Cobalt-based Heusler compounds in magnetic tunnel junctions [Elektronische Ressource] / Daniel Ebke. Fakultät für Physik - Abt. Experimentalphysik : Dünne Schichten und Nanostrukturen

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Publié par

DANIEL EBKECOBALT-BASEDHEUSLERCOMPOUNDSINMAGNETIC TUNNELJUNCTIONSBIELEFELD UNIVERSITYThis work was done by myself. Text and figures were partly taken from corresponding publi-cations originate directly from this work.(Daniel Ebke)Reviewers:Prof. Dr. Andreas HüttenProf. Dr. Walter PfeifferCopyright © 2010 Daniel Ebkebielefeld university, department of physicsthin films & physics of nanostructuresPh.D. thesisFirst printing, July 2010ContentsPublications and Conferences 9The optimal seed layer system 13Optimization of the Heusler layer 21Various Co-based Heusler compounds 29Transport properties 55Industrial applicability 79Conclusions 87Appendix 913Schaffe, schaffe Heusler baue!5IntroductionSpintronic devices have attracted a lot of attention in recent yearsdue to possible new applications, e.g., a magnetic random access1 1memory (MRAM), logic and sensors. The spin of the electrons S. A. Wolf, Science 294,1488(2001); and G. A. Prinz, Sci-is used as an additional degree of freedom in contrast to com-ence 282, 1660 (1998)mon electronic devices. The main constituent of many spintronicdevices is the magnetic tunnel junction (MTJ) where two ferro-2 2magnets are separated by a thin insulating tunnel barrier. The J. S. Moodera et al., Phys.Rev. Lett. 74,3273 (1995); andresistance of such a device depends on the magnetic orientationM. Julliere, Phys. Lett. A 54,of the ferromagnets.
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DANIELEBKE

COBALT-BASE

HEUSLER

COMPOUNDS

NI

MAGNETICTU

JUNCTIONS

BIELEFELDUNIVERSITY

D

N

N

E

L

Thisworkwasdonebymyself.Textandfigureswerepartlytakenfromcorrespondingpubli-
cationsoriginatedirectlyfromthiswork.

Ebke)(Daniel

vieReers:w

Prof.Dr.AndreasHütten
Prof.Dr.WalterPfeiffer

Copyright©2010DanielEbke

bielefelduniversity,

thinfilms&

thesisPh.D.

phprinting,FirstJuly

ysic2010sdoefapnratnmoesntrtuocftpuhryessics

Contents

encesConferandPublications

optimalThesystemlayerseed

layerHeuslertheofOptimization

9

13VcompoundsHeuslerCo-basedarious

opertiesprransportT

Industrialapplicability

Conclusions

Appendix

918755792129

3

Schaffe,

schaffe

Heusler

baue!

5

oductionIntr

Spintronicdeviceshaveattractedalotofattentioninrecentyears
duetopossiblenewapplications,e.g.,amagneticrandomaccess
1ismemorusedyasan(MRAM),additionallogicanddegreesensors.offreedomThespininofcontrastthetoelectrcom-ons
monelectronicdevices.Themainconstituentofmanyspintronic
demagnetsvicesisaretheseparatedmagneticbyatunnelthinjunctioninsulating(MTJ)tunnelwherebarriertwo.2ferrTheo-
resistanceofsuchadevicedependsonthemagneticorientation
RofPthe(parallel)ferrandomagnets.atunnelUsually,magnetorRAPesistance(antiparallel)(TMR)iscanhigherbethande-
finedasTMR=RAPRP−RP.Forsmallvoltagestheresistanceiscon-
levnectedeloftothetheferrspinomagnets.dependentHence,densitytheofTMRstatesvalue(DOS)isalsoatthegivenFerbmiy
TMR=12−PP11PP223withthespinpolarizationP1,2.
forThereforapplications.e,Amaterialshalfwithmetallicahighbehaspinvior,i.e.,polarizationtheyareare100%eligiblespin
dictedpolarizedforatsometheFeroxidemilevelcompoundsEFwhichsuchhasasFebeen3O4theorandCrOetically24,prpere--
65ovskitescompounds(e.g.7.InLaSrMnOparticular3),,Co-basedzinc-blende-typeHeuslerCrAsandcompoundsHeuslerare
prhighomisingCuriematerialstemperaturforesTspintrC.8onicHereaapplicationsHeuslerduecompoundtotherisequirgivened
byists.theXandYcompositionareXtransition2YZandmetalacrelementsystallographicandZisL2a1grstructuroupIII,eex-IV
element.VorrIneported2004,forroomMgO-basedtemperatureMTJs.9TMRratiosRecentlyofmorIkedaeprthanesented100%wTMRere
ratiostemperaturofoveser10600%foraatrsingleoomMgOtemperaturtunneleandbarrierov.erWith1100%theatlocon-w
ceptTMRofaratiosdoubleofmorebarrierthansystem1000%attheservoomaluescantemperaturbeincrehaveasedebeenand
11eported.r

7

1S.A.Wolf,Science294,1488
(2001);andG.A.Prinz,Sci-
ence282,1660(1998)
2J.S.Mooderaetal.,Phys.
Rev.Lett.74,3273(1995);and
M.Julliere,Phys.Lett.A54,
)1975(225

3M.Julliere,Phys.Lett.A54,
)1975(2254J.Coeyetal.,J.Appl.Phys.
91,8345(2002)
5W.Pickettetal.,JMagn
MagnMater172,237(1997)
6H.Akinagaetal.,JpnJAppl
Phys239,L1118(2000)
7R.D.Grootetal.,Phys.Rev.
Lett.50,2024(1983)
8P.Webster,JPhysChem
Solids32,1221(1971)
9S.Yuasaetal.,NatMater3,
868(2004);andS.S.P.Parkin
etal.,NatMater3,862(2004)
10S.Ikedaetal.,ApplPhys
Lett93,082508(2008)
11L.Jiangetal.,Applied
PhysicsExpress(2009)

12LettS.T93,sunegi112506et(al.,2008)ApplPhys
13LettN.T94,ezuka162504et(al.,2009Appl)Phys

14Mostcommondevicefor
amplifyingandswitchingsignals.onicelectr15g.com/.physorhttp://www.html179572434wsne

HighroomtemperatureTMRratioshavealsobeenreported
forMTJscontainingHeuslercompoundsaselectrodes:217%for
Co2MnSi12andveryrecently386%forCo2Fe0.5Al0.5Si13.Thelat-
terwasgrownbyusingmolecularbeamepitaxyinplaceofsput-
teringdeposition.However,sputteringisthepreferredandes-
tablishedmethodforindustrialapplications.Fromatechnologi-
calpointofview,theaimisalsotoachievehighTMRratiosby
sputtering.Theactualityofthistopiccanbyrecognizedbyre-
centpressreleases.Forexample,Toshibaannouncedthedevelop-
mentofaspintransportelectronicsbasedmetaloxidesemicon-
ductorfield-effecttransistor(MOSFET)14cellwithafullHeusler
compound.15However,thepredictedhalf-metallicityforHeusler
compoundsshouldleadtomuchhigherTMRratios.
Nevertheless,onehastomeettwochallengestoachievehalf
metallicity:crystallizationoftheHeuslerelectrode(s)inL21structure
coherentinterfacesoftheHeuslercompoundandtheMgOtun-
barriernel

InthisworkwehaveinvestigateddifferentCo-basedHeusler
compounds.Wehaveintegratedthemintosocalledhalfjunctions
toinvestigatethecrystalgrowthandmagneticpropertiesofthe
HeuslerelectrodeandintofullMTJsforthetransportproperties.
Wedescribetheoptimizationofarequiredseedlayersystemto
inducethepreferred(001)textureoftheHeuslerthinfilms.Fur-
thermore,wehaveoptimizedtheHeuslerlayerinanattemptto
achieveahighatomicordering,representedbyahighmagnetic
momentandamaximum(001)texture.Weinvestigatedthetrans-
portpropertiesofthefulljunctionsatroomtemperatureandlow
temperature(13K)respectively,anddiscussthemintermsofan-
nealingtemperature,biasvoltageandtemperaturedependence.
Finally,theindustrialapplicabilityandintegrationofHeusler
compoundelectrodesintoconventionalGMR/TMRsystemswill
beverified.Consequently,HeuslerjunctionspreparedbySingu-
lusNDTGmbHwillbecomparedtooursamples.Inparticular,
thegrowthpropertiesoftheHeuslerlayerwillbeaddressedtode-
terminedifferenceswithinthesputteringprocessoftheHeusler
films.thin

8

encesconferandPublications

Publications

20061.A.Hütten,J.Schmalhorst,A.Thomas,S.Kämmerer,M.Sacher,
D.Ebke,N.-N.Liu,X.KouandG.Reiss:Spin-electronicde-
viceswithhalf-metallicHeusleralloys,JournalofAlloysandCom-
pounds,423,148(2006)
2.D.Ebke,J.Schmalhorst,N.-N.Liu,A.Thomas,G.Reissand
A.Hütten:Largetunnelmagnetoresistanceintunneljunctionswith
Co2MnSi/Co2FeSimultilayerelectrode,Appl.Phys.Lett.,89,
)(20061625063.A.Thomas,D.Meyners,D.Ebke,N.-N.Liu,M.D.Sacher,J.
Schmalhorst,G.Reiss,H.EbertandA.Hütten:Invertedspin
polarizationofHeusleralloysforspintronicdevices,Appl.Phys.
Lett.,89,012502(2006)

20074.J.Schmalhorst,A.Thomas,S.Kämmerer,O.Schebaum,D.
Ebke,M.D.Sacher,G.Reiss,A.Hütten,A.Turchanin,A.Gölz-
häuserandE.Arenholz:Transportpropertiesofmagnetictunnel
junctionswithCo2MnSielectrodes:Theinuenceoftemperature-
dependentinterfacemagnetizationandelectronicbandstructure,Phys-
icalReviewB(CondensedMatterandMaterialsPhysics),75,
)(20070144035.J.Schmalhorst,D.Ebke,M.D.Sacher,N.-N.Liu,A.Thomas,G.
Reiss,A.Hütten,andE.Arenholz,Chemicalandinterfaceproper-
tiesoftunneljunctionswithCo2MnSi/Co2FeSimultilayerelectrode
showinglargetunnelmagnetoresistance,IEEETrans.Magn.,43,
)(20072806

9

6.A.Castrup,S.Dasgupta,T.Scherer,H.Rösner,J.Ellrich,R.
Kruk,M.Ghafari,H.Hahn,A.Hütten,D.Ebke,N.-N.Liu,I.
Ennen,A.Thomas,J.SchmalhorstandG.Reiss:Half-metallic
Co2MnSi/Co2FeSimultilayeredHeuslerelectrodesinmagnetictun-
neljunctions,J.Magn.Magn.Mater.310,2009(2007)

20087.A.Thomas,A.Weddemann,D.Ebke,N.-N.Liu,A.Hütten,
J.SchmalhorstandG.Reiss:Evidenceforbandstructureeffects
inthemagnetoresistanceofCo-basedHeuslercompounds,J.Appl.
Phys.,103,023903(2008)
8.D.Ebke,A.Thomas,A.Hütten,B.Balke,C.Felser,J.Schmal-
horstandG.Reiss:PreparationofHeuslerthinfilms:Thequater-
naryalloyCo2Mn0.5Fe0.5Si,PhysicaStatusSolidi(a),205,2298
)(20089.J.Schmalhorst,D.Ebke,A.Weddemann,A.Hütten,A.Thomas,
G.Reiss,A.Turchanin,A.Gölzhäuser,B.BalkeandC.Felser,
Ontheinuenceofbandstructureontransportpropertiesofmag-
netictunneljunctionswithCo2Mn1−xFexSisingleandmultilayer
electrode,J.Appl.Phys,104,043918(2008)
10.S.Dasgupta,R.Kruk,D.Ebke,A.Hütten,C.BansalandH.
Hahn:Electricfieldinducedreversibletuningofresistanceofthin
goldfilms,J.Appl.Phys.,104,103707(2008)

200911.J.Schmalhorst,D.Ebke,M.Meinert,A.Thomas,G.Reissand
E.Arenholz:Element-specificstudyofthetemperaturedependent
magnetizationofCo-Mn-Sbthinfilms,J.Appl.Phys,105,053906
)(200912.S.Wurmehl,J.T.Kohlhepp,H.J.M.Swagten,B.Koopmans,
C.G.F.Blum,V.Ksenofontov,H.Schneider,G.Jakob,D.Ebke
andG.Reiss:Off-stoichiometryinCo2FeSithinfilmssputteredfrom
stoichiometrictargetsrevealedbynuclearmagneticresonance,Jour-
nalofPhysicsD:AppliedPhysics,42,084017(2009)
13.D.Ebke,V.Drewello,M.Schäfers,G.ReissandA.Thomas:
TunnelingspectroscopyofMgObasedmagnetictunneljunctionswith
Co2FeAlelectrode,Appl.Phys.Lett.,95,232510(2009)

10

201014.D.Ebke,P.Thomas,O.Schebaum,M.Schäfers,D.Nissen,V.
Drewello,A.HüttenandA.Thomas,LowB2crystallizationtem-
peratureandhightunnelmagnetoresistanceinCo2FeAl/MgO/Co-
Femagnetictunneljunctions,J.Magn.Magn.Mat.,322,996-998
)(201015.M.Meinert,J.Schmalhorst,D.Ebke,N.-N.Liu,A.Thomas,
G.Reiss,J.Kanak,T.StobieckiandE.Arenholz:Structuraland
magneticpropertiesofCo-Mn-Sbthinfilms,J.Appl.Phys.,107,
)(201006390116.D.Ebke,Z.Kugler,P.Thomas,O.Schebaum,M.Schäfers,D.
Nissen,J.Schmalhorst,A.Hütten,E.ArenholzandA.Thomas,
X-rayabsorptionandmagneticcirculardichroismstudiesofCo2FeAl
inmagnetictunneljunctions,IEEETrans.Magn.,46,1925-1928
)(201017.O.Schebaum,D.Ebke,A.Niemeyer,G.ReissandA.Thomas,
DirectmeasurementofthespinpolarizationofCo2FeAlincombina-
tionwithMgOtunnelbarriers,J.Appl.Phys.,107,09C717(2010)

11

encesConfer1.D.Ebke,N.-N.Liu,M.Sacher,J.Schmalhorst,G.Reissand
A.Hütten,Co2FeSi-analternativefortheCo2MnSiHeuslerelec-
trodeintegratedinmagnetictunneljunctions,DPGspringmeeting,
Dresden(2006),MA3.5
2.D.Ebke,J.Schmalhorst,A.Thomas,S.Kämmerer,O.Schebaum,
M.Sacher,A.HüttenandG.Reiss,Transportpropertiesofmag-
netictunneljunctionswithCo2MnSielectrode:inuenceoftemperature-
dependentinterfacemagnetizationandelectronicbandstructure,SNI-
meeting,Hamburg(2006),D-P255
3.D.Ebke,J.Schmalhorst,A.Hütten,G.Reiss,B.BalkeandC.
Felser,Transportpropertiesofmagnetictunneljunctionswiththe
quaternaryHeusleralloyCo2Mn0.5Fe0.5Si,DPGspringmeeting,
Regensburg(2007),MA12.2
4.D.Ebke,J.Schmalhorst,M.Sacher,N.-N.Liu,A.Thomas,A.
Hütten,E.ArenholzandG.Reiss,Chemicalandmagneticinter-
facepropertiesoftunneljunctionswithCo2MnSi/Co2FeSimultilayer
electrode,DPGspringmeeting,Regensburg(2007),MA15.98
5.D.Ebke,J.Schmalhorst,A.Thomas,A.HüttenandG.Reiss,
MagneticpropertiesofCo2FeSithinfilmsdepositedbymagnetron
sputteringusingdifferenttargetcompositions,424.WE-Heraeus-
Semiar,MagnetismmeetsSemiconductor,BadHonnef(2009)
6.D.Ebke,J.Schmalhorst,A.Thomas,A.HüttenandG.Reiss,
MagneticpropertiesofCo2FeSithinfilmsdepositedbymagnetron
sputteringusingdifferenttargetcompositions,DPGspringmeeting,
Dresden(2009),MA40.108
7.D.Ebke,P.Thomas,O.Schebaum,M.Schäfers,D.Nissen,A.
HüttenandA.Thomas,Loworderingtemperatureandhightunnel
magnetoresistanceinCo2FeAl/MgO/Co-Femagnetictunneljunc-
tions,11thJointMMM-IntermagConference,Washington,DC
BA),(1220108.D.Ebke,Z.Kugler,P.Thomas,O.Schebaum,M.Schäfers,D.
Nissen,J.Schmalhorst,A.HüttenandA.Thomas,Magnetic
propertiesandhighroomtemperatureTMRratiosofCo2FeAlinmag-
netictunneljunctions,DPGspringmeeting,Dresden(2010),MA
.128

12

systemlayerseedoptimalThe

TorealizeahighspinpolarizedfullHeuslercompound(X2YZ)
thintypefilm,structuritise.AsnecessaryillustratedtoenforinceFigurthee1cr,thisystallizationisgiveninbyaLfour21
a.interEachpenetratingsub-latticefcccontainslatticesonethatarkindeofshiftedatom.by1An/4atomiclatticedisorconstantder
ofthetheXYandandYZcomponentscomponentsD0is3,rdefinedespectivasBely2.ThestructurA2e,typeadisorstructurderofe
hasnoorderedsub-lattices,i.e.itisasimplebcclattice.
toItisadditionalreportedstatesbyatPicozzitheFerthatmilevsomeelEFtypeandofthedisorspindermightpolarizationlead
16educed.risInadditiontotheatomicorderthecrystallographicorienta-
tionOoganeof17thethatHeuslera(001)thintexturfilmeisofaalsoCo-basedimportant.ItHeuslerisreportedcompoundby
isessentialbecauseCoatomsattheinterfacemaybreakthehalf
metallicityin(011)texturedHeuslerlayers.
orIndertogeneralinduceonethehascrtoystalgrconsiderowththeandbasetheforprtheeferredHeuslerlaorientation.yerin
TheHeuslerpreviousfilm.18worksHere,oftheourlalabyerswerweerefoundeddepositedonaon(011)Vbuftexturfereded
thermallyoxidizedSiwafertoinduceamaximumtexture.Later,
wepound,realizedwhichfirstaredescribedexperimentsinofthea(001)MasterstexturthesisedofHeuslerKeseberg.com-19
TheVbufferwasreplacedbyaMgO/Crbuffertoachieveamore
tur(001)edpartsorientedofgrtheowth.HeuslerPrlaobablyyerwdueeretostilltheprSiesent.substrate,(011)tex-
thinInfilmsthiswwillorkbewewilloptimizedcreatelayaermorbyelaefyer.fectivTherelayeforerethestacking.firststepThe
istoorientedfindaneHeuslerwandcompound.simpleseedForlayerdiffersystementrtoeasonsgeta20ithighlyis(001)most
usedsuitableontopassinsulatingonaMgOmetallicasanseedlaadditionalyer,suchseedaslayCrer.21whichisoften

13

L21Figuretype1:Crstructureystallographicofa
com-HeuslerMnSiCo2pound.B16S.69,1(Picozzi2004)etal.,Phys.Rev.
17M.IFCAMOoganeInteretnationalal.,ATIWork-and
hokushoponUniversitySpin-Curr(2007)ents,To-
18S.BielefeldKämmerUniver,ersityPhD(2004thesis,);
J.Schmalhorstetal.,Phys.
ReD.v.EbkeBet75,al.,1Appl(2007Phys);Lettand
89,162506(2006)
19F.Keseberg,Masters
(thesis,2007)BielefeldUniversity
20additionalexcludingforsourcestheofladifyerfusion;rstackingequiredto
rnelingealizeintospinsuperpolarizedconductortun-
experiments21Y.Sakurabaetal.,Appl
S.PhysTsunegiLettet88,al.,192508Appl(2006Phys);
Lett93,112506(2008);and
K.nol.AdvInomata.etMateral.,.9S,ci.T014101ech-
)2008(

22LettS.T93,sunegi112506et(al.,2008Appl)Phys

23LettG.X.93,Miao142511et(al.,2008)ApplPhys
24ReferenceDatabase,Inter-
tionnationalData(Centr1999e)forDiffrac-

Themostpromisingreportedmethodstoformahighly(001)
texturedthinfilmarethefollowing:
MgOsubstrates:(001)MgOsubstratesareused.22Theexpected
latticemismatchofabout5%tothe45degreesrotatedMgO
(aMgO=4.21Å×√2=5.95Å)allowsaepitaxialgrowthof
theHeuslercompound(aHeusler≈5.70Å).Thedisadvantage
ofMgOsubstratesaretheirhighcostswhencomparedtothe
establishedSiO2wafer.Thisiscausedbythesmallstandard
substratesizesandthereforeadirectindustrialapplicationis
challenging.Furthermore,MgOsubstratesareverysensitiveto
moistureandrequirespecialstorage.
Sisubstrates:TheSilatticeconstantofaSi=5.43Åisalsoingood
agreementwiththetreatedHeuslercompoundsofthiswork
(aHeusler=5.65Å-5.70Å).ThedisadvantageofSiistheelabo-
ratecleaningprocessofthesurface.Itisreported,thatthiscan
bedonesubsequentlybyisopropyl,dilutedH2SO4andHF.23
ThelattermethodwasusedforFethinfilmsinsteadofHeusler
compounds.Neverthelessitiscomparativebecausethelattice
constantofFe(aFe=2.866Å)24isalmosthalfofthelatticecon-
stantoftheinvestigatedHeuslercompounds(aHeusler≈5.70Å).
Additionally,Fegrowthsin(011)textureonstandardthermally
oxidizedSiO2wafer,too.
Therefore,weusedFeinthefirststep,insteadofaHeusler
compound,inordertofindanoptimumseedlayersystemwith
regardtotherequired(001)texture.Itsadvantageisthatanan-
nealingprocessisnotessentialforatomicordering,asitisinthe
caseofHeuslerthinfilms.Furthermore,aconfusionoftexture
andorderrelateddiffractionpeakscanbeavoidedinX-raydiffrac-
tion(XRD)measurements.Topreventsurfacecontaminations,all
substrateswerecoveredbyvariousthicknessesofaMgObuffer
.erylaTherefore,wepreparedthefollowingtwosystemsondiffer-
entsubstratesandperformedXRDmeasurementstoestimatethe
operties:prwthogr1.MgOsubstrate/MgO(xnm)/Fe(20nm)
2.Sisubstrate/MgO(xnm)/Fe(20nm)
Here,xisavaryingMgObufferthicknessof1,2,5,10,15,or
20nm.OneandtwonmwereonlyinvestigatedfortheMgOsub-
strates.Allpreparedsamplesweremeasuredintheasprepared

14

state◦and◦aftersubsequent◦annealingfor1hinstepsof200◦C,
300C,400Cand500C.
WeuseDC/RFsputteringforthepreparationofthethinfilms.
Thisistheestablishedmethodforindustrialapplications.All
filmsweredepositedatroomtemperature.Abasepressureof
1.0×10−7mbarofthesputteringsystemwasused;theArgonpro-
cesspressurewasbetween1.5×10−2mbarand1.5×10−3mbar.
XRDisastandardmethodforthecharacterizationofthecrys-
talgrowthpropertiesofthinfilms.Itispossibletodetermine
thelayerthicknessandtoconcludeorientationandtextureofthe
sputteredfilms.Theconstructiveinterfer25enceofradiationina
latticecanbedescribedbyBraggslaw:

nλ=2dhklsinθ(1)
wherenisaninteger,λisthewavelengthoftheincidentbeam
anddhklthelatticeplanedistancein(hkl)direction.TheXRD
measurementswereperformedbyanXpertPROMPDdiffrac-

15

Figure2:OverviewofXRD
patternfor20nmFeonMgO
sub-(bottom)Siand(top)strate.Thesamples◦werean-
nealedfor1hat400C.

