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Determination of the cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron spectroscopy

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Determination of the cation site distribution of the

spinel in multiferroic CoFe2O4 / BaTiO3 layers by

X-ray photoelectron spectroscopy

T. Aghavnian, J.-B. Moussy, D. Stanescu, Rachid Belkhou, N Jedrecy, Hélène

Magnan, P. Ohresser, Arrio M.-A., Ph. Sainctavit, Antoine Barbier

To cite this version:

T. Aghavnian, J.-B. Moussy, D. Stanescu, Rachid Belkhou, N Jedrecy, et al.. Determination of the

cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron

spectroscopy . Journal of Electron Spectroscopy and Related Phenomena, Elsevier, 2015, 202,

pp.16-21. �10.1016/j.elspec.2015.02.006�. �cea-01365822�

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ContentslistsavailableatScienceDirect

Journal

of

Electron

Spectroscopy

and

Related

Phenomena

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m /l o c a t e / e l s p e c

Determination

of

the

cation

site

distribution

of

the

spinel

in

multiferroic

CoFe

2

O

4

/BaTiO

3

layers

by

X-ray

photoelectron

spectroscopy

T.

Aghavnian

a,b

,

J.-B.

Moussy

a

,

D.

Stanescu

a

,

R.

Belkhou

b

,

N.

Jedrecy

c

,

H.

Magnan

a

,

P.

Ohresser

b

,

M.-A.

Arrio

d

,

Ph.

Sainctavit

d

,

A.

Barbier

a,∗

aCEA/Saclay,DSM/IRAMIS/SPEC,CNRSUMR3680,F-91191Gif-sur-Yvette,France bSynchrotronSOLEIL,L’OrmedesMerisiersSaint-Aubin,F-91192Gif-sur-Yvette,France cSorbonneUniversités,UPMCUnivParis06,UMR7588,INSP,F-75005Paris,France dIMPMC,F-75015Paris,France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received24October2014

Receivedinrevisedform3February2015 Accepted7February2015

Availableonline16February2015 Keywords:

Oxides Ferrites

Cationdistribution

X-rayphotoelectronspectroscopy(XPS) X-raymagneticcirculardichroism(XMCD)

a

b

s

t

r

a

c

t

ThepropertiesofCoFe2O4/BaTiO3artificialmultiferroicmultilayersstronglydependonthecrystalline

structure,thestoichiometryandthecationdistributionbetweenoctahedral(Oh)andtetrahedral(Td)sites (inversionfactor).Inthepresentstudy,wehaveinvestigatedepitaxialCoFe2O4layersgrownonBaTiO3,

withdifferentCo/Feratios.WedeterminedthecationdistributioninoursamplesbyX-raymagnetic circulardichroism(XMCD),awellacceptedmethodtodoso,andbyX-rayphotoelectronspectroscopy (XPS),usingafittingmethodbasedonphysicalconsiderations.WeobservedthatourXPSapproach convergedonresultsconsistentwithXMCDmeasurementsmadeonthesamesamples.Thus,withina carefuldecompositionbasedonindividualchemicalenvironmentsitisshownthatXPSisfullyableto determinetheactualinversionfactor.

©2015ElsevierB.V.Allrightsreserved.

1. Introduction

TheferroelectricperovskiteBaTiO3coupledwiththe

ferrimag-neticspinelCoFe2O4 isawellreferencedsystemforthestudyof

themagneticstatesuponmultiferroiccouplings[1,2].Suchastudy requirestofullyanalyzethemagneticpropertiesofthespinelof suchsystems,amongthemthespinelinversionfactor.

WealreadyknowthatbulkCoFe2O4hasseveralkeyproperties

suchasstrongmagneto-crystallineanisotropy,high magnetostric-tionandsaturationmagnetization,aswellashighcoercivefields atroomtemperature[3,4].BulkCoFe2O4crystallizesinthespinel

structurewithalatticeparameterof0.838nm.Theminimalcubic unitcell(i.e.gatheringthe4rhombohedralunitcellsimposedby thefcclattice)contains8formulaunitsandthespinelspacegroup isFd ¯3m.TheCoandFecationscanoccupyeithertetrahedralTd oroctahedralOhsitesoftheoxygensublattice.Theirdistribution followstheformula [FeyCo1−y]Td[Fe2−yCoy]OhO4,withthespinel

inversionparameterybeing0fora normalspineland1 foran inverseone. Latticeenergycalculations [5]showthatCo2+ions

∗ Correspondingauthor.

