Enhanced Spin and Electronic Reconstructions at the Cuprate-Manganite Interface

0.68 MB·English

by  B. A. Gray

Enhanced Spin and Electronic Reconstructions at the Cuprate-Manganite Interface
B. A. Gray,1,a) E. J. Moon,1 I. C. Tung,2,3 M. Kareev,1 Jian Liu,4 D. J. Meyers,1 M. J.
Bedzyk,3 J. W. Freeland,2 and J. Chakhalian1
1)Department of Physics, University of Arkansas, Fayetteville, Arkansas 70701,
USA
2)Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439,
3
USA
1
0 3)MaterialsScienceandEngineering,NorthwesternUniversity, Evanston, Illinois,60208,
2

n USA
a
J 4)Department of Physics, University of California, Berkeley, California 94720,

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1 USA

]
l
e We report on a resonant soft X-ray spectroscopy study of the electronic and magnetic
-
r
t structureofthecuprate-manganiteinterface. PolarizedX-rayspectroscopymeasurements
s
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t takenattheCuLedgerevealuptoafive-foldincreaseinthedichroicsignalascomparedto
a
m
pastexperimentalandtheoreticalvalues. Furthermoreanincreaseinthedegreeofinterlayer
-
d
charge transfer up to 0.25e (where e is charge of an electron) per copper ion is observed
n
o
c leadingto aprofoundreconstructionin theorbitalschemefor theseinterfacialcopperions.
[
1 ItisinferredthattheseenhancementarerelatedtoanincreaseinTMI observedformanganite
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layersgrownwithrapidlymodulatedflux.
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a)Electronicmail: [email protected]
1
The confluence of multiple competing order parameters stemming from the rich spectrum of
groundstatesaccessibleinartificialtransitionmetaloxidemultilayersfacilitatestheexplorationof
unique quantum states and phenomena at the interface. A quintessential example is the junction
between the high-T cuprate YBa Cu O (YBCO) and the colossal magnetoresistance manganite
c 2 3 7
La Ca MnO (LCMO),forwhichtheexactnatureoftheelectronicandmagneticstructureat
2/3 1/3 3
and near the interface remains actively debated. On one hand, proximity effects (PE) are excepted
to play a dominate role in determining properties,1 and indeed prior extensive measurements of
YBCO/LCMO heterostructures have revealed suppressions in the critical temperature and free
carrierresponseoperatingoveralengthscalefarexceedingtheanticipatedsuperconductive(SC)
condensatepenetrationdepthξ .2,3 OnepossibleexplanationfortheobservedlongrangePEisa
F
triplet pairing component of the SC condensate due to the presence of magnetic inhomogeneity
neartheinterface.4
Throughthe combined utilizationofpolarizedneutron reflectivityandX-raymagnetic circular
dichroism (XMCD), a definitive picture of the microscopic magnetic profile in YBCO/LCMO
multilayerswasobtainedwhichcoincidedwiththeaforementionedtheoreticalpredictionsincluding
aninducednetmomentintheSClayerattheinterfacewithanantiferromagneticcouplingtothe
ferromagnetic (FM)layer.5–7 However, subsequent X-raylinear dichroism (XLD)measurements
established the role of orbital reconstruction in which a strong covalent Cu-O-Mn bond at the
interfacepromoteschargetransferbetweenlayersandarearrangementoftheCud occupation.8
z2−r2
ThesignofthemagneticinteractionbetweenCuandMnissimplydeterminedbytheGoodenough-
Kanamorirules,whilechargetransferacrosstheinterfaceoffersanalternativeexplanationtothe
previouslyseenlongrangePE.3,9 Despitetheseadvancements,recenttheoreticalandexperimental
results have reproduced the magnetic dichroic signal on Cu but failed to account for the orbital
reconstruction.10,11 Furthermore,recentlyevidenceforatripletSCcomponentinYBCO/LCMO
heterostructureshasbeenemerging.12
In this letter, we report on a resonant X-ray absorption spectroscopy (XAS) study of the
electronicandmagneticprofileofthecuprate-manganiteinterfaceintherepresentative[YBCO(9
u.c.)/LCMO(26u.c.)]×3superlattice(SL).CircularlypolarizedX-raysareusedtoacquireelement