25Fes-dieinEinführungtkörOldenbourperphysik,gVerlagC.Kittel,MünchenR.
)2006(ienW

Figure3:XRDpatternofthe
Fe(002)peakfor1nm,2nm
15andnm5andnm20(left)nmand(right)10MgOnm,
MgOonthicknessferbuf(bot-substratesSiand(top)tom).

26Theobservedintensityisa
superpositionofdifferentef-
fectsandisinuencedbydis-
ordereffects,too.Forthe
investigatedFethisisne-
glected.

tometerbyPhilips.Thecopperanodeemitsawavelengthof
λ=1.54056ÅforCuKα1andtheaverageofKα1andKα2isdeter-
minedtoλ=1.54184Å.Withtheangleofconstructiveinterfer-
enceθtheplanedistanceisgivenby
λndhkl=2sinθ.(2)
Hence,thelatticeconstantoftheinvestigatedmaterialcanbe
y:bcalculated

a=h2+k2+l2∙dhkl.(3)
Theobtainedpeakintensities,orratherthepeaknetareas,rep-
resent,inafirstapproximation,thedegreeoftextureofthesput-
teredfilmandareusedtoestimatetheoptimumseedlayer.26
Figure2depictsanoverviewoftheobtainedXRDpatternfor
a20nmthickFefilm,depositedonaMgOsubstrate(top)andSi
substrate(bottom),respectively.TheMgObufferlayerthicknessis
5nm.Bothsampleswereex-situvacuumannealedfor1hat400◦C.
TheMgOsubstratepeakat42.9degreesonthetopofFigure2
andtheSisubstratepeakat33and69.1degreesonthebottomare
clearlyvisible.Thecorrespondingsmallerkβsubstratepeaksare
alsolabeled.TheFe(002)peakcanbeidentifiedat65.4degrees

16

forbothtypesofsubstrate.ThepeakofthesputteredMgOlayer
isoverlaidbytheMgOsubstratepeakbutcanbefoundincaseof
theSisubstrateat42.5degrees.
Forfurthercharacterizationofthecrystalgrowthpropertiesof
theFelayer,theanalysisisfocusedontheFe(002)peak.Thisis
shownfordifferentMgObufferthicknessesandforvaryingan-
nealingtemperaturesinFigure3.TheFe(002)peakintensitiesare
loweredforallMgObufferlayerthicknessesincaseoftheSisub-
stratecomparedtotheMgOsubstrate.Thehighestintensitiescan
befoundfor5nmand20nmMgObufferedFeonMgOsubstrate.
FortheSisubstrate,thehighestpeakcanbefoundfor5nmMgO,
aswell.Buttheheightisonlyafourthofthehighestcorrespond-
ingheightontheMgOsubstrate.Duetothestronglydegraded
growthoftheFelayeronSiwafers,MgOsubstrateswereusedin
ork.wwingfollotheTodeterminetheoptimumMgObufferlayerthickness,thenet
areaoftheFe(002),asafunctionofannealingtemperature,is
showninFigure4.Anincreasingnetareaforincreasingannealing
temperaturescanbefoundforallMgOthicknesseswhichindicate
animprovementof(001)texture.Onlythenetareaofthe2nm
MgObufferedFeisnearlyonaconstantlevel.The5nmbuffered
Feclearlyresultsinthelargestpeaknetareaforallannealing

17

peakFigureas4:anetfunctionareaofofFeanneal-(002)
ingentMgOtemperaturbuffereforthicknesses.differ-
onAll(001)samplesMgOweresubstratesdepositedand
annealed.subsequentlyexsituvacuum

27D:K.Appl.InomataPhys.et39,al.,816J.(2006Phys.)

Figurtensitiese5:forFe5nm(002)thickpeakMgOin-
bufferdepositedonaheated
MgOsubstrate.

temperatures.Hence,thisthicknessisassumedtobethebest
seedlayeranditwillbeusedforallHeuslercompoundsthat
wereinvestigatedinthiswork.
Theobtainedshiftinthemaximumpeakintensities,inFigure
3,canbeattributedtoachangeoftheFelatticeconstant.This
isshownforvaryingannealingtemperaturesinFigure6.All
achievedvaluesareclosetotheFebulkvalue,representedby
thedashedline.ThedeviationislargerforthickerMgOlayers
intheaspreparedstate.Withincreasingannealingtemperatures
alllatticeconstantsapproachthebulkvalue.Thismightindi-
cateaslightlydegradedcrystalgrowthofthethickerMgOlayers
whichcanbecompensatedbymoderateannealing.Remarkably,
allchangesareonlyintheorderof1/100Å.
Furthermore,itisreportedthatfilmdepositiononheatedMgO
substratescanimprovethecrystalgrowthproperties.27There-
fore,theoptimizedlayersystemwasdepositedonaninsitu◦an-
nealedMgOsubstrate.Amaximumtemperatureofabout400C
couldbeachievedinthecurrentsputteringsystem.Forasimilar
treatment,thedepositedlayerswerealsosubsequentlyex-situan-
nealedatvaryingtemperatures.ThecorrespondingXRDpattern
oftheFe(002)peakisshowninFigure5.Comparedtothesame
layerstackingshowninFigure3,noimprovementoftheFefilms
canbeachievedforanyannealingtemperature.Themaximum
peakintensityisonlyaboutathirdofthehighestintensityfor
thesimilarnonpre-annealedMgOsubstrate.Furthermore,nearly
nodifferenceinpeakintensitycanbefoundwithincreasingan-
nealingtemperature,asmightbeexpectedbythe400◦Cinsitu
e.temperaturannealing

18

Insummary,therequiredseedlayerforHeuslercompoundswas
28optimized.compound,FeBecausewasofusedtosimilarestimategrowththeproptimumopertiesseedtoalayer.HeuslerThe
todifferachieentveaseedlaymaximumerswere(002)invFeestigatedpeakbyheightXRDandwithnetinaranea,rattemptespec-
prtivelyeferr.edAccorseeddinglaytoertheon(001)obtainedMgOresults,substratea5nminorMgOderbuftoferobtainisthea
(001)texturedHeuslerthinfilm.

19

Figure6:Latticeconstants
fordifferentMgObuffered
Felayersasafunctionof
Thee.temperaturannealingdashedlinerepresentsthe
alue.vbulk

28halfcubicofthelattice,HeuslerFelatticelattice,is
(011)standardorientedSiO2wafergrowthon

layerHeuslertheofOptimization

ThischapterdealswiththeoptimizationoftheHeuslerlayerwith
regardtogettingahighmagneticmomentandagood(001)tex-
turedgrowth.Theinuenceoflayerthickness,aswellasexsitu
annealingtemperature,willbeinvestigated.Here,thewellknown
compoundCo2MnSiisusedasarepresentativeforallHeusler
compoundsthatwillbeinvestigatedinthiswork.Fromearlier
works,itisknownthatCo2MnSiisamorphousintheasprepared
state.Therefore,anannealingprocessisrequiredtoinitiatethe
crystallizationandtoinducetheatomicordering.Foundedon
previousresults,aninitialannealingtemperatureof400◦Cwas
29chosen.

dependencethicknessCo2MnSithinfilmswithavaryinglayerthicknessweredeposited
ontheoptimizedseedlayersystemMgO(001)/MgO(5nm)toin-
vestigatethicknessdependentcrystallinegrowthandthemag-
neticpropertiesoftheHeuslercompound.Thesampleswerepre-
paredunderthesameconditionsasdescribedpreviouslyforthe
films.thinFeFigure7showstheobtainedXRDpatternof10nm,20nm,
50nmand100nmthickCo2MnSilayers.Alllayerswereexsitu
vacuumannealedfor1hat450◦C,forcrystallizationandatomic
orderingoftheHeuslercompound.Thebaselinesoftheachieved
patternswereshiftedwithregardtotheHeuslerthickness.As
previouslydescribed,theMgOsubstratepeakcanbefoundat
42.9degrees(kα)and38.6degrees(kβ).TheCo2MnSi(004)peaks
at66.3degreesandthe(002)peaksat31.8degreesarevisiblein
allcases.Asexpected,theobtainedpeakintensitiesareincreased
byincreasingCo2MnSithickness.Forthe50nmand100nmthick
layers,aslight(022)peakat24.4degreescanbeimagined.Thein-
duced(001)growthdirectionbytheMgOseedprobablyvanishes

21

29S.Kämmerer,PhDthesis,
BielefeldUniversity(2004);
andA.Hüttenetal.,Journal
CompoundsandysAlloof423,148(2006)

Figure7:Co2MnSiXRDpat-
ternfordifferentCo2MnSi
annealinganatthicknesstemperatureof450◦C.The
baselinesareshiftedwithre-
gardtotheannealingtemper-
e.atur

difFigurfereent8:THeuslerexturedthicknefractionssesof
andvaryingannealingtem-
es.peratur

30Theintensityofthe5nm
thickMgObuffercanbene-
glected.

forThethicklapeakyersandintensitiestheareHeuslerprfilmoportionalformstopartialthelay(011)ercrthickness.ystals.
DuetendedtothepeakdifferanalysisentisrdepositedequiredtothicknessesdistinguishofCo2theMnSi,anoptimumex-
peakHeuslerisdividedthickness.bytheThercorrefore,espondingthenetlayarereaofthicknesstheandHeuslerdefined(004)
fraction:edtexturtheaspeaknetarea
texturedfraction=layerthickness(4)
TheresultsareshowninFigure8asafunctionofannealing
temperature.Thehighesttexturedfractioncanbefoundforthe
20nmthickCoMnSiandallinvestigatedannealingtemperatures
of400◦C,450◦C2and500◦C.Thevaluesofthe50nmthicklayer
wereslightlyreduced.Forthe400◦Cand500◦Cannealed10nm
Co2MnSifilms,noclear(004)peakwasdetectableandthenet
areacouldnotev◦enbeestimated.Thesamebehaviorwasaalso
foundhigherfortheannealing400Ctemperaturannealedes100wernmerthickequiredHeuslerforcrsample.ystallization.Here,
ThesubstrateepitaxialcangrbeoprwthovofenthebyCoan2XRDMnSipoleontothefigurebufferscan,ed(001which)wMgOas
rtakenotatedbyonanaEulertiltingcradle.stage.HerThee,difthefractionsampleanglecanofθadditionallyissettobea
afixeddifvfractionalue.Forpatterevernyisangletakenψ(0withtoa90vardegryingees)ofangletheφ(0tiltedtostage360
thedegrees)obtainedofthepolerotatingfigurescanssample.oftheFigureMgO9depictssubstratethe30r(left)esultsandof
20nmthickCo2MnSilayer(right)fora500◦Csubsequentlyan-
nealedsample.ThefixedanglewassettotheMgO(022)peak
positionof2θ=62.3degreesandtotheCo2MnSi(022)peakposi-
22

tionof45.5degrees.Thefourvisibleareaswithhigherintensities
showthe(022)peakoftheMgOsubstrateandrepresentthecubic
crystalstructure.Consequently,thecorresponding(022)peakof
theCo2MnSifilmisrotatedbyφ=45degrees,i.e.,theintended
epitaxialgrowthoftheHeuslercompoundin(001)directionwas
present.Asexpected,thelatticewasrotatedby45degreestothe
MgOsubstrateandthebufferdidnotaltertheintendedgrowth,
duetothelowlatticemismatch.
Inadditiontoagood(001)texture,ahighmagneticmoment
oftheHeuslerthinfilmsshouldbeachieved.Aloweredmoment
couldbeattributedtoacertainatomicdisorder.

Roomtemperaturealternatinggradientmagnetometer
(AGM)measurementswereperformedtoinvestigatethemag-
neticmomentoftheCo2MnSithinfilms.Here,samplesofabout
3mmx3mmwereplacedontoasampleholderthatwascon-
nectedtoapiezocrystal.Inamagneticfieldofapairofcoils,the
samplebehaveslikeadipole.Anadditionaloscillatingmagnetic
gradientfieldleadstoavibrationofthesample.Thegenerated
voltageofthepiezocrystalcanbemonitoredwithalock-inam-
plifier.Themagneticstateofthesamplewillbechangedbythe
variationofthehomogeneousfield,whichleadstoadifferentsig-
nalatthelock-inamplifier.Calibratedwithaknownmagnetic
moment,themomentoftheinvestigatedsamplecanbedeter-
mined.ThemagnetizationMcanbecalculatedfromtherelation
M=m/V,wheremisthemeasuredmagneticmomentandV
themagneticvolumeoftheinvestigatedmagneticmaterial.The
volumecanbecalculatedfromtheknownthicknessofthesput-
teredfilmsandthesamplearea.Thelattercanbeestimatedvia
thesubstratedensityandtheweightofthetreatedsample.

23

Figure9:Left:polefigureof
fixedThesubstrate.MgOtheangleissettotheMgO(022)
peakpositionat2θ=62.3
espond-corrRight:ees.degringpolefigureofthe20nm
thickCo2MnSilayer.Thean-
gleissettothe(022)peak
positionat45.5degrees.The
an-subsequentlyaswsampletem-maximumatonealedperatureof500◦C.

Figure10:Roomtemperature
MnSiCoofmagnetization2tionmeasurofedbysupposedAGM(graseay)func-and
ck-thiHeuslerect(black)corrness.Thelayers◦werean-
nealedfor1hat400C.

3171I.,1(2005Galanakis,)Phys.Rev.B

InshownFigurefor10diftheferrentesultsCoofMnSithefilmsperformedwhichwAGMeremeasurdepositedementsonarthee
2optimizedseedlayer.TheobtainedmagnetizationoftheCo2MnSi
layerisgivenasafunctionofthickness(greydots).Allsamples
wereexsituannealedfor1hat400◦C.Theplottedvaluesarethe
averagevaluesofthreesimilarsamples.Theerrorbarsarees-
measurtimatedfrements.omtheThecorrdashedespondinglinereprstandaresentsdthedeprviationedictedofthefullthrbulkee
magneticmomentof1026kA/m(5.008µB31andassumingtheex-
perimentallatticeconstantof5.64Å).ForaCo2MnSilayerthicker
than15nm,amagnetizationofabut900kA/mcanbereached
whichcorrespondstoabout88%ofthepredictedbulkvalue.The
magnetizationisalmostconstant,withintherangeof15nmto
100nmHeuslerthickness.Thinnerlayersshowastronglyre-
ducedmagnetizationwhichindicatesanatomicdisorderorab-
senceofthecrystalstructure.Thepresentloweredexperimental
magnetizationofCo2MnSi,comparedtothepredictedbulkmag-
netization,canbeexplainedbyanoverestimatedthicknessofthe
Heuslerlayer.Thethicknessofthesputteredfilmsaregivenbya
certaindepositiontimethatisdeterminedfromacalibrationsam-
ple.Here,athicklayerisdepositedforanexacttime(∼100s).
AfterwardsthethicknessisdeterminedbyX-rayreection(XRR)
oratomicforcemicroscopy(AFM)onthenon-annealedsample.
BecauseoftheamorphousgrowingofCo2MnSi,adifferenceof
about20%inlayerthicknesscanbefoundforthenon-annealed
andannealedsample,respectively.Thisresultsina20%lower
moment.magneticcalculatedThecorrectedvalues,representedbytheblackdotsinFig-
24

ure10,measureuptothepredictedfullbulkmagnetizationof
1026kA/m(correspondingtoabout5µB)foracorrectedfilmthick-
nessof12nmandabove.

Inadditiontothemagnetization,thecoercivefieldHC,ofthees-
timatedsoftmagneticCo2MnSilayers,canbedeterminedfrom
AGMmeasurements,aswell.InFigure11thecoercivefieldis
depictedasafunctionofCo2MnSithickness.AminimumHCof
3Oecanbefoundforthe15nmthickCo2MnSilayers.Within-
creasinglayerthickness,thecoercivefieldincreases,aswell.With
regardtotheXRDresults,thechangeofcoercivefieldmightbe
attributedtothecrystalqualityoftheHeuslercompound.The
lowestvaluesofHCarefoundforthehighesttexturedfractionof
Co2MnSi.Therefore,theprogressionofcoercivefieldcanbeused
asafirstapproximationtoestimatetheHeuslerquality.

Insummary,theperformedXRDandAGMmeasurementsof
theCo2MnSilayers,depositedontheoptimizedseedlayersys-
temMgO(001)/MgO(5nm),showanoptimalHeuslerthickness
ofabout20nm,withregardtotheatomicorderingandcrystal
growthin(001)direction.Below15nm,astronglydegradedmag-
neticmomentisfound,aswellasanincreasedcoercivefield.The
crystallinegrowthofa10nmCo2MnSilayerislowerwhencom-
paredto20nm.ThickerHeuslerlayersshowanincreasedcoercive
fieldandadegradedcrystallinegrowth,aswell.Therefore,weas-
sumedaHeuslerthicknessof20nmastheoptimallayerthickness
forthefollowingexperimentsandallothercompoundsthatwe
willinvestigateinthiswork.

dependenceetemperaturannealingTheatomicorannealingderingandtemperaturcreofystallization.theHeuslerAnalogouslayertoistheimportantabovedis-for
cussedinuenceofHeuslerlayerthickness,structuralandmag-
neticmeasurementswereperformedtoverifytheoptimalanneal-
ingtemperature.Figure12depictstheXRDpatternofa20nm
thickCo2MnSilayer,thatwasdepositedontheoptimizedseed
layersystemfordifferentannealingtemperatures.Thebaselines
ofthepatternswere◦shiftedwithregardtothistemperature.Prior
toannealingat350C,no(002)and(004)Heuslerpeaksat31.8
degreesand66.3degreescanbefound.Forhighertemperatures,
bothpeaksarevisible.The(002)kαandkβpeakoftheMgOsub-
25

Figure11:Coercivefield
atofCo4002◦CMnSiasaannealedfunctionfor1hof
thickness.Heusler

Figure12:XRDpatternof
20nmthickCo2MnSilayers
annealedatvaryingtemper-
ystallizationcrThees.aturprocessstartsatabout350◦C.

Figure13:(004)peaknetarea
of20nmthickCo2MnSias
aperaturfunctione.Theofcrannealingystallizationtem-
processstartsatabout350◦C.

Figuremagnetization14:RoomofCotemperatur2MnSi,e
tionmeasurofedbyannealingAGM,asatempera-func-
laturyeer.forThe20grnmeythickmarkerrHeuslerefers
mea-espondingcorrthetosurnessementsdependence.ofCo2MnSithick-

stratearelocatedat42.9and38.6degrees,respectively.There-
quiredannealingtemperaturetoinitiatethecrystalgrowthofthe
HeuslercompoundCo2MnSiin(001)directioncanbeestimated
fromthe(004)peaknetarea.Theachievedpatternsaresho◦wnin
Figure13asafunctionofannealingtemperature.Below300Cthe
Co2MnSilayeris◦amor◦phousand,therefore,nopeakispresent.In
therangeof300Cto375Cthecrystallizationprocesstakesplace
andthe(004)peaknetareaincreaseswithincreasingtemperature.
Higherannealingresultsinafairly◦constantnetarea.Themaxi-
mumvaluecanbefoundforthe425Cannealedsample.
Asimilarsteplikebehaviorispresentintheobtainedmagnetiza-
tionoftheHeuslerthinfilms.Theresultsoftheroomtemperature

26

AGMmeasurementsareillustratedinFigure14.Thedashedline
representsthepredictedbulkmagnetizationof1026kA/m(corre-
spondingto5.008µB32).Withinthetemperaturerangeof375◦Cto
500◦C,themagnetizationisalmostconstantandamaximumof
1039kA/mcanbeachievedforthe400◦Cannealedsample.
TheoverestimatedthicknessoftheCo2MnSilayerswasalready
takenintoaccount.Thegreymarkerreferstothecorresponding
measurementsofCo2MnSithicknessdependence.Itisslightly
loweredbutwithinexperimentalaccuracy.

ThecoercivefieldsoftheHeuslercompoundwereinvestigatedby
AGM,aswell.TheobtainedresultsaredepictedinFigure15as
afunctionofannealingtemperature.AveryhighHCispresent
forthe350◦Cannealedsample.Inadditiontothepreviouslydis-
cussedXRDmeasurements,itindicatesthestructuralchangeat
thistemperature.Fortemperaturesabove375◦Calowcoercive
fieldcanbeachievedforthecrystalizedandatomicallyordered
Co2MnSilayers.
Insummary,thegrowthdirectionoftheCo2MnSilayerwassuc-
cessfullychangedfrom(011)to(001)orientationwiththenew
seedlayersystem,whichwaspreviouslyintroducedandinves-
tigatedforFe.Thefullpredictedbulkmagnetizationandthus
themagneticmomentoftheHeuslercompoundwasreached.
Thebestresults,withregardtotheinducedatomicorderingand
anoptimal(001)texturedgrowth,wereachievedfor20nmthick
Co2MnSilayerdepositedon5nmMgObuffered(001)MgOsub-
strates,whichwereannealedfor1hat400◦C.

27

3271,I.1(2005Galanakis,)Phys.Rev.B

20Figurnme15thick:CoerCo2civMnSiefieldlayersof
annealedatvaryingtemper-
es.atur

compoundsHeuslerCo-basedariousV

Thehalf-metalicbehaviorhasbeentheoreticallypredictedfordif-
33fercouldentnotHeuslerbeobservcompoundsed.Ther,buteasonstheforrthisesultingcanbegiantmanifoldTMRratiosand
somearelistedinthebelow:
FabricationoftherequiredL21structure:Oftenthehalf-met-
allicityispredictedonlyforL2typestructure.Thedensity
ofstates(DOS)canbesensitive1tothecrystalstructureandso
adisorderedcompound(34B2-,A2-typestructure)mayhavea
polarizationspineducedrCorrectfilmcomposition:ThecalculatedDOSofcompounds
withmaterials,adeviationadditionalfromstatestheX2closeYZtothecompositionFermileshovel.ws,Tforhismasomey
leadtoareducedspinpolarization,aswell.
Mnoxideproblem:HeuslercompoundsthatcontainMnoften
showadecreasedmagneticmoment.Inparticular,thebarrier
MnO,interfacewhichmomentmightalsomightrbeeducelowtheeredspinduetopolarization.thefor35mationof
Coherentgrowth:Coherentgrowthofthebarrierispreferredto
getcoherenttunneling.Therefore,materialswithalowlattice
mismatcharemostpromising.
Latomyerrelectroughness:odecanArreduceoughthesurfacequalityoftheofthesputtertunneledHeuslerbarrierbot-and
wouldleadtoareducedTMRratio.
ThechallengeistofindaHeuslercompoundwiththebest
larpr,theopertiesfolloregarwingdingthecompoundsabovearevermentionedyprdifomising.ficulties.TheInprosparticu-and
conswillbepointedoutindetail.
Co2MnSi(CMS)istheHeuslercompoundwiththehighestre-
36talportedandTMRtheorveticalaluesatlopublicationswtemperaturcanbees.foundSevanderalitisverexperimen-ywell

29

33R.D.Grootetal.,Phys.
I.Rev.Lett.Galanakis50,et2024al.,(1983Phys.);Reandv.
B66,1(2002)

34BS.69,1(Picozzi2004)etal.,Phys.Rev.