E-mailaddress:abarbier@cea.fr(A.Barbier).

favorOhsitesinCoFe2O4,thusaninversespinelstructureis

pre-dicted.Inpracticeyisusuallysmallerthan1,especiallyinthinfilms

[6].

Tothebestsofourknowledge,apreciseanalysisofthecation distributioninsidethespinelofBaTiO3–CoFe2O4epitaxialthin

lay-ersmultiferroicshasnotbeenlookedintobymostofthemain references on the subject of magneto-electric coupling [2,7,8]. However,thedistributionofthosecationswithinthespinel struc-turestronglyaffectstheelectricandmagneticpropertiesofthe samples.Forinstance,theresultingmagneticmomentwillbe dif-ferentdependingonthecationdistribution(anillustrationofthe conceptisdetailedforMnFe2O4ferriteinRef.[9]).

Itisthereforeofhighimportancetodeterminethisionsite dis-tributioninordertodeducetheglobalmagneticmoment,aswellas foranymagneticcharacterization,andinparticularinthecontext of magnetoelectriccoupling. X-raymagnetic circular dichroism (XMCD)isconsideredthemostprecisetechniquetodeterminethe inversionparameter;howeveritrequiresaccesstoasynchrotron beamline.Alternatively,authorsoftenchoosetoestimatethecation distributionusingX-rayphotoelectronspectroscopy(XPS),asitisa commonapparatusforspectroscopyinlaboratories,andsinceitis obviousthatthespectrahavetocontainallnecessaryinformation likeintheX-rayabsorptionspectroscopy(XAS)case.Nevertheless,

http://dx.doi.org/10.1016/j.elspec.2015.02.006

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frompreviousreportsnoclearandreliablestrategyfortheXPS fit-tingprocedurewasestablished.Itisstillunclearhowtocorrectly exploitXPSspectratoextractthatdata,especiallyconsideringthat agivenfittingmethodtendstohaveanon-negligibleinfluenceon theresult,andmethodsseemtovaryfrompapertopaper[10,11], sometimesleadingtocontroversies[12,13].Thelackofdirect com-parisonwithXMCDdoesnotallowtoconcludeonthepertinence oftheproposedapproaches.

OuraimistodefineareliablestrategyforXPSdataprocessing basedonphysicalconsiderationsandconsistentwithXMCD mea-surements,inordertoextracttheadequatecationdistributionin CoFe2O4thinfilms.Forthatpurpose,weusewell-defined

epitax-ialthinfilmsofCoFe2O4(001)grownonBaTiO3underlayers.The

choiceofanalyzingspinelsampleswithdifferentCo/Feratioswas adoptedtoinsurethestabilityofthemethodforaseriesof sam-plesforwhichthecationorganizationislikelytovary.Weexpect thatthepresentmethodcanbeextendedtoalargevarietyofother spinels.

Maghemiteistheoxidizedformofmagnetitewhereallthe diva-lentironhavebeenreplacedbytrivalentones.IndeedthreeFe(II) ionscanbereplacedbytwoFe(III)ionsandavacancy. Magnemi-tizationofspinelisobservedwhenpartofFe(II)ionsarereplaced byFe(III).Byconvention,wediscussthedegreeofspinelinversion independentlyofthedegreeofmaghemitization,whichbecomes anadditionalparameterwhenCo/Fediffersfrom0.5.Inthesame manner,wedefinethecationicstoichiometryastheCo/Feratio regardlessofthedivalent/trivalentdistribution.

2. Experimental

ThreesamplesofCoFe2O4/BaTiO3/SrTiO3(001)wereprepared

by Atomic-Oxygen assisted Molecular Beam Epitaxy (AO-MBE)

[14–16].

