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DOI:10.1016/j.apsusc.2013.05.160
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To cite this version :
Pasquet, Isabelle and Presmanes, Lionel and Bonningue, Corine and
Tailhades, Philippe Patterned ferrimagnetic thin films of spinel
ferrites obtained directly by laser irradiation. (2013) Applied
Surface Science, vol. 283 . pp. 283-289. ISSN 0169-4332
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Patterned
ferrimagnetic
thin
films
of
spinel
ferrites
obtained
directly
by
laser
irradiation
I.
Pasquet
a,b,
L.
Presmanes
a,b,
C.
Bonningue
a,b,
Ph.
Tailhades
a,b,∗aUniversitédeToulouse,UPS,INPT,InstitutCarnotCIRIMAT,118,routedeNarbonne,F-31062ToulouseCedex9,France bCNRS,InstitutCarnotCirimat,F-31062Toulouse,France
Keywords: Laserpatterning Laserirradiation Spinelferrite Ironoxide Magnoniccrystals Magneto-photoniccrystals
a
b
s
t
r
a
c
t
Somespinelferritescanbeoxidizedortransformedatmoderatetemperatures.Suchmodificationswere carriedoutonthinfilmsofmixedcobaltcopperferritesandmaghemite,byheatingsmallregionswitha low-powerlaserspotappliedforabout100ns.Theverysimplelaserheatingprocess,whichcanbedone directlywithaconventionalphotolithographicmachine,madeitpossibletogeneratetwo-dimensional magnetizationheterogeneitiesinferrimagneticfilms.Suchperiodicstructurescoulddisplaythespecific propertiesofmagneto-photonicormagnoniccrystals.
1. Introduction
FromthepioneeringworksofYablonovitchh[1]andJohn[2]
opticalperiodicstructures,calledphotoniccrystals,haveattracted muchattention,notonlybecauseoftheir fundamentalinterest, butalsobecauseoftheirpotentialtechnologicalapplicationsdue totheir original collective properties[3]. Makingsuch periodic structureswithferroorferrimagneticmaterials,isalsovery attrac-tiveforseveralreasons.Firstly,opticalindicescanbetailoredby anexternal magnetic field in such structures, due tothe mag-neticbirefringenceand dichroicpropertiesofthecorematerial. Tunableopticaldevices,whichcanbecalled“magneto-photonic crystals”,canthusbeimagined[4,5].Thesecondreason isthat two-dimensionalmagnetizationheterogeneitiesinaferroor ferri-magneticmaterial,canleadtoastructureabletomanagespinwave propagation.Thesemagneticcounterpartsofphotoniccrystalsare generallycalled“magnoniccrystals”[6–8].Thetwo-dimensional magnetizationheterogeneitiescanbeholesorasecondmaterial, havingdifferentmagneticpropertiesthanthematrixinsidewhich
∗Correspondingauthorat:UniversitédeToulouse,UPS,INPT,InstitutCarnot CIR-IMAT,118,routedeNarbonne,F-31062ToulouseCedex9,France.
Tel.:+330561556174;fax:+330561556163.
E-mailaddress:tailhade@chimie.ups-tlse.fr(Ph.Tailhades).
itisinserted.Suchmagnonicdevicescouldfindtechnological appli-cationsinnarrow-bandopticalormicrowavefiltersorhighspeed switches[9].