35A.Hüttenetal.,Journalof
AlloysandCompounds423,
)2006(148

36S.Appl.TsunegiPhys.(et2009al.,)J.Phys.D:

understood.Itispredictedtobeahalfmetalbutthepositionof
theFermilevelisreportedtobeclosetotheedgeofthevalence
band.ThismightreducetheroomtemperatureTMRbecause
37B.Balkeetal.,JMagnofthermalsmearing.37Someyearsofexperienceinpreparing
MagnMater310,1823(2007)Co2MnSithinfilmsarepresentinBielefeld.Unfortunately,the
containingMnissupposedtoreducethespinpolarizationdue
38A.Hüttenetal.,JournaloftotheformationofMnOatthebarrierinterface.38
AlloysandCompounds423,
148(2006)Co2FeAl(CFA)containsthesamenumberofelectrons(29)as
39I.Galanakisetal.,Phys.Co2MnSi,andregardingtheSlater-Paulingbehavior39,thesame
Rev.B66,1(2002)magneticmomentisexpected.Halfmetallicityispredictedfor
bandstructurecalculationsusingtheSPR-KKRprogrampack-
40http://olymp.cup.uni-ageofH.Ebert40.Thespinpolarizationisloweredforthe
muenchen.de/ak/ebert/SPRKKR/approachofAkai,usedbyMiura.41TheFPLOmethodleads
4169Y,.144413Miura(et2004al.,)Phys.Rev.Balsotoareducedspinpolarization.42Incontrasttoothercom-
42S.Wurmehletal.,J.Phys.pounds,thehighspinpolarizationisalmostconserved,even
(D:2008)Appl.Phys.41,115007forB2typestructuredCo2FeAl.
Co2FeSi(CFS)hasthehighestreportedCurietemperatureTCand
43S.Wurmehletal.,Phys.bulkmagneticmomentof6µB.43Furthermore,itisreported
44Rev.B72,1(2005)tobeeasilyfabricatedintherequiredL21structure.44With
LettN.T89,ezuka112514et(al.,2006Appl)Physregardtothelattertwocompounds,thepositionoftheFermi
levelisshiftedtohigherenergies.ForsomeDOScalculations,
itendsupintheconductanceminoritybandandthepredicted
halfmetallicitycannotbeconserved.Buttheexactpositionof
45A.Thomasetal.,J.Appl.EFisdisputed.45
B.Phys.Balke103,etal.,023903(Phys.2008)Re;v.andBCo2MnAl(CMA)contains28valenceelectrons.Accordingtothe
74,1(2006)Slater-Paulingbehavior,ithasthelowestmagneticmoment
withinthislist,whichisinterestingforcurrentinducedswitch-
ing.ThepositionofEFisshiftedtolowerenergies,withregard
totheabovelistedcompounds.Veryfewpublicationscanbe
foundaboutthiscompound.TheincludedMnmightoxidize,
asreportedforCo2MnSi,buttheroleofAlorSi,respectively,
withregardtopreventionofoxidationisunclear.
Co2Mn0.5Fe0.5Si(CMFS)ispredictedtohaveEFlocatedinthemid-
46B.Balkeetal.,JMagndleofthebandgap.46Therefore,highTMRratiosatlowtem-
MagnMater310,1823(2007)peraturearealsoexpectedtobeconservedatroomtempera-
e.tur

45A.Thomasetal.,J.Appl.
B.Phys.Balke103,etal.,023903(Phys.2008)Re;v.andB
74,1(2006)

46MagnB.BalkeMateret310,al.,1823J(2007Magn)

Insummary,Heuslercompoundswith28(Co2MnAl)to30(Co2FeSi)
valenceelectronswillbeinvestigated.Thecorrespondingpre-
dictedmetallicbulkbehaviormagneticcanbemomentpredictedisinaforallrangeof4compoundsµBto6andµB.theAdif-half

30

ferentnumberofvalenceelectronsleadtodifferentpositionsof
theFermilevel,withregardtothebandgap.47Withinthislistof
Heuslercompounds,theroleofthedifferentelementscanprob-
ablybeverified,aswellastheconnectiontoFermilevel,bias
voltageandtemperaturedependence.Asimilarlatticeconstantis
expectedforallcompoundsandtheforCo2MnSioptimizedseed
layerwillbeused,aswell.First,halfjunctions(seeFigure16)
ofallcompoundswerepreparedtoinvestigatethecrystalgrowth
andmagneticpropertiesoftheHeuslerthinfilm.Thiswasnec-
essaryforevaluationandinordertounderstandtheresultsof
correspondingfulljunctions,whichwillbediscussedlaterinthis
ork.w

stoichiometryfilmDuetodifferentstickingcoefficientsthefilmcompositionmight
bedifferenttothetargetcomposition.However,thecorrectfilm
compositionisessentialfortheatomicorderingandahighspin
polarization.Therefore,sputteredlayersofallHeuslercompounds
wereanalyzedbyinductivelycoupledplasmaopticalemission
spectrometry(ICP-OES)toverifytheexactfilmcomposition.Here,
thesputteredfilmsofabout600nmweredepositedonGaAswafers
forHeuslercompoundscontainingSiandonSiO2waferforcom-
poundswithoutSi.Asubstratesizeofabout30mmx30mmwas
usedtoprovideenoughmaterialtoverifythefilmstoichiometry.
TheICPanalysiswasrealizedbyCurrentaGmbH&Co.OHG.
TheobtainedstoichiometriesfordifferentHeuslercompoundsare
giveninTable1aswellasthecorrespondingtargetcomposition.

compoundtargetcomposition(at%)filmcomposition(at%)
Co2MnAlCo2Mn1Al1Co2Mn0.957Al0.8
CoCo22FeAlMnSiCo2MnCo21.28Fe1SiAl1.291CoCo22MnFe0.980.985AlSi0.9850.968
Co2FeSiCo2Fe1Si1Co2Fe0.953Si0.925
Co2Mn0.5Fe0.5SiCo2Mn0.5Fe0.5Si1Co2Fe0.473Mn0.383Si0.912

ThebestagreementoffilmstoichiometryandintendedHeusler
composition(X2YZ)wasfoundforCo2FeAlandCo2MnSi.The
lattercompositionwasinvestigatedinearlierexperiments,which
allowedanadjustmentofthetargetcomposition,withregardto
anoptimizedfilmcomposition.Thesetwocompoundsarethe
mostpromising.TherequiredL21structuremightbeformed.A
highmagneticmomentandhighTMRratiosareexpected,be-

31

47ThepositionoftheFermi
rleveloughlyEFbycanthebenumberassumedof
ons.electr

aFigurhalfe16:magneticLayertunnelstackingjunc-of
electrtionsode.withabottomHeusler

Table1:comparisonofused
compositionsgettartheandcompositionsfilmesultingr

theFigure(002)17:andXRD(004)patterpeaknofof
atdiffer400◦entCHeuslerannealingcompoundtemper-s
is:ature.MgO(001)The/layerMgO(5stackingnm)
/Heusler(20nm)/MgO
nm).(1.8

causeoff-stoichiometricalHeuslerfilmsmightshowareduced
momentpolarization.spinand

owthgrcrystallineThecrystallinegrowthpropertiesofthedifferentHeuslercom-
poundswereinvestigatedbyXRD,aspreviouslydiscussedfor
Co2MnSi.Theoptimizedlayerstackofa5nmMgObuffered
20nmthickHeuslercompoundwasdepositedunderthesame
conditions,asmentionedbeforeonsinglecrystallineMgO(001)
substrates.ThelayerswerecoveredwithanadditionalMgOlayer
of1.8nmtopreventsurfacecontaminations.Alllayerswereex-
situvacuumannealedfor1hatdifferenttemperatures.

theFigur400e◦17Cdepictssamples,theincludingobtainedtheprHeuslereviously(004)anddiscussed(002)Co2peaksMnSi.for
Ashiftinthemaximumpeakintensitiesfordifferentcomposi-
tions,whichisattributedtoaslightlydifferentlatticeconstant,
isclearlyvisible.Theintensityofthepeaksiscorrelatedwith
layerthickness,ontheonehand,andontheotherhandwiththe
amountofcrystallinity.Here,thehighest(004)peakcanbefound
forButCothe2FeAlindentedandthe(001)highestorientation(002)ispeakprforesentCo2forMnSiallrinvespectivestigatedely.
compounds.Furthermore,Figure18showsthe(004)peaknetareaasafunction
arofecrannealingystallinepriortemperature.annealing.ExceptForforCoCo22MnAl,MnSi,theallnetarcompoundseais
nearlyunaffectedbyannealingtemperaturewithintheinvesti-
32

gatedrange.Forallothercompounds,anincreaseoftheHeusler
(004)peaknetareacanbefoundforincreasingannealingtem-
peratures.Thisincreaseindicatesaimprovedcrystalgrowth.In
contrasttothesteplikebehaviorofCo2MnSi,thecompounds
Co2FeAlandCo2Mn0.5Fe0.5Sishowacontinuosincreasewith◦an-
nealingtemperature.ForCo2FeSi,onlyhighannealingat500C
createsacrystallographicimprovement.

Figure19representstheprogressionofthelatticeconstantforin-
creasingannealingtemperaturesanddifferentHeuslercompounds.
Thedashedlinesrepresenttheparticularpredictedvaluesfound
inliterature(Co2MnSi48,Co2MnAlandCo2FeAl49,
Co2FeSi50,Co2Mn0.5Fe0.5Si51).Thelatticeconstantwasobtained
byapplyingtheBraggequationtothecorresponding(002)and
(004)peaksandaveragingthem.Therefore,thelatticeconstantre-

33

ofFigurtheedif18:fer(004ent)peakHeuslernetarcom-ea
ingpoundstemperaturforvares.yinganneal-

Figure19:Latticeconstant
Heuslerestigatedinvallforcompoundsatdifferentan-
temperaturnealingThees.dashedlinesrepresentthere-
alues.vbulkported

48MagnU.MaterGeiersbach240,et546al.,(J2002Magn)
49MagnK.BuschoMaterw38,et1(al.,1983J)Magn
50ReS.v.BW72ur,1mehl(2005et)al.,Phys.
51D:M.Appl.KallmaPhys.yer39et,al.,786J.(2006Phys.)

52RietvPoweldderMetDifhodfraction:andThethe
TwSpringero-StageVerlagMethod,(2006)G.Will,

ectsonlytheoutofplanecomponentofthelattice.Ifnotmarked
otherwise,acubiclatticeisassumed.
AsshowninFigure19thelatticeconstantsofthecompounds
Co2MnSi,Co2FeSiandCo2FeAlarealmostindependentofanneal-
ingtemperature.ThelatticeconstantofCo2MnSimeasuresupto
thepredictedvalueof5.64Å.ForCo2FeSithevaluesareslightly
raised,whencomparedtothereference,probablyduetothesto-
ichiometricaldisagreement.Bycontrast,thelatticeofCo2FeAlis
clearlyreduced.Thismightbeduetoadeformationofthelat-
tice.ThelatticeoftheneighboringtopandbottomMgOgrowths
45degreesrotatedto√theHeuslerlattice.ThisresultsinaMgO
bufferlatticesizeof2xaMgO=5.95Åthatmightspreadout
theinplanelatticeoftheHeusler(aHeusler≈5.70Å)togetcoher-
entinterfaces.AssumingaconstantvolumeoftheHeuslercell
thiswouldreducethemeasuredoutofplanelatticeconstant.
Thestrongestchangeinlatticeconstantcanbedetectedfor
Co2MnAl.Here,thelatticeislargerthanthereferencevalueinthe
aspreparedstateanddecreaseswithincreasingannealingtemper-
atures.Forannealingtemperaturesof400◦Candabove,thelattice
constantdropsclearlybelowtheliteraturevalue.Inthecaseofthe
quaternarycompoundCo2Mn0.5Fe0.5Si,theexperimentallatticeis
slightlyhigherthanthereportedreferenceforannealingtemper-
aturesto300◦C.Forhighertemperaturesthelatticedecreasesand
measuresuptothereferenceforthe500◦Cannealedsample.

TodeterminethecrystallographicHeuslerstructure,polefigure
scanshavetobeperformed,asdiscussedpreviously.Butanother
approachwouldbetousetheobtaineddiffractionpeaks.Inthe
caseofe.g.,anfcccell,thecenteredatomscauseanadditional
reectionplaneatthehalfdistance.Thiscanmakesomepeaks
vanishbydestructiveinterference.Toestimatetheallowedpeaks
ofanXRDpatternonehastoconsidertheinternalstructureof
theunitcell.ThetotalintensityIhklthatisregisteredbythedetec-
torisdirectlypr52oportionaltothesquaresofthecrystallographic
structurefactorFhkl.
Ihkl∝|Fhkl|2(5)
ThestructurefactorFhklisacomplexquantitywhichisgiven
y:b

34

NFhkl=∑fkexp(2πi(huk+kvk+lwk))(6)
1=k

withfk,theatomicscatteringfactorofthedifferentatomsk,
(hkl)theMillerindicesanduk,vkandwk,therelativpositions
oftheatomsintheunitcell.Nisthecorrespondingnumberof
atomswithinintheunitcell.
Inthecaseofafull53Heuslercompound,threedifferenttypes
ofreectioncanbeidentifiedfromthefourinterpenetratingfcc
lattices.Planes(hkl)withallevenMillerindicesoralloddin-
dicesleadtoanon-vanishingreectedintensity.Theevenindices
canfurtherbedistinguishedintoevenandoddratiosof(hkl)/2.
Therefore,thefollowingpeakscanbeconnectedtothecorre-
spondingstructurefactors,whichrepresentthecrystallographic
54e.structuroftypeallodd(hkl)peaksrepresentthepresenceofaL21structure(e.g.
(111),(113),(133),(333),...)andthestructurefactorisgivenby:
|Fhkl|2=16(fA−fC)2+(fB−fD)2withh+k+l=(2n+1)
allodd(hkl)/2peaksrepresentthepresenceofaB2structure(e.g.
(002),(222),(024),(006),...)andthestructurefactorisgivenby:
|Fhkl|2=16[(fA+fC)−(fB+fD)]2withh+k+l=2(2n−1)
alleven(hkl)/2peaksrepresentthepresenceofaA2structure(e.g.
(022),(004),(224),(044),...)andthestructurefactorisgivenby:
|Fhkl|2=16[(fA+fB+fC+fD)]2withh+k+l=4n
However,the(004)peakisfundamentalfortheA2typestruc-
ture.Here,theHeusleratomsarerandomlydistributedinabcc
lattice.Theexistenceofanadditional(002)peakindicatesaB2
typestructure.Here,onlythesub-latticesYandZofthecom-
poundX2YZaredisordered.TheproofofaL21structurecanbe

35

53AhalforafullHeusler
XYZcompoundoraX2YarZegivencomposition,bya
.elyespectivr

54S.BielefeldKämmerUniver,ersityPhD(2004)thesis,

Figure20:Calculatedareara-
tioofthe(002)to(004)peaks
Heuslerestigatedinvtheforcompoundsatdifferentan-
es.temperaturnealing

55nationalReferenceCentreDatabase,forDifInterfrac--
)1999(Datation56H.thesis,WBielefeldulfmeier,UniverDiplomasity
)2010(

Figure21:Magnetizationof
forcompoundsHeuslerthedifferentannealingtempera-
tures.Thedashedlinesrep-
resentthevaluesfoundinlit-
e.eratur

givenbydetectionofthe(111)peak.AnEulercradleisrequired
to(002r)/ealize(004)thesepeakmeasurratiotoements.powderAlterdifnativfractionely,adataiscomparisonoftenusedoftheas
anTheindicatorcorrforespondingtheL21ratiostructurforalle.investigatedHeuslercompounds
asafunctionofannealingtemperatureisgiveninFigure20.The
dashedstructure.55linesrTheeprbestesentagrtheeementpredictedcanberatio,foundwhichforCohints2MnSi.toaLThe21
robserequirvededfr(111)ompetheakpoletoprfiguroveethescans.L21Buttypeitwasstructurfoundeincouldmornotede-be
tailedannealedscansforwith1hathigher400◦rC.56esolutionForallonothersimilarcompounds,samplesthatthewratioere
issumed.clearlyInlowerparticularedand,noan(111)absentpeaksL21wertypeefoundstructurforethecanCo2beFeAlas-
halfjunctions.However,theobtained(002)peaksforthedifferent
compoundsindicateatleastaB2typestructure.

opertiesprmagneticInthissection,themagneticpropertiesoftheHeuslerhalfjunc-
tionswillbediscussedindetail.ThecoercivefieldHCaswell
asthebulkmagnetizationweredeterminedbyroomtemperature
ements.measurAGM

Figure21depictsthemagnetizationofthe20nmthickHeusler
layersfordifferentannealingtemperatures.Thegivenvalueswere
obtainedfromaveragingthreemeasurementsofsimilarhalfjunc-
tions.Theerrorbarsaregivenbythestandarddeviation.Thethe-
oreticallypredictedvaluesofthemagnetization,representedby

36

thedashedlines,weregivenbyGalanakisforCo2MnAl
(800kA/m),Co2MnSi(1026kA/m)andCo2FeAl(1007kA/m).57
ThecorrespondingvaluesforCo2FeSi(1219kA/m)and
Co2Mn0.5Fe0.5Si(1120kA/m)werereportedbyWurmehl58and
Balke59respectively.TocalculatethemagneticmomentinµBor
themagnetizationinkA/m,theexperimentalvaluesofthelattice
parameterwereused.AnidealL21typestructurewasassumedin
allcases.ThesteplikeprogressionoftheCo2MnSimagnetization
hasalreadybeendiscussedprivously.Bycontrast,allotherinves-
tigatedcompoundsshowamagneticbehavior,evenforthenon
annealedsample,whichcanbeattributedtothecrystalstructure
intheaspreparedstate.
ForCo2FeAlthereportedbulkvalueof4.99µBisreachedal-
mostindependentlyofannealingtemperature.
ThemagnetizationofCo2FeSiincreasesslightlywithincreas-
ingannealingtemperature,whichindicatesanimprovedatomic
ordering.Amaximumvalueof1090kA/mcanbeachievedfor
500◦C.Thisvalueishigher,whencomparedtosimilar(011)tex-
turedCo2FeSifilms.60Here,wereachedamaximumof901kA/m
at400◦C.Furtherannealingdecreasedthemagnetization,proba-
blyduetodiffusionoftheVseedlayer.
Asimilarbehaviorispresentforthequaternarycompound
Co2Mn0.5Fe0.5Si.Here,amaximummagnetizationof941kA/m
canalsobereachedfor500◦C.Thisvalueisalmostidenticaltothe
previouslyreported,for(011)texturedCo2Mn0.5Fe0.5Sihalfjunc-
tions61,butthevalueisonlyabout84%ofthereportedbulkvalue.
Thisresultisprobablyrelatedtothestrongdeviationinthefilm
stoichiometrie.Thelowestmagnetizationofallinvestigatedcompoundsis
presentforCo2MnAl.AspreviouslyfoundintheXRDmeasure-
ments,thevaluesarenearlyindependentofannealingtempera-
tureandamaximumof535kA/mcanbereachedat500◦C.This
islessthan70%ofthecorrespondingpredictedbulkvalueand
canprobablybeattributedtothedisagreementofthefilmstoi-

ratiomaximummaximumofmagneticmagnetizationcompound(kA/m)moment(µB)predictedvalue
Co2MnAl5352.700.68
Co2MnSi10395.021.00
Co2FeAl10995.451.09
Co2FeSi10905.360.89
Co2Mn0.5Fe0.5Si9414.620.84
37

57I.Galanakis,Phys.Rev.B
71,1(2005)
58S.Wurmehletal.,Phys.
Rev.B72,1(2005)
59B.Balkeetal.,Phys.Rev.B
74,1(2006)

60LettD.89,Ebke162506etal.,(2006)ApplPhys

61sol.D.(a)Ebke205,et2298al.,(2008phys.)stat.

Table2:Achievedmaximum
spondingmagnetizationmagneticandmomentcorre-
intheµBrineportedcomparisonbulkvalueswith
Heuslerestigatedinvallforcompounds.

ofFigurthee22inv:CoerestigatedcivefieldHeuslerHC
compoundsforvaryingan-
es.temperaturnealing

chiometrie,whichmightleadtoadisorderedHeuslerlayer.
TheeffectoftheoverestimatedthicknessincaseoftheCo2MnSi
layerhasnotbeentakenintoaccountfortheotherinvestigated
Heuslercompounds.Thesecompoundsarealreadycrystallinein
theaspreparedstateandtheassumeddepositionratesyieldthe
thickness.erylaintendedTheachievedmaximumvaluesofmagnetizationforthediffer-
entHeuslercompoundsaresummarizedinTable2aswellasthe
correspondingmagneticmomentsinµB.Asmentionedabove,the
experimentallatticeconstantsandaL21structurewereassumed
calculation.thefor

InFigure22,thecoercivefieldisgivenasafunctionofannealing
temperature.WiththeexceptionofCo2MnSi,thesoftmagnetic
characteroftheHeuslercompoundswiththecoercivefieldsbelow
20Oecanbefoundinallcases.Aspreviouslydescribed,ahigh
HCcanbedetectedforCo2MnSiintheannealingtemperature
rangeofthecrystallizationprocess.Forannealingtemperatures
higherthan375◦C,itdropsclearlybelow10Oe.Apartfromthis
drop,theHCofCo2FeSi,Co2MnAlandCo2Mn0.5Fe0.5Siisabout
5Oeforallannealingtemperatures.ThecoercivefieldofCo2FeAl
times.otwaboutisBesidethediscussedbulkmagneticmoment,theproper-
tiesatthebarrierinterfaceareveryimportant.Theseinterfacial
atomsdeterminethetunnelingcurrent.Therefore,theelement
specificmagneticmomentoftheHeuslerlayers,atthebarrierin-
terface,wereexaminedasafunctionofannealingtemperature
byX-rayabsorptionspectroscopy(XAS)andX-raymagneticcir-
culardichroism(XMCD)respectively.Themeasurementswere
38

performedatbeamline6.3.1oftheAdvancedLightSourceofthe
LawrenceBerkeleyLaboratoryatBerkeley,California,USA.The
Co-,Mn-andFe-L3,2edgeswereinvestigated.Surfacesensitive
totalelectronyield(TEY)wasrecordedwithagrazingangleofin-
cidenceϕof30degreestothesamplesurface.TheXMCDspectra
wereobtainedbyapplyingamagneticfieldofupto±2Talong
thex-raybeamdirection,usingellipticallypolarizedradiation
withapolarizationPof60%.TheXASintensityandtheXMCD
effectaredefinedas(I++I−)/2andI+−I−,respectively.Here,
I+andI−nametheintensity,measuredwithparallel/antiparallel
orientationofthephotonspintothemagneticfield.Themeasure-
mentswereachievedatroomtemperaturewithasamplingdepth
62nm.2aboutofTheobtainedspectraoftheinvestigatedHeuslercompound
Co2FeAlwillbediscussedindetail,followedbyadiscussionof
theresultsforCo2FeSi,Co2MnSiandCo2Mn0.5Fe0.5Si.Thecor-
respondingspectraofthesecompoundscanbefoundintheap-
pendix.ForCo2MnAl,noXASdataareavailableatthepresent
time.

