Silica coated nanoparticles: synthesis,magnetic properties and spin structure

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Silica coated nanoparticles: synthesis,magnetic

properties and spin structure

Frederic Mazaleyrat, Mehdi Ammar, Martino Lobue, Jean-Pierre Bonnet,

Pierre Audebert, Guillaume Wang, Y. Champion, Martin Hÿtch, Etienne

Snoeck

To cite this version:

Frederic Mazaleyrat, Mehdi Ammar, Martino Lobue, Jean-Pierre Bonnet, Pierre Audebert, et al..

Silica coated nanoparticles: synthesis,magnetic properties and spin structure. ISMANAM, Aug 2007,

Corfou, Greece. pp.473-478, �10.1016/j.jallcom.2008.08.121�. �hal-00662962�

properties and spin stru ture

F. Mazaleyrat

SATIE,ENS Ca han, CNRS, UniverSud, 61av President Wilson,F-94230 CACHAN,Fran e.

M. Ammar, M. LoBue

SATIE,ENS Ca han, CNRS, UniverSud, 61av President Wilson,F-94230 CACHAN,Fran e.

J-P. Bonnet, P. Audebert

PPSM, ENS Ca han, CNRS,UniverSud, 61av President Wilson,F-94230 CACHAN,Fran e.

Y. Champion

ICMPE,CNRS, 15rue Georges Urbain, F-94070Vitry-sur-Seine, Fran e

M. H¸t h, E. Snoe k

CEMES, CNRS, rue Jeanne Marvig, F-31055 Toulouse, Fran e

Abstra t

In the re ent years magneti nanoparti les have been extensively studied for their superparamagneti properties providing useful labels inbiology or for fundamental aspe ts in luding the size dependen e of magneti atomi moment and the ee t of surfa e anisotropy. In most ases, the parti les were smaller than 10 nm and interestingly,thesizesrangingbetween10and100nmhavebeenpoorlyinvestigated until now. This is mainly due to the fa t that usual hemi al routes produ e 5-10 nm oxide or metalli parti les or eventually 20 nm at most. On the over side, atomization te hniques yields parti les in the mi rometer range. Metalli parti les are parti ularly interesting for better magneti propeties ompared to oxides, but they have two big drawba ks: they arenot bio ompatible and they are ondu ting ele tri ity.Consequently,it'sne essarytoprodu e ore-shellparti les,for wit hthe shell is bio ompatible and insulating and with a perfe t ontrol of thi kness and uniformityofthatshell. Inthis work,we study metalli parti lessynthetizedbyan original evaporation- ondensation te hniques that produ es parti lesof sevral tens

ryogeni metltingpro ess oatedbyasili ashellusingsol-gelmetod.Morphologi al and magneti properties are presented, showing the e ien y of ultrasoni sol-gel pro ess forthat purpose.

Key words: Core-shell parti les, nanomagnetism, ultrasoni synthesis PACS: 81.20.Fw, 7550.Tt, 7525.+z

1 Introdu tion

Magneti nanoparti les have attra ted in reasing interest inthe re ent years. The reasons are a tually fundental as well as applied: many issues on fund-mentalmagnetism of nanoparti les are still unsolved and experiments at the s ale of individual parti le is la king. For exemple, although theory is quite wellestablished,systemati experimentsonthe riti lesizeatwhi hthe mag-neti stru ture hanges from single-domain to vortex or multi-domain is still missing . From pra ti al point of view, magneti nanoparti les have a great potentialin biomedi alappli ations and high frequen y ele troni s.

