Quantitative Magnetic Analysis of...

Quantitative Magnetic Quantitative Magnetic ggAnalysis of NanostructuresAnalysis of Nanostructures

M AngelakerisM AngelakerisM. Angelakeris, M. Angelakeris, C. Boubeta, K. Simeonidis, A. DelimitisC. Boubeta, K. Simeonidis, A. Delimitis

Department of PhysicsDepartment of PhysicsAristotle University of ThessalonikiAristotle University of Thessaloniki--GreeceGreece

I i l S S h l & 4I i l S S h l & 4thth W k hW k hInternational Summer School & 4International Summer School & 4thth Workshop onWorkshop onSynthesis and Orbital Magnetism of coreSynthesis and Orbital Magnetism of core--shell nanoparticlesshell nanoparticles

26/9 26/9 –– 1/10/2006, Thessaloniki, Greece1/10/2006, Thessaloniki, Greece

OutlineOutline

Synthesis & StructureSynthesis & StructureMagnetics & AnalysisMagnetics & AnalysisConclusionsConclusionsConclusionsConclusions

SYNTORBMAG 26/9 SYNTORBMAG 26/9 –– 1/10/2006, Thessaloniki, Greece1/10/2006, Thessaloniki, Greece

IntroductionIntroduction

IronIron oxideoxide basedbased nanoparticles,nanoparticles, ee..gg.. magnetitemagnetite orormaghemitemaghemite areare commonlycommonly usedused asas thethe magneticmagneticmaghemite,maghemite, areare commonlycommonly usedused asas thethe magneticmagneticcomponentcomponent ofof commerciallycommercially availableavailable productsproducts forforbiomedicalbiomedical applicationsapplicationsbiomedicalbiomedical applicationsapplications..InIn thisthis talktalk wewe willwill considerconsider experimentalexperimental resultsresultsonon thethe effecteffect ofof microstructuremicrostructure (particle(particle size,size,structuralstructural order,order, internalinternal inhomogeneities,inhomogeneities, surfacesurfaceggroughness)roughness) andand surfacesurface coatingcoating (interparticle(interparticlespacingspacing andand surfacesurface modification)modification) inin thethe overalloverallspacingspacing andand surfacesurface modification)modification) inin thethe overalloverallmagneticmagnetic behaviorbehavior ofof nanoparticlesnanoparticles..


SynthesisSynthesisTh l d iti f th i t b lTh l d iti f th i t b lThermal decomposition of the iron pentacarbonylThermal decomposition of the iron pentacarbonyl

oleic acidDioctyl ether

10 ml +oleic acid

1.28 g

surfactant

100 oC

solvent

F (CO)

surfactant

Fe(CO)5

0.2 ml

organometallic

reflux 280 oCFe(CO)5 : oleic acid

1:4 → 13 nm

1 2 8organometallic

Fe precursor60 min1:2 → 8nm

1:1 → 5nm

colorchange


Monodisperse Fe nanoparticles

SynthesisSynthesisV i tiV i ti ii th ith i dd dd NPNP ithithVariationVariation inin synthesissynthesis procedureprocedure engenderengender NPsNPs withwith averageaveragediameterdiameter rangingranging fromfrom 55 nmnm toto 1313 nmnm..TheThe particleparticle sizesize waswas mostmost sensitivesensitive toto thethe surfactantsurfactant toto ironironTheThe particleparticle sizesize waswas mostmost sensitivesensitive toto thethe surfactantsurfactant toto ironironmolarmolar ratioratio..TheThe timetime ofof refluxreflux waswas alsoalso aa determiningdetermining factorfactor inin uniformityuniformity ofofTheThe timetime ofof refluxreflux waswas alsoalso aa determiningdetermining factorfactor inin uniformityuniformity ofofparticleparticle shapeshape andand sizesize monodispersitymonodispersity..TheThe synthesissynthesis methodmethod selectedselected itselfitself facilitatesfacilitates precipitationprecipitation ofofyy p pp pveryvery uniformuniform nanoparticlesnanoparticles withwith veryvery lowlow polydispersitypolydispersity..