Figurspectrae23forshothewsasprtheeparFe-edandandCo-XASannealed(top)Co2andFeAlXMCDhalfjunctions.(bottom)

39

turFigureeXAS23:(top)Roomandtempera-XMCD
(bottom)spectraattheFe−
L3,2andCo−L3,2edge,re-
spectivelyfordifferentan-
HeuslernealingcompoundtemperaturesCo2ofFeAl.the
ThemalizedXAStotheintensitiesvalueswerepriornorto-
theL3edge.Forcompari-
son,thebaselineofthemea-
surementisshiftedinregard
toThethearrowsannealingmarkprtemperaturominente.
es.featurXAS

62R.Nakajimaetal.,Phys.
Rev.B59,6421(1999);and
Y.Idzerdaetal.,Nuclear
PhysicsInstrumentsResearandch-SMethodsectionAin
Only347,134(1994)

63ReJ.v.SB70,chmalhorst1(2004)etal.,Phys.

64BT.64,J.1(Regan2001)etal.,Phys.Rev.

65B.Tholeetal.,Phys.Rev.B
31,6856(1985)
66thesis,DiplomaEbke,D.BielefeldUniversity(2007)
67B.Tholeetal.,Phys.
Rev.B31,6856(1985);and
J.Schmalhorstetal.,Phys.
Rev.B70,1(2004)

ThemeasuredXASintensitywasnormalizedtotheintensityprior
totheFe-L3andCo-L3edge,respectively.Thearrowsindicate
prominentfeaturesintheXAS.InthecaseofCo,theshoulderat
about4eVabovetheL3edge,canbeattributedtoacertainatomic
andmagneticorderoftheCo2FeAlcompoundatthebarrierinter-
face,aspreviouslyreportedforCo2MnSi.63Thisfeatureisalready
presentforthenon-annealedsampleandbecomespronounced
withincreasingannealingtemperatures.ThecorrespondingCo
spectraofCo2FeSi,Co2MnSiandCo2Mn0.5Fe0.5Si(seeappendix)
alsoshowatypicalmetallicbehaviorforallannealingtempera-
tures.Wefoundacertainatomicordering,representedbythe
shoulderatabout780eV,forCo2Mn0.5Fe0.5Sialreadyintheas
preparedstate.Thisshoulderbecomespronouncedwithincreas-
ingannealingtemperatures.ForCo2FeSi,theshoulderisonly
presentforthe500◦Csample.IncaseofCo2MnSiitispresentfor
375◦Candbecomespronouncedwithincreasingannealingtem-
peratures.Thisisingoodagreementwiththeobtainedresults
fromXRDinvestigations.Forthe500◦Csample,thisfeaturedis-
appearsagain,probablyduetooxidation.
Theshoulderatabout2eVabovetheL3edgeoftheFe-XAS
(Figure23)andthenon-annealedCo2FeAllayerisattributedto
FeO.64TheFemightbeoxidizedduringthesputteringofthe
MgObarrier.Forannealingtemperatureshigherthan200◦C,no
FeOcanbefound.Theabsencemightbeexplainedbythecrys-
tallizationprocessofthebarrierandthebindingofO.TheFe
spectra,ofthe200◦Cand300◦CCo2FeSisamples,alsoshowthe
fingerprintofoxidizedFe.Fortheaspreparedandhigheran-
nealedlayers,ametallicFespectraispresent.Thisalsoobtains
fortheFeatomsatthebarrierinterfaceoftheCo2Mn0.5Fe0.5Si
layers.Here,wefoundametallicbehaviorforallannealingtem-
peratures.Bycontrast,theassociatedMnspectrashowthetypical
multipletstructureofMnOforthewholeinvestigatedrangeof
annealingtemperatures.65Mostlikely,theMnpreventstheoxi-
dationofFebecauseofahigheroxygenaffinity.Wehadprevi-
ouslyfounda66similarbehaviorfor(011)texturedCo2Mn0.5Fe0.5Si
halfjunctions.Thesamemultipletstructureisalsopresentin
theMn-XASofCo2MnSiwhichindicatesanoxidationoftheMn
atoms67foralltemperatures,excepttherangeof375◦Cto425◦C.
Here,theMnspectrashowmetallicbehavior.
TheFe-andCo-XMCDofCo2FeAlinFigure23(bottom)show
anasymmetryforallannealingtemperatures,i.e.,amagneticin-
terfacemomentcanbeachievedevenforthenon-annealedHeusler

40

layer.AsexpectedfromtheAGMinvestigations,anasymmetryis
alsopresentintheCo2FeSiandCo2Mn0.5Fe0.5Sispectra(seeap-
pendix)forallannealingtemperatures.Bycontrast,anXMCDef-
fectfortheCo2MnSilayerscanonlybeobservedfortemperatures
higherthan350◦C.InthecaseofMntheeffectisagainreduced
forthe500◦C68sample.Thismightbeattributedtotheformation
ofMnxSiyO.

Figure24(top)showstheFe-andCo-XASintensityattheL3
edge(IL3,asdefinedinFigure23)asafunctionof◦annealing
temperature.Atannealingtemperatureshigherthan200C,the
XASintensitydecreasesforCoandFerespectively.Thismight
beduetoanAlsegregationtowardthebarrierinterface,toim-
provetheHeuslerstructure.Incontrasttotheprogressionfound
forCo2FeAl,aroughlyconstantlevelcanbefoundfortheCoand
FeintensitiesofCo2FeSi,andtheCo,MnandFeintensitiesof
Co2Mn0.5Fe0.5Si.Thisindicatesthatastrongchangeintheatomic
structureisnotpresentintheinvestigatedrangeofannealingtem-
peratures.Thisobservationisincontrasttoourpreviouslyinves-
tigatedVbuffered(011)texturedCo2Mn0.5Fe0.5Sihalfjunctions.
Here,wefoundastrongMndiffusiontowar◦d69thebarrierinterface
forannealingtemperatureshigherthan400C.

41

Figure24:Top:roomtem-
peratureXASintensityatthe
L3edgeofCo,FeandMn
y-laMgO/Heusler/MgOofersasafunctionofanneal-
ingementtemperaturspecifice.momentMiddle:(orel--
bital+spin)ofthebarrierin-
espond-corrtheandterfaceingtotalmoment(2×mCo+
mX,X=Fe,Mn).Bottom:
Heuslertheofmomentbulk.eryla

68A.Hüttenetal.,Journalof
AlloysandCompounds423,
)2006(148

69sol.D.(a)Ebke205,et2298al.,(2008phys.)stat.

70A.Hüttenetal.,Journalof
AlloysandCompounds423,
)2006(14871C.Chenetal.,Phys.Rev.
Lett.75,152(1995)

andInthetheMncaseIofcurCov2ewMnSi,aspranesent.oppositeThisbehabehaviorvioroftheindicatesCoIaL3strcurongve
L3structuralchangeatthebarrierinterfacewithvaryingannealing
temperatures.Forannealingtemperaturesof375◦Cto425◦C,we
foundamaximumofCo-andaminimumofMn-intensity.For
thesetemperatureswefoundafingerprintofmetallicMn.Most
likelytheCoatoms◦attheinterfacepreventtheMnatomsfromox-
idation.Forthe500CsampletheCo-aswellastheMn-intensity
attheL3edgedrops,probablyduetoSidiffusiontowardthebar-
rierinterfaceandtheformationofMnxSiy-oxides,asreportedfor
70junctions.similarTheelementspecificmagneticmomentcanbecalculatedfromthe
XASandXMCDbyapplyingthesumrules.71Theintegralsr,p
andqofthespectraaredefinedas:

p=L(I+−I−)dE(7)
3q=L3+L2(I+−I−)dE(8)
r=I++I−−fdE(9)
L3+L22
Thefunctionfisatwo-stepfunction.Thethresholdissettothe
peakpositionsoftheL3andL2absorptionedgesandtherela-
tivestepheightissetto2/3and1/3respectively,oftheaverage
intensityabovetheL2edge.
Thecorrespondingorbitalandspinmagneticmomentcanbe
y:bmineddeter

41qmorb=−Pcosϕ6rnd(10)
mspin=−Pcos1ϕ(6p2−r4q)nd(11)
ThepolarizationPandtheangleofincidenceϕaresetaslisted
above.Thenumberof3dholesaregiveninTable3.Thesewere
72http://olymp.cup.uni-carriedoutfromSPR-KKRbandstructurecalculations.72
muenchen.de/ak/ebert/SPRKKR/Theresultingelementspecificmagneticmoments(orbital+spin)
aredepictedinFigure24(middle).Thetotalmomentwasdeter-
minedby2×mCo+mX,(X=Fe,Mn).
42

compoundnd(Co)nd(Fe)nd(Mn)
CoCo22FeAlFeSi21..289333..4829--
Co2MnCo0.52Fe0.5MnSiSi22..33243.-3944..4452
ForCo2FeAlandCo2FeSi,themagneticmomentoftheCoatoms
measureuptothepredictedvalueof1.14µBand1.54µB73,respec-
tively,forthefullrangeofannealingtemperature.Thereference
valuesarerepresentedbythedashedlines.ForCo2MnSi,aCo
interfacemomentispresentforannealingtemperaturesof375◦C
andabove.Thevaluesmeasureuptothepredictedvaluefound
inliterature,aswell.74Bycontrast,theCointerfacemomentof
Co2Mn0.5Fe0.5Siishigherthanthepredictedvalueof1.15µB.75
Theinterfacemomentstaysalmostconstantforallannealingtem-
peratures.TheComomentfitsverywellwiththetheoreticalval-
uesoftheabovediscussedcompositions.Therefore,anoveresti-
matednumberof3dholes,becauseofadeviatingcomposition,
canbeassumedforCo2Mn0.5Fe0.5Si.
TheFemagneticmomentinCo2FeAlincreaseswithincreas-
ingannealingtemperaturesandisclosetothepredictedvalue
of2.81µBforthe500◦Cannealedsample.76Thisismostlikely
duetothereductionofFeO.Asimilarbehaviorwasfoundfor
thetotalmomentresultingfrom2xmCo+mFe.Thepredicted
bulkvalueof4.99µBcanbereachedforannealingtemperatures
higherthan400◦C.InCo2FeSi,theFemomentisreducedfor
annealingtemperaturesbelow300◦Candfitswiththepredicted
valueof3.3µBfor400◦Candabove.77Thiscanbeexplainedby
thepreviouslydiscussedpresenceofFeOforthesetemperatures.
Asimilarbehaviorcanbefoundforthetotalinterfacemoment
(2×mCo+mFe),whichmeasuresuptothepredictedvalueof
78.6µBAreducedinterfacemoment,becauseofoxidation,canalsobe
foundfortheMncontainingcompounds.Wefoundanoptimal
temperatureforCo2MnSi,withrespecttoahighMninterfacemo-
ment,tobeintherangeof400◦Cto450◦C.Here,wefounda
metallicfingerprintintheXASofMn.Thetotalinterfacemo-
mentgivenby2×mCo+mMnisinitsmaximumandreachesthe
predictedvalueofabout5µB.79ThereducedMn-andtotalinter-
facemomentsbelowthistemperature,andforthe500◦Csample,
areattributedtotheoxidationofMnatoms.Asexpectedfrom
previouslydiscussedAGMinvestigations,wefoundnointerface

43

Table3:Assumednumber
of3dholesfortheinves-
compounds.HeuslertigatedThevaluesweredetermined
bySPR-KKRbandstructure
calculations.73I.Galanakis,Phys.Rev.B
71,1(2005);andS.Wurmehl
etal.,Phys.Rev.B72,1(2005)

7471I.,1(2005Galanakis,)Phys.Rev.B
7574,B.1(Balke2006)etal.,Phys.Rev.B

7671I.,1(2005Galanakis,)Phys.Rev.B

77S.Wurmehletal.,Phys.
Rev.B72,1(2005)
78ReS.v.BW72ur,1mehl(2005et)al.,Phys.

7971,I.1(2005Galanakis,)Phys.Rev.B

momentfortheamorphousCo2MnSilayerbelow350◦C.
TheMnmomentattheCo2Mn0.5Fe0.5Siinterfaceisstrongly
reduced,whencomparedtothereportedvalueof3.19µB.Most
likelythisisbecauseofthestrongpresenceofMnOforallanneal-
ingtemperatures,whichwecanconcludefromtheXAS.Aslight
increaseoftheinterfacemomentispresentonlyforthe500◦C
sample.Bycontrast,theachievedFemomentmeasuresuptothe
predictedvalueof2.79µB.Here,wefoundametallicfingerprint
intheFe-XASforalltemperatures.
DuetotheroughlyconstantleveloftheComomentsofthevar-
iousHeuslerlayers,thetotalinterfacemomentsaremostlydeter-
minedbytheFeor/andMnmoments.ForCo2FeAlandCo2FeSi,
thepredictedvaluescanbereachedforannealingtemperatures
C.◦400higherHowever,theresultingtotalmomentgivenby2mCo+½mMn+
½mFefitswiththepredictedvalueof5.5µB.Here,thestrongly
reducedMnmomentwascompensatedbytheprobablyoveresti-
matedComoment.Theoverestimationmightissuefromadevia-
tionintheatomicconcentrationofthefilms.

Forabettercomparison,thepreviouslydiscussedbulkmoments
oftheHeuslerhalfjunctionsarealsoshowninFigure24(bot-
tom).Wefoundanalmostidenticalprogressionofbulkandtotal
interfacemomentforCo2FeSi,Co2MnSiandCo2Mn0.5Fe0.5Si.The
predictedinterfacemomentswerereachedbutthebulkmoment
ofCo2FeSiandCo2Mn0.5Fe0.5Siwerelowered.Thedeviationcan
beexplainedbytheassumednumberof3d-holes,whichmightbe
differentforthefoundfilmstoichiometries.Therefore,anover-
estimatedinterfacemomentcanbeassumedforthelattertwo
compounds.ForCo2MnSiweobservednearlythesamevalues
ofinterfaceandbulkmoment.Theloweredinterfacemomentof
the500◦CCo2MnSisample,incomparisontothebulkmoment,
confirmsthepresenceofMnOatthebarrierinterface.Theroughly
constantbulkmomentsoftheCo2FeAllayers,areincontrastwith
thetotalinterfacemoment,whichincreaseswithincreasingtem-
peratures.ThiscanbeexplainedbythedetectedoxidizedFeat
thebarrierinterface,whichwasformedduringthedepositionof
MgO.

Insummary,wehaveinvestigatedtheinterfacialmagneticmo-
mentsofthecompoundsCo2FeAl,Co2FeSi,Co2MnSiand
Co2Mn0.5Fe0.5Sifordifferentannealingtemperatures.Wefound

44

anoxidationofMnatomsforallMncontainingcompounds,which
ledtoareducedinterfacemagneticmoment.Wefound,foronly
asmallannealingtemperaturerangeof(400◦C-450◦C),afinger-
printofmetallicMnintheXASofCo2MnSi.Thismightindicate
anatomicallyorderedHeuslerforthesetemperatures.
Bycontrast,theobtainedFe-XASspectramostlyshowafin-
gerprintofmetallicFe.RemarkablywefoundnooxidizedFein
thehigherannealedFecontainingHeuslercompounds.Thepre-
dictedinterfacialFemomentwasreached.Adirectcomparison
oftheFe-andMn-XAS,inthecaseofthequaternarycompound
Co2Mn0.5Fe0.5SileadstotheassumptionthattheMnatomsoxi-
dizemoreeasilyandcanhindertheFefromoxidation.
TheinvestigatedCo-XASspectraareverysimilarforallcom-
canpounds.befoundAforshoulderall,samplocatedles.Itatisaboutfaintly4eprVaboesent,veevtheenCoforL3theedge,non-
prannealedonouncedsampleswithincr(Co2easingFeAlandannealingCo2Mn0.5temperaturFe0.5Si),es.andThisfeaturbecomese
indicatesacertainatomicordering.ItisreportedbyTsunegi80
thatthisfeatureiscorrelatedtotheL21structureoftheHeusler
compound.ButweobservedaL21structureonlyinCo2MnSihalf
junctions.WefoundthatneitherSinorAlpreventsFefromoxidationat
vloerwysimilarannealing.Butwtemperaturededucees.aTheXASconnectionofCo2ofFeSitheandfoundCo2FeOFeAlarande
theatomicordering,representedbythemagneticmomentand
theshoulderintheCo-XAS.Adirectcomparisonoftheresultsfor
Co2FeAlandCo2FeSishowsthatoxidizedFedisappearsatlower
annealingtemperatures(forCo2FeAl).Correspondingly,the(004)
peaknetareaincreases(seeXRD),themagneticmomentincreases
(seeAGM)andtheshoulderintheCo-XASappears.

45

80S.Appl.TsunegiPhys.(et2009al.,)J.Phys.D:

81ReS.v.BW72ur,1mehl(2005et)al.,Phys.
82Phys.N.T(2006ezuka)etal.,J.Appl.

83BS.69,1(Picozzi2004)etal.,Phys.Rev.

84perforThemedmeasurbyementsCurrwentaere
OHGCo.&GmbHTable4:Filmcompositions
oftarthegetsthrveeerifieddifferbyentICPCo-Fe-Sianaly-
sis.

Co-Fe-SiofcompositionsentdifferOneofthemostpromisingHeuslercompoundsforspintronic
applicationsisCo2FeSi.IntheclassofCo-basedHeuslercom-
pounds,ithasthehighestpredictedmagneticmomentof6µB
andthehighestCurietemperatureofabout1100K.81Further-
more,itisreportedtobeeasilyfabricatedinL21structure82.We
canconcludefromtheXASandXMCDmeasurements,thataFe
containingcompositionispreferredtoMn,duetothereduced
formationofoxidesattheinterfacewhichmightreducethespin
polarization.

DOScalculationsofHeuslercompounds(inparticularCo2MnSi)
haveshownthatantisitedefectsmightdestroythehalfmetallicity
duetoadditionalstatesattheFermilevel.83Consequently,the
compositionoftheHeuslerisveryimportant.Theinvestigation
ofoff-stoichiometricalfilmsmighthelptorevealmagneticand
crystallographicproperties.Therefore,theinuenceofHeusler
filmcompositionwasinvestigated.WeusedtwoadditionalCo-
Fe-Sitargetsthatresultedina
stronglyFeenrichedfilmcompositionanda
stronglySienrichedfilmcomposition.
ThespecificfilmcompositionsweredeterminedbyICP-OES84
andaregiveninTable4.

targetcomposition(at%)filmcomposition(at%)
balancedCo2Fe1Si1Co2Fe0.953Si0.925
FeenrichedCo2Fe1.43Si1.22Co2Fe1.313Si0.894
SienrichedCo2Fe1.09Si1.37Co2Fe1.019Si1.205

Theoff-stoichiometricallyHeuslerlayerswereinvestigatedwith
rtions,egardtotheconsistingcrystalof20gronmwthofandCo-Fe-Simagneticandpr1.8nmoperties.ofMgO,Halfwjunc-ere
depositedon5nmMgObuffered(001)MgOsubstrates.

Figure25depictstheobtained(002)and(004)XRDpeaksatabout
32and66degrees,respectively,forthethreedifferentCo-Fe-Si
compositions.Thesampleswereannealedfor1hat400◦C.For
theSienrichedfilmcomposition,bothpeaksnearlyvanished.
Thecrystallizationwasstronglyreducedwhencomparedtothe

46

balancedandFeenrichedfilmcomposition.Nearlythesamein-
tensitieswithasimilarpeakshapewerepresentforthelattertwo
compositions.The(022)Heuslerpeakatabout45degrees,could
notbefoundinanycase.

InFigure26theparticular(004)peaknetareaisshownasafunc-
tionofannealingtemperature.TheSienrichedlayerisamorphous
intheaspreparedstate.Withanincreasingannealingtempera-
tureaslightincreaseofnetareacanbeestimatedbuttheachieved
intensitiesareconsiderablylowerwhencomparedtoallotherin-
vestigatedHeuslercompounds.Bycontrast,thebalancedandFe
enrichedlayerswerealreadycrystallinepriorannealing.Wede-
tectedanincreasingnetareawithincreasingannealingtempera-
tures.TheslopeoftheFeenriched(004)peaknetareawasslightly
47

Figure25:XRDpattern
overviewforthreediffer-
entnealedCo2atFeSi400◦C.compositionsThelayan-er
stackingis:MgO(001)/MgO
(5nm)/Co2FeSi(20nm)/
MgO(1.8nm).

arFigureaefor26the:dif(004fer)entpeakCo2FeSinet
ingHeuslerannealingcompoundstemperaturatvares.y-

Figure27:Latticeconstant
forthedifferentCo2FeSi
Heuslercompoundsatvary-
es.temperaturannealingingThedashedlinerepresents
thereportedbulkvalue.

85ReS.v.BWur72,1mehl(2005et)al.,Phys.

86Phys.N.T99,ezuka08Tet314(al.,2006J.)Appl.

strongerwhencomparedtothebalancedfilm,whichindicatesan
improvedcrystalgrowthwithouttakingintoaccountthepresent
e.structuroftype

Theannealingtemperaturedependenceoftheoutofplanelat-
ticeconstantisillustratedinFigure27.ThebalancedandFe
enrichedlayersshowsimilarbehavior.Bothlatticesareslightly
higherthanthereportedbulkvalue85andkeepalmostconstant
forthewholerangeofannealingtemperatures.TheFeenriched
latticefaintlyapproachesthebulkvaluewithincreasingannealing
temperatures.Thisisincontrasttothebehaviorof◦theSienriched◦
latticeconstant.Forannealingtemperaturesof200Cand500C,
thelatticeconstantisclosetothe◦bulkvalue.◦Apartfromthis,it
isstronglyreducedforthe300Cand400Cannealedsamples.
Thiseffectcanbeexplainedbyastrongcrystallographicchange
cell.unitdistortedorToestimatethepresenttypeofcrystalstructureforthedifferent
compounds,the(002)and(004)peakratioswerecalculatedand
aredepictedinFigure28fordifferentannealingtemperatures.
ThedashedlinerepresentstheexpectedvalueforaL21structure.
ThepeakratiosofthebalancedandFeenrichedHeuslerlayers
areconsiderablylowerandaB2typestructurecanbeassumed
forthesecompounds.Bycontrast,theratiooftheSienriched
layerincreaseswithincreasingannealingtemperatureandmea-
suresuptotheexpectedvalueforaL21structure.Asreported
byTezuka86,itseemsthatSicansomehowprovidethepreferred
L21structure.Butthefoundpeakintensitiesleadstotheassump-
tionthatamostlyamorphousfilmispresentinthecaseoftheSi
junctions.halfenriched48

However,theproofcanonlybedeterminedbypolefigurescans
andthedetectionofthe(111)Heuslerpeak.Duetothelow
peakintensities,thescanswereskippedatthispointbutplaned
forhigherannealedsamples.Highertemperaturesmightleadto
higherintensitiesandanimprovedcrystalHeuslerlayer.

WehaveperformedroomtemperatureAGMmeasurementstoin-
vestigatethebulkmagnetizationandcoercivefieldoftheCo-Fe-Si
compounds.TheobtainedresultsaregiveninFigure29fordiffer-
entannealingtemperatures.Thedashedlinesrepresentthepre-
dictedbulkvalues,assumingL21structureandtheexperimentally
determinedlatticeconstant.Thepredictedoff-soichiometrically
valueswereroughlyestimatedbytakingtheparticularelement
specificmomentofCo2FeSi87andtheobtainedfilmstoichiome-
.ytr49

Figur(002)/(004)e28:peakRatioareaof
forHeuslerthedifcompoundsferentatCov2arFeSiy-
es.temperaturannealingingprTheedicteddashedvlinealuereprofesentsL21
e?structur

theFigurdifefer29:entCo-Fe-SiMagnetizationcom-of
poundsasafunctionofan-
Thee.temperaturnealingdashedlinesrepresentthe
valuesfoundinliterature.