2Dperiodicstructurationofmagneticfilms,hasalreadybeen performed to make magnonic crystals.However, the materials usedweresinglegarnetfilmsormagneticalloysandthemethod carriedout resortedtoquiteheavyopticalor electron lithogra-phyprocessing[9–12].Thispaper proposesa very simplelaser processingofspinelferritefilms,withtheaimtofabricatenew mag-neticdevices,forinstancemagneto-photonicormagnoniccrystals. Laserbeamsorspotshavealreadybeenusedtoanneal[13–16], sinter[17]orpattern[18–24]alotofoxides,notablyspineloxides
[25,26].Ofcourse,spinelferriteswerechosenforthiswork,because oftheirferrimagneticproperties,whichcanbeeasilyadjustedby propercationicsubstitutions.Buttheotherreason,whichisthe keytosuccessfulpatterning withlow-powerlaserspots,isthat spinelferritethinfilmscanoftendisplayareal“thermalreactivity” atmoderatetemperatures.“Thermalreactivity”meansreactivity towardsoxygen for spinel containingcations capableofhigher valencestates[27],ormetastabilityforstronglynon-stoichiometric ferrites [28,29]or quitelow sinteringtemperatures, mainlyfor coppersubstitutedferrites[30,31].Suchthermalsensitivitiesare alreadyusedforopticaldatastorage[32–34].
Thispaperwillmainlyfocusonthinfilmsofmixedcobaltcopper ferrites,becauseoftheirhighsensitivitytolaserirradiation.Itwill alsogiveanotherexamplewithlaserpatternedg-Fe2O3thinfilms.
2. Experimental
2.1. Samplepreparation
Thinfilmsofspinelferriteswerepreparedbyradio-frequency sputteringof10cmdiameteroxidetargets.Formixedcobaltcopper ferrites,anoxidetargethavingCo:Cu:Fecationsintheproportions 0.15:0.85:2wasused.Thesecondtargetwasmadeofmagnetite Fe3O4. The sputtering machine was an Alcatel A450 equipped
witharadio-frequency-generator(13.56MHz)deviceaswellasa pumpingsystem(amechanicalpumpcoupledwithaturbo molec-ularpump)whichreachesresidualpressuresdownto10−5Pa,agas
flowcontroller,awatercooledtargetholderandtwowatercooled sampleholders.Thefilmsweredepositedonglasssubstrateswith anaveragearithmeticroughnesslowerthan0.5nm.
Conventionally,aresidualvacuumof5×10−5Pawasreached in the sputtering chamber before introducing the argon depo-sition gas. In order to obtain various microstructures for Co0.15Cu0.85Fe2O4, target-substratedistances of 5 and8cm and
argonpressuresof0.5 and 2Pa, wereused.Moreover,for each experimental condition, the targets were sputtered for 20min beforestartingfilmdepositionontheglasssubstrate.The sput-teringpowerwasmaintainedatabout3Wcm−2 foreach ofthe
sputteringconditionsused.
Below, the mixed cobalt copper ferrite samples are named “Pxdy”withxthevalueofargonpressureinPascal,andythesample targetdistanceincentimetres.
Magnetitefilmswereobtainedbymagnetitetargetsputtering P0.5d5conditions.Thesesampleswereoxidizedat300◦Cfor2hin
ordertoformg-Fe2O3metastablephase. 2.2. Laserpatterning
MostofthepatterningexperimentsweredoneusingaDWL200 machinefromHeidelbergInstrumentsMikroTechnik.Thismachine isgenerallydedicatedtomask manufactureforoptical lithogra-phy.Itisahighprecisiontoolusingpixelgenerationtechnology byHe–Cdlaser scanning(=442nm,maximalpower125mW). Thewritingspeedwasabout1mm2/sor10Mpixels/s.The
aver-agedurationoflaserinsolationforeachpixel,iscloseto100nsand themaximallightenergyis7.8J/cm2.Theopticalsystemismadeof
anAutofocus,whichhasapneumaticservo-controltocorrectthe flatnessdefects.ThetuningrangeoftheAutofocusis70mmandits
zresolutionis100nm.Theworkingdistancebetweenthesample andtheobjectivelenswas100mm.
Otherexperimentswerecarriedoutwithamachinedesigned fortheproductionofmastersforopticaldiscmanufacturing.The 476nmwavelengthofanArlaserwasfocusedbyanobjectivelens withanumericalapertureof0.8,flyingoverthesampleata dis-tancecloseto1mm.Thewritingtimeforeachpixelwascloseto 100nsand themaximallight poweratthesamplesurfacewas about20mW.