Superparamati parti les(SPMP) are widly used in in biology. Indeed, their are many appli ation of superparamagneti beads as image ontrast agent in magneti resonan e imaging and hyperthermia therapy but essentially for magnetophoresis (drug arriers, magneti on entration...). For all appli a-tions,superparamagneti behaviorisne essary be ausea zeronet magnetiza-tion underzero magneti eldprevents agglomerationofmagneti beads. For magnetophoresis, be ause relatively high DC magneti eld an be produ ed by permanentmagnetsystems, essentiallythe volumeofmagneti parti lesis important,so lassi albeads omposed of50%of 5nmindiametermagnetite and/ormaghemite parti lesare perfe tlytted. Moving beads by appli ation ofa gradienteldisone thing butitwould beeven moreinterestingtolo ate and/or quantify this beads. Various prin iples for the dete tion of magneti beads have been proposed (1; 2;3;4) but allthese systems are limted in sen-sitivity by the magneti properties of the parti les. As it was demontrated by Néel (5), the su eptibilty of aSPMP at a given temperature isessentially dependent onits volume and saturation magnetization:

χ =

J

2

S

V

3µ

0

k

B

T

= αJ

2

S

D

3

.

(1)

This formulais validif two onditions are satised:

Email address: mazaleyratsatie.ens- a han.fr(F. Mazaleyrat). URL: http://www.satie.ens- a han.fr(F. Mazaleyrat).

•

the volume of the SPMP has to be small enough for the magnetization to be relaxed, i.e. if the thermal energy of the parti le exeeds the anisotropy enregy

K

1

πD

3

SP

6

= ln

f

0

f

k

B

T

(2)

•

the parti le has to be single domain (SD), i.e. its diameteris smaller than the riti al one,

D

SD

, where vortex and domainformation are expe ted to be energeti ally favourable;

D

SD

an be estimated e.g. following Bertotti (6) for asoft parti le

D

SD

≈ 4

√

3ℓ

ex

,

(3)

where

ℓ

ex

=

q

2µ

0

A/J

_S

2

. Indeed, onsidering the diversity of soft magneti materials and many fa tors ae ting their behavior, nding the ideal SPMP is not trivial. On Fig. 1, the values of

D

SP

and

D

SD

for most ommon soft magneti mateirals have been reported as a fun tion of saturation magneti-zation together with iso-sus eptibility urves plotted using equation (1). The best materials are that having their lower riti al diameters on the highest iso-sus eptibility urve. It is lear that metalli materials leads to the best ompromisebetweensize,magnetizationandanisotropy.Frommagneti point ofview,FeNialloysare parti ularlysuitableforthatpurposeinparti ulardue totheir lowanistropy.In ontrast, be ause of the toxi ity of Ni, the parti les haveto be oatedby astable bio ompatible layer.

Con erninghighfrequen y ele troni s,nanoparti leshavepotentialespe ially in power appli ations. It is well known that metalli thin lms exhibits high permeability athigh frequen y, but appli ationsare limitedto lowpower ap-pli ations be ause the volume of the material is ne essarilly very small. In ontrast, nanoparti les an be used for realization of bulk omponents. The frequen y limitsof magneti parti les are mainlydue tothree phenomena:

•

gyroresonan ewhi his ontrolledby magnetizationand anisotropy only;

•

eddy urrentrelaxation wi h an be ontrolledusing parti lessmaller than the skindepth;

•

domain wall resonan e whi h an by suppressed by using single domain parti les.

Again metalli parti les are interesting be ause of their high magnetization and the possibility to hange the anisotropy onstant. If large permeabilty is required,ultrasoft materialssu h aspermalloy anbeused.On the ontrary, if highfrequen y isneeded, higheranisotropy materialssu has obalt an be hosenbut inall ase, insulationwillbene essary.However, insulationwitha nonmagneti layerae ts thepermeabilityof thematerial.Ifthe relative vol-ume ofnon magneti oatingissmall, NonMagneti Grain Boundarymodels

0,0 0,5 1,0 1,5 2,0 2,5 0 20 40 60 80 100 120 140 F e F e N i4 4 F e N i7 0 F e N i8 0 F e N i9 0 N i C o F e C o M a g h e m i t e F e3 O 4 N i Z n f e r r it e M n Z n f e r r i t e 100 1500 500 C r i t i ca l D i a m e t e r s ( n m ) Saturation Polarisation, J S (T) D SP D SD 10

Figure 1. Criti al parti le size for dierent soft magneti materials. Open squares, superparamagneti limit; olsed ir lessingledomainlimit; ontinuouslines, iso-sus- eptibility urves omputed fromeq. 1(labelsarefor thevalues of

χ

).

applies:

χ

ef f

=

χ

i

1 + χ

i

_D

t

(4)

where

g

is the average gap between parti les, i.e. approximatively twi e the thi kness oating.This equation learly demonstrate that the oating has to be mu hsmaller than the parti lesize. Considering that parti lesare smaller than 100 nm, it is lear that oating must embed ea h parti le individually with perfe t homogeneity and nanometer ontroll of itsthi kness.