87ReS.v.BWur72,1mehl(2005et)al.,Phys.

Table5:Magneticmoments
fordifferentCo-Fe-Sifilm
-experThestoichiometries.imentalparametervwaluesereofusedthetolatticecal-
momentmagnetictheculate.µinB

theFigurdifefer30:entCoercivCo-Fe-Siefieldcom-of
poundsnealingasatemperaturfunctione.ofan-

totheTheachiebalancedvedlayerformagnetizationtheasofprtheeparFeedenrichedstate.Wlayithersincrissimilareasing
imumannealingmagnetizationtemperaturesofa1310continuouskA/mcanslopeberispreachedesentforandthea500max-◦C
paredannealedtothesample.balancedThereforHeuslere,alayhighererispresent,magnetizationwhichwhenmightcom-be
attributedtothehigheramountofFe.
Incontrast,wefoundaroughlyconstantmagnetizationofthe
prSiesentenrichedforthelayers.300◦CAannealedmaximumlayers.magnetizationThisvalueofis811onlykA/maboutis
60%oftheestimatedmagnetization.Higherannealingresulted
inXRDardecresultseaseofthishintsmagnetization.atacertainInatomiccombinationdisorderwith.Athemagneticobtained
SimomentenrichedisalrHeuslereadyprlayesenterswineretheasamorpreparphous.edThisstateinalthoughcontrasttheto
theresultswefoundfortheCo2MnSihalfjunctions.

ratiomaximummaximumofmagneticmagnetizationcompound(kA/m)moment(µB)predictedvalue
balanced10905.360.89
Feenriched13106.420.92
Sienriched8113.470.58

TheidealL2achie1vtypeedstructurhighestevandaluestheareexperimentalsummarizedlatticeinTableconstant5.werThee
assumedtodeterminethecorrespondingmagneticmomentin
/f.u.µB

50

TheFe-SicorrlayersarespondingedepictedcoercivinefieldsFigureof30theasofafunctionf-stoichiometricallyofannealingCo-
ancedtemperaturCo2eFeSiandlayarers.eAcomparsimilaredtolowtheHCachiecanvedbevfoundaluesoffortheallbal-an-
nealingtemperatures.Inparticular,thecoercivefieldoftheamor-
withphousthegrownachievSiedhighenrichedHClayoferstheiscrbelowystalizing5Oe,Co2whichMnSi.contrastsOnlya
slightincreaseispresentforthe500◦Cannealedsample.

Theelementspecificpropertiesatthebarrierinterfacewere
investigatedbyXASandXMCD.TheobtainedspectraoftheFe-
andSi-enrichedCo-Fe-Sifilmscanbefoundintheappendix.We
foundafingerprintofmetallicFeintheXASforbothoffstoichio-
metriccompoundsandallinvestigatedannealingtemperatures.
Thisincontrasttotheresultswefoundpreviouslyforthebal-
ancedCo2FeSilayers.TheCo-XASalsoshowametallicbehavior
forallannealingtemperatures.Wefoundashoulderatabout4eV
abovetheL3edgeforthe500◦Csamples,whichcanbeattributed
toacertainatomicandmagneticorder.88Forlowertemperatures
theshoulderisclearlyweakened.AsexpectedfromtheAGM
measurements,aFe-andCo-XMCDeffectwasachievedforall
es.temperaturannealing

Thecorrespondingprogressionofthecalculatedinterfacemo-
ments,aswellastheXASintensities,areillustratedinFigure31
(middle)and(top),respectively.Forbettercomparison,theresults
ofthepreviouslydiscussedbalancedCo2FeSiarealsogiven.
TheCo-XASintensitiesarealmostconstantforallannealing
temperatures,exceptadipat300◦CfortheSienrichedsample.
WealsofoundasimilardipinthecorrespondingFe-XAS,which
mightindicateahigheramountofSiatthebarrierinterface.How-
ever,wealsoobserveastructuralchangefromtheXRDinvestiga-
e.temperaturthisattionsAnincreasingFe-XASintensityinconnectionwithincreasing
temperaturescanbeassumedfortheFeenrichedHeuslerhalf
junctions.Mostlikely,thecontainingsmalleramountofSiforms
theHeuslerstructureinsidethelayerandexcessiveFesegregates
atthebarrierinterfaceorrathertheSisegregatesintothelayer.
ButcomparedtothepriviouslydiscussedXASoftheothercom-
pounds,especiallytoCo2FeAlandCo2MnSi,thechangeswere
range.smallainonly

51

88ReJ.v.SB70,1chmalhorst(2004)etal.,Phys.

Figure31:Top:roomtemper-
atureXASintensityattheL3
edgeofCoandFeofMgO/
Co-Fe-Si/MgOlayersasa
tem-annealingoffunctionelementMiddle:e.peraturspin)specificofthemomentbarrier(orbitalinterface+
to-espondingcorrtheandtalmoment(2×mCo+mFe).
theofmomentbulkBottom:.erylaCo-Fe-Si

89Lett.C.75,Chen152et(al.,1995)Phys.Rev.

90ReS.v.BWur72,1mehl(2005et)al.,Phys.

Wehaveestimatedthe89interfacemoments(Figure31(middle))by
applyingthesumrules.Wehaveassumedthesamenumber
of3dholesaspreviouslyforthebalancedCo2FeSi,thatistosay,
nd=2.28forCoandnd=3.48forFe.
FortheFeenrichedlayers,wefound,thatthemagneticinter-
facemoment90ofComeasureduptothepredictedvalueof1.54µB
forCo2FeSi.Bycontrast,theinterfacemomentofFewaslower
thanthepredictedmoment.Aslightincreasewithincreasingan-
nealingtemperaturecanbeestimated.Thetotalinterfacemoment
(2×mCo+mFe)behavedsimilarly.ThepredictedvalueofCo2FeSi
wasreachedfortemperatureshigher300◦C.Thisisincontrastto
thecontinuousincreaseofthebulkmomentwithincreasingan-
nealingtemperatures,whichwefoundpreviouslyintheAGM
ements.measurFortheSienrichedlayers,wefoundamaximumofabout5.2µB
for◦thetotalinterfacemoment(2×mCo+mFe)afterannealingat
300C.TheCo,aswellastheFeinterfacemomentsareintheir
maximum.WithregardtotheobtainedXAS,thehighestmoment
canbefoundforthelowestCoandFeabsorptionintensities.A
higheramountofSiattheinterfacecanbeconcluded.
Wefoundthesamebehaviorfortheinvestigatedbulkmoment,
asdepictedinthebottomofFigure31.Theachievedlowerval-
uesleadtotheassumptionthattheestimatednumberof3dholes

52

resultsinanoverestimatedinterfacemoment.Nevertheless,the
trendoftheelementspecificmomentsareingoodagreementwith
alues.vbulkobtainedthe

Insummarywehaveinvestigatedtwooff-stoichiometricalCo-Fe-
Sihalfjunctions.Withregardtocrystalgrowth,theFeenriched
layersshowcomparablebehavior,aspreviouslyfoundforthebal-
ancedCo2FeSifilms.Bycontrast,theSienrichedCo-Fe-Silayers
areamorphousintheaspreparedstateandonlyaweakcrystal
structurecanbeobtainedafterannealing.
Wefoundasimilarbehaviorforthemagnetizationand(inter-
face)magneticmoments.Weobserveincreasingmomentsbyin-
creasingannealingtemperaturesfortheFeenrichedlayers.By
contrast,themagnetizationandinterfacemomentoftheSien-
richedlayerswerereduced.Wefoundbothmaximaforthe300◦C
annealedsample.NoFeOwasfoundintheXASoftheFeen-
richedlayersnorintheSienrichedlayerswhichisincontrastto
film.balancedtheWecanconcludethattheHeuslerCo2FeSiisconsiderablyinu-
encedbyadditionalSiatoms,whereasaddedFeledtoanalmost
identicalbehaviorwhencomparedtothebalancedcompound.
OurresultscontrastwithsimilarMgO/CrbufferedCo-Fe-Si
halfjunctionsthatwedepositedfromidenticaltargets.Apartfrom
theCo-Fe-Si(004)and(002)peaks,wealsodetecta(022)peakin
theXRDpattern.Thisleadstotheassumptionthatalowerdegree
of(001)texturewaspresentduetothebufferlayerweused.Fur-
thermore,abulkmagnetizationofaboutonly800kA/mcouldbe
achievedbyAGMmeasurements,independentoftargetandan-
nealingtemperature.Theobservedcorrespondinghighcoercive
fieldsofabout100OeleadsustoexpectalessorderedHeusler
film.

53

opertiesprransportT

Thetransportpropertiesoffullmagnetictunneljunctionswith
differentHeuslercompoundelectrodeswereinvestigatedandthe
resultsarediscussedinthischapter.AsshowninFigure32,
thefulltunneljunctionscontaintheoptimizedlower20nmthick
Heuslerelectrodeona5nmMgObuffered(001)MgOsubstrate.
AnMgObarrierthicknessof1.8nmor2.1nmwasused.The
counterelectrodewasformedbya5nmCo-Felayerthatwas
pinnedtoa10nmantiferromagneticMn-Irlayer.Forgoodcon-
ductancetheselayerswerecoveredwitha40nmRuand20nm
Aucappinglayer.Thefinaljunctionsweresubsequentlyex-situ
vacuumannealedfor1htoinducecrystallizationandordering
ofthelowerlayerstackuptotheCo-Feelectrode.Thesamples
werecooledinamagneticfieldof0.65Tinordertosettheex-
changebiasofthepinnedelectrodeandwerepatterned,byop-
ticallithographyandionbeametching,tojunctionswithasize
of7.5µm×7.5µm,12.5µm×12.5µmand22.5µm×22.5µm.The
transportpropertiesofthemagnetictunneljunctionsweremea-
suredasafunctionofthemagneticfield.Alltransportcharac-
terizationswerecarriedoutbyconventional2-terminalmeasure-
ments.Exceptasnotedotherwise,allmeasurementsweredoneat
roomtemperature.Theappliedbiasvoltagewas10mVandthe
annealingtimewas1h.

stabilityetemperaturannealing

TheresultingroomtemperatureTMRratiosforthesubsequently
annealedfullHeuslerjunctionsareshowninFigure33.Anin-
creasingTMRratiowithincreasingannealingtemperature,most
likelyduetoanimprovementofthecrystallinityoftheHeusler/
tunnelbarrierinterface,wasfoundinallcases.Thisiscommonly
foundinconventionalCo-Fe-B/MgO/Co-Fe-BMTJs,too.91The
highestratiosofabout150%wereachievedforCo2FeAljunctions

55

ofFigurtheefull32:Latunnelyerjunctionsstacking
forallinvestigateddifferent
compounds.Heusler

91LettS.93Ikeda,082508etal.,(2008)ApplPhys

Figurtemperature33:eAchieTMRvedrratiosoom
contain-MTJsfullofingCo2FeAl,Co2MnSi,Co2MnAlCo2FeSi,and
Co2ex-situMn0.5Fe0.5annealingSiatvartemperying-
linesdashedThees.aturrMgOepresentbarriera,1.8thenmsolidthick
2.1nmMgO,respectively.
Theindicatesquartheestwandodiffetrianglesrent
annealedsubsequentlyCo1.82nmFeAlthicksamplesMgObarrierfor.the

92PhysJ.HaLettyaka89w,aet232510al.,(2006Appl)
93PPhys..V.97,10PaluskarC925et(al.,2005J.);C.Appl.H.
Kantetal.,ApplPhysLett84,
et1141al.,(J2004);MagnandC.MagnH.MaterKant
286,154(2005)

94D:M.Appl.OoganePhys.et39,al.,834J.(2006Phys.)

witha2.1nmMgObarrierthatwereannealedat450◦C.Thejunc-
tionsshowgoodannealingtemperaturestability.TheTMRra-
tiostaysroughlyconstantfortemperaturesbetween410◦Cand
470◦C.ThisisincontrasttootherstudieswheretheMndiffusion
oftheMn-Ir,towardtheupperbarrierinterface,wasthoughtto
beresponsibleforthedecreaseoftheTMRratiobeyond400◦C.92
Ontheotherhand,similarbehaviorwasreportedbyKantand
93PaluskerFortheCo2FeSijunctions,amaximumTMRratioofabout80%
canbefoundforannealingtemperaturesintherangeof350◦Cto
400◦C.Thedifferencesbetweenthe1.8nmand2.1nmthickMgO
barriersweresmall.Highertemperaturesledtoadecreaseofthe
theTMR.upperThisdecrbarriereaseinterface.mightbeThedecaattributedyistostrMnongerdifforfusionthetowthickerards
.barrierTheCo2MnAljunctionsshowanearlyconstantTMRratio,on
alowlevelofabout60%,withvaryingannealingtemperatures.
Theratiosofthe2.1nmthickbarrierarehigherthanforthe1.8nm
thickMgO.Astrongdecayoccuredfortemperatureshigherthan
◦460similarC,toandthelikerwiseeportedforvCoalues2ofFeAl.OoganeTheatachieel.vedforAlOTMRratiosbasedarB2e
Xordered(001)Co2MnAljunctions,buttheannealingtemperature
stabilityismuchbetter.94
TheexpectedhighTMRvaluesofthequaternarycompound
Co2Mn0.5Fe0.5Si,duetoashiftoftheFermilevelintotheband
gap,couldnotbefound.Ratiosofabout50%wereachievedfor
the350◦Cannealedjunctions.
ThelowestTMRratioswereachievedforCo2MnSialthough
56

itisacompositionwithreportedhighroomtemperatureTMR
values95.Here,theTMRratiosremainbelow20%,whereashigh
magneticbulkandinterfacemomentswerepresent.Aslightmax-
imumwasfoundforanannealingtemperatureof366◦C.Aphase
changeispresentforthistemperatureregionasweobservedin
theXRDpattern.Furthermore,theXASshowedapresenceofox-
idizedMnatthebarrierinterface.Higherannealingtemperatures
wouldimprovethecrystalstructure.Buthighertemperaturesalso
ledtoMndiffusiontowardthebarrierinterfacewhichloweredthe
ratios.TMRInconclusion,compoundscontainingFeleadtohigherratios
thansimilarcompoundscontainingMn.HeuslerscontainingAl
leadtohigherTMRthanSi.TheTMRratiosofSicontaining
compoundsarelessstableagainstannealingthansimilarcom-
poundscontainingAl.Here,adecayoftheTMRispresentfor
annealingtemperaturesabove470◦C.Mostlikely,theAlcontain-
ingcompoundsformlargergrainsatlowerannealingtempera-
turesincomparisonwiththeSicontainingcompounds.Because
grainboundarydiffusionoccursmuchfasterthanbulkdiffusion
thisseemstoleadtomoretemperaturestableTMRratios.

ThecorrespondingproductsofarearesistancearegiveninFig-
ure34fordifferentHeuslerelectrodesandannealingtempera-
tures.TheMgObarrierthicknessis1.8nm.Thevaluesareap-
proximately100KΩµm2forCo2FeSiandCo2FeAl,andmorethan
100MΩµm2forCo2MnAl.Anincreaseofresistancecanbefound
forallcompounds.Thisincreaseindicatesanimprovementofthe
barrierquality.Inparticular,nostrongdecaycanbefoundwhich
wouldindicateabreakwithinthebarrierorshortcuts.Theob-
57

95Appl.S.TPhys.sunegi(et2009al.,)J.Phys.D:

(AR)Figureof34the:ArHeuslerearfullesistanceMTJs
annealingoffunctionaasMgOtemperaturbarriere.fora1The.8nmsquarthickes
twandodiffertrianglesentindicatesubsequentlythe
samples.FeAlCoannealed2

Figure35:Bottom:model
forinuenceofatomicorder-
inganddiffusiontotheob-
tainedlowTMRvalueinthe
caseofCo2MnSi.Ashift
(dashedline)ofthediffusion
annealinghighertoocessprtemperatureswouldincrease
theTMR,whichcanbeesti-
matedbytheproduct(top)of
fusion.difandderingor

tainedhigherresistiantforMncontainingcompoundscanbeex-
plainedbyanadditionalthininsulatingMnOlayer,atthelower
interface.barrier

ThelowTMRratioofCo2MnSicanbeexplainedbythefollowing
model:ThecrystallizationprocessofCo2MnSiisintheannealing
temperaturerangeof350◦Cto400◦C.ThelowTMRratiosmay
beattributabletodiffusiontakingplacebeforethecrystallization
iscompleted.Figure35illustratestheimpactofatomicordering
(greenline)anddiffusion(redline)ontheTMRratios.TheTMR
ratiocanbeestimatedbytheproduct(black)ofbothlines.As
shownbythedashedlines,arelativeshiftofoneeffectwould
leadtohigherTMRvalues.Therefore,thefollowingprocedures
toimprovetheTMRratiosareimaginable:
Temperaturestablecounterelectrode:Higherex-situannealingtem-
peraturesarenecessarytoinducetherequiredatomicorder-
ingandcrystallizationoftheHeuslercompoundCo2MnSi.To
generateamorestablelayerstackingandtosuppresspossible
Mndiffusion,theantiferromagneticMn-Ircouldbeomitted.
ThefreeCo-Fecounterelectrodeissupposedtoshowadiffer-
entmagneticswitchingbehavior,whichisinfactrequiredto
achieveanantiparallelstateoftheelectrodesandtoobtaina
ratio.TMRIn-situannealing:Theintegrationofanin-situannealingstepfor
thecrystallizationoftheCo2MnSielectrode,priortothedepo-
sitionofMn-Ir,mightreduceMndiffusiontowardtheupper
barrierinterface,whichissupposedtolimittheTMR.

58

Loweringthecrystallizationtemperature:AreductionoftheCo2MnSi
crystallizationandorderingtemperaturemightleadtohigher
ratios.TMR

temperaturestablecounterelectrode
AsecondMgOlayerof1nmisdepositedontopoftheupperelec-
trodetoeliminatepotentialdiffusionofthefinalRuconductance
layer.DuetothedifferentcoercivefieldsofCo-FeandCo2MnSi,
anantiparallelstateoftheelectrodescanbeachieved.Amajor
loopofa370◦Cand400◦CannealedsampleisgiveninFigure36.
Theloopsareveryspikyandaclearantiparallelstatecouldnotbe
achieved.ComparedtotheCo2MnSijunctionsabove,nohigher
TMRratioscouldbefoundandadecayoftheTMRratiowasalso
presentwithincreasingannealingtemperature.
Tocreateanantiparalellstateofthetwoelectrodes,theCo-
Fecounterelectrodewaspinnedtoasecond6nmthickCo-Fe
layer.Thesewereseparatedbya0.92nmRuspacerlayerwhich
resultedinaantiferromagneticcouplingbetweenthetwoCo-Fe
layers.Themajorloopofa400◦Cannealedsampleisdepicted
inFigure37.Aswasthecasefortheaboveinvestigatedsamples,
noclearantiparallelstateoftheelectrodeswaspresentandalow
TMRratioofabout12%couldbereached.Thesupposeddiffusion
ofMnassourceforaloweredTMRcannotbeconcludedfromthis
experiment.

annealingin-situTheutilizedLeyboldsputteringsystemalsoprovidesaninsitu
heatingsystem.Here,aheatingwireismountedinasecond
processchamber(oxidationchamber).Annealingtemperatures
ofabout350◦Cto400◦Ccanbereachedwhicharesufficientfor
thecrystallizationandorderingofCo2MnSi.Tokeepthebarrier
interfacecleantheannealingstepwasaddedafterdepositingthe
Co-Fecounter(sampleA)andwithinhalfoftheCo-Feelectrode
(sampleB).Hereafter,Mn-IrandRuweredepositedforpinning
andconductance.Thefulljunctionswereexsituannealedfor
10minutesat275◦Candcooledinamagneticfieldtoactivate
theexchangebias.Thesamplesweresubsequentlyannealedat
differenttemperaturesforanidenticalsampletreatment.
Incontrasttothesamplesdiscussedabove,aclearantiparallel
statewaspresent,whichallowedadeterminationoftheTMRratio
(Figure38).ThevaluesareshowninFigure39asafunctionofex
59

withFigure36:unpinnedCo2MnSiCo-Fecounterjunction
electrodetorealizehard/soft
switching.

Figure37:Co2MnSijunction
Co-Fepinnedartificiallywithode.electrcounter

Figure38:Majorloopof
anin-situannealedCo2MnSi
junction.

Figure39:Comparisonof
annealedin-situotwthejunctions.MnSiCo2

96D.Ebkeetal.,JMagn
MagnMater322,996(2010)
97D.Ebkeetal.,ApplPhys
Lett89,162506(2006)
Figure40:X-raydiffrac-
tionpatternofmultilayered
Co2FeAl/Co2MnSihalfjunc-
tionsatdifferentannealing
es.temperatur

situannealingtemperature.WeobservedamaximumTMRof
39%forthesampleA(triangles).Furtherannealingledtolower
TMRratiosandtheeffectnearlyvanishedfor450◦Cannealing.
ThestrongdecayofTMRatabout375◦Cmaybeattributableto
Mndiffusiontowardstheupperbarrierinterface.
Incomparison,sampleBshowedareducedTMRvalueevenfor
thelowestannealingtemperature.Theinitialratioofabout20%
decreasedwithincreasingannealingtemperatureandthedecay
at375◦C,whichhintstoMndiffusion,couldalsobefound.

etemperaturcrystallizationtheloweringMultilayeredhalfjunctionswerepreparedtolowerthecrystal-
lizationtemperatureoftheCo2MnSi.Here,thelowerelectrode
consistedofCo2FeAl(5nm)/Co2MnSi(5nm)/Co2FeAl(5nm)/
Co2MnSi(5nm)wascappedbya1.8nmthickMgOlayer.The
ideawastoinducethecrystallizationofCo2MnSi,bycombination
withthelowtemperaturecrystallizingCo2FeAl.96Weperformed
asimilarexperimentfor(011)orientedmultilayeredCo2MnSiand
Co2FeSiHeuslercompounds.97

Figure40showstheobtainedXRDpatternofthepreparedmul-
tilayeredHeuslerhalfjunctions.Thebaselinesofthepatternsare
shiftedwithregardtotheappliedannealingtemperatures.A
cleardoublepeakstructurewaspresentforalltemperaturesat
theHeusler(004)and(002)peakpositions,whichindicatesthat

60

nointermixingofthetwocompoundstookplace.Thepeaksat
31.5degreesand65.6degreescanbeattributedtotheCo2FeAl
(002)and(004)peak,respectively.Thesevaluesareactuallycom-
parabletothepreviouslydiscussedvaluesofthesingleCo2FeAl
thinfilm.Incontrast,thecorrespondingCo2MnSi(002)and(004)
peaksat32.3and66.7degreesshiftedtolargeranglesof2θ.This
canbeexplainedbythecorrespondinglatticeconstants.