The growth was performed in a dedicated AO-MBE setup workinginthe10−10mbarpressurerangeequippedwitha radio-frequency plasmasource,anAugerelectron spectroscopy(AES) analyzer,areflectionhighenergyelectrondiffraction(RHEED)gun andconnectedinultrahighvacuumconditionstoaseparate ultra-highvacuumchamberwithanX-rayphotoelectronspectroscopy apparatus.Duringgrowth,thesamplesrotatecontinuouslyaround thesurfacenormaltoobtainhomogenousfilms.

Themainfeatures ofthesamplesatdifferentstagesof their growtharedetailedinTable1.Theymainlydifferbyhaving dif-ferentCo/Feratios.TwooftheSrTiO3(001)substrateswere1%Nb

doped(forsamplesnumber1and2),thelastwasundoped(sample 3).Allthreesubstrateswereannealedinairat900◦Cfor3hafew weeksbeforedepositioninordertorestorethenominal stoichiom-etryandgetridofthepolishinginduceddefects.Wefirstperformed theepitaxialgrowthofBaTiO3(001)onSrTiO3(001)inthe

con-ditionsdetailedin[14].BaandTiweredepositedfromindividual Knudsencells.Thegrowthprocesswasmonitoredinrealtimeby RHEED,whichisatechniquethat permitstocontrolthecrystal quality.ThestoichiometryofthesampleswascheckedbyAESand XPSafterthegrowth.

Table1

MaincharacteristicsoftheCoFe2O4/BaTiO3/SrTiO3samples.

Sample 1 2 3

BaTiO3thickness(nm) 15 13 11

CoFe2O4thickness(nm) 6 6 6

XPSCo/Fe 0.28 0.48 0.63

SpinelinversionparameteryXMCD 0.90 0.80 0.90

SpinelinversionparameteryXPS 0.808 0.806 0.824

%Fe3+

OhbyXMCD 55% 60% 55%

%Fe3+

OhbyXPS 59.6% 59.7% 58.8%

AftertheBaTiO3 (001)crystallinegrowththesampleswere

extractedfrom theAO-MBEchamber andmounted ona differ-entsampleholderoptimizedforCoFe2O4 growth,thenlater

re-introducedinthesameepitaxychamber.Thecarbonobservedby AESontheBaTiO3,duetoair-exposure,wasremovedby45min

highbrillianceoxygenplasmaexposure.Thentheferritegrowth wasperformedasinRef.[15],andatatemperatureof450◦C.Then again,realtimeRHEEDandpost-growthinsituAESandXPS tech-niqueswereused.

TheXPSapparatususestwinanodeswithaXR3X-raysourceand thehemisphericalanalyserAlpha110fromsocietyThermoElectron Corporation.XPSspectrawereobtainedbyusingAl-K˛(1486.6eV) andMg-K˛(1253.6eV)X-raysources.Theprecautionofusingtwo sourcesistoinsurethattheAugerorphotoemissionpeaks can-not beconfused.A widescan of thesampleswasfirst realized (0–1200eV)todetectallthespeciesinthesample.Thenwefocus onFe2p,Co2pandO1s.Thosespecificenergyintervalswerethen scannedmultipletimes–typically10scansforCoandFe,5forO–in ordertomaximizethesignaltonoiseratio.Theenergyresolutionof theXPSspectraistypicallyaround0.1eV.Analysiswasperformed usingthesoftware“AvantageDataSpectrumProcessing”TM.

TheXMCDmeasurementswereperformedonDEIMOS (Dichro-ismExperimentalInstallationforMagneto-OpticalSpectroscopy) beamlineatsynchrotronSOLEIL(Saint-Aubin,France).DEIMOSis abeam-linededicatedtothestudyofthemagneticandelectronic propertiesusingpolarizedlight.ThemainadvantageofXMCDover othertechniquestoinvestigatemagneticbehaviorisitschemical and orbitalselectivity.Theenergyrange is 350–2500eV,which allowsthestudyofL2,3edgesofFeandCoandtheKedgeofO.The

energyresolutionisE/Efrom6000to10,000.Theoverall prop-ertiesofthebeamline[17]giveusveryclearspectradespitethe smallthicknessofthethinfilmofspinelof6nm.Measurements wereperformedat4K.Thecryo-magnetdelivers±7Talongthe beamdirectionand±2Tperpendiculartothebeam.AllourXMCD measurementswecarriedoutat4K,withanincidenceangleof30◦ withrespecttothesurfaceandamagneticfieldof5Talongthe beamdirection.

