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Appendix 1

List of publications 1 communicated articles

1. STRUCTURAL AND MAGNETIC PROPERTIES OF Col.,Zn,Fq04 NANOPARTICLES BY CO-PRECIPITATION METHOD G. Vaidyanathan, S. Sendhilnathan, R. Arulmurugan, J. Magn. Magn. Mater, 3 13 (2007) 295:

2. FTIR AND EPR STUDY OF Col.,Zn,Fq04 NANOPARTICLES FOR FERROFLUID PREPARATION BY CO-PRECIPITATION METHOD G. Vaidyanathan, S. Sendhihathan. (Accepted in Physica B)

3. STUDY OF THERMAL AND MAGNETIC PROPERTIES OF Col.,Zn,Fe20c NANOPARTICLES BY CO-PRECIPITATION METHOD G. Vaidyanathan, S. Sendhihathan. (Communicated to Materials Science and Engineering A)

4. SYNTHESIS AND SPECTRAL STUDIES OF Col.,Zn,Fe204 OF TRANSFORMER OIL BASED MAGNETIC FLUID G. Vaidyanathan, S. Sendhilnathnn. (Communicated to Journal of Magnetism and Magnetic Materials)

5. SYNTHESIS AND MAGNETIC PROPERTIES OF Col.,Zn,Fe20r OF TRANSFORMER OIL BASED MAGNETIC FLUID G. Vaidyanathan, S. Sendhihathan. (Accepted in Journal of Magnetism and Magnetic Materials)

6. THE EFFECT OF MAGNETIC FIELD ON VISCOSITY OF Col.,Zn,Fq04 OF TRANSFORMER OIL BASED MAGNETIC FLUID G. Vaidyanathan, S. Sendhilnathan. (Communicated Journal of Magnetism and Magnetic Materials )

7 . STUDY OF SMALL ANGLE NEUTRON SCATTERING (SANS) Co-Zn MAGNETIC FLUID G. Vaidyanathan, S. Sendhilnathan, V. K. Aswal (Accepted in Joumal of Magnetism and Magnetic Materials )

Journal ol Milgnet~m and Magncllc Malrnsls I t ) lXKI7l !01-2W

Structural and magnetic properties of Col-,Zn,Fe20 articles by co-precipitation method

G. V a i d y a n a t h a n " S, ~ e n d h i l n a t h a n ~ ~ ~ , R , Aru 'Depurtmnl oj Phnlrs Pondirhrrr, Englmring C0hk Pondickrr! M

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ferrites (Cc-Zn) chosen hert IS highly sensitive to temperature. Ferrofluids conslituted by these ferriles may be good cand~dates to be used as liquid carriers in heat exchangers using magneto caloric energy conversion [10,1 I]. It is well known that the magnetic property can be altered by the addition of zinc. Addition of zinc also afccts the lattice parameter (h). Various preparation techniques, such as sol-gel pyrolysis method [I l l hydro. thermal technique [I31 and mechanical alloying [I41 are used to prepare ferrite nanoparticles. But co.prccipitation method is considered to be an economical way of producing fine particles [15,16]. The physical properties of nanoparticles are of current interest due to the size dependent behavior observed in the nanometer length scale and high crystallinity. Preparation and properties of Cc-Zn ferrites have been reported only for a particular value of x or limited values. Jeyadevan et al. [I71 successfully prepared needy monodiapcrsd single-domain cobalt ferrite particles, which wuld be uacd for the high. density recording media. Morais et al. [I81 npcncd (he

h J I d ~ 1 i a CI a1 1 l o r n 1 n/ Mawrum and Mpncrir Materurlr I11 lWU7l ID>-!!YJ 205

computed using the '6 value and w~lh their respective a,,6 ( h k h parameters. Analysis of the difraction pattern

conforms the formation of cubic spinel structure lor all 8.45 the samples. The strongest reflectlon comes from the (3 1 I) plane, which denotes thc spinel phase. All the compositions $ had a spinel structure. Thc peaks indexed to (2 2 0). (3 1 I), 2 43 (4001, (4221, ( 5 1 I ) and (440) planes of a cubic unit cell, *. correspond to cubic spinel structure. The calculated latt~ce 842 constant (o,), identified the samples lo be cubic spinel and

shown In Fig. I. The broad XRD lines Indicate that the

f was refined using Powder X. The XRD pattern for '"' Col-,Zn,Fe204 with 1 = 0 , 0.2, 0.4, 0.6, 0.8 and I is 840 4 particles are of nanosizc range. The peaks of (220). (3 1 I), 8.38

(400), (422), (5 1 I) and (440) have been deconvoluted to 8 3 a

Lorentian curves, using Peak Fit softwart for the determination of FWHM value of the indexed peaks 1241. he lattice constant was found to increase from 8.384 to 8 451 A with the increarc in zinc concentration. The lattlce

FIE 2 V8mu&&uf!m con8hnl(uJ wlib Zn wnwnir811un for constant (ao) increased with the Increase In Zn content. col -JnxFek yaryln8 I" t ,u

which suggested the formation of a compositionally homogeneous solid solution and was found tb be within he range of the lattice constants of ZnFc20r and CoFe204 ferritcs reported by Jln.Ho Lee et al. This lncrease could be also attributed to the substitution o ur values. Fig. 2 shows that the lattice he large sized I n cation for the small sized Co cation. Th h increase ~n zinc conccntral~on. The lattice constant obta~ned for C O F C ~ O ~ 1,8.384A) is close t was estimated by the Debye-Scher. the known of bulk CoFelOl (8.39JA) 131. The lattic the FWHM value of the respective

average crystallite size (Drvrxk)