Figure41showstheaveragedlatticeconstantsthatwerecalcu-
latedfromthe(004)and(002)Heuslerpeaksfordifferentanneal-
ingtemperatures.Aspreviouslydiscussed,onlytheoutofplane
componentofthelatticeconstantisabletobedeterminedbythe
performedθ/2θscans.Asexpectedfromthepeakpositions,the
latticeconstantsofthesingleandmultilayeredCo2FeAlwereal-
mostidentical.Bycontrast,thelatticeconstantofthemultilayered
Co2MnSiwasgreatlyreducedwhencomparedtotheoutofplane
latticeconstantofthesingleCo2MnSilayer.Assumingaconstant
latticevolumeofVCMS=aC3MS(SL),(aCMS(SL)istheoutofplane
componentofthesingleCo2MnSilayer)theinplanecomponent
aipcanbeestimatedby:

3aip=aCMS(SL)(12)
aCMS(ML)
Asexpected,theinplanelatticeconstantofCo2MnSiwasclose
totheCo2FeAllatticeconstant,i.e.,theCo2MnSilatticewasdis-
tortedtofittheCo2FeAlbuffer.Theeffecttothespinpolarization
wasestimatedbySPR-KKRDOScalculations,foracubicanddis-
tortedCo2MnSilatticeandtheexperimentallatticeconstants.As
61

Figure41:Outofplane
sin-theofconstantslatticegleCo2FeAlandCo2MnSi
junctionscomparison(dashedwiththelines)outinof
planelatticeofmultilayered
Co2FeAlandCo2MnSilattice
inestimatedTheline).(solidplaneCo2MnSilatticeofthe
multilayerisgiveninred.

Figure42:Polarizationof
s-electronsforacubicand
distortedCo2MnSilatticeob-
calcu-SPR-KKRomfrtainedlations.

Figure43:Roomtempera-
tureTMRratiosoffullMTJs
containingCo2MnSi,Co2FeAl
andCo2FeSiwiththecor-
respondingmultilayered
Co2FeAl/Co2MnSiand
Co2FeSi/Co2MnSisystems
an-ex-situoffunctionasThees.temperaturnealingdashedlinerepresents1.8nm
solidtheandbarrierMgO2.1nm,respectively.

forshownmationinandFigurethe42,spinadditionalpolarizationstatesofappearthedues-electrtoonstheislatticeslightlyde-
rserveduced.edattheNevFermiertheless,levelaandhighhighpolaTMRrizationratiosofcanaboutbe95%expected.iscon-

ToverifytheinuenceoftheloweredCo2MnSicrystallization
temperature,correspondingfulltunneljunctionswereprepared.
Inaddition,asecond,multilayeredsystemcontainingaCo2FeSi
(5nm)/Co2MnSi(5nm)/Co2FeSi(5nm)/Co2MnSi(5nm)bottom
electrodewaspreparedforcomparison.TheresultingTMRratios
aregiveninFigure43incomparisonwiththesinglelayeredjunc-
tionsofCo2FeSi,Co2FeAlandCo2MnSi.AnMgObarrierthick-
nessof1.8nmwasusedinallcases.TheTMRofthemultilayered
junctionswasincreasedwithregardto◦theplainCo2MnSilayer.
Forannealingtemperaturesbelow350C,theratioswerefound
tobecomparablewithCo2FeAl.Fortemperaturesabove400◦C,
theTMRratiosreachedtheCo2FeSivalues.Amaximumvalueof
about82%and77%wasachievedat425◦CfortheCo2FeAland
Co2FeSibasedmultilayers.

Insummary,differentapproachesweretestedtoincreasethe
wloerwerTMReachedratiobyofrtheeducingCo2theMnSiCoMnSijunctions.crTheystallizationhighestvtempera-alues
2ture.ThiswasaccomplishedbyCo2FeAlandCo2FeSibuffered
multilayerjunctions.RoomtemperatureTMRratiosofabout80%
eached.reerw62

temperaturedependenceoftheTMRratio
forGiantMTJslowcontainingtemperaturaeCoTMRMnSiratiosHeuslerofoverelectr750%ode.wButerertheeportedcorre-
298prspondingoximityrofoomtheFermitemperaturleveletovthealueswereconductanceonlyoraboutvalance220%.bandTheis
99thethoughtorigintoofbertheesponsibletemperaturforethedependencetemperaturisestilldependence.underdiscus-But
sion.InsteadoftheplainCo2MnSijunctions,themultilayered
Co2FeAl/Co2MnSijunctionswereinvestigatedbecauseofthe
previouslydiscussedhigherTMRvalues.Insteadofsubsequently
annealing,identicalsampleswereannealedattheoptimaltemper-
aturagaineanddeposited.patternedAsbshoywne-beaminFigurelithography33,the.Auhighestcontactroompadstemperwer-e
aturTheesameTMRratiosjunctionsofshoaboutwed150%aboutwere260%foundTMRforatCo213FeAlK.Thejunctions.corre-
electrspondingodeallomajorwsaloopswellcanbedefinedseenindeterFigurminatione44.ofThethepinnedTMRupperratio
foralltemperatures.Themajorloopsoftheotherinvestigated
Heuslerfulljunctionsarealsogiven.Thehighestratioat13Kof
about330%canbeachievedforCoMnSi.Thisisincontrastto
theobservedroomtemperatureTMR2valuesbutreproducedthe
reportedcompound.str100ongtemperaturedependenceoftheTMRratioforthis
Allratios(left)dependencesandthearecorrgiveninespondingFigurenor45malizedfortheTMRmeasur(right).edTMRWe
(RT)defineTMRthevratioalueasofΓlo.wTheachietemperaturvedeTMR(13Kv)toaluesroomarealsotemperatursumma-e
rizedTheinTabletemperatur6.edependenceoftheFecontainingcompounds
Co2FeAlandCo2FeSi(ΓCFA=ΓCFS=1.7)wasasweakasre-
63

Figure44:Lowtemperature
(13K)androomtemperature
majorloopsoftheinvesti-
junctions.fullHeuslergated98S.Tsunegietal.,J.Phys.D:
Phys.Appl.)2009(99K.Inomataetal.,Phys.Rev.
)2008(B

100PhysS.TLettsunegi93,et112506al.,(2008Appl)

Figure45:Temperaturede-
forratiosTMRofpendencedifferentHeuslercompounds
ing(left)norandmalizedthecorrTMRespond-ratio
(right).

101LettW.95,Wang182502et(al.,2009Appl)Phys
102PhysN.TLettezuka94,et162504al.,(2009)Appl

Table6:Summaryoffound
roomandlowtemper-
atureTMRvaluesand
theratioespondingcorrΓ=TMR(LT)/TMR(RT)
com-estigatedinvtheforpounds.

103S.Tsunegietal.,J.Phys.D:
2009(Phys.Appl.)104PhysS.TLettsunegi93,et112506al.,(2008)Appl

portedbyWang(ΓCFA=2.1)101andTezuka(ΓCFAS=2.1)102for
B2structuredCo2FeAlandidealL21structuredCo2FeAl0.5Si0.5.
ThereportedhighroomtemperatureTMRratiosofthelattercom-
poundarethoughttobecausedbyashiftoftheFermilevelinto
themiddleofthebandgap.

compoundTMR(13K)TMR(RT)Γ
CoCo22FeAlFeSi134%261%153%78%1.71.7
Co2MnAl157%63%2.5
CoCo22MnMnSi0.5Fe0.5(ML)Si330%144%107%68%2.13.1
TheachievedtemperaturedependencesofourMncontainingjunc-
tionswerehigher.ForCo2MnAljunctionsitwasdeterminedtobe
ΓCMA=2.5andforCo2MnSitobeΓCMS=3.1.Aweakdepen-
denceisexpectedforthequaternarycompoundCo2Mn0.5Fe0.5Si.
ThepredictedshiftoftheFermilevelintothemiddleoftheband
gapshouldhaveledtoalowerΓ,asfoundforCo2MnSiand
Co2FeSi.OurratioofΓCMFS=2.1wasobviouslylowerthan
ΓCMSbuthigherthanΓCFS.Incontrasttothelinearprogression
oftheweaklytemperaturedependentslopes,aconvexTMR(T)
curvewaspresentforthestrongerdependenciesofCo2MnAland
Co2MnSi.Asimilarconvexbehaviorwasreportedforepitaxial
(001)texturedCo2MnSitunneljunctionsinL21structure.103Here,
thereportedtemperaturedependencieswereconsiderablyhigher.
ForasputteredMgObarrierΓspitwasdeterminedtobe4.1and
forane-beamevaporatedMgObarrierΓebitwas3.5.104Thedif-
ferenceismostlikelyattributabletobarrier(interface)properties.
Probably,disorderislessimportantforthetemperaturedepen-
dence.64

OnecanassumethatacertainpositionoftheFermilevelcan
notbethe(only)reasonfortemperatur105edependence.Thiscanbe
calculations.SPR-KKRybillustratedTheparticularDOSwereobtainedbyassumingL21structure
andtheexperimentallydeterminedlatticeconstants.TheseDOS
areincontrasttothecitedtheoreticalpublications,106whichare
basedonalatticeconstantdeterminedbyenergyminimization
calculations.Figure46depictsthespinpolarizationofs-electrons
oftheinvestigatedHeuslercompounds.Theseelectronsdominate
107ocess.prtunnelingtheThedashedlinesat±140meV(=±5.4kBT108withregardto
theFermilevel)representtheestimatedeffectofbroadeningdue
tothermalsmearing.Accordingtothisassumption,neitherthe
valencebandnortheconductancebandiswithintherangeof
thermalsmearing.Thestrongesteffectwouldbeexpectedfor
Co2Mn0.5Fe0.5SifollowedbyCo2FeSiandCo2FeAlbecauseofthe
proximityoftheFermileveltotheconductanceband.Butthis
sequenceisincontrasttothedependencieswefoundintheex-
periments.Therefore,ithastobereconsiderediftheoriginofthere-
portedweaktemperaturedependenceisreallyrelatedtoashift
oftheFermilevelintothegapaswaspreviouslyreportedby
Tezuka109andBalke110,orifthetemperaturedependenceismore
likelygeneratedbythebarrier(interface)properties.Veryrecently
Sakuraba111reportedthatthelargetemperaturedependenceof
Co2MnAlxSi1−xjunctionsisnotconnectedtothepositionofEF,
butismostlikelyconnectedtointerfacestates.

65

electrFigureons46:foridealPolarizationL21ofstruc-s-
compoundsHeusleredturCoand2CoFeAl,2MnAl.Co2FeSi,TheCo2dashedMnSi
brlinesroadeningepresentduethetotherassumedmal
smearing.

105http://olymp.cup.uni-muenchen.de/ak/ebert/SPRKKR/

106I.Galanakis,J.Phys.:Con-
dens.Matter16,3089(2004);
S.Picozzietal.,Phys.Rev.B
69,1(2004);andH.C.Kand-
paletal.,Phys.Rev.B73,1
)2006(107J.Hertzetal.,Phys.Rev.B
8,3252(1973)
108J.Kleinetal.,Phys.Rev.B
7,2336(1973)

109N.Tezukaetal.,Appl
PhysLett94,162504(2009)
110B.Balkeetal.,Phys.Rev.B
74,1(2006)
111Y.Sakurabaetal.,Phys.
Rev.B81,144422(2010)

112J.D.Teresaetal.,Phys.
Rev.Lett.82,4288(1999)
113A.Bratkovsky,Phys.Rev.
B56,2344(1997)

114RevJ..SB75,1chmalhorst(2007)etal.,Phys.
115A.Thomasetal.,Appl
PhysLett89,012502(2006)
116Fthesis,.KeseberBielefeldg,UnivMasterersitys
)2007(

Table7:Summaryoffound
biasvoltageswheretheTMR
ratiosdropstothehalfofthe
foundvalueatzerobias.The
wereachievedat13K.

biasvoltagedependenceoftheTMRratio

Differenteffectsarereported,whichareinuencingthebiasvolt-
agedependenceofmagnetictunneljunctions.Ononehand,the
insulatingbarriercanstronglyinuencethespinpolarizationof
tunnelingelectrons.112Ontheotherhand,impurity-assistedtun-
nelingandspin-ipscatteringfromdefectstateswilldecreasethe
TMRratiowithincreasingbias.113Therefore,thebiasvoltagede-
pendenceoriginatesfromthesuperpositionofthetransportprop-
ertiesinthecomplexsystemofelectrode/barrier/electrode,
andtheelectrodesbelongingDOS.However,thebiasvoltagede-
pendencefordifferentHeuslerjunctionsalsoreectsDOSrelated
keyfeatures.Forexample,anachievedkinkintheTMR(V)curve
ofCo2MnSiatabout300mVisreportedtobeastrongchangeof
theeffectivespinpolarizationofs-electrons.114Atevenhigher
biasvoltagesofmorethan1000mV,theTMRratiocanevenbe
inverted,asreportedforAlOXbased(011)texturedCo2MnSi115
and(001)texturedCo2FeSi116junctions.AninverseTMRratiois
notpresentin(001)texturedMgObasedHeuslerjunctionsandis
assumedtobesuppressedincoherenttunnelingprocesses.
However,thebiasvoltagedependencemighthelptoestimate
thepositionoftheFermilevelwithregardtothecalculatedDOS.
Weusedthelowtemperaturebiasvoltagedependenciestoreduce
theeffectofthermalsmearing.Thesedependenciesarecompared
tothecalculatedDOS.Thedependenciesofthe(001)textured
HeuslerjunctionsareshowninFigure47.Thecurvesarenor-
malizedtothehighestTMRvalues.Theelectronsaretunneling
intotheHeuslerlayeratpositivebiasvoltagesandintotheCo-Fe
counterelectrodeatnegative.Therefore,featuresintheDOSof
unoccupiedHeuslers-electronstatescanbedeterminedatposi-
tivevoltages.Asexpectedforacoherenttunnelingprocess,aclear
positiveTMRispresentforthemeasurablerangeofbiasvoltage.
Anasymmetrictunnelingcharacteristicwasfoundduetothevar-
iouselectrodes.TheparticularbiasvoltagesV±1/2,wherethe
TMRratiodroppedto50%oftheTMRratiosfoundatzerobias,
arelistedinTable7.

66

V−1/2V+1/2
Co2FeAl−151mV234mV
CoCo22MnAlFeSi−−556355mVmV>700285mVmV
Co2MnSi(ML)−44mV48mV
Co2Mn0.5Fe0.5Si−175mV315mV

DuetothepreviouslydiscussedlowTMRvaluesoftheplain
Co2MnSijunctions,themultilayeredCo2FeAl/Co2MnSijunctions
wereinvestigatedinstead.Here,thestrongestdependenceof
allcompoundswaspresent.Theabovementionedkinkinthe
TMR(V)ofCo2MnSijunctionsatabout+300mVwerealsofound.
Inaddition,asecondshoulderwaspresentfornegativebiasat
biasaboutv−oltages600mVof.lessThethanTMR50ratiomV.Aalrsimileadyardrstroppedongbiastovtheoltagehalfforde-
pendenceofCo2MnSiwaspreviouslyfoundbyDaibouandis
reportedtobeanarrowhalf-metalgapneartheFermilevel.117
Asimilarbehavior,butwithaloweredandlessspikyvolt-
agedependence,wasfoundforCo2FeAl.Twoshoulderswere
−pr150esentmVatandaboutV++1/2250atmVaboutand+−230300mVmV..WThisefoundbehaVvior−1/2isatinaboutcon-
trasttothereportedTMR(V)curvebyWangforsimilarjunctions
whichindicatesaslightly118differentDOSduetothereported,dif-
ferentfilmstoichiometry.
TheobtainedTMR(V)curveofthequaternaryHeuslercom-
poundCo2Mn0.5Fe0.5SiisalmostcoincidenttotheCo2FeAlcurve
fornegativebiasvoltages.Thedependenceforpositivebiasvolt-
ageswasagainlowered.Aslightshoulderwasfoundatabout
.mV500+Bycontrast,theTMR(V)ofCo2FeSishowedanalmosttriangu-
larbehaviori.e.,theTMRratiodroppedlinearlywithincreasing
biasvoltages.Wefoundnofeaturesinthedependenceofthein-
vhigherestigatedvoltagesbiaswhenrange.Thecomparedjunctionstoallwerotherelessrjunctionsesistantandagainstactu-
allybrokeatabout±600mV.Thismightindicatearoughbarrier
67

Figure47:Lowtempera-
ture(13K)biasvoltagede-
tioforpendencedifferofentthefullTMRHeuslerra-
junctions.Thevaluesarenor-
malizedtotheachievedmax-
alue.vTMRimum

117MagnT.DaibouMateret310,al.,1926J(2007Magn)

118LettW.95,Wang182502et(al.,2009)ApplPhys

Figure48:Biasvoltagede-
pendenceat13Kandroom
temperature,respectively,for
differentHeuslerjunctions
po-thecomparison,In(top).onss-electrtheoflarizationthatwerecarriedoutbyDOS
calculationsforoptimalL21
(solidline)andB2structure
line).(dashed

interfaceorevenbarrierdefectssuchaspinholes.
ThebiasvoltagedependenceofCo2MnAlwasdifferent.The
normalizedratioswerehigherincomparisontoallothercom-
pounds.V−1/2wasreachedatabout−550mVandV+1/2could
notbereachedintherangeof+700mV.Furthermore,wefound
asecondmaximumatabout+500mV.Wealsoverifiedtheval-
uesbymajorloopsathigherbiasvoltages.Bycontrast,thenor-
malizedratiosdroppednearlylinearlyfornegativebiasvoltages
higherthan−100mV.

Toillustratetheconnectionofbandstructureandvoltagedepen-
dence,wehavedepictedtheTMR(V)curvesat13Kandroom
temperatureincomparisonwiththecalculatedpolarizationofthe
s-electrons(Figure48).TheabovediscussedDOSoftheopti-
mumcompositionX2YZinL21structurearerepresentedbythe
solidline.Assumingtheexperimentalfilmstoichiometriesand
B2structure,leadstothepolarizationwhichisrepresentedbythe
dashedline.Thecalculationsarebasedonthepreviouslydeter-
constants.latticeminedThespikybiasvoltagedependenceoftheCo2MnSijunctionsat
13Kisnotconservedatroomtemperature,probablyduetother-

68

malsmearing.Asexpectedfromthetemperaturedependence,
considerablyloweredTMRratioswerefoundforlowbiasvolt-
ages.Higherbiasresultedinsimilarvaluesindependentoftem-
perature.Theobservedspikybehaviorat13Kmightbeexplained
bythecorrespondingDOScalculationsandexperimentalvalues
(lattice,composition).Incontrasttoabroadgapintheminor-
itybandfortheoptimalL21structuredcompound,theDOSof
theB2structured119Co2MnSishownonvanishingstatesatthe
Fermilevel,representedbyacleardipinthepolarizationofthe
s-electronsinFigure48.Anassumed,slightlyshiftedposition
ofEF120toahalf-metallicpositionmightexplaintheinvestigated
strongbiasvoltagedependence.
ThebiasvoltagedependenceofCo2MnAlisconsiderablydif-
ferentfromtheabovediscusseddependenceofCo2MnSi.The
TMRratioat13Kwashigherwhencomparedtotheobtained
valuesatroomtemperature,independentofbiasvoltage.The
secondmaximum,atabout+500mV,isalsoconservedatroom
temperature.Thecorrespondingpolarizationofs-electronsshows
analmosthalf-metallicbehaviorfortheL21structuredcompound,
whereasaconsiderablyloweredpolarizationofaboutPB2=75%
canbefoundforB2typestructureandtheexperimentallydeter-
minedfilmstoichiometry(Co2Mn0.957Al0.8).Thepresenceofthe
secondmaximumintheTMR(V)curvescanbeexplainedbythe
highpolarizationofabout75%ofthes-electronsatabout0.5eV
to0.7eVwhichmightleadagaintoariseofTMRratioforhigher
biasvoltages.However,thefeaturecannotbeexplainedbythe
performedcalculationsoftheL21structuredsystem.
ThevoltagedependenciesoftheCo2FeAljunctionsshowvery
similarbehaviorfor13Kandroomtemperature.Thelowtem-
peratureTMRratioswereincreasedonlyinasmallrangefrom
−130mVto+130mV.Thepresentshoulderdeterminedinthe
TMR(V)curvesoftheCo2FeAljunctionscanbeexplainedaswas
explainedpreviouslyfortheCo2MnSisystem.Astrongdropof
thecalculatedspinpolarizationforthedisorderedB2structured
systemcanbeestimatedatthesameenergies.
ThealmosttriangularbehavioroftheCo2FeSiTMR(V)curves
at13Kwasalsoconservedatroomtemperature.Strikingfeatures
werenotpresent.Inparticular,noshoulderscouldbefound,
asonewouldexpectfromthestrongdecayofspinpolarization
atabout+300mVofthecalculatedDOSforidealL21structure.
Thecorrespondingpolarizationofs-electronsandB2structurein
X2YZcomposition121isconsiderablydifferent.Areducedgap

69

119structurAlthougheinweourvCoerifyMnSiL21
2latedjunctions,thewDOSehaforveB2,alsotoshocalcu-w
dis-(partly)ofinuencethe.deror120ThepositionoftheFermi
toleveltheisachiehighlyveddifdisputed,ferentpo-due
sitionsofEFfordifferent
ingappliedtheDOS,modelse.g.forSPR-KKR,calculat-
LDA+U.,FLAPW

121theofassumptionThemineddeterexperimentallyfilmstoichiometryleadstoa
nonconvergentsolutionfor
DOS.calculatedthe

12254,M.225(Jullier1975)e,;andPhys.J.S.Lett.Mood-A
200era,et248al.,(J1999Magn)MagnMater
Re123v.W.BH.63,1(Butler2001)etal.,Phys.

widthledtoalowpolarizationofabout25%attheFermilevel.
ForthequaternarycompoundCo2Mn0.5Fe0.5Si,theTMRvalue
at13Kincreasedwhencomparedtotheroomtemperatureval-
uesintheinvestigatedrangeofbiasvoltage.Wefoundashoul-
deratabout+500mV.Astrongdropofspinpolarizationcanbe
foundatabouttheseenergiesforthecalculatedDOSinL21and
B2typestructureinX2YZstoichiometrie.Thedifferencesbetween
theseDOSvanishedandwefoundahighspinpolarizationofs-
97%.aboutofonselectr

Inconclusion,wefoundfeaturesinthebiasvoltagedepen-
denceoftheHeuslerjunctions,whichcanbeexplainedbythe
calculatedspinpolarizationofs-electrons.Forthecompounds
Co2FeAlandCo2Mn0.5Fe0.5Sithefeaturescanbeexplainedwith
theDOSofaL21oraB2structuredHeusler.Here,thecalculated
DOSareonlyslightlyinuencedbydisorder.Thisisincontrastto
thedependencesofCo2MnSiandCo2MnAl.Thecorresponding
DOSofthedisorderedcompounds,showareducedspinpolar-
izationattheFermilevelwhencomparedtotheDOSofanideal
L21structuredcompound.TheobservedfeaturesintheTMR(V)
curvesaremostlikelyrelatedtothepresenceofB2structure.

polarizationspinThemeasuredTMRvalueisasuperpositionofthetransportprop-
ertiesinthecomplexsystemelectrode/barrier/electrode.For
anamorphoustunnelbarrierlikeAlOX,thespinpolarizationof
theinvolvedelectrodescanbeestimatedbytheJullieremodel.122
ForacrystallineMgObarrier,thetunnelingprocessiscoherent.
Here,thetunnelingcurrentdependsonthesymmetryofBloch
statesintheelectrodesandtheevanescentstatesofthetunneling
barrier.AdifferentdecaylengthfordifferentsymmetriesofBloch
statescangreatlyenhancetheresultingTMRratio.123Therefore,
theJullieremodelcannotbeappliedtoaMgObasedtunneljunc-
tioninordertodeterminethespinpolarizationofthecontaining
electrodes.ToestimatetheHeuslerqualityinsuchasystemwithregard
toahighpolarization,itisrequiredtodirectlymeasurethespin
polarization.Thisisverychallengingandtwomethodswillbe
discussedinthefollowingsection.Thesemethodswereapplied
toCo2FeAljunctionsbecauseofthehighroomtemperatureTMR
ratiowhichwasachievedforthiscompound.