5 nm5 nm 8 nm8 nm 13 nm13 nm


SynthesisSynthesis 60 5nm 8nm 13nm

ØØ(nm)(nm)Assuming aAssuming a

40P (%

)

0808±±0 40 41313±±0.40.4

(nm)(nm)Assuming a Assuming a spherical shape of spherical shape of the particles we the particles we

20

0505±±0.30.30808±±0.40.4pp

may estimate the may estimate the mean volume mean volume <V><V>

d ld l2 4 6 8 10 12 14 16

0

Diameter (nm)<V><V>

and later on and later on approximate approximate KK ≈≈ 2525 kk TT //<V><V> Diameter (nm)KKeffeff ≈≈ 25 25 kkBBTTBB//<V><V>MMss==μ/μ/<<V>V>

5 nm5 nm 8 nm8 nm 13 nm13 nm


SynthesisSynthesisOxidationOxidation ofof FeFe startsstarts takingtaking placeplace duringduring synthesissynthesis duedue totothethe presencepresence ofof anan oxygenoxygen--containingcontaining ligandligand likelike oleicoleicacidacid..SuchSuch mechanismmechanism leadsleads toto surfacesurface oxidizedoxidized nanoparticlesnanoparticlesppatat aa variedvaried shellshell depthdepth dependingdepending onon refluxreflux temperaturetemperatureandand durationduration..Additionally,Additionally, tthehe useuse ofof aa solventsolvent suchsuch asas octyloctyl etherether withwithaa highhigh boilingboiling pointpoint ((287287ooC)C) allowsallows thethe rearrangementrearrangement ofofgg gg pp (( )) ggthethe atomsatoms withinwithin thethe nanocrystalnanocrystal duringduring thethe synthesis,synthesis,givinggiving riserise toto highlyhighly crystallinecrystalline nanoparticlesnanoparticles..g gg g g yg y yy ppTheThe initiallyinitially formedformed oxidesoxides havehave aa largelarge Fe/OFe/O ratioratiocomparingcomparing toto maghemitemaghemite andand magnetitemagnetite..


comparingcomparing toto maghemitemaghemite andand magnetitemagnetite..

SynthesisSynthesisOxidationOxidation mainlymainly occursoccurs afterafter exposureexposure inin atmosphereatmosphere followingfollowingOxidationOxidation mainlymainly occursoccurs afterafter exposureexposure inin atmosphereatmosphere followingfollowingthisthis routeroute:: anan instantinstant totaltotal oxidationoxidation ofof thethe nanoparticlenanoparticle andand aaveryvery slowslow oxidationoxidation ofof thethe formedformed oxidesoxides toto maghemitemaghemite (after(after

800

veryvery slowslow oxidationoxidation ofof thethe formedformed oxidesoxides toto maghemitemaghemite (after(aftersomesome months)months) asas shownshown byby XRDXRD experimentsexperiments..Finally, the oleic Finally, the oleic

maghemite magnetie

acid (used as a acid (used as a surfactant) surfactant)

6002 months

FeOps

)seems to be seems to be strongly bound strongly bound to particleto particle

400

5 days

2 hr

I (cpto particle to particle

surface leading surface leading to magneticto magnetic

1 hr

to magnetic to magnetic isolation (~3nm) isolation (~3nm) and further oxide and further oxide

SYNTORBMAG 26/9 SYNTORBMAG 26/9 –– 1/10/2006, Thessaloniki, Greece1/10/2006, Thessaloniki, Greece40 60 80

200

2θisolation. isolation.

P blP bl P iP i d t i tid t i ti ff hhStructureStructure

ProblemProblem:: PrecisePrecise determinationdetermination ofof phasephaseNPsNPs consistconsist ofof aa mixturemixture ofof γγ--FeFe22OO33 andand FeFe33OO44 oxidesoxides whichwhichhavehave nearlynearly thethe samesame spinelspinel crystalcrystal structurestructure withwith ~~ 11%%differencedifference inin thethe cubiccubic latticelattice constantconstant.. TheThe typetype ofof thethe oxideoxidethatthat isis formedformed isis anan importantimportant issueissue forfor thethe macroscopicmacroscopicthatthat isis formedformed isis anan importantimportant issueissue forfor thethe macroscopicmacroscopicmagneticmagnetic behaviorbehavior ofof thethe samplessamples becausebecause::

Magnetite is Magnetite is ferrimagnetic below 860 ferrimagnetic below 860 K with a saturationK with a saturation

In maghemite the net In maghemite the net magnetic moment magnetic moment originates from Feoriginates from Fe3+3+ ionsionsK, with a saturation K, with a saturation

magnetization value of magnetization value of ~~90 emu/g90 emu/g, the net , the net

originates from Feoriginates from Fe33 ions, ions, with a magnetic orderwith a magnetic order--disorder transition disorder transition 90 e u/g90 e u/g, t e et, t e et

magnetization reflecting magnetization reflecting the Fethe Fe2+2+ ions in the ions in the

temperature close to 1020 temperature close to 1020 K and a bulk magnetization K and a bulk magnetization ff 80 /80 /


octahedral sites. octahedral sites. of ~of ~80 emu/g80 emu/g..