sensitiveinformationaboutthemagneticstructureoftheinterface,whilelinearpolarizationsprobe
the electronicstructure. Temperature dependent dctransport measurements explorethe electronic
andmagneticqualitiesofthefilms. Weconcludethatbothspinandelectronicreconstructionsare
presentandmarkedlyenhanced. Furthermore,theseenhancementsarelinkedtothestructuraland
2
chemicalpropertiesofthemanganite-cupratelayersobtainedbyrapidlymodulatedflux.
SamplefabricationwascarriedoutinarecentlydevelopedPulsedLaserEpitaxy(PLE)facility
featuringinfraredlasersubstrateheatingandin-situgrowthmonitoringviahigh-pressureReflection
HighEnergyElectronDiffraction(HPRHEED).Duringdeposition,stoichiometrictargetsofYBCO
andLCMOwereablatedbyaKrFlaser(λ =248nm). Thesubstratetemperaturewasheldat750
◦C, and a partial oxygen pressure of 250 mTorr was maintained inside the chamber throughout the
deposition. Immediatelyafterdeposition,thesubstratetemperaturewasloweredto530◦C,andthe
samplewasannealedin500Torrofultra-pureoxygenfor1hour.
Amutuallycompatiblegrowthregime(i.e. temperatureandpressure)for YBCO andLCMO
was achieved through interval deposition in which material is deposited by high frequency pulses
followedbyanextendeddwelltimebetweenthedepositionofeachunitcell(u.c.).13 Undamped
RHEED specular intensity oscillations (not shown) were observed for both YBCO and LCMO
layersoveralargenumberofcyclesallowingforalayer-by-layergrowthwithu.c. control. Theleft
inset to Fig. 1 shows the post growth RHEED image for the SL along the (0 0 1) direction. The
presenceofunbrokencrystaltruncationrodsuptothesecondordercombinedwithwelldefined
specular(00)andoffspecularBraggreflectionstestifiestothequalityofthe2-Dgrowth.
To determine the structural properties of the SL, we performed X-ray scattering at beamline
5-BM-D ofthe AdvancedPhoton Source(APS) atArgonne National Laboratory. Figure1 shows
thespecularX-raydiffractionalongthe(00L)crystaltruncationrodasafunctionofthemagnitude
of the out-of-plane momentum transfer vector, Q . As seen in Fig. 1, the SL shows all expected
z
Braggreflectionsforc-axisorientedlayersandKiessigfringes testifying tothequalityofthefilm
andsharpnessoftheinterfaces. BasedonthepositionoftheYBCO(005)reflection,theaverage
c-axislatticeconstantfortheYBCOlayersintheSLis11.70A˚,whichagreeswiththereported
bulkvalueforoptimalstoichiometry.14
In order to establish the electronic qualities of the samples, we investigated the dc transport
properties of single layer YBCO and LCMO films and the SL. Figure 2 shows resistivity versus
temperatureforsinglelayersofYBCO(leftaxis)andLCMO(rightaxis). AsseenfortheYBCO
film, thesuperconducting transitionT takes place at 93K attestingto theproper optimallydoped
c
stoichiometry. InthecaseoftheLCMOsample,themetal-to-insulatortransitiontakesplaceatT
MI
= 212 K (inflection point). Note, the previously reported values of T for LCMO films grown
MI
undersimilarconditions(i.e. substrate,thicknessetc.) butwithouttheuseofintervaldepositionare
typically<170K.11 InaconventionalgrowthtoachieveanelevatedT asinourfilms,significantly
MI
3
increasedfabricationtemperatureswouldberequiredwhich isadetrimentalconstraintoncuprate
growth.15 Thestabilizationofmaterialphasesoutsideofregionsofthermodynamicstabilityisa
hallmarkofintervaldeposition,andtheelectronicandmagneticpropertiesofLCMOareknownto
dependsensitivelyonstoichiometryandthepresenceofdefects.16,17 Together,thesehelpexplain
theoriginoftheenhancementsseeninthespectroscopydatabelow. Inaddition,thetemperature
dependent resistance for the SL is plotted on the bottom left axis of Fig. 2. A suppression in the
criticaltemperatureT =56.7Kisobservedandismostlikelytheresultofholedepletionthrough
c
interlayerchargetransferascorroboratedbytheXASdatabelow.
Next we explored the electronic and magnetic structure with XLD and XMCD resonant soft
X-rayspectroscopies. Theexperimentswerecarriedoutatthe4-ID-CbeamlineoftheAdvanced
Photon Source in Argonne National Laboratory. In the XLD studies at the Cu L edge, we inves-