70

Spinpolarizedtunnelingintosuperconductor(SPT):Thecurrentsof
aspin-transportdeviceweredeterminedbytunnelingintoasu-
perconductor.124Wereplacedtheupperelectrodeofourstack
layoutwithathin(4nmto7nm)superconductinglayerofAl-
Si.Thesamplesweresputteredthroughshadowmaskswith
crossstripgeometry.Wecarriedouttheconductancevs.volt-
agecurveswithfourprobemeasurementsina3Hecryostatat
0.49Kandaninplanemagneticfieldofupto2.9T
DuetotheZeemansplitting,anasymmetrictunnelingcon-
ductanceispresentandthespinpolarizationPcanbecalcu-
y:blatedP=(σ4−σ2)−(σ1−σ3)(13)
(σ4−σ2)+(σ1−σ3)
Here,theσirepresentfeaturesintheconductanceofthenor-
malizeddI/dVcurve,whichisshowninFigure49forour
FeAl.CocompoundHeusler2

FortheinvestigatedCo2FeAl,aspinpolarizationof60±2%can
beestimatedfromthisequation.However,theeffectsofspin-
orbitscatteringandorbitaldepairingwereneglected.TheMaki
theory125canleadtoamoreprecisevaluebyanalyzingthe
dI/dVcurves.FromthistheorywecanconcludeacorrectedP
valueof55%,whichisingoodagreementtothereportedexper-
imentalvalueofP=56.2%byInomata126andthetheoretical
predictedvalueforB2structuredCo2FeAlofP=60.7%.127
Moredetailscanbefoundinourrelatedpublication,reported
128chebaum.Syb71

124RepR.238,Meser173ve(y1994et)al.,Phys

dI/FigurdVeof49:ourNorHeuslermalized
Co2ductancesFeAlwithσtomarkedcalculatedcon-
iwtheastakenpolarization.fromourTherFigurelatede
ybeportedrpublication,S09C717chebaum)(JAP107(2010),

125P.Fuldeetal.,PhysicalRe-
)1966(wvie126determinedbypoint
contactAndreevreection
(PCAR)127K.Inomataetal.,Sci.Tech-
nol.Adv.Mater.9,014101
)2008(128O.Schebaumetal.,Direct
measurementofthespinpo-
larizationofco2fealincombi-
nationwithmgotunnelbar-
2010riers,

129G.M.Muelleretal.,Nat
Mater8,56(2009)
130E.Beaurepaireetal.,Phys.
Rev.Lett.(1996)

131GerUnivmanyersityofGöttingen,
132Uni-GöttingenMann,A.versity,unpublishedresults

133Fortheusedlaserwave-
lengthofabout800nm,a
probedepthofabout15nm
assumed.is

Magnetizationdynamics:Recently,Müller129establishedamethod
toestimatethespinpolarizationPfromameasureddemag-
netizationtimeτM.130Thecalculationsarebasedonathree-
temperaturemodelwheretheelectrons,spinsandlatticeare
assumedtodifferent,independenttemperatures.Theintercon-
nectionisgivenbytheparticularrelaxationratesbetweenspins
andelectrons,electronsandlatticeandlatticeandspins.With-
outgoingintodetail,thedemagnetizationismeasuredbya
pumpprobeexperiment.Inaferromagneticmaterial,thelaser
pulseexcitationleadstoanincreaseofthetemperatureinthe
electronsystemandalmostinstantaneouslyalsoleadstoan
increaseoftemperatureinthespinsystem.Bycontrast,thedi-
rectchannelisblockedinahalf-metallicsystemsuchasthatof
Heuslercompounds.Theenergyhastobetransferredthrough
latticeexcitationswhichisrelativelyslow.Thespinpolarization
canthenbeassumedfromthedeterminationofthedemagne-
time.tizationTheexperimentwasperformedbythegroupofMünzen-
berg131onourpreviouslydiscussedHeuslerhalfjunctions.
Inthisexperiment,thespinpolarizationofCo2FeAlwases-
timatedtoPCFA=77±3%132whichishigherwhencompared
tothevaluesdeterminedforsimilarjunctionsabove.

Thedifferentobtainedspinpolarizationsfromthetwoexperi-
mentsfortheHeuslercompoundCo2FeAlcanbeexplainedbythe
investigateddifferenceofprobedepth.Incaseofthepumpprobe
experiment,thespinpolarizationofsomenmcanbeassumed.133
Therefore,themeasurementsrepresentmorelikelythespinpolar-
izationofthebulkcompound.Bycontrast,thedescribedmethod
ofMeserveyandTedrowevaluatesthespinpolarizedtunneling
current.Thisalsoincludesthebarrierproperties.Inparticular,
theinvestigatedtunnelingelectronsoriginatefromtheverylast
layeroftheelectrodematerial.Here,thespinpolarizationmight
bestronglyreducedduetointerfacedefectstates,oxidesordisor-
.der

compositionoff-stoichiometricalTheinuenceofcompoundstoichiometryhasalreadybeendis-
cussedaboveforthehalfjunctions.Wepreparedcorresponding
fulljunctions,containingthreedifferentcompositionsofCo-Fe-Si,
toinvestigatethetransportproperties.

72

Figure50depictstheachievedroomtemperatureTMRratiosof
theoff-stoichiometricallyCo-Fe-Situnneljunctionsincompari-
sonwiththebalancedcompoundfordifferentannealingtemper-
atures.Allsamplesweresubsequentlyvacuumannealed.The
MgObarrierthicknesswassetto1.8nm.
TheTMRratiosoftheFeenrichedHeuslerjunctionswerecom-
parabletothebalancedHeuslerjunctionsforlowannealingtem-
peraturesupto350◦C.Higherannealingtemperaturesreduced
theTMRandtheobtainedvalueswereconsiderablyloweredin
comparisontothebalancedCo-Fe-Si.ThehighestTMRvalueof
about75%wasachievedfor325◦Cannealing.
BycontrasttheSienrichedHeuslerjunctionshowedaconsider-
ablylowerTMRratio,evenforthelowestannealingtemperature
of275◦C.134Here,amaximumvalueofabout50%wasachieved.
Higherannealingledtoastrongdecrease.Thiscanbeexplained
bythepreviouslydiscussedcorrespondinghalfjunctions,which
showaclearcrystallographicchangewithinthesetemperatures.
Thischangemightleadtostressatthebarrierinterface.Ade-
creaseoftheinterfacialmagneticmomentwasfoundfromXMCD
forannealingtemperaturesabove300◦C,whichmightbeattributed
toadisorderedHeuslercompound.Furthermore,lowcrystalliza-
tionwasfoundinXRDscans.AsdiscussedforCo2FeAl,diffusion
tookplacewithinthegrainboundaries.Thereducedcrystalline
growthoftheSienrichedHeuslerjunctionmightallowdiffusion
atlowerannealingtemperatures.

Thesamejunctionsshowabout116%(Feenriched)and91%(Si
enriched)TMRratioat13K.Thetemperaturedependenceofthe
obtainedTMRvaluesisgiveninFigure51(left)and(right)for

73

difFigurfereent50:annealingTMRratiotemperfor-
aturCo-Fe-Si.esandcompositionsof

134Barriercanimperfectionsbeassumedforlowanneal-
ingtemperatures.Therefore,
wesettheinitial◦annealing
C.275toetemperatur

Figure51:Temperaturede-
forratiosTMRofpendencecomposi-Co-Fe-Sientferdiftions(left)andthecorre-
TMRmalizednorsponding(right).

Table8:Summaryoffound
roomandlowtemper-
atureTMRvaluesand
ratioespondingcorrtheΓ=TMR(LT)/TMR(RT)
fortheinvestigatedCo-Fe-Si
compounds.

thenormalizedratios.Incontrasttotheabovediscussedtemper-
aturedependences,wefoundonlysmalldeviationsforthedif-
ferentCo-Fe-Sicompositions.TheratioΓ=TMR(LT)/TMR(RT)
wasslightlyweakerfortheFeenrichedjunctions(ΓFe=1.6)when
comparedtothebalanced(ΓCFS=1.7),andslightlystrongerfor
theSienrichedjunctions(ΓSi=1.8).Theachievedvaluesatroom
temperatureand13KaresummarizedinTable8.

compoundTMR(13K)TMR(RT)Γ
1.778%134%balancedSiFeenrichedenriched116%91%49%74%1.81.6

Theseresultsleadsagaintotheassumptionthatthetemperature
propertiesdependenceandofnotTMRbyratiotheriseportedmoreshiftlikelyofdetertheFerminedmilebyvel,interfacewhich
canbeprecipitatedbychangingthecomposition.

Incontrasttothepreviouslydiscussed,considerablydifferentbias
voltagedependencesforthedifferentHeuslercompounds,the
founddependenciesoftheCo-Fe-Sijunctionsarequitesimilar.
TheachievedTMR(V)curvesat13KareillustratedinFigure52.
ThebestagreementcanbefoundforthebalancedandtheFeen-
richedfilms.Incontrasttothemorelikelytriangularslopeof
thebalancedcomposition,theTMR(V)curvesoftheFeenriched
junctionsshowsimilarfeaturesatabout−400mVand+500mV,
aspreviouslydiscussedfortheothercompounds.Thebiasvolt-
agedependenceoftheSienrichedjunctionsvariesonlysparsely
andisslightlystronger,i.e.,thenormalizedTMRislowerfor

74

thesamebias,exceptforvoltagesbelow−600mV.Remarkably,
theoff-stoichiometricallycompoundjunctionsaremoreresistant
againsthighvoltagesthanthejunctionsofthebalancedfilms.

junctionsHeuslerdoubleWereplacedtheCo-FecounterelectrodewithasecondHeusler
electrodetogetcoherentinterfacesandhigherTMRratios.The
growthconditionsofthesecondelectrodewereinvestigatedby
XRD,aspreviouslydescribed.Thestudiedlayerstackingcon-
tainstheoptimizedbufferandHeuslerlayerthatwefoundbe-
fore.TheinvestigatedlayerstructureisgiveninFigure53.We
usedCo2MnSiagainasarepresentativeHeuslercompoundtoop-
timizethesecondHeuslerelectrodewithregardtocrystalgrowth
properties.XRDmeasurementswereperformedforsamplescon-
tainingatopHeuslerlayerthicknessof5nm,10nm,20nmand
30nm.Alllayerswereexsitusubsequentlyvacuumannealedat
400◦C,450◦Cand500◦C.ToestimatethequalityoftheHeusler
topelectrodefordifferentlayerthicknesses,thetexturedfraction
wasdefinedasfollowed:
peaknetareatop
texturedfractiontop=layerthicknesstop
=peaknetareatotal−peaknetareabottom
layerthicknesstop
Weassumedourpreviouslyfoundpeaknetareabottomvalues.
TheresultsareshowninFigure54fordifferentannealingtemper-
atures.Wefoundthehighesttexturedfractionforthe5nmthick
topelectrodeandallinvestigatedtemperatures.Thickerlayers
showedaloweredtexturedfraction.
75

biasFigurev52oltage:Lowdependencetemperaturofe
diftions.ferentCo-Fe-Sicomposi-

Figure53:Layerstacking
forXRDinvestigationsofthe
doublejunction.Heusler

Figure54:Calculatedtex-
turedfractionfordifferent
Co2MnSitopelectrodesat
tempera-annealingyingarves.tur

Figure55:TMRratioofa
dou-annealedsubsequentlycontain-junctionHeuslerbleingCo2FeAlelectrodesanda
1.8nmthickMgOtunnelbar-
.rier

135MagnW.C.44,Lim2595et(2008al.,)IeeeT

Figure56:TMRratioof
aCo2FeAldoubleHeusler
junctionwitha2.1nmthick
MgO◦barrier,annealedfor1h
C.410at

FullmagnetictunneljunctionscontainingtwoHeuslercom-
poundelectrodeswereprepared.WeusedtheHeuslercompound
Co2FeAlbecauseitresultedinthehighestroomtemperatureTMR
ratiosofourinvestigatedcompounds.Thelayerstackingdepicted
inFigure53wascompletedwithMn-Ir(10nm),Ru(40nm)and
nm).(20uAFigure55depictstheobtainedTMRratiosforasubsequently
annealedjunction,containinga5nmthickHeuslertopelectrode.
TheMgObarrierthicknesswassetto1.8nmandthetopelectrode
waspinnedtoMn-Ir.AroughlyconstantTMRratioofabout
80%wasachievedforthewholeinvestigatedrangeofannealing
e.temperaturIncontrasttotheexpectedimprovementofTMRratiodueto
thecoherentinterfacesandthehigherspinpolarizedsecondelec-
trode,theTMRwasreducedtothehalfwhencomparedtothesin-
gleHeuslerjunction.Thismightbeattributedtodegradedcrys-
talgrowthofthetopelectrodewhichaffectstheTMRratiomore
stronglyforaHeuslercompoundthanforCo-Fe.Furthermore,
barrierinterfaceimperfections,duetoadifferentmicrostructure,
arepossible.ThisimperfectionswouldalsoreducetheTMRratio,
becauseofeliminatedcoherenttunneling.Thisisalsoreported
byLimfor(double)HeuslerjunctionscontainingCo2FeSielec-
135ode(s).trTheconstantlevelofTMRratioisincontrasttothepreviously
founddecayatannealingtemperaturesofabout450◦C.Probably,
thetopHeusleractsasadiffusionbarrierforthesupposedMn.

AjunctionsgentlevalsorariationesultedofintheacountermaximumelectrTMRoderatioforthicknessthe5innmthethickfull
secondHeuslerelectrode(Figure56).Thisisingoodagreement
withourXRDinvestigations.Duetotheresultsfoundforthesin-
gleCo2FeAlelectrode,hereaMgO◦barrierthicknessof2.1nmand
anannealingtemperatureof410Cwasused.Theoptimizedbar-
rierthicknessmightexplaintheenhancedTMRratiowhencom-
paredtotheresultsaboveinFigure55.Nevertheless,thevalues
arebelowthevaluesoftheplainCo2FeAljunctionswithCo-Fe
ode.electrcounterTheabovediscussedcrystalgrowthpropertiesofthecounterelec-
trodeinuencedthemagneticbehaviorofthesecondHeusler
laForyer.the2Thenmcorrand3nmespondingthickmajorsecondloopsHeuslerarelayplotteder,abrinoadFigureswitch-57.

76

ingispresent,whichmightindicateacertaincrystallographicde-
cayandatomicdisorder,aspreviouslyfoundforthedisordered
Co2MnSi.Forlayersthickerthan5nm,thecoercivefieldofthe
pinnedcounterelectrodewasbelow80OeandamaximumTMR
ratioofabout115%wasachieved.

InsummarytheHeuslercompoundsoftheinvestigatedhalf
junctionswereintegratedintofullmagnetictunneljunctionsto
analyzethetransportproperties.Amaximumroomtemperature
TMRratioofabout150%(261%at13K)wasachievedforB2struc-
turedCo2FeAljunctions.Theobtainedarearesistancesvaryfrom
lessthan100KΩµm2forCo2FeAltomorethan100MΩµm2for
Co2MnAl.OnecanconcludethathighroomtemperatureTMR
values,lowarearesistancesandalowΓwerefoundforcom-
poundswithoutMn(Co2FeAlandCo2FeSi)andviceversafor
compoundscontainingMn.Inparticular,thediscussedoriginof
temperaturedependenceseemedtobemorerelatedtointerface
propertiesandnottothereportedshiftoftheFermilevelintoa
DOS.theofgapFromthedifferentcompositionsofCo-Fe-Siwefoundanal-
mostidenticalΓ,independentofstoichiometry.Furthermore,we
canconcludethatanexcessofFeislesscriticaltothetransport
propertiesthananexcessofSi.Asseenbefore,theexcessofSi
stronglyinuencesthecrystallinegrowth,aswell.
Finally,wehavealsointegratedthepromisingHeuslercom-
poundCo2FeAlassecondelectrodeintotunneljunctions,inor-
dertogeneratecoherentinterfacesandtoenhancetheTMRratio.
ProbablyduetoadegradedcrystalgrowthontopoftheMgO
barrierorbarrierinterfaceimperfections,alowerTMRratiois
presentwhencomparedtoaplainCo2FeAljunctionwithCo-Fe
ode.electrcounter

77

ofFiguredouble57:HeuslerMajorjunctionsloops
trwithodevarthickness.yingcounterelec-

applicabilityIndustrial

WithintheBMBF136projectHeuSpin,acollaborationwithpart-
nersfromindustrywasfoundedtotransfertheknowledgeof
Heuslerpreparation.ThepossibleintegrationofHeuslerjunc-
tionsintorelevantapplicationswillbeevaluated.Thischapter
willdescribehowthelimitingconditionscanbehandled.Prepa-
rationtechniquesandtheresultingjunctionpropertiesoflaband
industrysampleswillalsobecompared.

verificationlayerseedTheintegrationofHeuslercompoundstoexistingsystems,e.g.,
GMR/TMRsensors,isverychallengingduetolimitingcondi-
tions.Aspreviouslydiscussed,theseedlayersystemoftheHeusler
isessentialfortheoptimumgrowthconditionswithregardtoes-
tablishthepreferred(001)textureofthecompoundaswellas
toinduceahighatomicorderinginthecrystalstructure.For
theintegrationintoexistingsystemsfabricatedbySiemensAG,
onemustverifywhethertheoptimizedseedlayercanbeused,
aswell.TheexistingsensorswerecoveredwithSiN/SiO2and
polishedbychemicalmechanicalpolishing(CMP).Therefore,we
usedstandardSiO2waferstoinvestigatethetransportproperties
oftheexpected,degradedcrystalgrowthoftheHeusler.Co2FeAl
isthepreferredcompoundbecauseofthehighroomtemperature
TMRratiosachievedontheMgOsubstrate.
Figure58depictstheobtainedTMRratios,asafunctionof
MgObufferlayer◦thickness.Allsampleswereex-situvacuum
annealedfor1hat410C.Clearlyvisible,isthenecessityofaspe-
cificMgObuffer.WithoutanMgObuffer,TMRratiosofabout
20%canbereached.Inthebufferthicknessrangeof2.5nmto
10nmtheachievedratioisalmostonaconstantlevelofabout
100%.Forathickerseedlayer,theTMRvaluesdecreasedagain
toabouthalf.TheloweredTMR,whencomparedtothejunc-
79

136BundesministeriumForschngundBildung

für

Figure58:TMRratioof
Costrates2FeAlwithMTJsdifonferentSiOMgOsub-
bufferlayerthickness.

137.com/w.dohttp://www.htm402440od/clotene/prcy138canThetakefullsecurveeraltimedaofysBCBde-
bakingpendingontemperaturthicknesse.and

tionsthatweredepositedon(001)MgOsubstrates,isreasonable.
Theimpairedcrystalgrowthofsimilarjunctionshasalreadybeen
discussedpreviously.Fortheintegrationintoexistingsystemsan
optimumMgObufferof5nmthicknesswasassumed.Thefirstre-
sultsofourHeuslerlayers,depositedonSiO2waferswhichwere
furtherprocessedatSiemensAG,areverypromising.ATMR
ratioofabout80%canbeachievedforjunctionsthatweremoder-
atelyannealedat250◦C.Furtherannealingisplannedandhigher
expected.earaluesv

Frmicrequentlyoelectr,onicBenzocyprocessing.clobuteneThe(BCB)basedphotosensitivepolymersBCBisareusedusedforin
wpassivaferlevation,elorapplications,whereathinwhereadielectricprotectivlayereislaryerequirised.needed137Onefor
peraturlimitingethatconditioncanbeofthisappliedtosubstratetheisBCB,therdependingestrictedonannealingthecure.tem-138
TheMgOrequirbarrieredandforannealinginducingtemperaturtheesatomicfororcrderingystallizationoftheofHeuslerthe
compoundwouldprobablymelttheBCB.
However,wehavepreparedfulljunctionsof5nmMgObuffered
Co2FeAltoinvestigatethetransportproperties.Thejunctions
◦atwrereoomannealedtemperaturfore,1hatwhich360isC.WecomparablefoundtoatheTMRachieratiovedofvalues58%
oncausesMgOthersubstrates.elativelyPrhighobablyTMR,thevratios.erysmoothFurthermorsurfacee,ofthesubsequentBCB
annealingofthejunctionsisplanned.

Insummary,highTMRratiosinCo2FeAljunctionscouldbe
crachieystalvedgroevwthenofontheSiO2HeuslerandlaBCByerwouldsubstrates,bewherexpected.eaTheprdegradedevi-
ouslysubstratesfoundBcould2orprderingobablyatbelowconserannealingvedonothertemperaturessubstrates,onMgOas
wcallyell.releDuevanttothesubstrates,easyitisfabricationaninterofCoesting2FeAlmaterialontointhetechnologi-class
compounds.Heuslerof

80

propertiesofindustriallypreparedHeuslerlayers
ForanoptimalcomparisonofHeuslerjunctionswhichwerepre-
paredinthelabandunderindustrialconditions,Co2FeSitargets
fromthesamesupplier139wereused.Halfandfulljunctionsfrom
anidenticallycomposedHeuslertargetwerepreparedbySingu-
lusNDTGmbH.Alowerbasepressureaswellasahigherfilm
homogeneitycanbeachievedwithinthesputteringsystemwhich
probablyleadstoanimprovedcrystallinegrowthofthebuffered
Heuslerthinfilmandsmootherinterfaces.
ThedetailedlayerstructuresofthejunctionsaregiveninFig-
ure59.SiO2aswellasMgOsubstrateswereused.Incontrastto
ourhalfjunctions,theSingulushalfjunctionsendedupwiththe
HeuslerlayerinsteadofaMgOcappinglayer.Fortheintendedin-
vestigationsbyX-raydiffraction,toestimatethequalityofcrystal
growth,thisdifferencecanbeignored.
Figure60showsanoverviewofaXRDpatternoftheSingu-
lushalfjunctionsofthetwodifferentsubstrates.Inthecaseof
theMgOsubstrate(top),aclearCo2FeSi(004)and(002)peakwas
foundat65.9degreesand31.5degrees,respectively.The(002)
peakofthesinglecrystallineMgOsubstrateislocatedat42.9de-
grees.ThisisincontrasttotheobtainedpatternfromtheSiO2
basedhalfjunctions.Here,no(004)and(002)Co2FeSipeaksare
visibleattheexpectedpositions.Instead,averyslight(022)peak
canbeestimatedat45.1degrees.Theclear(004)and(002)sub-
stratepeaksarelocatedat69.2degreesand33degrees,respec-
tively.Aweak(002)peakofthesputtered5nmMgObufferlayer
canbeobservedat42.5degrees.