Forthesepurposes,ourappro histostartfrommetalli parti lesprodu edby ryogeni melting (derived from lassi al evaporation- ondensation method) and to grow an inorgani diele tri layer by sol-gel te hnique. In order to rea h the obje tive in terms of ontrol of homogeneity and thi kness of the diele tri layer, aimprovementofStöbermethodby useofhigh powerdensity ultrasounds has been developped.

Synthesis of nanoparti les. Co and FeNi parti les have been produ ed by ryogeni melting, a method onsisting in overmelting a droplet of metal by indu tion kipping it in levitation in the ryogeni media (usually LN

2

). The vapor ondensate in the alefa tion layer around the droplet and fur-ther oales en eyieldssolidi ationinto20-100nm parti les,the averagesize depending on the metal, the ryogeni medium and the temperature of the melt. Parti les are then olle ted in hexane where a passivation layer of 2-4 nm formsatthe surfa e of the parti les. Cobalthavebeen hosen foritshigh anisotropyfavorabletohighfrequen y appli ations.Co parti lesare f ,have an average size of 52 nm (standart deviation of 23 nm) and a surfa e layer omposedofCoOandhydroxides. Permalloywas hosenforitslowanisotropy favorabletohighpermeabilityandSPMproperties.Con erningFeNi,be ause metals have dierentvaportension, the omposition of vapor (and so thatof parti les) may dierfromthat of melt.To avoidthe omposition driftduring thepro ess, wehave hoosen thehomoazeotropi ompositionFe

29.5

Ni

70.5

(7). The average grain size is 50 nm (20 nm standart deviation) and the surfa e layeris omposedmainly of NiOand ni kel hydroxides.

synthesis of sili a oating. Thesili ashell ontoparti leswassynthesized a ordingtotheStöbermethod[23℄(sol-gelrea tion).Firstly, obaltparti les (typi ally 80 mg) have been dispersed in y loexane, a polar solvant mis i-blewith APTES (Amino-Propyl-Triethoxysilane) and olei a id (whi hplays the role of surfa tant) during 90 minutes under a ontrolled ultrasoni ex i-tation of 3 W. m

−3

powered by a 200 W horn. The surfa tant is ne essary todisperse obalt parti lesdue tothe strong magneti for es. Then, APTES and olei a id are introdu edinorder tofun tionalize the surfa e with amine groups(NH

2

)whi hpresent a goodanity with metaland to start the poly-merization with the sili a pre ursor xed on the end of the APTES hains. Inathirdstep, dierentquantities oftetraethylorthosili ate(TEOS)and am-monia were su essively introdu ed into the suspensions to grow the sili a layer. Soni ation (0.5 W. m

−3

) was maintained during the sol-gel rea tion (90mn). Finally the suspensions are entrifuged at 3000 rpm for 10 minutes, the solvent is dis arded, the nanoparti les are again ultrasoni ally dispersed inethanoland dryed forsamplepreparation.In the ase ofFeNiparti les,the fun tionalization step was not ne essary. Indeed, ni kel hydroxides have an isoele tri point at pH=12, whereas the mixture has a pH=8. Thus, the sur-fa eof parti lesispositively hargedand have agoodanity withnegatively harged sili a.Consequently, dispersion was ondu ted simplyinethanoland the polymerisationstarts in the se od step with hydrolizationof TEOS.