3. Results

TheRHEEDpatterns wererecordedduringthedepositionof both BaTiO3 and CoFe2O4.Anexample offinal RHEED patterns

is reportedin Fig.1. It indicatesthat thesamplesare epitaxial and single crystalline.Also, we can observefrom the compari-sonofsample3(non-stoichiometric,Co/Fe=0.63)withareference stoichiometricsample(Co/Fe=0.50)thattheRHEEDpatternsare similar.Moderatestoichiometrychangesdonotseemtoaffectthe observationsmadeusingtheRHEEDtechnique.X-rayreflectivity measurementsperformedat8keVgaveusanaveragedeposition rateof0.06nm/minforBaTiO3andof0.13nm/minforCoFe2O4.The

deducedthicknessesofdepositionaregiveninTable1.

Becauseouraimistodeterminethecationdistributionbetween octahedralandtetrahedralsitesinthespinelusingXPS,itis nec-essarytohaveareliableandwellacceptedbenchmark.Thusthis distributionwasfirstdeterminedusingXMCDmeasurementson DEIMOSbeamline.ThemeasurementswereperformedattheL3

edgeofFearound716.7eV.Theexperimentalplotsarepresented inFig.2, left,forthedifferentsamples.The similarformof the XMCDplotsindicatesthattheaverageelectronicstructureforFe issimilarforthethreesamples.Inaddition,onenoticesthatthe samplesareinversespinel.Thoseplotswerefittedusingalinear combinationofthetheoretical[16,18]spectraofFe3+in

octahe-dralandtetrahedralsitesandofFe2+inoctahedralsites(Fig.2,

middle),asthesethreecomponentsaresufficientsinceFe2+isnot

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Fig.1.RHEEDpatternsafterthedepositionofCoFe2O4onBaTiO3–stoichiometricsample(top),sample3(bottom),[100]direction(left),[110]direction(right). absorptionspectratakenfrom[19,16]originatefromacrystalfield

atomicmultipletapproach,takingintoaccounttheoccupied3d electronconfigurations,thecrystalfield,thespin–orbitcoupling andelectron–electroninteractionswithinFe,andthehybridization of3delectronstoothervalenceelectrons.

Theleastsquaresmethodwasappliedusingtwoparameters, theFe3+proportiononthetotalofFe2+andFe3+cations(closeto

1asFe2+appearedneglectful),andtheFe3+Ohproportiononthe

totalofsitesFe3+OhandFe3+Td.Thismethodwasusedtoconclude

onthebestfittingvalues(Fig.2,right).

ThepercentageofFe3+inoctahedralsitesisrespectivelyof55%,

60%and55%forsamplesnumber1,2and3.Wethusconcludethat theCoFe2O4thinlayersgrownonBaTiO3crystallizemainlyinthe

inversespinelstructure.

Fig.2.ExperimentalXMCDspectraattheL2,3edgeofFeinthesamples(a);theoreticalXMCDplotsofFe3+Oh,Fe3+TdandFe2+Oh(b);fromRef.[19],fittedXMCDspectraof

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Fig.3.XPSspectrumofCoinCoFe2O4forsample3(a).XPSspectrumofFeinCoFe2O4forsample3(b).

TheXPSspectrathatwemeasuredforCoandFearepresentedin

Fig.3.Themaincomponentsarethepeakscorrespondingto elec-troniclevels2p3/2and2p1/2andtheassociatedshake-upsatellites

forbothmetals.Theshake-uppeaksareattributedtothetransition ofanelectronfroma3dorbitaltotheempty4sorbitalduringthe ejectionofthecore2pphotoelectron[20].

