意想不到的磁性纳米材料外文文献

意想不到的磁性纳米材料外文文献 ReviewUnexpected magism in nanomaterialsRanberSinghnInstitute ofPhysical Chemistryand Centerfor ComputationalSciences,Johannes GutenbergUniversity Mainz,Staudingerweg9,D-55128Mainz,Germanya rt ic le in foArticle history:Received9AprilxxReceived inrevised form24JunexxAvailable online9JulyxxKeywords:Nanothinf i lmNanowireNanoparticleSingle moleculemagMagismSuperparamagismDefectSpintronica bs tr a c tConventionalmagic order in a material requiresthe partially f illed dor fbands.The exchangeinteractions between the electrons in these partiallyf illed bands give rise to a magic order.However,the discoveryof unexpected magism observed in somenanomaterials,which havethe d and f shellseither pletely empty or full,has challengedour understandingof magism in conventionalmaterials.The magism in nanomaterialsshows theeffects of reduced dimensions,reducedcoordination of atoms at the surface and somequantum effectswhich dominateat low dimensions.In thisreview paperwe givea briefreview anddiscuss theunexpected magismexperimentallyobserved and/or theoreticallypredicted in nanomaterials of conventional magicand nonmagicbulk materials.&xxElsevier B.V.All rightsreserved.1.IntroductionThe nanostructured materials are the criticalbuilding blocksforthe fastemerging f i eldof nanoscienceand nanotechnology.The uniquestructural,physical,electronic and magic propertiesof nanostructured materialsobserved in experiments and/or pre-dicted theoreticallyhave attractedgreat interestin these materialsin lastcouple of decades16.The uniqueand unusualpropertiesof nanostructuredmaterials as pared to their correspondingbulk counterparts aremainly due to theemergence ofquantumeffects atnanoscale.The properties ofnanostructuredmaterialsare stronglyinf l uenced by the f i nitesize andmicrostructuraldetails ofboth coreand surface6,7.There arethree mainapproacheswhich areapplied tocreate newnanomaterials: (1)top-down, (2)bottom-up,and (3)virtual approach.Top-down approachhas beenthe traditionalapproach for minia-turization vialithographic tools.Bottom-up approachis theself-assembly frommolecular-precursor buildingblocks in the chemi-cal solutions.Virtual approachtechnique is the oneused by theputational theoristswho createnew materials in thepu-ter simulations.The experimentaltechniques for the growth ofultrathin f i lms1,2and for the selectiveproduction andanalysisof atomianoclusters36have improveddramatically overthelast coupleofdecades.This advancementin growth techniquesand measurementtools has lead to the enormousadvancement inunderstanding the uniqueproperties of nanomaterials and theirpotential uses in theconstruction ofnanodevices.It is nowpossible to grow artif i cialmaterials witha controlledprecisionconsisting of alternating verythin crystallinelayers of differentmaterials.The propertiesof these materials aredetermined by thestructure of interfaces and the interactionsof atoms at theinterfaces.The unexpected magism appearsin somematerials whenone or moredimensions arereduced.The reduction in oneormore dimensionstypically resultsin thereduction ofcoordinationnumber of atoms whichreduce thehoping tendencyof electronsfromsite tosite.Thus,the kiic energy(bandwidth)of electronsisreduced andCoulomb interactions/bandwidth ratiois enhanced,which enhancesthe tendencytowards theappearance ofmag-ism inmaterials withreduced dimensions.The appearanceofsurface andinterface states due to the reducedsymmetry andchangedboundary conditionsplays an important role in inducingthemagism in the materials ofreduced dimensions.Thelocalized defectstatesdue to dopantsand/or vacanciesalso playan important role in localizingthe electronswhich inducecollec-tive ferromagism.In thispaper,we mainlyfocus on theunexpected magism in the materials when the reduceddimen-sions arein nanometer scale.With thereduction inoneormoredimensions to the nanometerscale the materials startsto displayquantif i ableproperties due to theemergence ofquantum effects,which inturn play an importantrolein inducing the ferromagicstate in thesematerials.For instance,conventional bulkmagicmaterials have different magic properties in the nanometerscaledue to the reducedcoordination of atom atthe surfaceandconf inement quantumeffects.Magism in nanomaterials(also known as nanomagism)is one of thescientif ic disciplinesattheforefront offast emergingf i eldsof nanoscienceand nanotechnology.Recent progressin theContentslists availableat