PrecisePrecise determinationdetermination ofof phasephase:: StepStep 11StructureStructure

PrecisePrecise determinationdetermination ofof phasephase:: StepStep 11SelectedSelected AreaArea DiffractionDiffraction (SAD)(SAD) PatternsPatterns indicateindicate randomlyrandomlyorientedoriented singlesingle crystalscrystals DetailedDetailed measurementsmeasurements ofof thetheorientedoriented singlesingle crystalscrystals.. DetailedDetailed measurementsmeasurements ofof thetheobservedobserved latticelattice spacingspacing revealreveal thatthat thethe majoritymajority ofof particlesparticlesareare crystallizedcrystallized inin thethe magnetitemagnetite phasephase..areare crystallizedcrystallized inin thethe magnetitemagnetite phasephase..

SAD patterns SAD patterns provide only an provide only an indication of theindication of theindication of the indication of the coco--existence of existence of the oxide the oxide phasesphasesphases phases

HoweverHowever inin somesome casescases individualindividualHowever,However, inin somesome cases,cases, individualindividualreflectionsreflections byby bothboth phasesphases couldcould bebeeasilyeasily revealedrevealed..


yySAD pattern for the 13SAD pattern for the 13--nm NPsnm NPs


PrecisePrecise determinationdetermination ofof phasephase:: StepStep 22PrecisePrecise determinationdetermination ofof phasephase:: StepStep 22MossbauerMossbauer spectroscopyspectroscopy –– OO.. KalogirouKalogirouXX--rayray absorptionabsorption nearnear--edgeedge structurestructure (XANES)(XANES) –– HH WendeWende

PrecisePrecise determinationdetermination ofof phasephase:: StepStep 22

XX rayray absorptionabsorption nearnear edgeedge structurestructure (XANES)(XANES) HH.. WendeWendeHighHigh ResolutionResolution TransmissionTransmission ElectronElectron MicroscopyMicroscopy (HRTEM)(HRTEM)



PrecisePrecise determinationdetermination ofof phasephase:: StepStep 22PrecisePrecise determinationdetermination ofof phasephase:: StepStep 22EachEach individualindividual nanoparticlenanoparticle isis eithereither γγ--FeFe22OO33 oror FeFe33OO44..TheThe magnetitemagnetite particleparticle exhibitsexhibits itsits [[110110]] projectionprojection withwith ((22--2020))

PrecisePrecise determinationdetermination ofof phasephase:: StepStep 22

TheThe magnetitemagnetite particleparticle exhibitsexhibits itsits [[110110]] projection,projection, withwith ((22--2020))crystalcrystal planesplanes predominatelypredominately revealedrevealed;; highlyhighly facetedfaceted atat thethe{{111111}} andand {{100100}} planesplanes inin thethe formform ofof truncatedtruncated octahedronsoctahedrons..{{ }} {{ }} ppTheThe mainmain latticelattice planesplanes revealedrevealed forfor thethe maghemitemaghemite particlesparticlesareare ((11--1010)) andand ((1111--11),), hencehence thethe correspondingcorresponding zonezone axisaxis isis thethe[[112112]].. 1313 nmnm

In some cases, the {110} In some cases, the {110} γγ–– FeFe22OO33 lattice fringes were lattice fringes were

resolved which are only present resolved which are only present in maghemite due to in maghemite due to

the different space group the different space group of the two phases. of the two phases.


o e o p aseso e o p ases

StructureStructurePrecisePrecise determinationdetermination ofof phasephase:: StepStep 33

TheThe preciseprecise determinationdetermination ofof thethe crystalcrystal phasephase andand theirtheir relativerelativepercentagepercentage werewere performedperformed byby meansmeans ofof thethe FourierFourier TransformTransform

PrecisePrecise determinationdetermination ofof phasephase:: StepStep 33percentagepercentage werewere performedperformed byby meansmeans ofof thethe FourierFourier TransformTransform(FT)(FT) methodmethod inin eacheach individualindividual nanoparticle,nanoparticle, imagedimaged inin HRTEMHRTEMconditionsconditions..InIn moremore detail,detail, thethe TEMTEM negativesnegatives –– wherewhere anan adequateadequate numbernumberofof nanoparticlesnanoparticles waswas capturedcaptured-- werewere initiallyinitially scannedscanned inin aa highhigh--resolutionresolution scannerscanner and,and, consequently,consequently, thethe FTFT waswas obtainedobtained fromfromeacheach individualindividual nanoparticlenanoparticle (for(for moremore thanthan 100100 nanoparticles)nanoparticles)..

HRTEM images of two nanoparticles in the 13 nm sample, along with their corresponding FTs. The

lattice fringes resolved in the two particles correspond to the [112] projection of magnetite

(left) and the [012] projection of maghemite (right).