2.3. Characterizationtechniques 2.3.1. X-raydiffraction
Structuralcharacterizationsoffilmswereperformedbygrazing angle X-ray diffraction (˛=1◦) on a Siemens D 5000
diffrac-tometer equipped with a Brucker sol-X detector. The X-ray wavelengthwasthatofthecopperKaray(Ka1=0.15405nmand
Ka2=0.15443nm). 2.3.2. Ramanspectroscopy
Ramanspectrawerecollectedunderambientconditionsusinga HoribaScientificRamanmicroscopefittedwithalaserwavelength of532nmanda100×objectivelens.Duringthemeasurement,the
resultinglaserpoweratthesurfaceofthesamplewasadjustedto 1.1mW.Thefinalspectrumistheaverageofthree300s accumula-tions.Examinationofmultiplespotsshowedthatthesampleswere homogeneous.
2.3.3. Magneticmeasurement
Themagnetic propertiesweremeasured in theplaneof the films, witha SQUID magnetometerMPMSXL 7 from Quantum design. The maximal applied field for the measurements was 70kOe.Themagnetizationsofthesampleswerecorrectedfor sub-stratecontribution.
2.3.4. Thicknessmeasurementandmicroscopy
FilmthicknessesweremeasuredusingaDektak3030ST pro-filometer.Atomicforcemicroscopy(AFM)wascarriedoutwitha VeecoDimension3000atomicforcemicroscope,equippedwith a super sharp TESP-SS AppNano© tip (nominal resonance fre-quency320kHz,nominalradiusofcurvature2nm).MagneticForce Microscopy (MFM)observations were also performedwith the sameapparatususingmagnetizedtips(Co/Crcoating,nominal res-onancefrequency70kHz).AFMwasusedtorevealtheheatedareas ofthefilms,wherechangesinvolumeoccurredduetostress relax-ation,oxidationorcrystallization.MFMisnotreallyappropriatedto studysuchstrongtopographicdeformationsinferrimagneticfilms. Indeed,themagneticcontrastisgenerallylowandthetopographic signalduetoahighbumporadeephollow,isdifficulttoremove totallyfromthemagneticsignal.However,MFMisverypowerfulto revealchangesinlocalmagneticpropertieswhenthereisno topo-graphicalmodification.MFM wasthenusedonlytoreveallocal maghemite–hematitetransformations,whichcanoccur without topographicalchangeandwhichinvolvetheformationof antifer-romagneticzonesinferrimagneticferritefilms.
The microstructureof thesamples wasalso investigatedby scanning electron microscopy with a JEOL JSM 6700F appara-tus.TheproportionofcationswasdeterminedbyEDX(Princeton GammaTech).SomepatternswerealsoobservedwithaKeyence VHX-600digitalopticalmicroscopeusingaVH-Z100RorVH-Z500R objectivesystem,havingbothahighresolutionandalargedepth offield.
3. Resultsanddiscussion
3.1. Mixedcobaltcopperspinelferrites
Theferritethinfilmspreparedwerepoorlycrystallizedbutthey displayed themain X-ray diffraction(Fig.1)and Raman peaks (Fig.2)ofthespinelstructure.Moreovertheywereferromagnetic atroomtemperatureasrevealedbytheirM=f(H)hysteresiscurves (Fig.3).EDXanalysesalsoshowedthatthemetalchemical compo-sitionwas0.15Co:0.85Cu:2Fe,thesameasthatofthetarget.The thinfilmswerethenmadeofaCo0.15Cu0.85Fe2O4spinelferrite.The
samplespreparedat0.5Paargonpressureand5cmfromthetarget (samplesP0.5d5),hadX-raydiffractionpeaksshiftedtowardsthe smallanglescomparedtothepeakpositionsforapowderhaving thesamecomposition.Anin-planecompressivestress,makingthe reticulardistanceslargerinadirectionclosetotheperpendicular ofthefilmsurface,wasassumedtoberesponsibleforthisshift. Bycontrast,theP2d8samplesweresubmittedtoaslighttensile stress(Fig.1andTable1).Atomicforcemicroscopyclearlyshows thepolycrystallinestructureofthefilms(Fig.4),whichweremade ofsmallcrystallitesofabout25nmand40nmfor100nmand1mm thicksamplesrespectively.Thesamplespreparedatahighargon pressure(P2d8)displayedcrystallitesaggregatedinlargergrains separated byporosity. This porosity clearly appeared for P2d8, mainlyfor1mmthickfilms(Fig.4).