Chara terization. The magneti properties have been measure between room and LN

2

temperature using standart VSM. Morphology and

omposi-Ele tron Energy Loss Spe tra (EELS) were re orded using a Gatan Image Filter atta hement. Holography experiments were performed on a Spheri al Aberration Corre ted TEM Te nai F20 (FEI) tted with an obje tive lens aberration orre tor (CEOS) and rotatable ele tron biprism. The eld emis-sion gun (FEG) provides the highly oherent ele tron sour e ne essary for ele tron holography. Hologramswere re orded with abiprismvoltage of90 V ona Gatan 1k

×

1k slow s an CCD amera and pro essed using adedi ated software, developed inthe Gatan Digital Mi rograph environment. Exposure times were typi ally ofthe order of 4 s.Obje tivelens wasused to magnetise the sample in two opposite dire tions by tilting the sample holder. The two holograms re orded at remanen e were substra ted to eliminate ele trostati potential ontributiontothephaseshiftofele tronspassingthroughthe sam-ple.Thephaseshitf

φ

M ag

issensitivetothein-plane(perpandi ulartoele tron beam) omponent of the indu tion, a ording to(8; 9):

φ

M ag

= −

e

~

Z

A(x, y)dz = −

_~

e

Z

Z

B

⊥

(x, y)dxdz

(5)

3 Results and dis ussion

After oating, parti les have been dryed and observed by TEM. Pi ture (a) inFig.2shows and exemplesofCo parti les oateda 8nmlayerof sili a.The layer is not perfe tly regular but globally uniform and ea h parti le appears separatly oated.Byin reasingtheTEOS on entration,the oating anbeas thi kas 80nm, parti lesare stillseparated and thelayer isuniform(b). This result is attributed to the the ee t of high power density ultrasounds whi h avoidparti leagglomerationand favours uniformityof the sol-gelrea tionon the parti les.Sameresultsare found qualitativlelywithFeNiparti lewithout surfa e fun tionalisation as it is shown on pi ture ( ) and (e). Additionally, Ele tronEnergy LossS atteringhas onrmthatsili onisex lusivelylo ated atthe surfa eof the parti lesand HRTEMgives eviden eofthe homogeneity and of the amorphous nature of sili a oat. The main dieren e ompared to the te hnique usinganAPTES fun tionalizationstep,is that the e ien y is redu ed at high TEOS on entration.

Fig.3showsthe hysteresisloopsof oatedCo nanoparti les.Thespe i mag-netizationde rease onsistentlywithin reasingthevolumeofTEOS.By om-paring the saturation values with respe t to raw parti les and onsidering spheri al parti les of averadge diameter

D

surrounded by a shell of thi k-ness

t

, it was possible to extra t

t

from the data (see Table 1). This values are in good agrement with TEM measurements whi h onrms that TEM samples are representative of the whole. As expe ted, Co parti les are ex-pe ted, parti les are strongly hystereti be ause all parti les are larger than the riti alsuperparamagneti size (

D

SP

= 7.5 nm

)and mainlysingledomain

andthi k(b)layerofsili a.FeNiparti leswithathinlayerofsili a:( ), onventional image,(d)sili onmapbyEELS.Thi kerlayeronFeNiparti les, onventionalTEM (e); HRTEM(f).

Figure3.Hysteresis loopsofCoSiO

2

nanoparti les. Inset: thesame urvesplotted withrespe t to redu edmagnetization

M/M

S

.

Table 1

Thi knessdependen eofsili ashellonTEOS on entrationmeasuredbyTEMand dedu edfrom spe i saturation magnetization

Co Fe

30

Ni

70

TEOS

t

T EM

t

M AG

TEOS

t

T EM

t

M AG

(

µℓ

) (nm) (nm) (

µℓ

) (nm) (nm) 100 8 7 50 3 4 200 13 12 100 8 9 300 44 43 200 15 17 400 80 71 500 33 24

(

D

SD

≈ 50 nm

, see Fig.1). In the inset of Fig.3, loops are plotted with re-spe t to redu ed magnetization showing learly that or ivity (55 kA/m) is independent of sili a shell thi kness. This indi ates that the main sour e of anisotropyin thissystem ismagneto rystalline.The theoreti al oer ivity for an enssembleof randomly oriented parti les isgiven for ubi parti les, after Néel (10), by

H

C

= 0.64K

1

/J

S

. This yields the value of 146

kJ.m

−3

for the anisotropy onstant of f -Co.

