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Nanocrystalline and Nanocomposite Magnetic Materials and Their Applications

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Nanocrystalline and Nanocomposite Magnetic Materials and Their Applications

    Nanocrystalline and Nanocomposite

    Magnetic Materials and Their

    Applications

    Availableonlineatwww.sciencedirect.com

    

    ScienceDirect

    JOURNALOFIRONANDSTEELRESEARCH,INTERNATIONAL.2007,14(4):69

    74

    Nan0crystallineandNanocompositeMagnetic

    MaterialsandTheirApplications

    RobertDShull

    (MagneticMaterialsGroup,NationalInstituteofStandardsandTechnology,GaithersburgMD20899,Maryland,USA)

    Abstract:Nanocrystallinematerialscanpossessbulkpropertiesquitedifferentfromthosecommonlyassociatedwith

    conventionallarge-grainedmaterials.Nanocomposites,asubsetofnanocrystallinematerials,inadditionhavebeen

    foundtopossessmagneticpropertieswhicharesimilarto,butdifferentfrom,thepropertiesoftheindividualconstit

    uents.Newmagneticphenomena,unusualpropertycombinations,andbothenhancedanddiminishedmagneticprop

    ertyvaluesarejustsomeofthechangesobservedinmagneticnanocompositesfromconventionalmagneticmaterials.

    Here,adescriptionwillbepresentedofsomeoftheexcitingnewpropertiesdiscoveredinnanomaterialsandthemag

    neticapplicationsenvisionedforthem.

Keywords:nanocomposite;ferromagnet;magnetoresistance;magneticrefrigeration;dom

    ainimaging

    Therearethreebasicreasonswhyonemight expecttoseechangesinpropertiesofmaterialsas somematerialdimensionisreducedtothenanometer leve1.(1)Quantumconfinement,wheretheenergy levelsoftheatom.electron,ornucleuschangeas thesizeoftheconfinedregionchanges;thewell

    knownparticleinaboxsituation.Sincethequan

    tizedenergylevels(E)varyinverselywiththesize ofthebox(L),smallchangesinLwhenthatsizeis smallamounttoverylargechangesintheenergy levels.SmallchangesinLwhenLislargedonotre

    sultinlargeshiftsintheenergylevels.Therefore anypropertywhichdependsonthevalueoftheen

    ergylevel,orthespacingbetweenenergylevels, willlikelychangewhenyouhaveamaterialstruc

    turewhichwillconfinethoseparticles,likeelectrons andatoms.(2)Highinterfacearea,wherethenum

    berofatomsataninterfacebecomesverylarge.For apolycrystallinematerial,whenthediameterofthe crystals(e.g.,grains)isontheorderofmicrons, thefractionofatomsattheinterfaceisonlyonthe orderofahalfpercent.Asthedimensionofthe grainisreduced.thefractionofatomsattheinter

    faceincreasesquickly.Forexample,Ref.E13shows thatatacrystalsizeof3nm,between40%and 60oftheatomsareattheinterface.Consequent

    ly,atlargegrainsizes,thepropertiesofthemateri

alwillbecloselyrelatedtothepropertiesoftheat

    omsinteriortothegrains.Conversely,atsmall graindiameters,thematerialpropertieswillbere

    latedtothepropertiesoftheinterfaceatoms.(3) Closenessofthemateriallengthstothecritical lengthscaleoftheproperty.Everypropertyhasa criticallengthscaleassociatedwiththatproperty (e.g.,themeanfreepathinelectricalandthermal conductivity,diffusionlengthinatomictransport, wavelengthinscatteringbehavior,penetrationdepth inabsorption,andhalflifeinradioactivedecay

    processes),andmaterialbehaviorwilltypically changewhenamateriallengthscalebecomescompa

    rablewiththatpropertylengthscale.Formagnet

    ism.Table1(fromJ.M.DCoey)givessomeof thesecriticallengthscalesandtheirvaluesfora common"soft"ferromagnetlikeFeanda"hard" ferromagnetlikeNd2Fel4B.Inthistable,Aisthe exchangeconstant,0isthepermeabilityoffree space,Jistheexchangeintegral,istheanisotro

    PY,kBistheBoltzmannconstant,andTisthetem

    perature.Notethatmostoftheselengthscalesare inthenanometerrange.

    Consequently,onewouldexpectthebulkmag

    neticcharacterofananostructuredferromagnetic Biography:RobertDShull(1946

    ),Male,Doctor,Engineer;E-mail:Shull@nist.gov;RevisedDate:November27,2006

?

    70?JournalofIronandSteelResearch,InternationalVo1.14

Table1Magneticlengthscalesnm

    materia1tochangefromthatofsuchamateria1with

    outthenanometersizedmateria1dimensions.The

    typesofmaterialdimensionswhichmightbeimpor

    tantwouldincludetheparticlesizeinaparticulate materia1,theseparationdistancebetweenthosepar

    ticles,thegrainsizeofamaterial,thelayerthick

    nessorseparationdistancebetween1ayersina1ay

    eredmateria1,thethicknessofasecondaryphaseat agrainboundary,thefiberdiameterandseparation distancebetweenfibersinafibrousmateria1,andthe surfaceirregularityatinterfaces.