0 1000 2000 3000 4000 5000 6000 25 35 45 55 In te n s it y 2 theta (degree) (2 2 0 ) (3 1 1 ) (4 0 0 ) (4 2 2 ) (511 ) P05D5 P2D8
Fig.1.X-raydiffractionpatternsofas-depositedCuCothinfilms(1mmthick)for twoexperimentalconditions:argonpressure2Paandtarget-substratedistance 8cm(P2d8);argonpressure0.5Paandtarget-substratedistance5cm(P05d5).
Fig.2.Ramanspectraofas-depositedCuCothinfilms(thickness:1mm)for exper-imentalconditionsP2d8andP05d5.
Fig.3.M=f(H)hysteresiscurve atroomtemperature for1mmthicksamples ofmixedcobaltcopperferritefilms,preparedindifferentconditions:(a)argon pressure:2Pa,target-substratedistance:8cm,(b)argonpressure:0.5Pa, target-substratedistance:5cm.
Preliminaryexperimentsoflaserirradiationwerecarriedouton as-depositedfilms,havingathicknessof70nm.Thelaserspotofthe masteringmachine(=476nm)wasusedfortheseexperiments. Twodifferentbehaviourswereobserved.Forthefilmspreparedat lowargonpressure,thelaserirradiationcreatedbumps,whereas crater-likeshapedholesappearedforfilmsobtainedathigh pres-sure(Fig.5).Thepatternswrittenwereveryregularinsizeand shapeforallsamples.Theirsize,however,increasedalittlebitwith thelaserpowerduetotheGaussianprofileoftheenergyofthe spot.Forinstance,thebumpsdisplayedanoveralldiameterclose to0.3mmandaheightof10nmfor5mW,butthesevaluesbecame 0.6mmand30nm,forapowerof15mW.
Someexperimentswerecarriedouttotrytounderstandthe phenomenarelatedtopatternformation.Atfirst,thinfilmswere heatedat450◦Cinair(AAsamples)orinpurenitrogen(NA
sam-ples).ItwasobservedthatforAAferritelayers,itwasnotpossible togetapatternwithamoderatelaserpower(5–15mW).ForNA samples,filmscanbewrittenon,usingaslightlyhigherlaserpower than for as-deposited ferrites. Pattern formation couldthen be relatedtoanoxidationphenomenoninducedbylaserheating.
Table1
Modificationsoftheout-of-planereticulardistancesduetointernalstressinferrite films.
Sample (311)Reticulardistance(nm)
Co0.15Cu0.85Fe2O4powder 0.25274
P0.5d5thinfilm 0.25539
Fig.4.AFMimage(1 × 1micrometers)ofthesurfaceof100nmand1mmthickfilms, preparedindifferentconditions:(a)argonpressure:2Pa,target-substratedistance: 8cm,(b)argonpressure:0.5Pa,target-substratedistance:5cm.