( ) [ ] p j g ( g )

John Tsiaoussis on Friday

StructureStructurePrecise determination of phase:Precise determination of phase: ConclusionConclusionConsequently,Consequently, wewe werewere ableable toto distinguishdistinguish thethe phasephase (Fe(Fe33OO44 oror

Precise determination of phase:Precise determination of phase: ConclusionConclusion

γγ–– FeFe22OO33)) eacheach individualindividual nanoparticlenanoparticle comprisedcomprised ofof andand theirtheirrelativerelative percentagepercentage..

(γ(γ–– FeFe22OO33))11--xx(Fe(Fe33OO44))xx

Ø (nm)Ø (nm) xx1313 0 70 7±±5%5%(γ(γ 22 33))11--xx(( 33 44))xx 1313 0.70.7±±5%5%

55 0.30.3±±5%5%

TheThe structurestructure andand oxidationoxidation statestate ofof thethe oxideoxide dependdepend ononparticleparticle sizesize SmallerSmaller particlesparticles areare richerricher onon maghemitemaghemite whilewhileparticleparticle sizesize.. SmallerSmaller particlesparticles areare richerricher onon maghemite,maghemite, whilewhileincreasingincreasing corecore meanmean size,size, thethe outerouter shellshell becomesbecomes mainlymainlycomposedcomposed ofof magnetitemagnetite..


pp gg

MagneticsMagneticsCritical TemperaturesCritical TemperaturesCritical TemperaturesCritical Temperatures

TheThe blockingblocking temperaturetemperature isis aa parameterparameter thatthat stronglystronglydependsdepends onon particleparticle sizesizedependsdepends onon particleparticle sizesize..Above,Above, blockingblocking temperature,temperature, typicaltypical characteristicscharacteristics ofofsuperparamagneticsuperparamagnetic behaviorbehavior areare observedobserved showingshowing almostalmostsuperparamagneticsuperparamagnetic behaviorbehavior areare observedobserved showingshowing almostalmostimmeasurableimmeasurable coercivitycoercivity andand remanence,remanence, whilewhile susceptibilitysusceptibilitygenerallygenerally increasesincreases withwith particleparticle sizesize..g yg y ppBelowBelow blockingblocking temperature,temperature, magnetizationmagnetization hashas aa hysteretichystereticfeaturefeature..T<TT<TBB:: AsAs thethe temperaturetemperature increases,increases, thermalthermal energyenergy increasesincreasesandand thethe nanoparticlesnanoparticles becomebecome unpinnedunpinned andand alignalign withwith thethepp pp ggappliedapplied fieldfield increasingincreasing thethe sample’ssample’s netnet magnetizationmagnetization..TT≥≥TTBB:: AtAt TTBB superparamagneticsuperparamagnetic particlesparticles becomebecome thermallythermally


BB B,B, p p gp p g pp yyunstableunstable andand thethe magnetizationmagnetization decreasesdecreases..

Magnetics Magnetics –– Blocking TemperatureBlocking Temperature

1.0

FCZFCZFC--FC curves, FC curves,

100 Oe,100 Oe,

.u.) 160 K

80 K19 K

100 Oe, 100 Oe, 1010→→300 K.300 K.

13 nm0.5

ZFC

M (a

.

8 nm

13 nm

ZFC

5 nm

0 50 100 150 200 250 3000.0

T(K)

ZFCZFC curvescurves exhibitexhibit aa typicaltypical blockingblocking processprocess ofof anan assemblyassembly ofofsuperparamagneticsuperparamagnetic nanoparticlesnanoparticles withwith aa distributiondistribution ofof blockingblocking

T(K)


superparamagneticsuperparamagnetic nanoparticlesnanoparticles withwith aa distributiondistribution ofof blockingblockingtemperaturestemperatures Kostas Simeonidis on Friday

Magnetics Magnetics –– Critical TemperaturesCritical TemperaturesTheThe temperaturetemperature dependencedependence ofof thethe saturationsaturation magnetizationmagnetization followsfollows aaTheThe temperaturetemperature dependencedependence ofof thethe saturationsaturation magnetizationmagnetization followsfollows aamonotonicmonotonic decreasedecrease differentdifferent fromfrom Bloch’sBloch’s TT33//22 lawlaw..M(T)M(T) isis wellwell describeddescribed byby aa powerpower lawlaw:: M=MM=M00((11--bTbTaa)) withwith aa BlochBloch exponentexponentaa 22a~a~22..Good agreement with theoretical predictions by Good agreement with theoretical predictions by HendriksenHendriksen et al. J. Phys.: et al. J. Phys.: yyCondens.MatterCondens.Matter 55, 5675 (1993), 5675 (1993)

at the nanometer scaleat the nanometer scale s (a

.u.)