teresis, the observed behavior is mut h dierent ompare to the ase of Co. First we remark that the oer ive eld is quite strong in spite of the fa t that most of the parti leshave a size below the superparamagneti limit(71 nm) is larger than most of parti les. Se ondly,

H

C

is mu h larger than ex-pe ted by applying above mentionned Néel formula (350 A/m). These two phenomenons an be explained by shape anisotropy that an be relevent for even fornearlysperi alparti lesbe auseofthenearlyzeromagneto rystalline anisotropy. Following Néel (11), the demagnetizing fa tor of a slightly elon-gatedspheroid,havingaratiobetweenlenghtsofsmallandlongaxes

γ

, anbe written

N

k

=

1

3



₉

5

−

4

5

γ



.Knowingthat

N

⊥

+ 2N

k

= 1

, it'seasytoshowthat the shape anisotropy is

K

S

= −

γ−1

5µ

0

J

2

S

. Consequently, the observed oer ivity an be explained usingNéel-Stoner result for uniaxial anisotropy (12):

H

C

= 0.96

K

S

J

S

= 0.192

(γ − 1)J

S

µ

0

(6)

if

γ − 1 ≈ 20%

. Refeering toTEM pi tures, this dieren ein length between longandshortaxisisa tuallyunrealisti ,showingthattheobserved oer ivity hasmore omplexorigin.Itisalsointerestingtonotethatthe oer ivityvaries sensitivelywiththi kness oating.Thede reaseobserved forthinersili alayer an be due to a diminution of dipole-dipoleintera tions as distan e between parti lesin rease. On the otherhand, the in rease observed forthi kerlayers is rearly related with stress applied by the oating as it has been shown by FourrierTransformInfra red spe tros opy (13).

A ordingtopreviousremarks,theadditionofashapeanisotropyterm hange the parti le energy balan e. As a onsequen e their spin stru ture an be deeplymodied(untilnowthey were supposed singledomain).Toinvestigate the spin stru ture, ele tron holography experiments have been ondu ted. TEM samples have been prepared by dispersing parti les by ultrasound in ethanol and a arbon grid was dipped in the suspention. This dispersion is ne essary to avoid long hain formation that inuen es deeply the magneti stru ture(14).Duetothemagneti intera tions,itwasnotpossibletoobserve individual parti les. However, it was possible to isolate very small groups of 2-3 parti les and to re ord holograms. Pi tures of the parti les and the or-respondinggreys ale phase plot are shown in Fig.5.In this plot, the in-plane omponent of indu tion  and thus magnetization  is tangential to the iso-phase lines showing a vortex onguration where spins urls in the plane. In the enter of the biggest parti le, the white olor indi ates a maximum in the phaserevealingthat in-planeindu tionis goingtozero whi hmeansthat spins turns out of plane. To go more insight the magneti stru ture, phase prole along a line going through the biggest parti les have been plotted as a fun tion of radius (the originis the middle of entral parti le)a ording to the line drawn on the pi ture. The in-plane omponent of indu tion (

B

θ

) is

Figure4. HysteresisloopsofFe

29

.5

Ni

70

.5

SiO

2

nanoparti les

dedu ed byderivationofthe phasetaking intoa ountthe depthvariation.If the parti leis onsidered perfe tly spheri alequ.5 yields:

B

θ

(ρ) = −

~

e

×

1

z(ρ)

×

dφ

M ag

(ρ)

dρ

.

(7)

The derivation of phase prole was possible due to the exeptional stability of the mi ros opeand exellent signal tonoise ratio of the a quisitionsystem. Fig.6 depi ts the phase shift dedu ed from holograms and the al ulated in du tion. Theshapeof indu tion distributiona rossthe diameteris onsistent with a vortex having an out-of-plane ore (

B

θ

= 0

inthe enter). Out of the entral ore,

B

θ

rapidly in reases and rea h the maximum value of 0.87 T, losetothatexpe tedby magneti measurements(thesaturationpolarisation of the ore is estimated to 1.0 T). In the oxide shell,

