
5.5 Gvs.VgraphcomparingArandXebarriers,T=2K 51
5.6 Gvs.VgraphcomparingXeandBr-etchedbarriers,T=43K 53
6.1 CircuitdiagramusedtomeasuretunnelingIasafunctionofV 55
VI
6.2 CircuitdiagramusedtomeasuretunnelingGasafunctionofV 57
6.3 TypicalIandGvs.Vgraphs,T=2K 59
6.4 Gvs.Vgraphfordifferentresistancejunctions,T=10K 61
6.5 ExponentialfitofconductancedataforV>100mV,T=10K 62
6.6 T=0BCSfittoGvs.Vgraph,T=2K 63
6.7 Lai-xCaxMn03ZerobiasGvs.Tgraph,T==20K-290K 65
6.8 Lai-xCaxMnOsZerobiasGvs.Tgraph,T=4K-140K 67
6.9 Lai-xCaxMnOjGvs.Vgraph,T=2K-265K 68
6.10 Lai-xCaxMn03Gvs.Vgraph,T=8K- 30K 70
6.11 NormalizedLai-xCaxMn03Gvs.Vgraph,T=10K-40K 71
6.12 Lai.xCaxMn03Gvs.Vgraph,T=33K-80K 73
6.13 NormalizedLai-xCaxMn03Gvs.Vgraph,T=10K-80K 74
6.14 Lai-xCaxMn03Gvs.Vgraph,T=150K-265K 76
6.15 Lai-xSrxMn03ZerobiasGvs.Vgraph,T=4K-190K 78
6.16 Lai.xSrxMn03Gvs.Vgraph,T=10K-250K 79
6.17 Lai.xSrxMn03Gvs.Vgraph,T=10K-30K 81
vii
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
ELECTRONTUNNELINGMEASUREMENTSONFERROMAGNETICALLY
DOPEDLANTHANUMMANGANITEFILMS
By
HeatherD.Hudspeth
May2000
Chairman: FredSharifi
MajorDepartment: Physics
AseriesoftunnelingandtransportmeasurementsonLao.67Cao.33Mn03and
Lao67Sro.33Mn03perovskitethinfilmsispresented. Planartunneljunctionswere
fabricatedusingthemanganitethinfilmasoneelectrode. Toensureahigh-quality
turmeljimction,awell-characterizedsuperconductor,Pb,isusedastheotherelectrode.
BelowthePbsuperconductingtransitiontemperature,Tc,theassociatedsuperconducting
gapandphononstructuresarefullyobserved. Measurementsoftunnelingcurrentand
conductanceasafunctionofbiasvoltagewereobtainedfortemperaturesrangingfrom
2Kto300K.
ForLao67Cao.33Mn03,ourtunnelingmeasurementsshowthedensityofelectronic
statesattheFermilevelincreasingslowlybelow250K,theferromagneticordering
temperatureoftheLao67Cao33Mn03film. Carriermobilityplaysadominantrolein
electrontransportasthematerialundergoesaferromagneticmetal-insulatortransitionat
viii
250K. OurmeasurementsareconsistentwithaFermilevelmovingthroughamobility
edgeasthismaterialordersferromagnetically. Below40K,weseeadepletioninthe
densityofstatescenteredattheFermilevelandextendingafewmeV. Theresultsare
consistentwithadepletionofonespinband. Wealsoseeevidenceforinelastic
excitationsinthedensityofstatesoverabroadenergyrange. Weattributethese
excitationstopolaronicallylocalizedelectronscoexistingwithmobilecarriersinthe
mixedvalencestate.
Finally,itshouldbenotedthatpreliminaryresultsonLao.evSro.asMnOssamples
areinagreementwiththoseobtainedforLao67Cao.33Mn03.
IX
CHAPTER
1
INTRODUCTION
Themanganiteperovskiteshavebeentherecentfocusofliteraryattention. With
variousdopants,inparticulartheseriesRi.xAxMnOs(whereRisarareearthelement
suchasLa,andAisadivalentalkalimetal,suchasCaorSr),thematerialundergoesa
ferromagneticmetal-insulatortransition.' ThesecompoundsarebasedontheAMO?
cubicperovskitestructurewiththeMnf;^-electronsplayingakeyroleinbothtransport
andmagneticorder. Onefeatureofthesematerialsisthattheircharacteristicsare
stronglydependentondopingconcentration,x,andtemperature. Forexample,the
undopedcompoundLaMnOa(x=0)isanantiferromagneticinsulatorwithMn ions.
ThecZ-orbitalsorder,distortingtheoriginalcubiclatticeintoanorthorhombicstructure.
Dopingthesematerialswithadivalentionintroducesholes,producingMn'^'^ions. With
sufficientdoping,themixedMn^^/Mn"*^valencyintroducedbytheLa^VA^"^substitution
transformsthesystemintoaferromagneticmetalatlowtemperatures. Overtheentire
dopingrange,thematerialsarespindisordered,paramagneticinsulatorsaboveacritical
temperature,Tc. Thespin-orderedstatesbelowthiscriticaltemperaturevarywithdoping
concentration. Forx>0.5andx<0.16thecompoundsareinsulatorswithavarietyof
magneticorderdependingonthedopingconcentration. Theonlymetallicphaseisfound
withferromagneticorderintheintermediatedopingrangeof0.16<x<0.5. Inthis
intermediatedopingregime,aferromagnetic-paramagnetictransitionisobservedata
1