13 nm

at the nanometer scale, at the nanometer scale, due to finitedue to finite--size effects, size effects, where the spin wave spectrum where the spin wave spectrum

3.6x10-3

5 nm

8 nm

Ms

is modified and the magnetization is modified and the magnetization is better accounted by a Tis better accounted by a Taa powerpowerlaw with a Bloch exponent a~2law with a Bloch exponent a~2

5 nm

law, with a Bloch exponent a 2.law, with a Bloch exponent a 2.

0.05.0x10-41.0x10-3

770 K820 K

540 K


0 50 100 150 200 250 300 600 8000.0

Magnetics Magnetics –– Critical TemperaturesCritical TemperaturesM ti tiM ti ti t tt t i ti t tt diff tdiff t li dli d fi ldfi ld hhMagnetizationMagnetization vsvs temperaturetemperature experimentsexperiments atat differentdifferent appliedapplied fieldsfields showshowthethe temperaturetemperature TTss,, atat whichwhich thethe magnetizationmagnetization deviatesdeviates fromfrom previouslypreviouslyexaminedexamined TT22 lawlaw increasesincreases fromfrom 3030 KK toto (measured(measured atat 2020 kOekOe)) toto 5050 KK

(measured(measured atat 1010 kOekOe)) andand extrapolatesextrapolates toto ~~6565 KK forfor coolingcooling atat lowlow fieldsfields..

4 0x10-3

13 nmWithWith decreasingdecreasing temperaturetemperature

s (a

.u.)

13 nm

4.0x10

s (a

.u.)

13 nm

kOe

kO

e

0 O

e

aa progressiveprogressive freezingfreezing ofof thethemomentsmoments ofof thethe surfacesurface layerlayeroccursoccurs accordingaccording toto thethe

3.6x10-3

5 nm

8 nm

Ms

3.8x10-3

8 nm

Ms

20

10 100gg

distributiondistribution ofof anisotropyanisotropyenergyenergy barriersbarriers..EventuallyEventually belowbelow thisthis weaklyweakly 5 nm 8 nmEventually,Eventually, belowbelow thisthis weaklyweaklydependentdependent temperaturetemperature TTss,, thethefreezingfreezing ofof thethe surfacesurface layerlayer ininaa spinspin glassglass statestate isis completecomplete

0.05.0x10-41.0x10-3

770 K820 K

540 K10 20 30 40 50 60 703.5x10-3

5 nmaa spinspin--glassglass statestate isis completecompleteandand thethe shellshell--corecore exchangeexchangecouplingcoupling resultsresults inin aa frozenfrozen

titi t tt t ff thth h lh l


0 50 100 150 200 250 300 600 8000.0

T (K)magneticmagnetic statestate forfor thethe wholewholesystemsystem..

MagneticMagnetic

OneOne ofof thethe mostmost controversialcontroversial issuesissues inin magneticmagnetic

Saturation MagnetizationSaturation MagnetizationOneOne ofof thethe mostmost controversialcontroversial issuesissues inin magneticmagneticnanoparticlesnanoparticles isis thethe observedobserved variationvariation ofof thethesaturationsaturation magnetizationmagnetization inin oxideoxide nanoparticlesnanoparticles withwithsaturationsaturation magnetizationmagnetization inin oxideoxide nanoparticlesnanoparticles withwithparticleparticle sizesize..AA generalgeneral rulerule isis thatthat thethe saturationsaturation magnetizationmagnetizationAA generalgeneral rulerule isis thatthat thethe saturationsaturation magnetizationmagnetizationdecreasesdecreases withwith thethe decreasedecrease ofof particleparticle sizesize..ToTo explainexplain thisthis reductionreduction therethere areare severalseveral argumentsargumentsToTo explainexplain thisthis reductionreduction therethere areare severalseveral argumentsargumentsinin favorfavor ofof eithereither surfacesurface originorigin (random(random cantingcanting ofof thethesurfacesurface spins)spins) oror finitefinite sizesize effectseffects..p )p )


Analysis Analysis --Saturation MagnetizationSaturation MagnetizationConsideringConsidering aa systemsystem ofof nonnon--interactinginteracting monodispersemonodisperse

μμ =M=M <<V>V>

gg yy gg ppparticles,particles, wewe maymay fitfit thesethese curvescurves byby LangevinLangevin--likelike functionsfunctions::M/MM/Mss==cothcoth((μΗμΗ//kkBBTT))--((kkBBTT//μΗμΗ))1.0