SciVerseScienceDirectjournal homepage:.elsevier./locate/jmmmJournal ofMagism andMagic Materials0304-8853/$-see frontmatter&xxElsevier B.V.All rightsreserved.dx.doi/10.1016/j.jmmm.xx.07.005n Tel.:+4961313924720.E-mail address:ranber14yahoo.Journal ofMagism andMagic Materials346 (xx)5873growthtechniquesofnanomaterialsand observationtools formagismhas createdenormous interestin magianostruc-tures for fundamental explorationsas wellas variouspotentialapplications.The nanoparticles of the conventional magicmaterialshavedifferentmagic moments per atomand mag-ic anisotropyas pared to their bulkcounterparts35.The size ofamagic nanoparticleis sosmall thatit maycontainonly onedomain insteadof largenumber ofdomains in aconventional bulkspecimen.Because ofthis singledomain amagianoparticle canhave superparamagism810,where the spin ofwhole nanoparticlereverses underan externalperturbationf i eld.The heterostructuremagic nanothinf i lmsand nanowires havegiant magicresistance(GMR)1113andlarge magocaloriceffect14.The discoveryof GMR11,12(which hasbeen rewardedwithxxNobel Prizeto AlbertFertand PeterGrnberg,its discoverers)hasleadto thehighly densemagicinformation devices.The f i nitemagic momentsin somenanomaterials ofother-wise nonmagicbulk materialshave also been observedinexperiments1519and theoreticalcalculations2028.The unexpectedmagism observedin unconventionalmaterials,which havedandfshellspletelyemptyor pletelyfull,hasbeen attributedto the reduced dimensions,disorder anddefects29,30.The magic propertiesofnanostructuredmaterialsarealso affectedby the chemical position,shape,size andlocaldetails atthe coreand surfaceof thesample,and interactionswithsurrounding environment35,31.Magic nanomaterialsarethe idealcandidates formaking the spintronic devicesandrealization ofspin-qubits forquantum informationprocessing.Spintronic devicesare speculatedto usemagic momentorspin of electrons in addition to their chargeand wouldbe moreeff i cientfor sendingand storinginformation32,33.In thispaperwe brief l yreview anddiscuss theunexpected magismobservedexperimentally and/or predictedtheoretically in nanoma-terials ofconventional magicand nonmagicbulk materials.2.Theoretical understandingof magismBeforediscussing theunexpectedmagisminnanomaterials,we givea brief introduction to the theoreticalunderstanding ofmagismin differentmaterials.Magism isone of the oldestandmost fundamentalscientif ic problemsnot fullyunderstood tilldate.Aording toWebers classicaltheory of magism allsubstancesare posedof tinymolecular mags,which alignincertain directionsdepending upon the chemicalenvironment.In aferromagic materialmost of its molecularmags linedupso that they effectivelypoint inone direction.Aording to thedomain theoryof magismthere existmagic domainsin aferromagicmaterial.A magicdomain is a regionwithin amagicmaterial,which hasuniform magization.The regionsseparatingmagic domainsare called domain walls,where themagizationrotates smoothlyfrom thedirection inone domaintothat inthe nextdomain.The magicdomains have beenobserved inreal experimentalsamples34,35.Above Curietemperature,a pieceof ferromagicmaterial undergoesa phasetransitionto paramagicphase andthe uniformmagizationwithin adomain spontaneouslydisappears.Magic domainstructureis responsible forthe magic behaviorof ferromag-ic materialsof Fe,Ni,Co andtheir alloys,ferrites,etc.Modern theoriesof magismare based on the electron spin.The universalcriterion for ferromagism isvery diff i culttoformulate becauseof theplexity of exchange interactionsandthe existenceof hugevariety of the magicsubstancesand structures.The microscopicorigin offerromagism is thestrong electronelectron interactions.The electronswith parallelspinstend to avoid each other spatiallydue to the Paulis exclusionprinciple.This gives rise to the exchange interactions whichinturn leadto the magic phenomenaaording to the quantumtheoryof magism.The originofmagismlies inthe orbitalandspin motionsof electrons and howthe electrons interact withoneanother.The magicallyactive electronswhich formthemagic momentcan belocalized oritinerant collectively.The magisminnanomaterialsin practicealso dependon thedetailedenvironment andadditional interactionssuch asspinorbit,screening effectsand interactionswith ligandsattached tothe surface.Ising model36is thesimplest semi-classical modeltodescribe themagism asa cooperativeeffect.It is given asH?ij?I ijS iS j;?1?where?ij?indicates thesum overthe nearestneighbours.I