Probing the magnetic moments of [Mn^III₆Cr^III]³⁺single-moleculemagnets : A cross comparison of XMCD and spin-resolvedelectron spectroscopy

Probing the magnetic moments of [Mn ^III ₆ Cr ^III ] ³⁺ single-molecule magnets—A cross comparison of XMCD and spin-resolved

electron spectroscopy

Andreas Helmstedt

, Niklas Dohmeier

, Norbert Müller

, Aaron Gryzia

, Armin Brechling

, Ulrich Heinzmann

, Veronika Hoeke

, Erich Krickemeyer

, Thorsten Glaser

, Philipp Leicht

, Mikhail Fonin

, Thomas Tietze

,

Loïc Joly

, Karsten Kuepper

aFacultyofPhysics,BielefeldUniversity,Universitätsstr.25,33615Bielefeld,Germany

bFacultyofChemistry,BielefeldUniversity,Universitätsstr.25,33615Bielefeld,Germany

cFachbereichPhysik,UniversitätKonstanz,Universitätsstr.10,78457Konstanz,Germany

dMaxPlanckInstituteforIntelligentSystems,Heisenbergstr.3,70569Stuttgart,Germany

eInstitutdePhysiqueetChimiedesMatériauxdeStrasbourg,UMR7504,CNRS-UniversitédeStrasbourg,BP43,23rueduLoess, F-67034StrasbourgCedex2,France

fInstitutfürFestkörperphysik,UniversitätUlm,89069Ulm,Germany

Keywords:

Spin-resolved

Photoelectronspectroscopy Augerelectronspectroscopy Single-moleculemagnets XMCD

a b s t r a c t

Single-moleculemagnets(SMM)ofthe[Mn^III6Cr^III]³⁺structuraltypepreparedonSiandgold-coated glasssubstrateshavebeeninvestigatedbyspin-resolvedelectronspectroscopy(SPES)andX-raymag- neticcirculardichroism(XMCD)attheMnL3,2edgeandinadditionbyXMCDattheCrL3,2edgeusing synchrotronradiation.Differencesbetweenthetwomethodsarediscussed.Despiteitsseverelimita- tionsfor3dtransitionmetals,aspinsumruleevaluationisneverthelessperformedfortheMn^IIIcentres inthe[Mn^III6Cr^III]³⁺SMMtoprovideasimplemeansofcomparingXMCDandspin-resolvedelectron spectroscopyresults.

1. Introduction

The annual amountof data being produced worldwide has reachedtheExabyte(10¹⁸byte)rangeandisincreasingrapidly[1].

Whiledatastoragemainlyreliesonharddiscdrives,fundamental limitations [2–4] of the magnetic recording principle define a maximumrecordingdensityofapprox1Tbit/inch² [2,5].Beyond thiscriticaldensity,thermalfluctuationsareabletoovercomethe energy barrieragainst magnetizationreversal,leading toa loss of storedinformation [2,3,6].Effortsare beingmadetoreplace bulk magnetic and semiconductor devices bysingle molecules, which canperformas memorycells, transistorsor even logical

∗Correspondingauthor.Tel.:+491784952881;fax:+495211066001.

E-mailaddress:helmstedt.andreas@gmail.com(A.Helmstedt).

1 Present address: Department of Physics, Osnabrück University, Barbarastraße7,49076Osnabrück,Germany.

devices [7–9]. A comprehensivereview of this field is given in [10].Single-moleculemagnets(abbrev.SMM)[11–13]represent a promising approach withregard toquantum computing and information storage [14–17]. Single-molecule magnets require individualaddressingtoretrievethestoredinformation—therefore firstsuccessfulattemptstodepositSMMonsurfaceswithapref- erentialorientation[18] orinperiodic3Darrays [19]represent importantstepstowardsfutureapplications.

A‘serendipitousapproach’[20]resultedinthediscoveryofthe firstsingle-moleculemagnetMn₁₂ac[21–23].However,adifferent methodinvolvingrationalbottom-upmoleculardesignwascho- sentoobtainthe[Mn^III₆Cr^III]³⁺single-moleculemagnet[24–26].

Itisrepresentativeofthegeneral[M^t₆M^c]ⁿ⁺structuretypethathas beenstudiedthoroughlyinthepastyears[24–31].

Thestabilityofthe[Mn^III₆Cr^III]³⁺SMMdeposited onvarious substratesagainstsoftX-rayirradiationwasinvestigatedinearlier work[32],indicatingtheimportantroleoftheanionsassociated withtheSMMcorefortheradiationstability.Firstspin-resolved

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Erschienen in: Journal of Electron Spectroscopy and Related Phenomena ; 198 (2015). - S. 12-19

electronspectroscopy(SPES)experimentsonSMMwereperformed [33] usingthemoststable [Mn^III₆Cr^III]³⁺ typewithperchlorate anions.Withthecontrolleddepositionofthe[Mn^III₆Cr^III]³⁺SMM onsurfacesbeinga centralquestionfor futureapplications,the depositionbehaviouronsurfaceswasstudiedaswellbyAFMand STM[34,35].Withintheworkpresentedhere,SPESmeasurements wereperformedattheL_3,2edgeoftheMnionsinthe[Mn^III₆Cr^III]³⁺

SMM,usingcircularlypolarizedsynchrotronradiationfortheexci- tationstep.XMCDmeasurementsattheMnandCrL3,2edgeswere carriedoutaswellinordertocrosscomparethemwiththeelectron spinpolarizationspectra.