tigated the orbital occupation by measuring the difference in absorption for polarizations in the
ab-planeandalongthec-axis,whiletheXMCDexperimentsattheCuandMnLedgesobtained
information regarding element specific magnetic moments by measuring the differences in the
absorptionofrightandleftcircularpolarizations. Spectrawererecordedatanincidentangleof15
degreesoutofplanesimultaneouslyinbothfluorescenceyield(FY)andtotalelectronyield(TEY)
acquisitionmodes.
Thelinearpolarization-dependentCuL absorptionspectraoftheSLispresentedinFig. 3(a).
3
FirstweconsiderthebulksensitiveFYdatasetwhichshowsthestrongXLDexpectedfortheCu2+
(3d9)state ofcuprates.18,19 Note,the in-planesignalstemsfrom formallydivalentCuwithin the
CuO sheets. Severalimportantfeaturescanbeidentifiedinthespectrumofwhichtheexcitonic
2
linenear931 eVisthemost prominent andcorrespondstotransitions fromtheCu2pcorelevels
into the unoccupied bands of mainly Cu d orbital character (2p63d9 → 2p53d10). A clear
x2−y2
shoulderisobservedonthehighenergysideofthewhiteline. Theshoulderisascribedtotransitions
coupled with Cu ligand hole states (2p63d9L → 2p53d10L) and is the well-known signature of
theZhang-Ricestate.20 AnotherimportantfeatureoftheFYspectrumisconnectedtothec-axis
polarization,wherethemaximumoftheabsorptionpeakisshiftedby0.3eVtohigherenergy;the
largernumberofligandholestatesinthechainstransferspectralweightfromtheexcitoniclineto
thehighenergyshoulder. Sincethed orbitaloftheCuO planesisoccupied,theoutofplane
z2−r2 2
polarizationmainlyprobesunoccupiedstatesfrommonovalentCuinthechainsofd orbital
y2−z2
character.
In the TEY data, distinct transformations in polarization dependence and lineshape manifest
4
in the spectra indicating charge transfer across the interface leading to a reconstruction in the
orbital scheme for interfacial Cu ions. As indicated in Fig. 3(a), the position of the Cu white
lineisshiftedby0.5eVtolowerenergyintheTEYspectrum,whichexceedsthevalue(0.4eV)
reportedin theprior study. This substantialchemical shift isthe markof interfacialchargetransfer.
Through acomparison to Cu1+ and Cu2+ reference materials,an approximate calculationyields a
chargetransferamplitudeof0.25epercopperion.21 Furthermore,theshoulderathighenergyisnot
observedintheTEYdataconfirmingthedepletionofholesfromtheYBCOregionoftheinterface.
WhiletheevolutionintheamplitudeofthehighenergyshoulderisacommonfeatureofYBCO
XASdopingprofiles,theshiftinthewhitelineisunexpected.18 Previously,thishasbeenattributed
tothereconstructionofthed orbitalthroughtheformationofacovalentbondwithMnthrough
z2−r2
apical oxygen across the interface.8,22 Confirming this picture, the large linear dichroism of the
bulk is no longer present, and the absorption along the c-axis now even surpasses that of the the
in-planefortheTEYspectrum.
Finally, we turn our attention to magnetism on Cu and Mn. Figure 3(b) shows the XMCD
spectra at both the Mn and Cu L-edges acquired in TEY mode. As anticipated for a FM system,
thestrongmagneticdichroismreachesamaximumvalueof37.4%attheMnL edge. OntheCu
3
L-edge,however,onlyamoderatemagneticsignalispresent,whichindicatesanuncompensated
magneticmomentonCuneartheinterface. FromthesignoftheXMCDonbothMnandCuedges,
an antiparallel alignment of Cu and Mn moments coupled across the interface can be deduced.
The magnitude of the dichroism (6.9 %) a the Cu L -edge is up to a factor five larger than the
3
previouslyreported values providing additional evidence for the high quality ofthe interface.7 The
measurements at the Cu L-edge were repeated at 100 mT without loss in the magnitude of the
magneticdichroism.
In summary, our resonant soft X-ray spectroscopy study of the cuprate-manganite interface
hasconfirmedthepresenceofbothspinandorbitalreconstructions. Weobservedincreasesinthe
dichroicsignalandthechemicalshiftattheCuLedgesignalinganenhancementintheinduced
momentonCuandthedegreeofinterlayerchargetransfer,respectively. Furthermore,anincreasein