    Forcompositematerialscontainingnanometer

    sizedspecies,oneofwhich(M)isferromagneticand oneofwhich(N)isnonmagneticmixedtogether,

    onecanevenplotamagneticphasediagramasshown inFig.1.NotethattheCuriepointdropsprecipi

    touslyatsomecompositionofN(typicallywhen percolationofthetwospeciesoccurs),andthatfor allgreaterconcentrationsofNthereisacompletely newmagneticstatecalled"superparamagnetism". Superparamagnetismrequiresnanometer——sizedferro——

    magneticentitiesmagneticallydecoupledfromeach other,aswouldoccurbyphysicallyseparatingthem viathenonmagneticspecies.Inthismagneticstate, thereisnopermanentmagneticmemoryafterapply

    ingamagneticfield,eventhoughitsmagneticSUS

    ceptibilitycanberatherlarge.Sincehighdensity magneticrecordingmediaisthistypeofcomposite structure,whentheferromagneticparticlesizegets

smallenough,thematerialwouldpossessthismag

    neticstateand1oseitsabilitytostoreanyinforma

    tion.Forotherapplications,likemagneticcooling, thistypeofmagneticstatecanactuallygiveanin

    creasedcoolingcapability.

    0neofthemainusesofaferromagnetisasthe coreinatransformer.Theareaenclosedinthehys

    teresis1oopforthatmateria1indicatestheworkre

    quiredtomakeonecompletecycleofthefield.Fora transformercore,thecurrentintheprimarycol1(l) isalternatingdirection,andthereforecreatinganal

    ternatingfielddirection.Consequently,themagneti

    zationofthetransformercoretracesoutahysteresis Magnetic

    specles

    (^

    0.30.50.7Non-magnetic

    Vo1.fract.Nspecies

    ()(,v)

    Fig.1Magneticphasediagram

    loopeachofthemanycyclesperseconddictatedby thefrequencyofil,andduringeachcycletheenergy indicatedbytheareaenclosedintheloopisconver

    tedintoheat.Thatheatmustsubsequentlybere

    moved.Consequently,onewantstouseaferromag

    netwhichhasvery1ittleareainsideitshysteresis 1oop,whichmeansitshouldhaveasmal1coercivity (Hc).Inordertohavea1owHcvalue,theferro

    magnetshouldbeabletoreverseitsmagnetization

easily.Theeasiestmechanismwherebythemagneti

    zationcanbereversedisbynucleatingasmal11oca1 region,calledadomain,havingthatreversedmag

    netizationandthenrotatingonlythemagneticspins attheboundaryof.thatdomainwhilethatboundary wal1movesthroughthemateria1.Therefore,the easeinreversingthetotalmagnetizationuponfield reversalisdependentuponhoweasilythedomain wallsaremoved.Ifsomethingstopsthemotionofa wall,itiscalledapinningsite,andveryeffective pinningsitesaregrainboundariesinconventional materials.Inordertoovercomethepinningforceof thosespecialsites,agreaterfieldisrequired,lead

    ingtoahighercoercivity.Sincegrainboundariesare goodpinningsites,ifthegrainsizeisreduced,the coercivityincreases(asshownatthelargegraindi

No.4NanocrystallineandNanocompositeMagneticMaterialsandTheirApplications

    ameterregioninFig.2[])duetothelargernumber ofpinningsites.However,whenthegrainsizebe

    comescomparabletothewidthofthedomainwall (e.g.,innanocrystaUinematerials)thenthegrain boundariesbecomelesseffectiveaspinningsites; furtherdecreaseingrainsizeresultsinalowering ofthecoercivitytosomeofthesmallestvaluesyet achieved.

    Anothermajorapplicationofferromagneticmateri

    alsisinmagneticrecordingdevices,bothasthemedia andinthereadandwriteheads.Forthemedia,one wantseachregionwhereininformationisstoredand

    retrievedtobeassmallaspossible.Atthesame timeeachregionofstoredinformationneedstobe decoupledfromneighboringregions.Thereisalow

    erlimit,however,onhowsmallonecanmakethese ferromagneticcells.Asdescribedabove,whenone decouplesnanometersizedferromagneticentities fromeachother.thematerialwillsoonerorlaterbe

    comeasuperparamagnet.Atthatpoint,thematerial wouldnotretainanymagneticinformationsinceit hasnoremanentmagnetizationwhentheexternal fieldisremoved.Consequently,thatsizerepresents alowerboundtothelateralsizeoftheferromagnetic regionsinthemedia.