Iftheobtainedspectracanbesimilarfordifferentlaboratories, theexploitationmethodtendstovaryfrompapertopaperin liter-ature.Zhouetal.[10]fitXPSspectraonbulkandpowderCoFe2O4

anddifferentiatesTdandOhsites,butdonotusethesame back-groundasusforironnorfitshake-upsatellites.Wangetal.[11]fit alsoCoandFewithtwopeaks,andaddthefittingoftheshake-up satellite,butonlyconsiderthe2p3/2ionlevelsandnotthe2p1/2

aswell.Thefollowingparagraphsdetailourapproachtowardsthe exploitationofthoseXPSspectra.

Firstofall,beyondthequestionofsiteoccupation,theXPS spec-traallowusalsotoestimatethestoichiometryofthesamples.This wasdoneusingaShirley-typebackgroundbyfittingtheCo2p3/2

andFe2p3/2peaks(octahedralsitesandtetrahedralsitesand

shake-up)anddeducingtheirarea.Letsnoticethatthisdecompositionhas morevaliditythantryingtohighlightaseparationbetween differ-entoxidationstatesofironasXMCDmeasurementshaveshown thatFe2+islessthan1%ofFe3+.Thisareaisthencorrectedusing

thecorrespondingScofieldfactors[21].Wechosetoestimatethe Co/FeratiobycalculatingtheonebetweentheScofield-corrected intensityareaofCo2p3/2andFe2p3/2.Thechoiceof2p3/2over2p1/2

wasbecauseofthehigherintensityofsignalgivingusmore pre-cision.Thisratioisfoundtoberespectivelyof0.28,0.48and0.63 (Table1)forsamples1,2and3.

Experimentally,itiseasiertodiscriminatethedifferent contrib-utionstothepeaksfortheCospectrathanfortheFespectra.This observationwasmostlyconfirmedduringthefittingprocesswith the“AvantageDataSpectrumProcessing”TMsoftware,whichwas

morereproducibleforCo.ThemainCo2p3/2peakwasdecomposed

intotwodifferentpeaks(Co2p3/2OhandCo2p3/2Td(Fig.4))

how-everthemainFe2p3/2 peakwastoobroadanddidnotallowan

obviousdiscriminationofindividualcomponents(Fe2p3/2Ohand

Fe2p3/2Td)bythisfittechnique.Asaconsequence,wechosethe

XPSspectraforCoasareferenceforestimatingthespinelinversion parameter,ratherthanFe.ThisdecompositioninOhandTdpeaks iscommononFe[22],andweproceededbyanalogytodothesame onCo.

WesearchamethodologyallowingtofittheCo2pXPS spec-truminareproduciblewaybasedonphysicalpropertiesinterms ofelectronicstructureinordertoquantitativelyassesstheratios ofOhandTdsites.Thismethodisbaseduponthefollowing con-straints.Firstofall,weuseaShirleybackgroundthatcoversallthe energyrangefromCo2p3/2toCo2p1/2.Asaguardrule,forallfits

ofallelements,weusedaShirleybackground.Thenextstepisto

imposeanidenticalspinelinversionparameterfortheCo2p3/2and

Co2p1/2states,whichimpliesthattheratiobetweentheareasof

theXPSpeaksofCoinTdandOhsites,Co-Td/Co-Oh,mustbethe sameforbothCo2p3/2 andCo2p1/2.Thisconstraintisrelaxedat

theendofthefitandthefitisacceptedifthereisnomorethan 2%shiftfroma centralvalue.Thethirdconstraint derivesfrom therigidbandapproximation[23]linkedtothecontinuityinthe perceivedenvironmentforthedifferentsites.Itimpliestokeep,for thedecompositionofCo2p3/2andCo2p1/2inpeaksassociatedtoOh

andTdsites,theenergygapbetweenbothsitesECo−Oh–ECo−Td con-stantforeachelectronicconfiguration.Thisconstraintisrelaxedat theendofthefitandthefitisacceptedifthereisnomorethan0.1eV shiftfromacentralvalue.Theshake-upisabroadpeakconsisting ofamixofdifferentpeaks,andisconsideredaswellrepresented byauniquepeak.Wefixnoconstraintsonitsenergypositioning forthesamereasons.