μμ =M=Mss<<V>V>dγ-Fe2O3

= 4.87 emu/gdFe3O4

= 5.20 emu/g

0.8 8 nm, 300 K

13 nm 8 nm 150 K

ØØ(nm)(nm)

MMss(emu/g)(emu/g)

0.6

M/M

s

8 nm, 150 K1313 868688 5555

0 2

0.4 5 nmM 88 555555 4343

8080 // F O

0.0

0.2 8080 emu/gemu/g– γ-Fe2O3

90 90 emu/gemu/g –Fe3O4

bulk valuesbulk values


0.05 0.10 0.15 0.200.0

H (104 Oe)@300 K@300 K

Analysis Analysis --Saturation MagnetizationSaturation Magnetization

Spin canting: sizeSpin canting: size--dependent featuredependent feature

reduced reduced coordinationcoordination

broken superexchange broken superexchange bonds between bonds between coordinationcoordinationsurface spinssurface spins

lt ti f th i t ti f th ti tlt ti f th i t ti f th ti talteration of the orientation of the magnetic momentalteration of the orientation of the magnetic moment

disordered spin configurationdisordered spin configuration

a reduction of the average magnetic momenta reduction of the average magnetic moment

disordered spin configurationdisordered spin configuration

a reduction of the average magnetic moment a reduction of the average magnetic moment


MagneticsMagneticsEffective anisotropyEffective anisotropy

SurfaceSurface effectseffects dominatedominate thethe coercivitycoercivity valuesvaluesofof thethe smallestsmallest particlesparticles atat lowlow temperaturestemperatures..ThusThus thethe anisotropyanisotropy increasesincreases asas thethe volumevolume isisreducedreduced duedue toto thethe contributioncontribution ofof surfacesurfaceanisotropyanisotropy..


Analysis Analysis --Effective AnisotropyEffective AnisotropyKK 25 k25 k TT /<V>/<V> Exchange anisotropy

ØØ TTBB KKeffeff

KKeffeff ≈≈ 25 k25 kBBTTBB/<V>/<V>Magnetocrystalline anisotropy

Exchange anisotropy(exchange interaction at FM/AFM interface)

(nm)(nm) (K)(K) (erg/cm(erg/cm33))

1313 160160 66××10105 5 Shape anisotropy(dipole dipole interaction)

(spin-orbit interaction)

1 1 ××101055 – γ-Fe2O3

88 8080 1313××10105 5

55 1919 1212××10105 5

(dipole-dipole interaction)

4 64 6 ××101044 –Fe3O4

bulk valuesbulk values

55 1919 1212××1010 4.6 4.6 ××1010 Fe3O4

SizeSizeexchange anisotropy requires:exchange anisotropy requires: KKAFMAFMttAFMAFM >J>Jintintwherewhere JJintint isis thethe exchangeexchange constantconstant acrossacross thetheFM/AFMFM/AFM interface,interface, KKAFMAFM andand ttAFMAFM areare thethe anisotropyanisotropy

TTBB KKeffeff

,, AFMAFM AFMAFM pypyandand thethe thicknessthickness ofof thethe AFMAFM layerlayer respectivelyrespectively..


BB effeff

MagneticsMagnetics

ProblemProblem:: ExchangeExchange biasbias effecteffect appearanceappearanceProblemProblem:: ExchangeExchange biasbias effecteffect appearanceappearanceAA necessarynecessary conditioncondition forfor thethe observationobservation ofof exchangeexchange biasbias isisthatthat thethe antiferromagneticantiferromagnetic phasephase (or(or ferrimagneticferrimagnetic oror spinspin--thatthat thethe antiferromagneticantiferromagnetic phasephase (or(or ferrimagneticferrimagnetic oror spinspinglass)glass) hashas aa largelarge magneticmagnetic anisotropy,anisotropy, soso thatthat itit cancan exertexert aapinningpinning actionaction onon thethe ferromagneticferromagnetic phasephase..

HintsHints forfor thethe existenceexistence ofof exchangeexchange biasbiasFieldField coolingcooling inducedinduced::FieldField coolingcooling inducedinduced::UniaxialUniaxial anisotropyanisotropy (coercivity(coercivity increase)increase)UnidirectionalUnidirectional anisotropyanisotropy (exchange(exchange bias,bias, looploop shift)shift)pypy ( g( g pp ))


1 0 BelowBelow TT samplessamples0 6

Magnetics Magnetics –– Exchange BiasExchange Bias

B l TB l T l hibil hibi0 5

1.0

HaHex10 K

BelowBelow TTBB,, samplessamplesexhibitexhibit hysteresishysteresis inin MMvsvs HH curvescurves..