2. Scientificbackground

Theavailabilityofhigh-qualitysynchrotronradiationsources withvariablepolarizationwasthekeyfactorfortheXMCDmethod tobecomea versatile,widelyused toolfor theinvestigationof themagneticpropertiesofsolids[36–38].However,thetechnique requiresthesamplestoexhibitamagneticorientation—usuallythis conditionisachievedbystrongexternalmagneticfieldsandlow sampletemperatures.

ThewidepracticalapplicabilityoftheXMCDmethodisbasedon thediscoveryoftheso-calledsumrulesfortheorbital(Tholeetal.

[39])andthespinmagneticmoment(Carraetal.[40]), relating themeasuredhelicity-dependentdifferenceinabsorptiontothe localspinandorbitalmagneticmomentsoftheabsorbingatom [41,42].Thisapproachallowstheelement-specificdetermination ofthelocalmagneticmomentspresentinthesample.However, theapproximationsusedtoobtainthesumruleslimittheaccuracy ofthederivedspinandorbitalmagneticmoments,see[43–47].

Especiallyinthecase of3dtransitionmetals,correctionstothe sumruleresultsarerequired,eitherbyapplyingcorrectionfactors (Teramuraetal.[48])orbycomparingtheXMCDdatatotheresults frommultipletcalculationsasshownbyPiamontezeetal.[49].The lattermethodhasbeenappliedsuccessfully toMn^IIIsystemsby Kuepperetal.[50].

TheapplicationofexternalmagneticfieldsintheXMCDexperi- mentrepresentsaninfluenceontheinvestigatedsystemsthathas thepotentialtochangetheconditionofthesampleswithrespectto theundisturbedstatewithoutexternalfields.Forthe[Mn^III6Cr^III]³⁺

SMMinvestigatedwithinthiswork,theapplicationoftheexter- nalmagneticfieldcauses distinctchangesin thespectral shape oftheMnL_3,2edgeabsorption,confirmingtheexistenceofsuch influences.

ExperimentsperformedonparamagneticGdin2001byMüller etal.[51]indicatethatspin-resolvedelectronspectroscopyofsam- plesintheparamagneticregimeabovetheCurietemperatureyields thesameinformationas XMCDdatawithouttheneed toapply externalmagneticfields:Theprimaryexcitationstepinvolvingcir- cularlypolarizedradiationisgovernedbytherelativisticdipole selection rules [52,53]for paramagnetic samplesas wellas for magneticallyorientedmaterials.Asaresult,theexcitationinboth casesgeneratesorientedcoreholesinthesample.Thecorehole orientationoriginating fromtheprimaryexcitation stepcanbe investigatedbyanalyzingthespinpolarizationofAugerelectrons followingthedecayoftheprimarycorehole.TheL2,3M2,3VAuger decayoftheMnconstituentsinthe[Mn^III₆Cr^III]³⁺SMMwasused toretrievetheorientationoftheprimarycoreholesfortheresults presentedhere,see[33].

AnXMCDexperimentperformedwithcircularlypolarizedlight leadstotwodifferentabsorptionyields[39]Y+and Y₋depend- ingonthelight helicity(thephoton angularmomentum)being parallelor antiparallel tothepreferentialmagnetic directionM presentinthesample,respectively.Whileinmagneticallyordered solidsMisdefinedbythemagnetization,forassembliesofnon- interactinglocalizedmagneticmoments,e.g.forSMM,anexternal

magneticfieldofsufficientstrengthBisnecessarytodefineM.As themajorityspinorientationofamagnetizedsampleisantiparal- leltoM,theavailabilityofempty3dvalencestatesleadstoY₊>Y₋ fortheL₃edgeofa3dtransitionmetalasinvestigatedwithinthis work.TheXMCDasymmetryA_XMCD=(Y+−Y₋)/(Y++Y₋)resulting fromtheXMCDexperimentiscloselyconnectedtothecorehole orientationcreatedinamagneticallynon-orderedsamplebyan excitationstepusingcircularlypolarizedradiation.Theexistence ofthiscloserelationshipbetweenMCDeffectsinphotoexcitation aswellasintheangulardistributionofphotoelectronsandthespin polarizationofelectronsemittedfromparamagneticsampleshas beendescribedwithintheframeworkoftheelectricdipoleapprox- imation,see[54].Theequivalencyofbothapproachesisrestricted tosituationsinwhichtheexchangesplittingisnotresolvedinthe MCDexperiment[55].Also,corelevelsplittingandothereffects causedbycore–valenceinteractioninmagneticsamples[56]need tobeconsidered.

3. Experimentalapproach 3.1. Samplepreparation

The [Mn^III₆Cr^III]³⁺ SMM investigated herewere synthesized according to the procedure described in earlier works [26,32].

Methanolic solutions of the SMM with a concentration of 9·10⁻⁵mol/lwereprepared.BasedontheresultsofanearlierXAS studyontheradiationsensitivityoftheinvestigatedSMM[32],the moststable[Mn^III₆Cr^III](ClO₄)₃SMMwithperchlorateanionswas chosenforthespin-resolvedelectronspectroscopymeasurements.

Anairbrushprocessusingacommerciallyavailableairbrushgun (GrafoT1,see[57])andhigh-puritynitrogenasworkinggaswas developedforthe[Mn^III₆Cr^III]³⁺samplepreparation:200␮lofthe methanolicSMMsolutionwereusedtocoatasquaresubstrateof approx.11mmedgelengthbyrepeatedlymovingtheairbrushnoz- zleacrossthesubstrateinaregularpattern.Thismethodallowed thedepositionofhomogeneousSMMdepositswithanareaofupto 100mm².Fortheresultspresentedwithinthiswork,Si(110)and gold-coatedglasssubstrates(see[58])wereused.Thelargehomo- geneoussampleareaallowedtheuseofsamplescanningduring thespinpolarizationmeasurements:Dataobtainedfromalarge numberofsamplepositionsaremergedintoonespinpolarization datapoint,therebyovercomingthelimitationofthemeasuringtime duetothe[Mn^III₆Cr^III]³⁺SMMradiationsensitivity.