Somespineloxides,mainlyferritescontainingcopper,havea lowexcessof cations(i.e.metalliccations/oxygenanions>3/4). Theycanbedescribedbyageneralformulasuchas:MxFe3−xO4−ı
(ı>0, M:metallic cations). For such ferrites,non-stoichiometry occursduetoexcessoflowvalencestatecations,whicharelocated inoxygeninterstices,normallyfreeofcationsinthespinel struc-ture.“Interstitial”cuprousionswerealreadyrevealedin copper
[35,36]ormixedcobaltcopperferrites[37].Thefilmsstudied prob-ablydisplaysuchnon-stoichiometry,makingcuprouscationsthe onlyoxidizableions,duetothehighstabilityoftheotherCu2+,Co2+
andFe3+cationicspecies.Theoxidationcouldthenbewritten:
Co0,15Cu0,85Fe2O4−ı+ı/2O2→Co0.15Cu0,85Fe2O4 (1) Reaction(1),whichisinducedbytheheatingeffectofthelaser irradiation,canoccurmoreorlessreadilyaccordingtothefilm crys-tallitesize.Foras-depositedfilms,reactivitytowardsoxygenwas highbecauseoflowvalencestatecopperionsandsmall crystal-litesize.Patternscouldthenbeformedatlowlaserpower.When annealedat450◦C ininertgas,cuprousionsremainedbuttheir
oxidationwasmoredifficultbecauseoflargercrystallitesize,i.e. becauseofthelowerareaofmaterialincontactwiththeair. Pat-ternsformation requiredhigherlaser power.Ofcourse, sample
Fig.6.AFMimage(phasemode)showingtheincreaseincrystallitesizeinthe laserheatedzones(cobaltcopperferritefilm,thickness70nm,P0.5d5).Imagesize 600nm × 600nm.
annealinginairat450◦C,wasresponsibleforcuprousions
oxi-dation,makingitimpossibletowritepatternswithmoderatelaser power.
Althoughoxidationseemstoplayafundamentalroleinthe for-mationofthepatterns,itcannotexplainwhyferritefilmsreact to thelaser irradiation,to give either bumps or holes. Maybe, however,theheatbroughtbythelaser,addedtoasmallheating effectduetotheexothermicoxidation,bringsenoughenergyto modifythelocalmicrostructureleadingtothegrowthandthe sin-teringofthecrystallites.Thiseffectcanbeespeciallypronounced forcopperferrites,whichcanbesinteredatquitelow tempera-tures[30,31].ThechangeincrystallitesizewasrevealedbyAFM imaging, forthe bumpsformedin P0.5d5samples(Fig.6). The microstructuralchangegoesalsowithtopographicalmodification duetothemechanicalstressdevelopedduringthegrowthofthe films.ForfilmsP0.5d5submittedtoacompressivestressintheir plane,themicrostructuralchangegoeswiththeformationof a bump.Bycontrast,tensilestressleadsto“holes”withcrater-like shapesinP2d8layers.
Fig.5.Patterningoffilmspreparedindifferentconditions:(a)argonpressure:0.5Pa,target-substratedistance:5cm(AFMImage10mm × 10mm);(b)argonpressure:2Pa, target-substratedistance:8cm(AFMImage7mm× 7mm).
Fig.7.AFMimagesshowingtheraised(bright)andhollow(dark)zonesforP0.5d5(a)andP2d8,respectively.Thethicknessofthefilmsis1mm.Theimagesizeis 10mm × 10mm.
Fig.8.(a)SEMimageoflaserpatternedP2d8film(thickness:1mm);(b–d)detailsofas-depositedandpatternedzones.
Verysimilarresultswereobtainedforthickersamples irradi-atedbythe442nmlaserspotoftheconventionalphotolithography machine.For1mmthicksamples,hollowzonescanbeobservedfor P2d8filmsandraisedzonesforP0.5d5films(Fig.7).Aspreviously shown,thelaserspotinvolvescrystallitegrowthandsintering.In theP2d8hollowzones,thecrystalliteswerealittlebitlargerthan thoseintheas-depositedregions(Fig.8).Moreover,the rough-nesswasdecreasedandtheporositydeletedintheselasertreated zones.IntheP0.5d5raisedzones,thecrystallitesizeswerealso increased. Unlike the P2d8 samples, which were submitted to differentinternal stresses,laser heatingdidleadhowever toan increaseinroughness.Theroughnesswasmainlyduetogrooves resultingfromthescanofthelaserspot,asclearlyrevealedbyAFM images(Fig.7a).