Thefitisperformedusingsymmetricpeaksinordertolimit thenumberoffittingparameters,evenifwecannotexcludesome asymmetry.Howeverthismethodissuitabletogiveustherelative areaofoctahedralandtetrahedralpeakswhichmakesaphysical sense.

Andindeedalltheseconstraintsputtogethergiveusaunique andperfectlyreproduciblefitfortheCo2pspectra(Table2). What-evertheinitialpositionbeforetheprocess,thefitalwaysconverges onthesamevalues.ThisfullyjustifiestotakeCoasareference. However,ifweusethesamefittingprocedureforfittingtheFe2p lines,weobservedthattheresultsarestronglydependentofthe initialparameter.Itismaybeduetolargerpeaksandpresenceof differentcomponents(Fe2+andFe3+).Moreover,inthesameenergy

range,thereisalsotheKLLAugerofoxygen(Figs.3and5)whichis mixedwiththeshake-upofFe2p1/2.

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Table2

BindingenergypositionofthefittingpeaksinXPSforsample3.

Sample3 Co Fe 2p3/2Oh 786.95eV 717.50eV 2p3/2Td 789.35eV 720.15eV 2p3/2shake-up 793.35eV 725.65eV 2p1/2Oh 802.35eV 731.65eV 2p1/2Td 804.6eV 733.05eV 2p1/2shake-up 809.5eV 740.6eV

Fig.5. XPSspectrumofFeinCoFe2O4–decompositioninindividualcontributions ofthebestfitforsample3.

Becauseaninversespinelhas100%ofitsCoinOhsites(y=1), andanormalspinel100%inTdsites(y=0),wededucethespinel inversionparameter(y)tobeequaltotheoccupationpercentage ofthecobaltoctahedralsites.FromthefitofCo2pspectra,this percentagewasfoundtobe,forsamplesnumber1,2and3,of80.8%, 80.6%and82.4%respectively.Thoseresultsarewithina2%error marginlinkedtotheshiftafterreleasingconstraints.

WetriedtofittheFe2pXPSspectrainthesamemanner,but thereproducibilitywasaproblem.Indeed,weobtainedgoodfits correspondingtosignificantlydifferentinversionratios.Asa con-sequence,thosespectracouldnotbeusedasthereferenceforour XPSmethod.However,asweexaminethesamesample,itis pos-sibletoimposethesupplementaryconstraintofhavingthesame spinelinversionparameterasdeterminedfromtheCo2pspectrum. Usingthisaddedconstraint,wecanfindagoodfitthatcorresponds totheexpectedresults,andwhichismoreoverunique(Fig.5and

Table2).

ConcerningtheFe2p fitting,wefoundthatitisimportantto determinethebackgroundoveralargerangeincludingthehuge peakat≈748eVcorrespondingtotheKLLAugerpeakofoxygen, whichcontributesmassivelytothebackground.Itisherea bet-tersolutionthanswitchingtoanAlK˛sourcetoeliminatethe AugercontributionofO,astheAugercontributionofCowouldthen overlapinenergywithFe2p.

4. Discussion

The difference of binding energy between octahedral and tetrahedralsitesforsamplesnumber1,2and3arerespectively of2.75eV,2.23eVand2.63eVforFe2p(averagevaluesofFe2p3/2

andFe2p1/2),withina0.1eVmargin,andof2.40eV,2.35eVand

2.38eV for Co2p. Thesevalues are closeto theone found for CoFe2O4 nanoparticles[10](3eVforFe2pand2.10eVforCo2p),

wheretheamountofnormalspinelstructureishigherthaninour samples.Comparingourresultswithotherexperimentsonsimilar samplescangivediscrepancies[24]sincesomedonottakeinto accounttheCo2p1/2peakorhavenotfittedtheshake-upsatellite.

Itsoseemsthattakingintoaccountallparameters–background,

electroniclevels2p3/2 and2p1/2,shake-upsatellites– iskey in

findingaunifiedmethodofXPSanalysis.