Although opticalmicroscopy of the deposited [Mn^III6Cr^III]³⁺

revealslabyrinth-like,irregularstructuresonsmallscalesbelow 0.2mm(seeFig.1right),theseirregularitiesareaveragedoutdue tothespotsizeofapprox.1mm²usedforthesynchrotronradi- ationexperiments.Thehomogeneityofthedepositionprocessas wellastheconditionofthedeposited[Mn^III₆Cr^III]³⁺SMMhave beencheckedbyMnL_3,2edgeXAScharacterization.AllXASresults presentedwithinthisworkhavebeenobtainedinthetotalelectron yield(TEY)modei.e.bymeasuringthesamplecurrentandthere- forethetotalnumberofphoto-andsecondaryelectronsleaving thesample.

3.2. Spin-resolvedelectronspectroscopy

Thespin-resolvedelectronspectroscopy(SPES)measurements presented here have been performed at the beamlines UE52- SGM and UE56/2-PGM2 of the BESSY-II storage ring at the Helmholtz-Zentrum Berlin (HZB). Both beamlines usethe syn- chrotronradiationofpermanentmagnetundulatorsoftheAPPLE-II type and provide a range of photonenergies that extendfrom approx.90eVto1300eV,coveringthe640eVregionoftheMn L_3,2 edge.Fordetailedinformationconcerningthespecifications

Fig.1. Left:XAScharacterizationofa[Mn^III₆Cr^III](ClO4)3SMMsamplepreparedbythedescribedairbrushmethod.MnL3,2edgeabsorptionspectrarecordedatdifferent samplepositionsacrossthefull11mmby11mmsampleareaareshown.Right:MicroscopyimageoftheSMMsample—theirregularitiesoftheSMMdepositionvisibleat thismagnificationareaveragedoutduetothesynchrotronradiationspotsizeusedintheexperiment(blacksquare).

ofbothbeamlinessee[59–61].Anoff-focussamplepositionwas chosentoreducethesynchrotronradiationfluxatthesampleto 10¹¹photons/s/100mAwithrespecttotheradiationsensitivityof the[Mn^III₆Cr^III]³⁺SMM[32].

The experimental setupused for the spin-resolved electron spectroscopyexperimentsconsistedofaUHVapparatusequipped witha 90^◦ sphericalfield electronspectrometerin normalinci- dence, normal emission geometry. After energy analysis the collectedelectronsweretransferredtoasphericalMottpolarime- teroftheRicetype[62],usingascatteringenergyofupto45kV.The polarizationsensitivity(Shermanfunction)oftheMottpolarime- terat25kV(45kV)wasS_eff=−0.17±0.02(S_eff=−0.23±0.02).Note thattheuncertaintyofthepolarizationsensitivityasascalefac- tor is not included in the error bars of the SPES results given in Figs. 3 and 4. Detectionof the scattered and retarded elec- tronswasperformed bychannel electron multipliers[63] with a circular entry funnel (diameter 25mm) to maximize detec- tion efficiency. The electron spectrometer as well as the Mott polarimeter has been used successfully before[33,51,53,64,65]

and has been described in more detail elsewhere, see[53,65].

TheUHVsetupwascompletedbyanelectricallyisolatedsample stage in order toperform total-electron-yield mode XAS mea- surements,a sampletransfer andpreparation systemproviding UHVstorageforupto6samplesand aload-lockwithapump- downtime of less than 1h, allowingfrequent sample changes madenecessarybythelimitedsamplelifetimeundersoftX-ray exposure[32].

3.3. XMCDmeasurements

ComparativeXMCDinvestigationsof[Mn^III₆Cr^III](ClO₄)₃SMM havebeenperformedattheSIMbeamlineoftheSLSstoragering at the Paul-Scherrer institute in Villigen, Switzerland. The SIM beamlineisbasedonadoubleundulatorcomparabletotheUE56 undulatorsattheBESSY-IIstoragering.Bothundulatorsareslightly tiltedagainsteachotherandcanbeoperatedwithoppositehelicity.

However,onlyoneundulatorwasusedforthemeasurementspre- sentedwithinthiswork.Theavailableenergyrangeof90–2000eV [66]coverstheMnandCrL_3,2 edgeswhichwerestudiedinour experiment.Thephotonfluxatthesamplepositionwasreduced bytuningthebeamlinemonochromatortoafix-focusconstantof approx.20insteadoftheusualvalueof2.25andaddinganalu- miniumfilterinordertotakeintoaccounttheradiationsensitivity ofthe[Mn^III₆Cr^III]³⁺SMM.ThelowtemperatureXMCDsetupofthe IPCMSStrasbourg[66]providingsampletemperaturesofapprox.

2Kandamagneticfieldofupto6.9TwasusedtoobtaintheXMCD datapresentedhere.AllXMCDmeasurementsweredoneinTEY modebymeasuringthesamplecurrent.

4. Resultsanddiscussion 4.1. XASsamplecharacterization

Prior to the spin-resolved electron spectroscopy measure- ments, the [Mn^III₆Cr^III](ClO₄)₃ samples were characterized by X-rayabsorptionspectroscopy.ThespectralshapeoftheMnL3,2

absorptionedgeisverysensitivetochangesintheoxidationstate oftheMnions,therebymakingXASausefultooltomonitorthe samplecondition.ReferencedataobtainedfrombulkMn^IIandMn^III oxidesprovideameanstodeterminetheMn^II/Mn^IIIratiopresentin the[Mn^III₆Cr^III]³⁺sample[32].Theintact[Mn^III₆Cr^III]³⁺molecule containssolelyMn^III,thereforethisratioservesasanindicatorof changestothemoleculeconditionduetopreparationorradiation exposure.