0 3

0.6M

s

Below TBelow TBB, samples exhibit , samples exhibit hysteresis in M vs H curveshysteresis in M vs H curves

0.0

0.5

Hc2Hc1

M/M

s

TheThe appearanceappearance ofofshiftedshifted hysteresishysteresis loopsloopsleadsleads usus toto considerconsider asas aa0 0

0.3

M/M

-0.5

0.0M particleparticle containingcontaining aamagneticallymagnetically disordereddisorderedsurfacesurface ofof aa certaincertain-0 3

0.0

4000 2000 0 2000 4000-1.0

±± 10 kOe10 kOe±± 50 kOe50 kOe

su acesu ace oo aa ce ace awidthwidth andand aa corecore moremore ororlessless orderedordered dependingdependingonon thethe sizesize ofof thethe-0 6

-0.3 13 nm @ 100 K 5 nm @ 10 K

-4000 -2000 0 2000 4000

H(Oe)∅∅ 13 nm13 nm

onon thethe sizesize ofof thetheparticleparticle andand thethetemperaturetemperature..

-200 -100 0 100 200-0.6

H(Oe)

ΗΗCC=(H=(Hc1c1--HHc2c2)/2)/2HH =(H=(H +H+H )/2)/2


HHexex=(H=(Hc1c1+H+Hc2c2)/2)/2

Magnetics Magnetics –– Exchange BiasExchange BiasHHexex appearsappears belowbelow 150150 KK inin correspondencecorrespondence withwith thethe freezingfreezing ofof mostmost ofofthethe momentsmoments ofof thethe oxideoxide regions,regions, mostmost probablyprobably duedue toto thethe progressiveprogressiveincreaseincrease ofof thethe coercivecoercive field,field, thethe anisotropyanisotropy andand thethe consequentconsequent freezingfreezing

500

,, pypy qq ggofof moremore andand moremore oxideoxide regionregion momentsmoments..

480 Hex Hc

Oe)

13 nm 11 nm 8 nm 5 nm

200c &

Hex

(O

TT

100

Hc

HHCC HHexex

0

HHCC exex


0 50 100 150 280 300

T (K)

NeglectingNeglecting thethe interparticleinterparticleMagnetics Magnetics –– Exchange BiasExchange Bias

NeglectingNeglecting thethe interparticleinterparticlecorrelationscorrelations andand consideringconsideringthatthat thethe magnetizationmagnetization reversalreversal HHcc increases rapidly with increases rapidly with

decreasing T reachingdecreasing T reachingggprocessprocess takestakes placeplacecoherently,coherently, thethe coercivitycoercivity isis

t dt d tt f llf ll thth l til ti

decreasing T reaching decreasing T reaching HHc0 c0 ≈≈ 670 670 ±± 40 Oe40 Oe

expectedexpected toto followfollow thethe relationrelation::HHcc(T)=H(T)=Hc0c0[1[1--(T/T(T/TBB))1/21/2]]

free ingfree ing e changee change

Why ?Why ?

400

500 13 nmfreezing freezing of shellof shell

exchange exchange couplingcoupling

200

300

400

c (O

e)

strong pinning strong pinning action against action against the reversal ofthe reversal of

0

100

200Hc the reversal of the reversal of

the core the core momentsmoments

SYNTORBMAG 26/9 SYNTORBMAG 26/9 –– 1/10/2006, Thessaloniki, Greece1/10/2006, Thessaloniki, Greece0.2 0.4 0.6 0.8 1.0

0

(T/TB)1/2

TheThe exchangeexchange biasbias--likelike propertiesproperties dependencedependence onon particleparticle sizesize requiresrequiresAnalysis Analysis –– Exchange BiasExchange Bias

gg p pp p pp pp qqconsiderationconsideration ofof 33 differentdifferent fundamentalfundamental processesprocesses inin thisthis systemsystem::

1.1. thermalthermal fluctuationsfluctuations ofof thethe superparamagneticsuperparamagnetic corecore22 chemicallychemically dependentdependent surfacesurface anisotropyanisotropy

2.2. chemicallychemically dependentdependent surfacesurface anisotropyanisotropy3.3. energyenergy competitioncompetition

400 Hc

H

10 K120 10 K

200

ex

Hex

(Oe) 80

x (O

e)

SizeSize

50

100

Hc &

H

40

HexSizeSize

0

50

0HHCC HHexex

SYNTORBMAG 26/9 SYNTORBMAG 26/9 –– 1/10/2006, Thessaloniki, Greece1/10/2006, Thessaloniki, Greece4 6 8 10 12 14

0

Size (nm)

4 6 8 10 12 140

particle diameter (nm)