T hasbeenobservedformanganitelayersgrownwithintervaldeposition,whichisfundamentally
MI
relatedtoitsmagneticandelectronicproperties.
J.C.wassupportedbytheDOD-AROunderGrantNo. 0402-17291andtheNSFunderGrant
No. DMR-0747808. Work at the APS is supported by the U.S. DOE under grant no. DEAC02-
06CH11357.
5
REFERENCES
1A.I.Buzdin,Rev.Mod.Phys.77,935(2005);F.S.Bergeret,A.F.Volkov,andK.B.Efetov,Rev.
Mod.Phys.77,1321(2005).
2Z. Sefrioui, D. Arias, V. Pena, J. E. Villegas, M. Varela, P. Prieto, C. Leon, J. L. Martinez,
and J. Santamaria, Phys. Rev. B 67, 214511 (2003); V. Pena, Z. Sefrioui, D. Arias, C. Leon, J.
Santamaria,M.Varela,S.J.Pennycook,andJ.L.Martinez,Phys.Rev.B69,224502(2004).
3T.Holden,H.-U. Habermeier, G.Cristiani,A.Boris, A.Pimenov,J.Humlic¨ek,O.I.Lebedev, G.
VanTendeloo,B.Keimer,andC.Bernhard,Phys.Rev.B69,064505(2004).
4F.S.Bergeret,A.F.Volkov,andK.B.Efetov,Phys.Rev.Lett.86,4096(2001).
5F.S.Bergeret,A.F.Volkov,andK.B.Efetov,Phys.Rev.B69,174504(2004).
6J. Stahn, J. Chakhalian, Ch. Niedermayer, J. Hoppler, T. Gutberlet, J. Voigt, F. Treubel, H-U.
Habermeier,G.Cristiani,B.Keimer,andC.Bernhard,Phys.Rev.B71,140509(2005).
7J.Chakhalian,J.W.Freeland,G.Srajer,J.Strempfer,G.Khaliullin,J.C.Cezar,T.Charlton,R.
Dalgliesh,C.Bernhard, G.Cristiani,H.-U.Habermeier,andB. Keimer, NaturePhysics2,244
(2006).
8J. Chakhalian, J.W. Freeland, H.-U. Habermeier, G. Cristiani, G.Khaliullin, M. vanVeenendaal,
andB.Keimer,Science318,1114(2007).
9J.W.Freeland,J.Chakhalian,H.-U.Habermeier,G.Cristiani,andB.Keimer,Appl.Phys.Lett.
90,242502(2007).
10X.Yang,A.Yaresko,V.Antonov,andO.Andersen,arXiv:0911.4349(unpublished).
11R.Werner,C.Raisch,A.Ruosi,B.A.Davidson,P.Nagel,M.Merz,S.Schuppler,M.Glaser,J.
Fujii,T.Chasse,R.Kleiner,andD.Koelle,Phys.Rev.B82224509(2010).
12Y.Kalcheim,T.Kirzhner,G.Koren,andO.Millo,Phys.Rev.B83064510(2011);K.Dybko,K.
Werner-Malento,P.Aleshkevych,M.Wojcik,M.Sawicki,andP.Przyslupski,Phys.Rev.B80
144504(2009);A.F.VolkovandK.B.Efetov,Phys.Rev.Lett.102077002(2009).
13G.Koster,G.J.H.M.Rijnders,D.H.A.Blank,andH.Ragalla,Appl.PhysLett.74,3729(1999);
M.Kareev,S.Prosandeev,B.Gray,J. Liu,P.Ryan,A.Kareev,E.J.Moon,andJ.Chakhalian,J.
Appl.Phys.109,114303(2011).
14R.Liang,D.A.Bonn,andW.N.Hardy,Phys.Rev.B73,180505(2006).
15O.I.Lebedev,G.vanTendeloo,andS.Amelinckx,Phys.Rev.B58,8065(1998).
16P.Schiffer,A.P.Ramirez,W.Bao,andS.-W.Cheong,Phys.Rev.Lett.75,3336(1995).
6
17J.Aarts,S.Freisem,R.Hendrikx,andH.W.Zandbergen,Appl.Phys.Lett.72,2975(1998).
18N. Nu¨cker, E. Pellegrin, P. Schweiss, J. Fink, S. L. Molodtsov, C. T. Simmons, G. Kaindl, W.
Frentrup,A.Erb,andG.Mu¨ller-Vogt,Phys.Rev.B51,8529(1995).
19C.T.Chen, F.Sette,Y.Ma,M.S.Hybertsen,E. B.Stechel,W.M.C.Foulkes,M.Schulter,S-W.
Cheong, A. S. Cooper, L. W. Rupp, Jr., B. Batlogg, Y. L. Soo, Z. H. Ming, A. Krol, and Y. H.
Kao,Phys.Rev.Lett.66,104(1991).
20F.C.ZhangandT.M.Rice,Phys.Rev.B37,3759(1998).
21F.M.F.deGroot,J.ElectronSpectrosc.Relat.Phenom.67,529(1194).
22JianLiu,B.J.Kirby,B.Gray,M.Kareev,H.-U.Habermeier,G.Cristiani,J.W.Freeland,andJ.
ChakhalianPhys.Rev.B84,092506(2011).
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101 O (002)
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( -3 L 7)
y 10 0
vit YBCO (005) O (0
flecti 10-5 YBCO (004) YBC O (008)
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R B
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2.0 2.5 3.0 3.5 4.0
Q (Å-1)
Z
FIG.1. (Coloronline)X-rayreflectivitydata(dots)anderrorbar(solidline)fora[YBCO(9u.c.)/LCMO
(26u.c.)]×10SLSrTiO . TheinsetontheleftshowstheaftergrowthRHEEDimageforthesameSLona
3
)
7
SrTiO300substrate. Whitetriangular markersindicatetheweakhalf-order signalwhichisattributedtothe
(
O
PbnmcCrystalsymmetryofthecappingLCMOlayer. Theright-handinsetisahighangleannulardarkfield
B
Y
scanningtransmissionelectronmicroscopyimageoftheYBCO/LCMOinterfacialregion. Labelsindicate
differentlayers.
8
) 50 R
m 160
150 u.c. LCMO e
c s
50 u.c. YBCO 40 i
Ω s
µ 120 [YBCO (9 u.c.)/LCMO (26 u.c.)]x3 it
v
y ( 30 yti
vit 80 20 1(
ti 0
s 3
esi 40 10 Ωµ
R