Thesamplespreparedbytheairbrushmethoddescribedbefore werecheckedbyXASwithregardtothehomogeneityofthedepo- sitionprocess:Thehomogeneityofthe[Mn^III₆Cr^III]³⁺samplewith respecttothemoleculeconditioniscrucialasthespinpolariza- tiondatafromdifferentsamplepositionsaremergedtoreducethe statisticalerroroftheresults.

The results of the XAS characterization reveal a very good homogeneityoftheabsorptionyieldmeasuredatdifferentsam- plepositions, seeFig.1.Furthermore,theoxidationstateofthe Mnionsinthe[Mn^III₆Cr^III](ClO₄)₃SMM,indicatedbythespectral shapeoftheMnL3,2absorption,isthesameoverthewholesample area,fulfillingtherequirementsfortheapplicationofthesample scanningduringthespinpolarizationmeasurements.

4.2. Monitoringofradiation-inducedreduction

BesidesthecharacterizationofSMMsamplesuponpreparation, XASwasalsousedtomonitorradiation-inducedchangestothe moleculecondition.Previousinvestigationsrevealedthattheoxi- dationstateoftheMnionsin[Mn^III₆Cr^III]³⁺SMMchangesfrom Mn^IIItoMn^IIunderexposuretosoftX-rays,withtherateofthe reductionprocessbeingstronglycorrelatedtothechoiceofanions [32].

Mn L_3,2 edge absorption spectra were recorded before and after the acquisition of one SPES datapoint to investigate the

Fig.2.Radiation-inducedchangeoftheMnoxidationstatein[Mn^III₆Cr^III](ClO4)3

SMM.TheinitialMn^IIIcontentof0.93isreducedto0.78bythereductionprocess occurringuponsoftX-rayexposure.Theradiationdamageshowncorrespondsto theacquisitionofasingleSPESdatapointataphotonenergyof642eV,clearly demonstratingthenecessityofsamplescanning.

radiation damage caused by themeasurement, see Fig. 2. The [Mn^III₆Cr^III](ClO₄)₃sampleshowedaninitialMn^IIIcontentof0.93, indicatingthatthemajorityofthedepositedSMMwereintheir intended original state. TheMn^III contentwas reduced to 0.78 by a soft X-ray exposure of approx. 60s, corresponding tothe acquisitionofasingleSPESdatapoint.TheSPESmeasurementwas performedattheMn^IIIabsorptionmaximumat642eV,leadingto thecreationofamaximumnumberofsecondaryelectronsinthe SMMsample.Thereforetheextentofradiation-inducedreduction showninFig.2canberegardedasaworst-caseestimate.Forexcita- tionenergieswithalowerabsorptioncrosssection(off-resonance), lessradiationdamagewasobserved.

4.3. SPESofmanganese(II)acetate

SPESmeasurementsofmanganese(II)acetatewereperformed inordertodemonstratethatSPESandXMCDresultscorrespond also for Mn ions in a chemical environment resembling the [Mn^III₆Cr^III]³⁺ SMM more closelythan MnO and Mn₂O₃. Based on the results of previousspin-resolved electron spectroscopy investigationsof Mnbulk oxidesand manganese(II)acetate(see [33]),thespinpolarizationoftheelectronsejectedfromaman- ganese(II)acetatereferencesampleasaresultoftheMnL2,3M2,3V Augertransitionhasbeenmeasuredfordifferentexcitationener- giescoveringthewholeMnL_3,2 region.Duetosinglet coupling beingpredominantintheinvestigatedAugertransition,thespin polarizationoftheAugerelectronsequalsthespinpolarizationof theorientedcoreholesfromtheprimaryexcitationprocesswith theoppositesign.

Insteadofrecordingfullspin-resolvedAugerspectraasshown in[33],adifferentapproachwaschosenhere:Afterlocatingthe appropriateAugerlineinanoverviewspectrum,thespin-resolved measurementswereperformedwithafixedelectronenergycor- respondingtotheselectedAugertransition.Theresultsobtained frommanganese(II)acetatearegivenin Fig.3,thebottompanel showsthecorrespondingabsorptionspectrumwiththeexcitation energiesmarked.TheintegrationtimeoftheSPESmeasurement wassetto300sperhelicityanddatapoint.Thebandwidthofthe excitationwaschosenindividuallyforeachdatapoint,representing a compromisebetweenresolutionand therequiredcountrates dependingonthevaryingelectronyield.Theselectedbandwidth is given by the horizontal error bars of the spin polarization

- 0.3 - 0.2 - 0.1 0.0 0.1 0.2 0.3

spin polarization

spin polarization Mn^IIacetate

638 640 642 644 646 648 650 652 654

0.0 0.1 0.2 0.3 0.4 0.5

TEY (arb u)

photon energy (eV)

absorption yield 0.1 0.05 0.00 - 0.05 - 0.10

XMCD asymmetry

Fig.3.SPESresultsofmanganese(II)acetate.Toppanel:Thespinpolarizationof theAugerelectronsresultingfromtheMnL2,3M2,3VAugertransitionisshown.The horizontalerrorbarsrepresentthebandwidthofthecircularlypolarizedradiation usedintheexcitationstep.Theverticalerrorbarscorrespondtothestatisticalerror ofthemeasurements.ThesolidlinerepresentsXMCDdataobtainedfromman- ganese(II)acetate(7T,approx.15K)forcomparison.Bottompanel:Acorresponding MnL3,2edgeabsorptionspectrumofmanganese(II)acetateisshown.

datapointsin Fig.3, while theverticalerrorbars representthe statisticalerrorofthemeasurements.