    Thereisanotherproblemwithverysmallsize recording"bits".Asthesizeofthoseregionsdecrea

    ses,sodoesthemagnetizationofthoseregionsand alsothemagnitudeandextentofthemagneticfields emanatingfromthem.Therefore,oneneedsadevice sensitiveenoughtoreadthosesmallerfieldvalues. Luckily,in1988,therewasthediscoveryofanew nanomagneticphenomenacalledthe"giantmagne

    toresistanceeffect"(GMR)whereinastructurecon- 10

    

    O.O1

    O.oo1

    Grainsize

    F2CoercivityVsgrainsizeforseveralmaterials sistingoftwothinferromagnetic(FM)layerssepa

ratedbyaverythin(1nm)nonmagneticlayerwas

    foundtopossessverylargeresistancechangeswhen themagnetizationdirectionofthetwoferromagnetic layerschangedfrombeingantiparalleltoparallelc. Theeffectarisesfromspindependentscatteringof theelectrons,resultinginverylongmeanfreepaths foronlythoseelectronswiththesamespinstateas theferromagnetinwhichitismoving.Ifbothferro- magneticlayersaresimilarlymagnetized,thestruc

    tureallowselectronswithonestatetoshortcircuit thedevice,leadingtoasignificantlyreducedresist

    ance.Thesestructuresarenowusedassensitivede

    tectorsofmagneticfields,andoperateastheread heads(ina"spinvalve"configuration)onpresent daymagneticharddisks.

    Onecriticalelementinthespinvalvestructure istheuseofanother"nanomagnetic"effect:athin

    antiferromagnetic(AF)layerplacednexttooneof theGMRferromagneticlayersinordertopinthedi

    rectionofmagnetizationofthatFMtosomefixeddi

    rection.Suchabilayercoupleisknowntoshiftthe magnetichysteresisloopoftheFMalongthefield axis,therebygivingthematerialamagneticfieldbi

    as.calledan"exchangebias"(comingfromthemag

    neticexchangeinteractionbetweentheAFand FM[]).

    Forpermanentmagnets,reductionofsomema

    terialparametertothenanoscaleisalsobeingfound tobeimportant.Theideahereistoincreasetheen

ergyproduct(i.e.,theareaenclosedinthehystere

    sisloop)ofa"hard"ferromagnetbyreplacingsome ofitshighcoercivitymaterialwhichhasalowsatu

    rationmagnetization(Ms)byalowcoercivity"soft" ferromagnetwhichhasahighMsvalue.Suchare

    placementisexpectedtogivealargerenergyproduct sincetheexchangeinteractionbetweenthehardand softferromagnetwouldbeexpectedtobesostrong thatmagnetizationreversalwouldnotoccurbythe nucleationoflocalregionsofreversedmagnetiza

    tion,butinsteadviathecoherentrotationofthe magneticmoments,amuchmoredifficultprocess[.

    Ifthethicknessesoftheindividualspeciesareonthe orderofthedoraainwallwidthofthehardferromag- net(i.e.,inthenanometerrangeasshowninTable1), thisrotationwouldbecontinuous.

    Proofofthisreversalpredictionwasfoundby meansofusingthemagnetoopticindicatorfilm

    (MOW)techniquedevelopedatNIST['.Inthis technique,apolarizedlightbeampassesthroughan

?

    72?JournalofIronandSteelResearch,InternationalVo1.14 inplanemagnetizedthinfilmwithalargeFaraday effectplacedontopofthesample.Wherethereis fluxleakagefromthesample.likeatadomainwall andsampleedge,thepolarizationofthatlightbeam isrotateddifferently(sincethefilmmagnetization willbelocallyperpendicular),andtherebyproviding acontrasttoneighboringregionswhenviewed

    throughapolarizedanalyzingcrysta1.Whenapplied toobservingthemagnetizationreversalinahard/ softferromagnetbilayerthepicturesshowninFig.3 forfields(H)slightlyofftheeasymagnetization (M)axisofthematerialwereobtained.Inthisfig

    ure,sincethereisnodomainwallperpendiculartothe filmplane.noimageisobtainedofthatdomainwal1. Consequently,aholewasdrilledthroughthefilm, andthedirectionofMisobtainedfromtheblackline drawnthroughtheholeconnectingthepointsof maximumdarknessandbrightnessontheinsideedge oftheholewheremagneticpolesofoppositesign havebeencreatedbythemagnetizationvectorofthe surroundingregion.Notethatwhenthereversing fieldisalignedslightlyofftheeasymagnetizationax

    is,Mrotatesinproportiontothefieldmagnitude, eitherclockwise(CW)orcounterclockwise(CCW), dependinguponwhetherHisorientedcounterclock

    wiseorclockwisewithrespecttoM:thisprovesthe coherentrotationmode1.Higherenergyproductsthan thebestNd2Fe14Bpermanentmagnetshaveeven beenobtainedusingthismethodology['.

    10degreesCW10degreesCCW

    Fig.3MOIFimagesofaFe/SmCobilayerasa

    reversedHisappliedinplaneoffilm

    Afuturelargeapplicationenvisionedformagnetic materialsistheareaofrefrigeration[.

    Themainadvan

    tageofmagneticrefrigerationisthatitisbasedonare

    versibleprocessandtherebymakesCarnotefficiencies

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