Spinelferriteswithperiodicpatternsengravedinside,canthen beobtainedbydirectlaserprocessing.Becauseoftheversatility ofthisprocess,periodicstructuresverysimilartothoseusedfor magnoniccrystalsmadeofgarnets[9],canbedirectlypatterned inP2d8spinelferrites.Itwillbeinterestingtostudythepotential magneto-photonicormagnoniceffectsofsuchstructured ferrimag-neticspineloxidesinthenearfuture.
Itisalsoimportanttoshowthatwiththeconventional pho-tolithography machine used, it is possible to write very small
Fig. 9.Submicronic patterns in P2d8 film (thickness of the film: 1mm) (10mm × 10mmAFMimage).
Fig.10.AFMimage(a)andthecorrespondingMFMimage(b)ofag-Fe2O3filmlocallytransformedintoa-Fe2O3bylaserheating(sizeoftheimages:8mm × 8mm).
patternsinfilmsofcobaltcopperspinelferrites.Forinstance, sub-micronicpatternswereobtainedinP2d8films(Fig.9).Thatmakes thepreparationofminiaturizedstructurespossible.
3.2. Maghemitefilms
Themaghemite(g-Fe2O3)isametastablespinelphaseobtained
byoxidationofmagnetite(Fe3O4).Itcanbetransformedintothe
thermodynamicallystablea-Fe2O3,whichhasthecorundum
struc-ture.Thistransformation,whichoccursatamoderatetemperature (#300–600◦C)varyingmainlywithcrystallitesizeandmechanical
stress[29],canbeachievednotonlyforfinepowdersbutalsofor thinfilmsofmaghemite.
Somewritingexperimentsweredone withthe476nmlaser spoton25nm thickmaghemite films obtained byoxidation at 300◦Cofmagnetitesamples.Forlaserpowerinbetween7.5and
10mW, no topographical modifications were revealed. Careful MFMobservations showed,however, magneticcontrasts repro-ducingthepatternsprogrammedonthelasermachine(Fig.10). Themagneticcontrastscamefromthelocaltransformationsofthe ferrimagneticg-Fe2O3film,whichinteractedwiththemagnetized
tipofthemicroscope,intoantiferromagnetica-Fe2O3 zones,for
whichthemagneticinteractionswiththetipwerelower.
Themetastabilityofsomespinelferritethinfilms,thusoffers anotherpossibilityto createperiodic structuresby avery sim-pleand directlaser irradiationprocess. Theperiodic “magnetic heterogeneities” created inside the ferromagnetic film could, alsogenerate the specificproperties of a magneto-photonic or magnoniccrystal.
4. Conclusion
Thinfilmsofmixedcobaltcopperspinelferritesandmaghemite displayedthermal reactivity,whichmakestheirpatterning pos-siblebya low-power laser spot.By controllingthe elaboration parameters or the ferrite composition, holes, bumps or local phase changes can be created inside spinel ferrite films, by this very simple laser-based process. Ferrimagnetic films with two-dimensionalperiodicheterogeneitiesofmagnetization, can then be prepared. Such films meet the basic requirements of magnonic or magneto-photonic crystals. Moreover,due to the hugepossibilitiesofcompositionandnon-stoichiometry,offered bythespinelferritefamily, patternedperiodicstructurescould bemadeusingasimpleprocess,withferriteshavingtherelevant propertiestomakeeffectivemagneto-photonicormagnonic crys-tals.
Acknowledgements
ThisworkwassupportedbytheRENATECHtechnological plat-formfromInstitutCarnotLAAS-CNRS,memberoftheRTBnetwork. TheauthorsparticularlythankDr.PierreFranc¸oisCalmon.
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