The Auger electron spin polarization observed in man- ganese(II)acetate reveals negative values in the Mn L₃ region.

Towardshigherphotonenergies,asignchangeoccursattheupper edgeof theMnL₃ multiplet.Thehighest positivevalues ofthe spin polarization areobserved around thesatellite structure at 641.5eVwhichisacharacteristicspectralfeatureofMn^II[32].The signchangeoccurringbetweenthemainabsorptionandthesatel- litestructurehasbeenobservedinXMCDinvestigationsofother molecularMn^IIcompounds[32,67].Whilenosignificantspinpolar- izationisfoundforthephotonenergyrangeinbetweentheL₃and L₂multipletstructures,asmallpositivespinpolarizationwithabso- lutevaluesbelow5%hasbeenmeasuredforthehigherexcitation energiesoftheL₂multiplet.

Themeasuredspinpolarizationstronglydependsontheexci- tationbandwidthintheproximityofsharppolarizationfeatures:

At an excitation energy of 641.6eV, two measurements were performedwithdifferentexcitationbandwidthsof250meVand 750meV.Whilethemeasuredspinpolarizationisclosetozerofor thelargerexcitationbandwidth,apolarizationof+12%correspond- ingtothesignchangeinXMCDobservedbyKhanraetal.[67]is obtainedforthesmallerexcitationbandwidth.

XMCDmeasurementsofmanganese(II)acetatehavebeenper- formedincooperationwiththegroupofE.GoeringattheMPIfor IntelligentSystemslocatedinStuttgartforcomparison(seeFig.3 toppanel,solidline).TheXMCDsampleshavebeenpreparedbya drop-castingprocessongoldcoatedglasssubstrates.Themeasure- mentswerecarriedoutusingamagneticfieldof7Tandcoolingthe sampletoatemperatureofapprox.15KattheBESSY-IIstoragering facilityoftheHZB(beamlinePM3).Thedifferenceinabsorption yieldsobtainedfromtheXMCDexperimenthasbeennormalized tothesumof thehelicity-dependentyieldstoderive AXMCD as describedinthetheoreticalbackground.TheXMCDasymmetryand thedetectedspinpolarizationoftheAugerelectronsexhibitoppo- sitesignsasseeninpreviousexperiments[33],whichhasbeen takenintoconsiderationbychangingthedirectionofthevertical axis.

Agoodqualitativeagreementbetweenthespinpolarizationof theAugerelectronsand theXMCDasymmetry isfoundforthe energyrangeofthewholeL₃regionuptotheonsetoftheL₂multi- pletat649.5eV.WithintheL₂edgeregion,somedifferencesoccur betweenthetwophysicalquantities.Ithastobenotedthatdespite

- 0.3 - 0.2 - 0.1 0.0 0.1 0.2

spin polarization

spin polarization

[Mn^III₆Cr^III](ClO₄)₃

638 640 642 644 646 648 650 652 654 656 658 660 0.0

0.2 0.4 0.6 0.8 1.0

TEY (arb u)

photon energy (eV)

absorption yield 0.6 0.4 0.2 0.0 - 0.2 - 0.4

XMCD asymmetry

Fig.4.SPESresultsof[Mn^III₆Cr^III](ClO4)3SMM.Toppanel:Spinpolarization resultsobtainedfrom[Mn^III₆Cr^III](ClO4)3SMMusingthedescribedmethodare presented.Themeaningofthehorizontalandverticalerrorbarsisgiveninthecap- tionofFig.3.ComparativeXMCDdataaregivenbythesolidline,seemaintextfor details.Bottompanel:MnL3,2edgeabsorptionspectrumof[Mn^III₆Cr^III](ClO4)3, theexcitationenergiesusedfortheSPESmeasurementsaremarkedbycircles(diam- eterequalsexcitationbandwidth).

thesatisfactoryqualitativeagreementfoundhere,theabsoluteval- uesoftheXMCDasymmetryareapproximately3timeslowerthan thecorrespondingspinpolarizationdata:Notethedifferentscaling oftheleftandrightverticalaxisofFig.3;weexpectthisdifference tobecorrelatedtoaninsufficientmagneticorderingoftheman- ganese(II)acetatesampleintheexternalmagneticfieldduringthe XMCDmeasurement.

4.4. SPESof[Mn^III6Cr^III]³⁺SMM

Duetotheradiationsensitivityofthe[Mn^III₆Cr^III]³⁺SMM[32], theaboveapproachhadtobemodified,astheoccurrenceofradi- ation damage limited the measuringtime. Therefore, a sample scanningmethodwasintroduced:Anew,previouslyunexposed samplepositionwasselectedaftereachmeasurementofapprox.

60s.Alargenumberofmeasurementswascombinedintoonespin polarizationdatapoint.VariationsoftheSMMlayerthicknesscan- celledout,asonlytheasymmetryofthecountratesofthetwo detectors wasevaluated. Instrument related asymmetries were cancelledoutbycombiningmeasurementswithdifferenthelic- ity ofthecircularly polarizedlight usedfor theexcitation. The [Mn^III₆Cr^III](ClO₄)₃SMMwiththeperchlorateanionwaschosen fortheSPES measurements,aspreviousinvestigationsrevealed its higher radiationstability compared tothe SMM typeswith tetraphenylborateandlactateanions[32].