81

Figure59:Layerstackingof
theSingulshalfandfulltun-
comparisoninjunctionsnelwiththepreviouslyinvesti-
junctions.Bielefeldgated

139WilliamsAdvancedMate-
rials

Figure60:XRDpattern
overviewofSingulussam-
ples.with520nmnmMgOCo2onFeSiMgObuffersub-ed
substrateSiOand(top)strateannealingwithout(bottom)ocess.pr

TheroughXRDoverviewoftheSingulussamplesconfirms,again,
thepreferableusageofMgOsubstrates.Nevertheless,adetailed
extractofthe(004)Co2FeSipeaksforallSingulushalfjunctions
isgiveninFigure61,todeterminetheoptimalMgObufferthick-
nessandsubstrate.Almostnodifferenceinintensity,for3nm,
5nmand10nmMgObuffer,wasobservedforthethreeinvesti-
gateddifferentHeuslerthicknessesof10nm,20nmand30nmand
MgOsubstrate.Thepeakintensityincreases,asexpected,with
increasinglayerthickness.Asmallshiftinthepeakmaximum
tosmalleranglesisnoticeablewithincreasingHeuslerthickness.
Thisshiftisattributedtoachangeofthelatticeconstantandwill
.wbelodiscussedbeBycontrastnoconsiderable(004)peakoftheCo2FeSilayeris
presentinallXRDpatternincaseoftheSiO2substrate.Asshown
inFigure61(bottom),onlythe(004)substratepeakwasdetected.
WefoundthattheMgOsubstrateispreferabletotheindustrially
preparedCo2FeSiHeuslerthinfilms,aswell.Furtherinvestiga-

82

tionsoftheSiO2substratebasedthinfilmsarerejectedinthis
ork.wTheinvestigated(004)peakshiftforCo2FeSiinthecaseofthe
MgOsubstrate(Figure61(top)),issuesfromachangeinthelattice
parameter.Figure62depictsthecorrespondinglattice,asafunc-
tionofCo2FeSithickness,fordifferentMgObufferthicknesses.
Thedashedlinerepresentsthebulkvalueof5.64Å.140Forthe
10nmthickHeuslerlayer,thelatticeconstantvariesslightlywith
MgObufferlayerthickness.Thelowestvaluewasfoundforthe
10nmthickbuffer;thehighestforthe5nmbuffer.Forthickerlay-
ersofCo2FeSi,thelatticewasunaffectedbythebufferthickness
andincreasedwithincreasingHeuslerlayerthickness.However,
thechangeoflatticeconstantwasonlyintherangeofhundreds
Å.of

TodeterminetheoptimumthicknessfortheCo2FeSilayer,thetex-
turedfractionwascalculatedfromthe(004)Heuslerpeaks.Figure
63showsthatthehighestvalueswerereachedforthe20nmthick
layer,independentofbufferthickness.Theseresultsconfirmthe
previouslyachievedresultsfromourjunctions.

Topointoutthereproducibilityoftheresults,theXRDpatternof
ourpreparedsamples(greenline)andthepreparedsamplesby
SingulusNDTGmbH(redline),areplottedinFigure64.Both
samplescontain20nmCo2FeSiona5nmMgObuffer.Noan-
nealingprocesswasapplied.Both(004)and(002)Heuslerpeaks
83

Figure61:((004)peakofthe
MgObuffered10nm,20nm
and30nmCo2FeSilayers,de-
substrateMgOonposited(top)andSiO2substrate(bot-
tom),respectiveley.

140RevS..BW72ur,1(mehl2005et)al.,Phys.

Figure62:latticeconstantof
MgObuffered10nm,20nm
and30nmCo2FeSilayers.

Figure63:Texturedfraction
carriedoutfromthe(004)
Co2FeSipeaknetareaofthe
MgObuffered10nm,20nm
and30nmHeuslerlayers.

Figure64:XRD(002)and
(004)peaksofCo2FeSihalf
Singulusjunctions.andBielefeldComparisonsam-of
ples.

ofFigureSingulus65:andComparisonBielefeld
junctions.halfFeSiCo2

arenearlycoextensive.Asmalldeviationinthecaseofthe(004)
peakcanbeestimatedwhichisattributedtoashiftinthelattice
constant.The(002)peakoftheSingulussamplewasslightlyen-
hanced,incomparisontooursample.Allinall,thedifferences
arenegligibleandanequalqualityofthedepositedCo2FeSilayers
concluded.becan

Thecorrespondinglatticeconstantsandthe(004)peaknetareas
ofbothsamplesareshowninFigure65(top)and(bottom),re-
spectivelys,asafunctionofannealingtemperature.Thedashed

84

linerepresentsthereportedCo2FeSibulklattice.141Theobtained
differenceofabout0.02Åforthecomparedsamplescanbeig-
nored.Nevertheless,thelatticeoftheSingulussamplesfitsthe
bulkvalueevenbetter.
Theabovementionedgoodreproducibilitycanbeconfirmed
bythecorrespondingnetareasthatwereobtainedandaregiven
inthebottomofthisFigur◦e.Theachieved(004)peaknetareas
oftheaspreparedand200CannealedSingulussamples,fitsthe
obtainedvaluesoftheBielefeldsamplesperfectly.Furtheranneal-
ingisplanedbut,duetothegoodreproducibility,noconsiderable
differenceareexpected.
Acomparableinvestigationofthebulkmagneticproperties,via
roomtemperatureAGMmeasurements,ispending.
TheSingulusfulljunctionswereinvestigatedwithregardto
thetransportproperties.IncontrasttotheBielefeldHeuslerfull
junctions,anartificiallypinnedCo-Fe-Blayerwasusedasthetop
counterelectrodeinsteadofCo-Fe.Theprovidedsampleswere
cappedwitha20nmAulayerafterannealingforbetterconduc-
tance,similartoourpreviouslydiscussedjunctions.Theachieved
TMRratiosaregiveninFigure66,incomparisonwithoursimilar
Co2FeSijunctions.Forexsituannealingtemperaturesof300◦C
and325◦CalowerTMRvalueofabout33%wasfoundalthough
thedepositedHeuslerthinfilmsshowidenticalproperties.

ThereasonofthereducedTMRvaluesmightbefoundinthe
correspondingmajorloops,whicharedepictedinFigure67for
theSingulussampleandinFigure68foroursample.Bothsam-
plescontaineda1.8nmthickMgObarrierandweresubsequently
85

141RevS..BW72ur,1mehl(2005et)al.,Phys.

ofFigureachiev66:edTMRComparisonratios
frCo2omFeSifullSingulusjunctions.andBielefeld

Figure67:Typicalmajorloop
ofaSingulusCo2◦FeSijunc-
C.325atannealedtion

ofFiguraeCo68:2TFeSiypicaljunctionmajorprloope-
parnealededatin325◦C.Bielefeldandan-

annealedfor1hat325◦C.ApartfromthereducedTMRratio,the
arearesistancewasloweredbyafactorofthree,comparedtosimi-
larsamplesthatwehaveprepared,despitethesamebarrierthick-
ness.Thelowerarearesistancecanprobablybeexplainedbya
slightlydifferentcompositionoftheMgObarrier,i.e.,theSingu-
lusbarriermighthavealeakofoxygen.Ononehandthisleakcan
resultinapreferred,lowarearesistanceofthetunneljunctions,
butontheotherhandmightalsoleadtoaloweredTMRratio,due
tobarrierimperfections.Asimilareffectwaspreviouslyfoundin
commonCo-Fe-B/MgOjunctionsthatwerepreparedbySingulus.
ThedepositionoftheMgOlayerinalightoxygenatmosphere
mightimprovethebarrierqualityandenhancetheTMRvalues.

InsummaryCo2FeSiHeuslerhalfandfulljunctionswerede-
positedbySingulusNDTGmbH.Thepreparationwasbasedon
thepreviouslydiscussedresultsofoursimilarjunctions.Per-
formedXRDmeasurementsshowedalmostidenticalgrowthprop-
ertiestothelayerswehadpreviouslyprepared.Bycontrast,the
transportpropertiesshowedaconsiderablyloweredTMRratio,
whichmightbemorerelatedtodifferencesinthetunnelingbar-
rierthantodifferencesintheHeuslerlayerproperties.Duetothe
successfulintegrationoftheindustriallypreparedHeuslerlayer,
wecanconcludeadirecttransferofknowledgefromlabtoindus-
.ytr

86

Conclusions

Insummary,wehaveintegrateddifferentHeuslercompounds
theintorhalfequiredandseedfulllayermagnetictoinducetunnelthejunctions.atomicorWehadering.veTheoptimizedpre-
bestferredresults(001)forgroa5wthnmdirMgOectionbufwferasedachievHeuslered.Wlayeer,havedepositedfoundtheon
substrates.(001)MgOprWeopertieshaveofinvtheestigatedHeuslerthethincrfilmsasystallographicwellastheandelementmagneticspecific,bulk
chemicalandmagneticpropertiesatthebarrierinterface.Except
infortheCo2asprMnSi,eparalledinvstateestigatedinB2typecompoundsstructurwe.ereAcrsimilarystalline,behaevvioren
wallasobsercompoundsvedforwertheeferrmagnetization.omagneticevenAgain,intheexceptasprforeparCoed2MnSi,state.
Furthernetization.annealingThehighestledtoanmoment,increasewhenofmagneticcomparedtomomenttheroreportedmag-
bulkvalues,wereachievedforCo2MnSiandCo2FeAl.Thismight
beexplainedbythegoodagreementoffilmcompositionsand
ofFurtherindentedmore,X2wYeZfoundHeuslerthatstoichiometrcompoundsyforcontainingtheseMncompounds.usually
fororiginmedofantheoxidizeddetectedbarrierloweredinterlayTMRer,whichratios,wtheeassumehighertoarbeearthee-
junctionssistancesofandthetheMnstrongercontainingtemperaturcompounds.edependenceByincontrast,thethetunnelFe
makescontainingthemHeuslermorejunctionscompetitiveshoforwalowindustrialerarearapplications.esistance,whichThe
highestroomtemperatureTMRratiosofabout150%wereachieved
junctions.FeAlCofor2readyFromsufaficienttechnologicalforrealpointapplicationsofview,suchtheseasTMRsensors.valuesTorareealizeal-
aHeuslerbasedApplicationSpecificIntegratedCircuit(ASIC),
weexistingdepositedcircuits,theprCoo2videdFeAlbyTMRlaPREMAyerandstackingfurtherontoprwafersocessedwithby

87

FigurTMRev69:aluesRoomofHeuslertemperaturjunc-e
oftionsvalencewithdifelectrferons.entnumbers

TMFigurR(e1370k):/TMFoundR(RTratio)asΓ=a
142Wfunction.Wangofvetalenceal.,Applelectrons.Phys
Lett95,182502(2009)
143LettD.95,Ebke232510et(al.,2009)ApplPhys

144B.Hülsenetal.,Phys.Rev.
Lett.103,046802(2009)
145Theinsitusubsequentde-
positionofathinCucapping
layerbetweenthemeasure-
mentsimprovestheinterface
obing.pr146P.Schattschneideretal.,
Nature441,486(2006)
147P.Schattschneideretal.,
Ultramicroscopy(2009)

SiemensAG.Thepreparationofademonstratorisplaned.
SimilarfulljunctionscontainingthepromisingCo2MnSicom-
poundresultedinunexpectedlowTMRvalues.Thismightorigi-
natefromdiffusionpriortocrystallizationofthecompoundlayer.
Weinvestigateddifferentsolutionstoremedyit(multilayer,insitu
annealing,temperaturestablecounterelectrode)andachievedan
increaseoftheTMRratiotoabout110%.Duetothepresent,
strongtemperaturedependence(Γ=3.1)thesamejunctions
showedthehighestTMRvaluesat13K,withinthelistofcom-
poundsthatweinvestigatedinthiswork.
AllroomtemperatureTMRratiosaresummarizedasafunc-
tionofvalenceelectronsinFigure69.ThetrendofhighestTMR
ratiosforHeuslercompoundswith29valenceelectrons(Co2MnSi
andCo2FeAl)canbeconfirmedbythereported,highestvalues
foundinliterature(opencircles).Bycontrast,wecannotconfirm
thesupposedconnectionoftemperaturedependenceandposition
oftheFermilevel(ornumbersofvalenceelectrons).Asdepicted
inFigure70,ourdeterminedΓisindependentofvalenceelec-
trons.Here,theoriginseemstobemorerelatedtobarrierinter-
faceproperties.Inparticular,theMncontainingcompoundsshow
aconsiderablystrongertemperaturedependence.
ItwasshownbyWangthatTMRvaluesofover300%canbe
realizedwiththeB2structuredHeuslercompositionCo2FeAl,
whichwasoptimizedinanattempttogetsmoothinterfaces.142
Therefore,theinvestigationofinterfacepropertiesisessentialto
understandthetemperaturedependence,theinuenceofrough-
nessandspinpolarizationforfuturepreparationofhighlyspin
polarizedmaterials.Asafirststep,weperformedtunnelingspec-
troscopyfortheHeuslercompoundCo2FeAl.143Similarmeasure-
mentsareplanedforothercompounds,aswell.Theevaluation
ofthesemeasurements,inadditiontothepresentdata,ishelpful
tounderstandtheinterfaceeffects.Furthermore,theinvestigation
ofXASandXMCDwillbeextended.Abinitioatomisticther-
modynamicsoftheinterfaceinaCo2MnSi/MgOsystemhave
shownavariantspinmagneticmomentoftheinterfacialatoms
fordifferentterminations.144SurfacesensitiveXMCDmeasure-
ments145,whichwereperformedbyKrumme,onoursamples
indeedconfirmedthepredictedinterfacetermination.In2006,
Schattenschneider146reportedtheexperimentaldetectionofmag-
neticcirculardichroisminaTEM(namedEMCD),whichallows
amappingofelementspecificmagneticmomentatthebarrier
interfaceonananoscale.147InitialEMCDmeasurementsofour

88

HeuslerhalfjunctionshavealreadybeenperformedbyEnnenand
arecurrentlyunderevaluation.
Thisworkshowedthatthepreparationofhighlyspinpolarized
materialsisverychallengingandthedemandofsuchmaterialsis
great.Inthefuture,wecanprepareimprovedjunctions.Veryre-
cently,anewsputterchamberwasputintooperation.Theavail-
abledepositiononheatedsubstrates(upto1000◦C)shoulden-
hancethecrystallinegrowth.Aco-depositionofseveralmaterials
isalsopossible.Inparticular,Heuslerlayerscanbepreparedwith
anadjustedfilmstoichiometryordustedinterfaces.Acrystalline
MgObarriercanbeformedbycommonRFsputteringorelectron
beamevaporation.Furthermore,wehavesuccessfullyprepared
ourfirstHeuslerthinfilmsbymolecularbeamepitaxy(MBE).
Theadjustablefilmcompositioncanberealizedandadeposition
onheatedsubstratesispossible.Inparticular,thedustingofinter-
facesmightbyveryinterestingfortheconservationofahighbulk
spinpolarization.Recently,Miurashowedthatthehalf-metallic
tunnelingconductanceofMgObasedCo2MnSitunneljunctions
canbeconserved,withaninsertionofathinCo2MnAlinterlayer
148interface.barriertheatEventhoughhigherroomtemperatureTMRratioscannowa-
daysberealizedwithoutHeuslercompoundelectrodes,thesema-
terialsareofgreatinterestfortheintegrationintomagnetictunnel
junctions.Forexample,itwasshownbyAlbonthatawiderrange
forthedetectionofnanoparticlescanberealizedwithHeusler
basedjunctions,duetotheverysmallanisotropyofthecom-
pounds.149Furthermore,thispropertyleadstoalessnoisysignal,
whichisessentialforthedetectionofsmallmagneticfields.
Additionally,thereportedlowdampingconstantofhalf-metallic
Heuslercompoundsisveryinterestingwithregardtospincurrent
inducedswitchingandtheexpectedlowcriticalcurrentdensities
junctions.suchwithinHowever,theinternationaltechnologyroadmapforsemicon-
ductorspredictsafurthershrinkingofthecellsincommonde-
vices,suchastheDRAM(dynamicrandomaccessmemory)within
thenextyears.Ahalfpitchsize,whichishalfthedistancebetween
cellsofamemorychip,of22nmissupposedfor2011-2012.From
aphysicalpointofview,thechangetoperpendicularymagnetized
materialsisabsolutelyessentialtorealizefuturemagnetictunnel
junctions.ThiswaspredictedbyDaalderop150formultilayersof
(Co)n/(X=Cu,Ag,Pd)andexperimentallyverifiedforCo/Pd
multilayersbyCarcia.151TheclassofHeuslercompoundsalso

89

148Y.Miuraetal.,Journal
ofPhysics:ConferenceSeries
200,052016(2010)

149C.BielefeldAlbon,UniversityPhD(2009thesis,)

150Rev.G.B42,Daalder7270op(et1990)al.,Phys.
151LettP.47Car,cia178(et1985al.,)ApplPhys

152S.urWmehletal.,Jour18of,6171(2006Physics-Condensed)

nalMatter

providespromisingmaterials.Mn3Gaispredictedtoshowhalf-
metallicityandperpendicularanisotropy.152Furthermore,Mn3Ga
isexpectedtobeacompletelycompensated-ferrimagnetic,i.e.,
nomagneticmomentispresent,whichisveryattractiveforde-
vicesbasedonspininducedmagnetizationswitching.Wehave
alreadyperformedinitialexperimentsandtheintegrationinto
planed.isjunctionstunnel

90

Appendix

summarydataSummaryofachievedexperimentaldataoftheinvestigatedHeusler
compounds.ThefirsttablesummarizestheresultsofCo2FeAl,
Co2FeSi,Co2MnAl,Co2MnSiandCo2Mn0.5Fe0.5Si.Theresultsof
thehalfjunctionsaregiveninthefirstpartofthetable,theresults
ofthefulljunctionsinthesecondpart,respectively.
Co2FeAlCo2FeSiCo2MnAlCo2MnSiCo2Mn0.5Fe0.5Si
#valenceelectrons2930282929.5
targetcompositionCo2Fe1Al1Co2Fe1Si1Co2Mn1Al1Co2Mn1.28Si1.29Co2Mn0.5Fe0.5Si1
filmcompositionCo2Fe0.98Al0.985Co2Fe0.953Si0.925Co2Mn0.957Al0.8Co2Mn0.985Si0.968Co2Fe0.473Mn0.383Si0.912
max.magnetization1099kA/m1090kA/m535kA/m1039kA/m941kA/m
atmax.annealingmagn.temp.moment5005.45◦µCB5005.36◦µCB5002.70◦µCB4005.02◦µCB5004.62◦µCB
lattice5.688Å5.671Å5.719Å5.637Å5.667Å
max.TMR(RT)153%78%63%107%68%
max.TMR(13K)261%134%157%330%144%
atannealingtemp.410◦C400◦C400◦C425◦C350◦C
Γ1.71.72.53.12.1
Co2FeSiCo-Fe-Si(Fe+)Co-Fe-Si(Si+)
#valenceelectrons30
targetcompositionCo2Fe1Si1Co2Fe1.43Si1.22Co2Fe1.09Si1.37
filmcompositionCo2Fe0.953Si0.925Co2Fe1.313Si0.894Co2Fe1.019Si1.205
max.magnetization1090kA/m1310kA/m811kA/m
atmax.annealingmagn.temp.moment5005.36◦µCB5006.42◦µCB3003.47◦µCB
lattice5.671Å5.665Å5.607Å
max.TMR(RT)78%74%49%
max.TMR(13K)134%116%91%
atannealingtemp.400◦C325◦C275◦C
1.81.61.7Γ

91

XMCDandXAS

TheroomtemperatureXASofthe

TheroomtemperatureXASoftheCo-,Mn-andFe-L3,2edgesfor
theHeuslercompoundsCo2FeSi,Co2MnSiandCo2Mn0.5Fe0.5Si
areillustratedinthefollowingFigures.Thespectraoftheoff-

stoichiometricallyCo-Fe-Sifilmsaregiven,too.ProminentXAS

featuresaremarkedwitharrows.Thecorresponding

each

92

compound

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of

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es.Figur

XMCD

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107

Danksagung

AndieserStellemöchteichallenmeinDankaussprechen,die
michbeiderErstellungdieserDoktorarbeitunterstützthaben.
MeinDankgiltzunächstmeinemDoktorväternProf.Dr.An-
dreasHüttenundPDDr.AndyThomas.VielenDankfür
Unterstützung!eeur

BedankenmöchteichmichauchbeiProf.Dr.GünterReiss,
dermirdieGelegenheitgegebenhatinseinemLaborzuArbeiten,
undbeiDr.JanSchmalhorst,dermitIdeen,konstruktiver
KritikundwissenschaftlichenDiskussionenzudieserArbeitbeige-
haben.tragen

DesweiterenmöchteichmichbeiallenTeilnehmernderchinesis-
chenWochebedanken:beiOliverSchebaumfürTMRMessun-
gen,beiMarkusSchäfersfürLithographieundbonding,bei
PatrickThomasfürzahlreicheXRD-MessungenundbeiAndy
Thomas,derdiesesehrerfolgreicheWocheinsLeben
gerufenhat.Eshättenochvieleweiteregebensollen!
OliverSchebaumseiebenfallsfürdiehiergezeigten
gedankt.-MessungenSPTMeinemBürokollegenVolkerDrewellodankeichfürdie
-Messungen.TLchgeführtendurBeiZoëKuglerbedankeichmichfürdiegemeinsameMesszeit
anderALSinBerkeley,inderdiemeistenhiergezeigtenXASund
XMCDSpektrenentstandensind.Dr.JanSchmalhorstund
MarkusMeinertseiebenfallsgedanktfürdieDurchführung
vonergänzendenXMCDMessungen.
DennisNissenseigedanktfürdieerstenErgebnissemit
Co2MnAlundderUnterstützungbeiTMR-MessungenanCo2FeAl.
AlexanderWeddemannundAlexanderAugemöchtenich
fürdiedurchgeführtenBandstrukturrechnungdanken.
DergutenSeelevonD2KarstenRottmöchteichmeinen

109

DankfürdieBetreuungundInstandhaltungnahezuallerLabor-
einrichtungendanken.Auchdennamentlichhiernichtaufge-
führtenMitgliedernderArbeitsgruppevonD2gebührtein
Dankeschönfürdieangenehme,nahezufamiliäreAtmosphäre
währendundnachderArbeit.

CBeilauderdiaF(Heuslerelse,rHeuslerbedanke,ichHeuslermich-)fürdieArbeitsgruppeinspiriervonendeProf.Zusam-Dr.
menarbeitindenletztenJahrenunddasbereitgestellte
Co2Mn0.5Fe0.5Sitarget.

AuchfürdieUnterstützungderAdvancedLightSourcein
BerkeleyzurDurchführungderhiergezeigtenXASundXMCD
Messungenandenbeamlines6.3.1und4.0.2seigedankt.Ins-
besonderegilteingroßerDankElkeArenholz,dieunsstets
umsorgendbetreuthatunddiedurchdieaußergewöhnlichgut
gewartetenMessaufbautenimmerfürerfolgreicheStrahlzeiten
gesorgthat.DieAdvancedLightSource,Berkeley,USA,wird
gefördertvondemOfficeofScience,OfficeofBasicEnergySci-
ences,oftheU.S.DepartmentofEnergyunterderVertragsnum-
mer:DE-AC02-05CH11231.

VielenDankauchdemBundesministeriumfürBildungund
Forschung(BmBF)fürdiefinanzielleUnterstützungimRahmen
desProjektsHeuSpin.HierseidenKollegenJürgenLangerund
BertholdOckervonderSingulusNDTGmbH,sowieMan-
fredRührigvonderSiemensAG,fürdieguteZusammenarbeit
unddieBereitstellungderProbengedankt.

AbschließendmöchtenichmichbeimeinerFrauWiebkeund
meinemSohnJohannfürdieliebevolleUnterstützungundihr
VerständnisindenletzenJahrenbedanken.

110

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