Thefactthatthedifferenceinbindingenergybetween octahe-dralsiteandtetrahedralsiteinXPSisbiggerthatthedifferenceof absorptionedgesofthesamesitesisnotreallysurprising.Indeed, thescreeningofthecoreholeisreflectedinthespectraina com-pletelydifferentmannerforthetwoprocesses.

TheinversionparameteryobtainedusingtheXPSmethod,of 80.8%,80.6%and82.4%forsamplesnumber1,2and3,canbelinked totheestimatedpercentageofFe3+inoctahedralandtetrahedral

sites.Ifweconsiderthatinaninversespinel,Fe3+isinsimilar

pro-portioninbothOhandTdsites,andthatinanormalspinelallFe3+

areinOh sites,wefindthat%Fe(Oh)=(y×0.5+(1−y)×1)×100 whichis59.6%,59.7%and58.8%.Eachofthoseresultsislessthan 5%differentfromresultsfromXMCD–55%,60%and55%for sam-plesnumber1,2and3–whichisintheerrormarginoftheXPS technique.

Wecanalsoobservethatthespinelinversionparameteryseems fairlyindependentoftheCo/Feratio,soofthestoichiometryofthe spinel.Thisresultisquitesurprising[25].Indeed,for stoichiomet-rieswithlowCo2+cationscontent,wewouldhaveexpectediron

cationstoreplacesomeoftheOhsites,modifyingthusthe inver-sionparameter.Oramaghemitizationofthecrystal,whichwould meancationicvacanciesbutwouldimplyalsoasmallchangeofthe inversionparameter.

TheresultsindicatethatthespinelofourBaTiO3–CoFe2O4

mul-tiferroicthinlayerscrystallizesinamostlyinverseconfiguration andthusofhighmagneticmoment.

IfXMCDremainsarecognizedmethodwhenlookingfora pre-cisedeterminationofthespinelinversionparametery,weshow thatXPSspectracoupledtophysicalconstraintsfortheanalysis, cangiveacomplementaryestimation.BothXMCDandXPS meth-odsyieldverysimilarresults.ThisconvincesusthatXPScanbean excellentsynchrotronfreeapproach,whena5%erroronthesought informationremainsanacceptableprecision.

5. Conclusion

We have grown CoFe2O4/BaTiO3/SrTiO3 samples by

oxygen-assisted molecular beamepitaxy, fully controlledin crystalline structureandstoichiometry.ThespinelfilmofCoFe2O4was

stud-iedbothinXMCDandXPS.Toperformthatstudy,wefocusedon theFespectrainXMCDandtheCospectrainXPS.Tofitcorrectly theFespectrainXPS,itappearednecessarytoperformthescanon alargebandofenergyaroundFetoobtaintheactualbackground. Usingphysicalconsiderationstoimposeconstraintstothefitof theXPSspectra,wemanagedtoobtainquantitativevaluesofthe distributionoftheCoandFecationsbetweenoctahedraland tetra-hedralsitesofthespinel.Thosevalueswerefullyconsistentwith theXMCDresultsobtainedonthesamesamples,therefore con-firmingourapproach.Wealsohighlightedtheinversenatureof thespinel.Thismethodcanbeofmajorutilitytoobtainroutinely thosevaluesinlaboratoriesequippedwithanXPSapparatus,rather thanhavingtoperformthedeterminationonasynchrotron beam-line.Itisalsolikelyapplicabletoanyotherspinelferritessuchas CrFe2O4,MnFe2O4,NiFe2O4,CuFe2O4.

Acknowledgements

WeacknowledgeSOLEILforprovisionofsynchrotronradiation facilities and are gratefulto theSOLEIL-DEIMOSbeamline staff memberswhohelpedachievethoseexperiments.Thisworkwas partlysupportedbytheCNano-MAEBAgrant,forwhichwearealso grateful.

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Figure

Fig. 2. Experimental XMCD spectra at the L 2,3 edge of Fe in the samples (a); theoretical XMCD plots of Fe 3+ Oh, Fe 3+ Td and Fe 2+ Oh (b); from Ref
Fig. 3. XPS spectrum of Co in CoFe 2 O 4 for sample 3 (a). XPS spectrum of Fe in CoFe 2 O 4 for sample 3 (b).

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