Spin polarization results obtained from [Mn^III6Cr^III](ClO4)3

samplesareshownintheupperpanelofFig.4.Theresultof a comparativeXMCD studyat B=−6.9Tand approx. 2K is given bythesolid lineinthetoppanel.Again,thelowerpanelofthe figure represents the corresponding absorption spectrum with theexcitationenergiesfor theSPES measurementsmarked.For [Mn^III6Cr^III](ClO4)3anegativespinpolarizationwithabsoluteval- uesupto20%ispredominantintheMnL₃edgeregion.Instrong contrasttotheresultsobtainedfrommanganese(II)acetatethespin polarizationremainsnegativerighttotheonsetoftheL2absorp- tionmultiplet—thedistinctchangeofsignobservedinMn^IIsystems isnotfoundhere.Apositivespinpolarizationofupto+8%was detectedinthefirstpeakstructureoftheL2multiplet.

Besidesthequalitativeagreementbetweenthespinpolarization datainFig.4andtheXMCDresultshownforcomparison,anumber ofdistinctdifferencesarevisible:TheabsolutevalueoftheXMCD asymmetryisapproximatelytwicetheabsolutevalueofthespin

Fig.5.MnL3,2 edgeXMCDdataobtained from[Mn^III₆Cr^III](ClO4)3SMMat B=−6.9Tand2K.Thehelicitydependentabsorptionspectra(toppanel)areshown togetherwiththeresultingXMCDdifference(bottompanel).

polarizationintheL₃ region,thoughtheoverallstructurewithin theL₃multipletiscomparable.Thelargepositiveexcursionofthe XMCDasymmetryatthehighenergysideoftheL3multipletisnot reproducedinthespinpolarizationdata.Acomparabledifference isalsofoundatthehighenergysideoftheL₂ multiplet.Amore detaileddiscussionofthedifferencesisgivenlaterfollowingthe XMCDresults.

4.5. XMCDresultsof[Mn^III6Cr^III](ClO4)3

XMCDmeasurementsatanexternalfieldof−6.9Tandatem- peratureofapprox.2KwereperformedattheSIMbeamlineofthe SLSstoragering(Villigen,Switzerland)on[Mn^III₆Cr^III](ClO₄)₃sam- plesforcomparison.Asindicatedbymagnetizationmeasurements withvariedtemperatureandexternalfield(VTVH),saturationis reachedfor[Mn^III₆Cr^III]³⁺withexternalfieldsofapprox.7T,see reference[29].

AseriesofeightconsecutiveXASscanswasperformedtoesti- matetheradiationdamage causedbytheXMCD measurement.

Nodiscernibleradiationdamageeffectswerefoundinthisseries.

Using thecontinuousmode of thebeamline monochromator, a setofthreeabsorptionspectrawasrecordedforeachhelicityand magneticfieldorientation.Theaveragedhelicity-dependentXAS spectraaswellastheresultingXMCDdifferenceareshowninFig.5.

Despitetheverylowsignal-to-noiseratio,XMCDmeasurements weresuccessfullyperformedalsoattheCrL_3,2edge.Usingthesame setupasbefore,seriesof5XASscanswereaveragedtoobtainthe resultsshowninFig.6.AdistinctdifferenceinthesignoftheXMCD effectisvisibleintheCrL_3,2XMCDdatacomparedtotheresults obtainedattheMnL_3,2edge(seeFig.5),indicatingthattheanti- ferromagneticcouplingbetweentheterminalMn^IIIionsandthe centralCr^III[24]isstilleffectiveatanexternalfieldof6.9T.

4.6. SumruleevaluationofSPESdata

Accordingtotheapproach describedinthetheoreticalback- ground,informationequivalenttoXMCDdatashouldbeobtainable fromtheSPESmeasurement.Thereforeasumruleanalysis(see [39–42])wasperformedforbothmethods.

Ithastobenotedthattheapplicabilityofthesumruleapproach to3d⁴ systemsliketheMn^III ionsinthe[Mn^III₆Cr^III]³⁺ SMMis severelylimitedbyanumberofphysicalconstraints,see[49]for a detaileddiscussion. However,theseconstraintsapply toboth experimentalmethods,thereforethesumruleapproachremains

Fig.6. CrL3,2 edgeXMCD dataobtained from [Mn^III₆Cr^III](ClO4)3 SMMat B=−6.9Tand2K.Toppanel:Helicitydependentabsorptionspectra.Bottompanel:

ResultingXMCDdifference.NotethedifferenceinthesignoftheXMCDeffect comparedtoFig.5,indicatinganantiferromagneticorientationoftheMnandCr magneticmoments.

ausefultoolforthecomparisonbetweenXMCDandspin-resolved electronspectroscopydata.Charge-transfermultipletcalculations asdescribedinreferences[49,50]arerequiredtoderiveabsolute valuesforthelocalmagneticmomentswithoutthelimitedaccu- racyofthesumruleformalism.

The linearly interpolated SPES data were multiplied point- wisewithabsorptiondatarecordedinconnectionwiththeSPES measurements withoutan external magnetic field as the mea- sured electronspin polarization corresponds totheasymmetry ofthehelicitydependentabsorptionintensities.Themultiplica- tionaccountsforthefactthatthedifferenceofthetwoabsorption yieldsY+andY₋insteadoftheasymmetryA_XMCDisrequiredfor theusualformulation ofthe XMCDsumrules. Additionallythe signoftheSPESdatawasinvertedinthissteptoaccountforthe signchangeintroducedintotheSPESmeasurementsbytheAuger processinvolved[33].