ijis thespinspin interactionparameter.Latter worksof Heisenberg,Dirac,van Vleckand othersbased upon the quantumtheory haveattributedthe ferromagism to the alignment ofelectron spinsdue to exchangeforces.Spin,the intrinsicangular momentum,ofelectrons in atoms in a solidgive rise tothefinite magicmomentdepending uponthechemicalenvironment ofindividualatoms.If there isnocoupling between the individualatomic ormolecularmagic moments,thematerialis paramagic.However,if there is coupling between individualatomic ormolecularmagic momentsthematerialhas ferromagic orantiferromagic order.The coupling,which ispurely quantummechanicalin natureand isknown as exchange interaction,is duetothe overlap of electronic orbitals of different atomsandthe Paulis exclusionprinciple.In conventional magic materialsthe electrons inunf illed dor fbands areresponsibleforthemagic order.The exchange interactions between the electronsinthese partiallyfilled dorfbandsgive rise tothe magic orderof electronic spins.There arevarious models of exchange interac-tions proposedin differentmaterials whichgiverise toamagicorder in a material37,namely:2.1.Direct exchange interactionIt ourswhen there is adirect overlapbetweenthe localizedorbitals of electrons on adjacent magic ion sites.The simplestmodelfor thiskind ofinteractions is the Heisenbergmodel ofexchange interactions givenas38H ex?JijS i!?S j!;?2?where J is thestrength ofexchange couplingbetween S i andS jspins of electronsat sites i and j,respectively.The graphicalrepresentation(also known as Bethe-Slater curve39)of J asa function of interatomic distance and radius ofpartiallyfi lledd-shell ofan atomin differentmaterials isshown inFig.1(a).When theinteratomic distance is small the electronsspent mostofthe timein betweenthe atomsand giverise tothe antiferromag-icorder due tothe Paulis exclusionprinciple.However,whenthe interatomicdistanceislarge,theelectronsspent mostof thetimeaway fromeachotherand giverisetothe ferromagicorder.In realmaterials thedirect exchangeis drivenby minimiz-ingthepotential energyduetoreduced overlapof electronwavefunctions.Heisenberg typemodel isnot areasonable work-able modelat allinternuclear separations.Therefore,there areothermodels whichhave beenformulated toaount forthemagic statesobservedinthe materials where theinternuclearseparations areparatively large.R.Singh/Journal ofMagism andMagic Materials346 (xx)5873592.2.Superexchange interactionIt is an indirect exchange interaction40,41between thelocalizedelectrons onmagic ionsseparated by the nonmag-icion or ligand.The overlapof orbitalsof the localizedelectrons onthe magic sites ismediated through their monnonmagic ion orligand.It isgiven asHex?ij2t2ijUS i!?S j!;?3?where tij and U arethe hoppingenergy andCoulomb repulsionbetweensites iandj,respectively.The strength of couplingisdetermined bythe amountof hoppingenergy andCoulombrepulsion.The superexchange interaction inMnO isshown sche-matically inFig.1(b).2.3.Double exchangeinteractionThis kind ofexchangeinteraction issimilar tothe super-exchangeinteraction.In superexchangeinteraction there is nomovement ofelectronsfrom one magicsiteto othermagicsite because the oxidation states of magic ions are same ordiffer bytwo.However,in double exchangeinteraction there ismovementofelectronsfrom onemagic iontotheothermagic ionbecause oxidation states oftwo nearestneighbourmagic ions differs by onesee Fig.1(c).Double exchangeinteraction is also knownasZener exchangeinteraction42.It isbasedonthe assumption that intraatomicexchangeinterac-tionsbetweenlocalized spin and delocalizedspins arevery strongwhichaligns thespinsofdelocalized electronsalways paralleltothe localized ion spin.It isdescribed bythe Hamiltonian43givenasH ex?ijst ija?isajs?J HiS i!?si!;?4?where thefirst termdescribes themotion ofan electronover thelatticesitesi,j with spin s,andthesecond termaounts fortheHunds exchangecoupling.J His theHundsexchangecouplingparameter,S iis thelocalizedion spinandsiis thedelocalizedelectron spingiven assi?12ss?a?is P ss?a is?;?5?where a?is?a is?isthecreation(annihilation)operator andPs?s isavector ofPauli spinmatrices.This modelof doubleexchangeinteractions suessfullydescribed theferromagic phaseofmetallic manganites.It doesnot takeinto aountitinerantcharacter ofelectronsandFriedel oscillationsof theelectronpolarization aroundthelocalizedspin.These effectsare explicitlytaken into aountin RudermanKittelKasuyaYosida(RKKY)exchangeinteractionmodel.2.4.RKKY exchangeinteractionThe RKKYexchangeinteraction4446isthelong rangeexchangecouplingbetweenthelocalizedmagic momentsFig.1.Schematic illustrationofdifferentmodelsofexchange interactions.