B

θ

goes again to zero rapidly be ause the shell isnot magneti . Aspredi ted, the vortex ongura-tion is learly attested here with welldened ore and urling zones. Indeed, it is possible to al ulate the sigle domaine riti al size from eq. 3. Taking

Fe

30

Ni

70

valuesforthe ex hange onstant

A

and thesaturationmagnetization

J

S

(

10 pJm

−1

,

1 T

and

700 Jm

−3

,respe tively)yields

ℓ

ex

=

q

2µ

0

A/J

_S

2

= 5 nm

, and wend that the parti le(40 nm) issensitivelylarger than

D

SD

≈ 35 nm

. Thisexperimentaldistributionofmagnetisationinthevortex have been om-pared to Usov model (15). This model is based on on Brown mi romag-neti approa h (16) onsidering a ylindri alparti lein the limitof vanishing anisotropy in whi h the magnetisazation distribution is divided in two

re-hasadiameter of40 nm.Bottom: omputed iso-phasemap. Whiteisfor maximum phaseshift orresponding here to out of planemagnetization, ba k isfor zero. The line showsthe path along whi htheindu tion is omputed.

magnetization (hereequatorial omponent

B

θ

) ompared withUsovmodel. Onthe border of parti le,magnetisation goesto zerodue to post-pre essing(thethi kness be omes zero).

gions:anouterregionwherethemagnetisation urlsin-planeanda oreregion wherethemagnetisationturnsgraduallyoutofplane.Underthese onditions, Brown's equation an be solved analyti ally and the magnetisation distribu-tion is given by:











m

θ

=

_a

2

2

₊

ar

_r

2

if

0 6 r 6 a

m

θ

=

1

if a < r 6 D/2

(8)

where

r

is the radius of the parti le and

a

the radius of the vortex ore. By tting experimental data with model, one an obtain the ee tive ex- hange lengthgiven by

πℓ

∗

ex

= 2a

,so that

ℓ

∗

ex

≈ 8 nm

.This ee tiveex hange length has tobe ompared with the intrinsi ex hange length

ℓ

ex

= 5 nm

and the domain wall parameter

ℓ

w

=

q

A/K

1

= 120 nm

(the magneto rystalline anisotropy of

Fe

30

Ni

70

is

700 Jm

−3

). This experiment shows that the vortex ongurationisnotruledby lassi alex hangelengthsbutbyanee tiveone thathaveanintermediatevalue,presumablydependentofthesizeofthe parti- leitself. The magneti pro esses involved are dierent fromthat of oherent rotation and domain wall displa ement. Previous mi romagneti simulation

vortex nu leates or ips (17) and sin e these elds are intimatly dependent to

ℓ

∗

ex

it an be inferred that the oer ivity is strongly size dependent inthis onguration.

Con lusion

Metalli ore-shellparti leshavebeensu essfuly produ edbysili a oatingof 50nm Co and FeNi parti les. Sol-gelsynthesis underhigh power ultrasounds allowed to oat ea hparti le individuallywith a nm- ontrolled uniform sili a layer. Magneti mesurement have shown that obalt parti les behaves glob-ally like single domain parti les as it was expe ted. In ontrast, the FeNi parti les do not exhibit superparamagnetism be ause the shape anisotropy dominates the magneto rystalline anisotropy. In addition, parti les are mag-neti allyharderthanexpe ted even taking the shapeanisotropy in onsidera-tion. Sin eele tron holography has onrmed the formationof spin vortex in the parti lesinthis size range,itissupposed thatthe high oer ivity ofthese extremly soft nanoparti lesis due to the pe uliar magneti pro esses inthat magneti onguration. Summerizing, we have demonstrated that nanos ale ontrolof sili a oatingon metalli parti le sipossiblewith very good homo-geneity and reprodu ibility.Co parti leshexibitedthe expe ted behaviorand aregood andidateforhighfrequen yappli ations.In ontrast,FeNiparti les, inspiteofthebio ompatible oating, annotbeusedwhensuperparamagneti behaviouris required.For this purpose, itwillbe ne essary toprodu e parti- les at least under40 nm and with redu ed size dispersion.

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29

.5

Ni

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.5

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