ItiswellknownthatthefullseparationoftheL₂andL₃ con- tributionsisdifficultfor3d⁴systemsasMn^III.Duetothecomplex structureoftheL₂andL₃multiplets,anattempttofittheabsorp- tiondatawiththeoreticalmodelswasnotperformed.Instead,the followingsimplifiedapproachwaschosen:TheXMCDdifference nearlyvanishesbetween650eVand652eV,creatingaregionin whichthevalueofthepintegralchangesonlyslightlywithpho- ton energy.Theupper limitof thep integrationwaschosen in thisregionforbothXMCDandspin-resolvedelectronspectroscopy data.Withthischoice,theresultingerrorduetotheinaccuratesep- arationoftheedgesisexpectedtobesmallcomparedtotheother contributionstothesumruleerrordescribedin[49].

Usingthemostcommonsumruleexpression[41,42]andcal- culatingtherequiredintegralsp(L₃edgeXMCDdifference),q(L_3,2 edgeXMCDdifference)andr(L_3,2edgehelicityaveragedabsorp- tion),thefollowingresultswerederivedfromSPESdata:

spin=−(3p−2q)×(10−n_3d)

r×kcorr =−1.8±0.4Bohr

orbit=−2×q×(10−n_3d)

3r =−0.60±0.12Bohr

ApossiblemagneticdipolecontributionTZtothespinmag- neticmomenthasbeenignoredgiventhefactthatthiseffectis smallfor3dtransitionmetals[48].DuetothenatureoftheMn^III- ligandbondpresentinthe[Mn^III₆Cr^III]³⁺SMMthesystemcanbe regardedasbeingpurely3d⁴.Anadditionalcorrectionfactorkcorr

Fig. 7.Comparison of XMCD and spin polarization data obtained from [Mn^III₆Cr^III](ClO4)3 SMM. Note that the sign of the spin polarization has beeninverted.Thespinpolarizationrawdataweremultipliedwithahelicity- independent absorption spectrum derived from the XMCD measurements to accountforthedifferencesintheintensitynormalizationusedbytheSPESand XMCDmethod.

hasbeenappliedtotheresult,followingtheapproachsuggestedby Piamontezeetal.[49].Thecorrectionfactorsuccessfullyderivedfor Mn^IIIionsbyKuepperetal.[50]wasused,althoughithastobecon- sideredthattheMn^IIIionsin[Mn^III₆Cr^III](ClO₄)₃areembeddedin adifferentchemicalenvironment.

Theerrorgiveninthespinandorbitalmagneticmomentresults wasderived usingthefollowingmethod:Eachspinpolarization datapointwasvariedwithanormaldistributionwithinitsuncer- taintylimits(seeerrorbarsinFig.4),leadingtoanewasymmetry curve.Theresultwasevaluatedusingthesumrulesasbeforeto obtainnewvaluesforspinandorbit.After500runs,thestandard deviationofthecalculatedvalueswasderivedtoestimatetheerror oftheresults.

Resultsforthelocalspinandorbitalmagneticmomentswere derivedaccordinglyfromtheXMCDdatagiveninFig.5.Notethat thecorrectionfactorforthespinmagneticmomentwasappliedas beforeandthemagneticdipolecontributionwasneglected:

spin=−(3p−2q)×(10−n_3d)

r×kcorr =−1.9±0.2Bohr

orbit=−2×q×(10−n_3d)

3r =−0.30±0.03Bohr

Theerrorofthederivedspinandorbitalmagneticmomentshas beenestimatedfollowingthesameapproachasusedfortheSPES results—theuncertainty ofeachXMCDdifferencedatapoint was derivedfromtheaveragingoftheoriginalspectra.Afterwards,the variationmethoddescribedbeforewasapplied.

4.7. DiscussionofdifferencesbetweenXMCDandspin-resolved electronspectroscopy

Despitethereasonableagreementbetweenthespinmagnetic momentobtainedviathesumrulemethodfromSPESandXMCD data,significantdifferencesbetweenthetwodatasetsexist.Adirect comparisonoftheXMCDdifferencesignalandthemeasuredspin polarizationisshowninFig.7.Notethatthesignofthespinpolar- izationhasbeeninvertedandthespinpolarizationdatahavebeen multipliedwithaspin-integratedabsorptionspectrumtoaccount forthedifferentintensitynormalizationofbothmethods.

ItisstrikingthatinthewholeMnL₃edgeregion,theXMCD signal reveals larger absolute values with respect to the spin

Fig.8. XASspectraof[Mn^III₆Cr^III](ClO4)3SMMobtainedunderdifferentexperi- mentalconditions:Measurementatroomtemperature(blacksolidline)andat2K withoutexternalfield(darkyellowsolidline).Spectrarecordedat2Kand−6.9T withoppositelighthelicity(dottedlines)havebeenaveragedtoobtainthespin independentabsorptionspectrum(redsolidline).Inset:Ashiftofapprox.300meV occursforthemainMn^IIIabsorptionfeature.(Forinterpretationofthereferencesto colourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

polarizationdata.Nevertheless,thestructureofthemultipletwith itsthreepeaksatapprox640eV,641eVandslightlyabove642eV isreproducedreasonablywellinbothmethods.Thesignchange andthelargenegativeexcursionoftheXMCDsignalatenergies abovetheL3multipletisnotreproducedinthespinpolarization data.The measuredspin polarization retainsitssignup tothe onset of theL₂ peak structure. Here, the XMCD signalreveals againrelativelylargenegativevalueswhicharenotreproducedin the(sign-corrected)spinpolarization.Duetothesecharacteristic differences,theintegral-basedsumruleevaluationleadstocom- parableresultsforthespinmagneticmoment:Thelargerpositive swingoftheXMCDsignalintheMnL₃regionandthenegative excursioninbetweentheL₃ andL₂multiplet regioncancelout, resultingincomparablevaluesfortheL3andL3,2integrals.