TheThe thermalthermal fluctuationsfluctuations onon thethe

Analysis Analysis –– Exchange BiasExchange Bias11 TheThe thermalthermal fluctuationsfluctuations onon thethe

superparamagneticsuperparamagnetic corecore withwith decreasingdecreasingNPsNPs diameterdiameter maymay destroydestroy thethe freezingfreezing ofofthth ff tt th hth h i di ti di t

11

thethe surfacesurface momentsmoments throughthrough indirectindirectexchangeexchange interactionsinteractions forfor thethe casecase TTss<T<TBB..

completelycompletely frozenfrozen statestateoxideoxide magneticmagnetic regionsregions becomebecomeprogressivelyprogressively unfrozen,unfrozen, accordingaccording toto thethedistributiondistribution ofof effectiveeffective anisotropyanisotropy energyenergybarriers,barriers, determineddetermined byby theirtheir sizesize andand thethestrengthstrength ofof thethe magneticmagnetic interactioninteraction withwiththethe surroundingsurrounding..OnceOnce thethe netnet momentmoment ofof thethe surfacesurface layerslayersbecomebecome ableable toto thermallythermally fluctuatefluctuate theytheytendtend toto bebe polarizedpolarized byby thethe corecore momentmoment..

EnergyEnergy competitioncompetition maymayalsoalso playplay aa rolerole sincesince thethe

hh

33TheThe chemicallychemically dependentdependent surfacesurface anisotropyanisotropymaymay bebe treatedtreated asas uniaxial,uniaxial, withwith thethe easyeasy axisaxisdefineddefined byby thethe dipoledipole momentmoment ofof thethe

22

exchangeexchange energyenergy(surface(surface term),term), becomesbecomeslargerlarger thanthan ZeemanZeeman

defineddefined byby thethe dipoledipole momentmoment ofof thetheneighboringneighboring ionsions.. HenceHence thethe easyeasy axisaxis forforthesethese ionsions isis approximatelyapproximately radialradial.. IfIf thethe spinsspinswerewere perfectlyperfectly alignedaligned thethe effecteffect ofof aa radialradial


energy,energy, (volume(volume term)term)..werewere perfectlyperfectly aligned,aligned, thethe effecteffect ofof aa radialradialsymmetricsymmetric surfacesurface anisotropyanisotropy wouldwould averageaveragetoto zerozero..

Magnetics Magnetics –– Exchange BiasExchange Bias

OnOn thisthis basisbasis wewe maymay convenientlyconveniently describedescribe ouroursystemsystem asas constitutedconstituted byby twotwo differentdifferent componentscomponentst lt l l dl d tt thth i t fi t fstronglystrongly coupledcoupled atat thethe interfaceinterface::

aa nonnon--relaxingrelaxing ferromagneticferromagnetic corecorea relaxing magnetically disordered surface layera relaxing magnetically disordered surface layera relaxing, magnetically disordered surface layer a relaxing, magnetically disordered surface layer

inin thethe FCFC process,process, aa preferredpreferred orientationorientation isis imposedimposed uponupon thethe spinspin--glassglasssurfacesurface spins,spins, whilewhile thethe FMFM core,core, withwith aa higherhigher orderingordering temperature,temperature, isissinglesingle domaindomain..whenwhen thethe fieldfield isis removed,removed, thethe FMFM corecore experiencesexperiences thethe fieldfield generatedgenerated bybythethe frozenfrozen surfacesurface layerlayer inin thethe directiondirection ofof thethe coolingcooling fieldfield originatingoriginating thethethethe frozenfrozen surfacesurface layerlayer inin thethe directiondirection ofof thethe coolingcooling field,field, originatingoriginating thetheobservationobservation ofof thethe offsetoffset ofof thethe hysteresishysteresis looploop..


ConclusionsConclusionsHomogeneous nucleation synthesis: Homogeneous nucleation synthesis:

monodisperse sized Femonodisperse sized Fe--OxidesOxidesSolventSolvent--surfactant effect:surfactant effect:

different oxidation ratiodifferent oxidation ratiopolydisperse stoichiometrypolydisperse stoichiometry

Highly crystallinity:Highly crystallinity:γγ--FeFe22OO33 is the dominant phase of the small 5 nm NPsis the dominant phase of the small 5 nm NPsFeFe33OO44 component increases on increasing the Np sizecomponent increases on increasing the Np size

Magnetics:Magnetics:Superparamagnetism @ RTSuperparamagnetism @ RTLow temperature surface spinLow temperature surface spin--glass layerglass layercritical diametercritical diameter--exchange phenomena exchange phenomena


Quantitative Magnetic Analysis of...

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