ELECTRONIC STRUCTURE aND maGNETISm

The European Synchrotron 15

TUNINGTHEMAGNETISMOF3D-METALPHTHALOCYANINEADLAYERSBYELECTRONDOPING

Thin phthalocyanine films havebeen incorporated in various devicesincludingfield effect transistors, light-emittingdevicesandphotovoltaiccells.Phthalocyanines can accommodate avarietyofmetalionsintheircentrethathave particular spin configurationsrendering this class of compoundsalso interesting for magnetic materials [1].Duetotheparticularcoordinationgeometry of the metal ions, the spinstates of the complexes are sensitiveto their local chemical environment.Bringing the molecules into contactwith metal electrodes could thereforehave significant influence on themagnetic properties of the metalcentres. Another effective route fortailoring the electronic and magneticcharacteristicsofmolecularmagnetsisofferedbydirectchemicaldoping.

The charging sequence of metal-phthalocyanines involves both ligandand metal orbitals and dependsstrongly on the nature of the centralmetal ion. Doping of phthalocyaninechemisorbed on a metal substrateis further complicated due to theinterplay of additional substrateinducedchargingandscreeningeffectsthat can alter the role of the chargedonortowardsthemolecules.Thestep-wise attachment of electron donor Liatoms to chemisorbedCuPc andNiPcmolecules was recently investigatedby scanning probe microscopy andDFT calculations [2]. The effects ofalkali metal doping on the magneticproperties of metal-phthalocyanines,however, have not been studied in asystematic way, neither in the bulknor at interfaces. These effects are ofparticular importance for molecularelectronicandspintronicapplications.

This study focuses on the effect of Lidopingontheelectronicandmagneticstatesofthemetalionsforaseriesof3d metal-phthalocyanine monolayers(MnPc, FePc, NiPc, CuPc) adsorbed onAg(001).Our systematic investigationemployed X-ray magnetic circular

dichroism (XMCD) at beamline ID08(now ID32) complemented by atomicmultipletcalculations,whichallowedustoaccuratelydeterminechangesofthespin,electronicvalence,andcrystalfieldin electron-doped phthalocyanines.Figure 13a shows a close-packedarrayofCuPc.TheadditionofasmallamountofLiatomsleadstotwomajortypesofLi-CuPccomplexeswheretheLiatomscanresideattheligandposition(denotedL)orclosetothemetalcentre(denotedM).TherelativeproportionofeachtypeofcomplexdependsontheLicoverage.With increasingLicoverage,the preference changes from L-typeto the M configuration even thoughthere are more ligand sites availablethan central positions (Figure 13b).The different Li dopant types lead tosite specific electron donation eithertothemetald-statesortotheorganicbackbone.

Principal publication and authorsS.Stepanow(a),A.LodiRizzini(b),C.Krull(b),J.Kavich(b),J.C.Cezar(c),F.Yakhou-Harris(c),P.M.Sheverdyaeva(d),P.Moras(d),C.Carbone(d),G.Ceballos(b),A.Mugarza(b)andP.Gambardella(a,b),J. Am. Chem. Soc. 136,5451-5459(2014).(a) Department of Materials, ETH Zürich (Switzerland)(b) Catalan Institute of Nanoscience and Nanotechnology (ICN2), Barcelona (Spain)(c) ESRF (d) Istituto di Struttura della Materia, CNR, Trieste (Italy)

Fig. 13:STMtopographyofaCuPcadlayerwithtwodopedcomplexesofdifferenttypes

(8.3nmx8.3nm).StatisticalanalysisofthetypeofdopantasafunctionofLi

coverageextractedfromtheSTMdata.Errorbarsaccountforundefinedconfigurations.

SCIENTIFIC HIGHLIGHTS ELECTRONIC STRUCTURE aND maGNETISm

ESRF16

Figure 14 shows theX-rayabsorptionand XMCD spectra of NiPc as afunction of Li doping.With increasingLi concentration, the pristine non-magnetic Ni ions, which have a spinsinglet d8 configuration, becomereduced and assume a d9configuration.Amagneticmomentcorresponding toS =1/2appearsat thesametime,asevidencedbytheXMCD.TheLidopinghas the opposite effect For CuPc: themagneticmomentcanbeturnedoffbyLidoping,duetothecompletefillingofthed-shell(fromd9 to d10).Incontrast,for MnPc, we observed no changesof thed5metal ionvalencestateasa

function of doping, but a transitionfrom an intermediate S=3/2toahighS =5/2spinstate.Thistransitionisduetoastrongreductionoftheligandfieldinduced by Li, which is also observedforCuPc,NiPc,andFePc.

When combined with STMstudies (Figure 13 and ref. [2]),these results provide a completepicture of chemically-doped metal-phthalocyaninesonsurfaces, inwhichstrong correlation effects induce site-specificchargingandmagneticeffects.Moreover, these results demonstratethat different spin states can berealised in metal-phthalocyaninelayers interfaced with metals bydopingwithLiatoms,offeringarouteto tune the magnetic properties ofsurface-supported molecular systemsemployingnon-magneticdopants.

Fig. 14:EvolutionofNiPcXASandXMCDatnormalincidencewithsubsequentLiatomdeposition.

References[1]J.Bartolomé,C.MontonandI.K.Schuller,inMolecular Magnets,J.Bartolomé,F.Luis,J.F.Fernandez(Eds.),Springer-Verlag,BerlinHeidelberg,221-245(2014).[2]C.Krull,R.Robles,A.MugarzaandP.Gambardella,Nature Mater. 12,337(2013).

EUROPIUMNITRIDE:ANOVELDILUTEDMAGNETICSEMICONDUCTOR

Ferromagnetism, where a materialspontaneously forms a netmagnetisation, is one of the oldestand most striking examples ofcorrelated electron phenomena. Mostferromagnetsaremetals,butundertherightconditionssomesemiconductorscanalsobecomeferromagneticiftheyaredopedwithasmallconcentrationofmagnetic ions. Such so-called dilutedmagnetic semiconductors (DMSs)

offer exciting possibilities in the fieldof spintronics,where the functionalityof electronic devices is enhancedby exploiting the intrinsic magneticmoment of electrons in addition to their charge. The most well studieddiluted magnetic semiconductors are formed by adding manganese to III-V semiconductors such as GaAs.Mobile charges in the semiconductorhost mediate magnetic interactions

Principal publication and authorsDoLeBinh(a),B.J.Ruck(a),F.Natali(a),H.Warring(a),H.J.Trodahl(a),E.-M.Anton(a),C.Meyer(b),L.Ranno(b),F.Wilhelm(c),andA.Rogalev(c),Phys. Rev. Lett. 111,167206(2013).(a) The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington (New Zealand)(b) Institut Néel, Centre National de la Recherche Scientifi que and Université Joseph Fourier, Grenoble (France)(c) ESRF