Withrespecttotheexperimentalconditionswhichthesample issubjectedto,twomajordifferencesexistbetweentheXMCDand SPESmethod:WhiletheXMCDmethodrequired(1)theapplication ofanexternalmagneticfieldof−6.9Tand(2)alowsampletem- peratureofapprox.2K,theSPESmeasurementswereperformedat roomtemperatureandwithallexternalmagneticfieldscompen- satedtonegligiblevalues.

Absorptionspectraobtainedunderdifferentexperimentalcon- ditionswerecomparedtoinvestigatetheinfluenceofthesetwo factors,seeFig.8.Twospectraobtainedatroomtemperature(MAX- lab beamline D1011, black solid line) and at approx. 2K (dark yellowsolidline,measuredatSLS)showaverygoodagreement andhintatanegligibleinfluenceofthesampletemperatureon theMnL_3,2 absorption.Twospectrarecordedat2Kand−6.9T withoppositehelicity(dottedlines)wereaveragedtoobtainthe spin-independentspectrum(redsolidline)showninFig.8.

Distinctdifferencesareobservedwithrespecttotheexperimen- talsituationwithoutexternalmagneticfields:TheMn^IIIabsorption maximum(leftmostarrow)revealsahigherrelativeabsorption.It appearstobeshiftedtowardshigherphotonenergiesbyapprox.

300meV.Thehigh-energyedgeoftheL₃multipletaswellasthe wholespectralregionbetweenL3andL2righttotheonsetofthe L₂multipletrevealahigherabsorptionyieldthanwithoutexternal fields.IthastobenotedthatthisdifferenceintheMnabsorption coincideswiththespectralregioninwhichthelargestdiscrepancy betweenXMCDandSPESresultsoccurs.Anon-negligibleinfluence

oftheexternalmagneticfieldontheelectronicstructureoftheMn^III ionspresentin[Mn^III₆Cr^III](ClO₄)₃SMMmaybeassumedtoplaya majorroleintheobserveddifferencesbetweenthetwomethods.

ThismaybecorrelatedtothefactthattheSt=21/2spingroundstate ofthe[Mn^III₆Cr^III]³⁺SMMisenergeticallynotwellseparated,but severalspinstatesofdifferentspinquantumnumbersarewithina fewwavenumbers[24,26].

Despitethereasonablygoodagreementofthespinmagnetic momentderivedviathesumruleapproachfromXMCDandSPES data,theobtainedresultsaresignificantlylowerthanthetheo- reticalvalueexpectedfortheMn^IIIandCr^IIIcouplingschemeand previousdatafromACandDCmagnetometry[26],indicatingthe physicallimitationsofthesumruleformalismfor3d⁴systemsas describedbyvariousauthors[43–50].

5. Summaryandconclusion

ComparativeXMCD(2K,7T)andspin-resolvedelectronspec- troscopymeasurements(roomtemperature,noexternalfield)at the Mn L edge have been performed on [Mn^III₆Cr^III]³⁺ single- moleculemagnetsdepositedonAuandSisubstrates.Aninfluence oftheexternalmagneticfieldontheabsorptionspectrumofthe SMMsampleshasbeenobserved.Despiteitslimitedapplicabil- ityto3d⁴ systems,asimplesumruleevaluationwasperformed toprovideameansofcomparingthetwoexperimentalmethods.

However,theabsolutevalueofthelocalspinmagneticmoment obtainedfromboth methods differssignificantlyfromprevious resultsmeasuredbyACandDC magnetometry.Furtherinvesti- gationsarerequired:Thecorrectionfactorusedinthesumrule evaluationneedstoberefinedfortheinvestigatedSMMsystem usingthechargetransfermultipletapproach[49]forcomparative calculationsoftheXMCDandspinpolarizationeffect.

Itisunlikelythatthedistinctdifferencesintheabsolutemagni- tudeofthespinpolarizationandtheXMCDeffectarecausedsolely bytheobservedinfluencesofthemagneticfield.Theresultsseemto indicatethattheinitialassumptionofadirectonetoonecorrespon- denceneedstoberefinedbytheoreticalinvestigations.However theexistenceofacorrespondenceisclearlyprovenbytheverygood agreementoftheenergydependencefoundfortheelectronspin polarizationandtheXMCDincludingtheirsignchange,whichhas beenshownforGdaswellasformanganese(II)acetate.Despitethe observeddifferences,aqualitativeagreementhasbeenconfirmed for[Mn^III₆Cr^III]³⁺SMMaswell.Itmightbepossiblethatthereisan energy-independentfactorofnegativesign,dependinguponthe angularmomentaJ,LandSfortheratioofelectronspinpolariza- tionandXMCDasymmetry.Thereforefurthertheoreticalworkis requiredtogainmoreinsightintothenatureoftheobservedcor- respondenceandtoderiveaquantitativerelationbetweenthetwo physicalquantities.

Asthemagnetizationstudies[26]probeonlythetotalmagneti- zationoftheSMMsamplewithoutprovidingelementalresolution, aninvestigationoftheCr^IIIcontributionisrequiredtoprovidea deeperunderstanding.

Acknowledgements

SpecialthanksareduetotheHelmholtzZentrumBerlin(BESSY II)andthePaulScherrerInstituteVilligen(SLS)andtheirbeamline staffforthecontinuoussupporttroughoutthenumerousbeam- times.ThanksarealsoduetoE.Goering(MaxPlanckInstitutefor IntelligentSystems,Stuttgart)forprovidingtheXMCDsetupused forthemanganese(II)acetatestudies.KKacknowledgesfinancial support by the SFB 569 funded by the Deutsche Forschungs- gemeinschaft(DFG). Thisworkwassupportedby theDeutsche Forschungsgemeinschaft(DFG)withinResearchUnit945.