(a)Graphical representationof thestrengthofdirect exchangecoupling asa functionofinteratomic distance(r ab)andradius(rd)of theunf illed d-shell ofan atomina material.It isalsoknownas Bethe-Slater curve.(b)Superexchange interactions in MnOsystem.In thiskindofinteractiontheoxidationstatesof magic ionsaresameordiffer bytwo.(c)Double exchange interactions inMnO system.In thiskind interactiontheoxidationstatesof magicionsdifferbyone.In thesuperexchange anddoubleexchangeinteractions theoverlapoflocalized electronicorbitals onmagicion sites ismediatedthroughtheirmon nonmagicionorligand(O ion in caseof MnO).In doubleexchangeinteractiontheelectronsmove fromonemagicionsiteto theother,whereas in superexchangeinteractionthereisno suchmovementofelectrons.(d)The RKKYexchange parameterJasa functionof interatomicdistance ofmagicatoms.The magic order isferromagic or antiferromagic depending upontheinteratomicdistance between magicions.(e)Schematic bandstructure ofStonermodel.The exchangeinteractions havesplit theelectronic densityof states(EDOS)withspinup andspin downstates inamagicmetal.At Fermilevel theEDOS arespinpolarized whichgivesrisetotheferromagism inamaterial.R.Singh/Journal ofMagism andMagic Materials346 (xx)587360mediated bythe conduction electronsinmetals.It isgiven asHex?ijJ2k Fcos?2k F?sin?2k F?2k?4Si!?S j!;?6?where?r ab?isthedistancebetweenthe interactingmagic ionsaand b,Jisthe interactionparameter which depends ontheeffective massof theconductionelectrons,and kF isthe Fermiwavevector of theelectrongas.It isthe dominantexchangeinteraction inmetals wherethereislittle orno directoverlapbetween theelectronicorbitalsof neighboringmagicions.A localizedionspinpolarizes thespin ofsurrounding conductionelectronswhich actas aneffective fi eldto influencethe polariza-tionofnearby localizedion spinswith thepolarization decayinginan oscillatorymannersee Fig.1(d).This oscillationmediateseither ferromagicorantiferromagicexchange couplingdependingupontheseparation betweenthe magicions.Thiskind ofexchangeinteractionsbees effi cientwhenthenumberof conductionelectrons islarge.It suessfullydescribed theGMRin magichetrostructures47.2.5.Stoner modelTheparallel alignmentoftheelectronicspinsleads toa gainofexchange energy.However,thealignmentalso causesa lossofkiic energy.Most ofthe solidstate systemsare non-magic,since thegain inexchange energyis dominatedbytheloss inkiicenergy.Aording toband theoryof solidsthe onsetofferromagism inamaterialis determinedbytheso-calledStoner criterionsee Fig.1(e),which isassociated witha peti-tion betweenexchange energyand kiicenergy effects48.None ofthesp-bonded elementsare magicin theirbulkcrystalline phasebecausethekiicenergydominates theexchangeenergy inthesematerials.The tendencytowards mag-isminnanomaterials ofmagicornonmagic materialsisconsiderably enhancedduetothereduceddimensions,defectsand reducedcoordination of atoms atthe surfaceor interface17,18,47,4959.There aresome modelHamiltonians developedtounderstand themagisminconventional delectron systemsandunconventional spelectron systems.The simplestmodel isthemean fi eldHubbard model21which isdescribed bytheHamiltonian(H mfi eld)given asH mfield?Ui?ni?ni?ni?ni?ni?ni?:?7?The resultingmagnitude ofmagic moment(M)inasystemdescribed byHmfieldisgivenasM?ij?ni?ni?j2;?8?where nis isthespinpolarized densityat sitei withspin sandUisthe magnitudeof on-site Coulombrepulsion.Conventionally thepartiallyfilleddorfstates giverisetononzero magic momentin amaterial.However,a newferromagic statehas beenobservedin somematerialswhered-orf-states haveno con-tribution in inducing themagic momentrather it is inducedbythe s-or p-states.This newstate ofmagism isknownasd0magismtoindicate thatthemagic moments resideins-orp-states.3.Magism innanomaterialsMagic materialsare usedin largenumber ofdaily lifeapplicationssuch asin switches,televisions,audio devices,motors,credit/debit/ATM cards,magic resonanceimaging(MRI)equipments,puter hard drives,etc.Some of theseapplications suchas puterharddriveshave leadto theminiaturizationof singlemagic unitsto increasethe densityofdata storageinagiven space.The magismin nanomaterialscanbe manipulatedvia size and growingthe artifi cialstructureswhich donot existin nature.The nanomaterialsare broadlyputinto threecategorie