c
0 0 m
)
) 60
Ω
(

e 40
c
n
a 20
t
s
i
s 0
e
R
0 50 100 150 200 250 300
Temperature (K)
FIG.2. (Coloronline)Temperature-dependentdctransportfor50u.c. YBCOand150u.c. LCMOsingle
layersandthe[YBCO(9u.c.)/LCMO(26u.c.)]×3SLonSrTiO substrates. Measurementswereperformed
3
intheconventionalvanderPawconfiguration.
9
(a)
Cu L Edge T = 15 K
6 3
Z.R.
Pol. in ab-plane
))
u.u.5 Pol. along c-axis
..
aa
((
4
nn
FY - Bulk
oo
ii
ptpt3
rr
oo
ss2
bb
AA
1
TEY - Interface
0
924 926 928 930 932 934 936
EEnneerrggyy ((eeVV))
(b)
10 x 2
)
% 0
(

D -10
Mn L Edge
C
M -20 Cu L Edge
X
-30 T = 15 K
H = 5 T
-40
630 640 650 660930 940 950 960
Energy (eV)
FIG.3. (Coloronline)SoftX-rayspectroscopiesofthe[9u.c. YBCO/26u.c. LCMO]×3SLonSrTiO .
3
(a) XLD measurements at the Cu L -edge. The top and bottom sets of spectra correspond to FY and
3
TEY detection modes, respectively. Due to TEY’s shallow probing depth, only Cu at the first interface
predominantlycontributestotheCuLintensityinthismode. (b)XMCDmeasurementsattheMn(left)and
Cu(right)L-edgesinTEY.Note,toruleoutartifactstheXMCDdatasetswerecheckedatbothmagnetic
fieldorientations.
10