CY. ~ h ~ o , to 6.92nm when the parl~al subst~tu. I 13111 x = 0-1.0). This value is close to the

crystalllte s m reported by Arulmurugan el a1 1201 Though all the s a m p b were prepared under ~dentlcal cond~tion, the crystalllte size was not the same for all Zn concentrations

,239 "3 Po u*04 This was orobablv due to the nreoaratlon cond~hon '*

followed here wh~ch gave rise to ditTirent rate of ferrite formation lor diferent concentrations of zinc, favoring the variation of cryslallile size. The variation of average crystallue size with the zinc concentration is given in Fig. 3. Ferrofluids can be conveniently prepared by making

a use of particles in this size range. The average crystallite rn - size (DavlX~) and the lattice constant (a,) of C O ~ - ~ Z ~ , .

f Fe204 with x varying from 0 to 1.0 is shown ~n Table 1.

3.2. Magnetic memremenrs

In the cubic system of ferrimagnctic apinels, the magnetic order is matnly due to a super exchange interaction mechanism occurring between the melel ions in the A and B sublattieco. The substitution of nonmagnetic ion such as Zn, which has a preferential A site w p a n c y results in the reduction of the exchange interaction between A and 0 sites. Hence, by varying the degra of zinc substitution, it is poasible to vary magnetic properties of the fine paliiclca. Fig. 4 shows the room temperature

# 1 0 5 0 B O 7 0 hysteresis loop of the powder aamplm for various zinc Z U W 1-1) substitutions. From Table 2 it can bc m that the

'18, I ,a X.ny dill* plncm Ccl-Pz~c@, r . 0, variation pattern of apecific ~aturat ion ma$netintion O1.04, 0.6.0.8 ~ p d 1.0. (M,) aa a function of Zn content shows an i r i a c ~ for

G V n i d p l h u n ul / J O U ~ M ~ U ] Mupnf~Lnt and Mmnr M~ftrbb113 iX07128-JW

i l k 3 Vnnnlion of thc average crystallln SIX W I I ~ Zn concrntrarlan for rn,.,Zn,FelO~ wlrh x vnrylng from 0 to 1.0.

Idhk I \unmary of averngc crystall~le slzr iD,,,xa) (nml nnd lnt l~ce constam (A1

5 ' ('u,,. ,)rln,Fe>O, w ~ t h r vurylng from 0 tu I O nnnoprnlclcs

iinpla Average cryr tdl l~te ria L n t t q conslant IDnv,xal (nm1 (u.) (AI

( 'I 0Fe20, 12.02 r.,,,&+ iofelO, 10 RS thesize Coo9Zno lFe,O, fine particles (10.85nm in size) i~,~&nawFe~O, 8.80 i a , , ; J ~ ioFelO, 8.08 8.195 ins hdnp~~Fe~O, R OS 8.411 nanoparticles from the bulk, influcnccn the temperature Lo,dbwFc~O, 7 4 1 [ L ~ I ~ ~ ~ o M F ~ O ~ 7.42 8413

dependena of magnetization The hysteresis curve (Fig. 4)

(4, ,&no ,oFe10, 7.25 R415 recorded at room temperature shows very low remanence, coercivity for large zinc concentration proves that the particles are super paramagnetic at room temperature.

3.3. S~ectral measuremenen

0 even at 1194.I5kAlm Figs. 6 and 7 show the nd coercivity (H,), which

of remanence (M,) and 'Oercivity (H,) for Col-,Zn,Fe204 w~th x varying from 0.4 :dl.0. The changes in magnetic properties such as M,, He, 'UI are due to the iduence of the cationic stoichiomelry ind their occupancy in the spccific sites. ln addition, Omation of dead laya on the surface, exiatena Of landom canting of particle surface spins (24.261, noose IUration e h t s due to rsndom diltribution of particle a'm.

The FTlR spectra for Fe,04 and for Col.,Zn,Fe20, with x = 0,O.j and I arc shown in Fig. 8. By overlayingthe FTlR spalra for FejO, and for Cot.,ZnPe204 with x = 0, 0.5 and I .O, the sppctral similarities arc observed. The main transmittana fnquencies observed in the region ~ O C A C C ~ C ~ - ~ of the FTlR spcctra for Col-,2n,Fe204 with x varying from 0 to 1.0 are summarkd in Table 3. The assignments of peaks arc listed in Table 3. The broad feature between 3441.43 and 3219.90m'l is due to 031 stretch (vI), which m ~ n d s to the hydroxyl group attached by the hydrogen bonds to the iron oxide turface and [he water molecules chemically adsorbed to the magnetic particle surface ( a s d a t e d water content) (271. From these rerulta, it apputn that the hydroxyl groups are retained in the sample during the preparation of the uncoated Col-Ja,FelO, spinel femtes prepared by co- precipitation method. Dey and Ghose 1281 reported that the presence of mme hydtoxyl ions are compldely m o v e d

Table 2 Sumwry or mom omprnturr magna!c prnyrbrm o lCql . ,Jn ,Rp, n a n o p m b

sampla Magnetr para mete^ a1 1194 I I ~ A , ~ A K ~ B ery~wi l~o h t 1 m mtutmm I A )

Preparation technique suitable for preparation of Co-Zn ubstituted ferrite nanoprticles is reported. Co~-,zn,~. Fe20, nanoparticles can be prepared by the co.precipita-

Fig 6 Vnriatton o l r m a n c m w~lh Zn concentration lor Co,.Jn,Fefl, wlth x varying lrom 0 lo 1.0 (~n+t Vartauon of nmnncooc w~th Zn concentratton for Col.L.Fe104 w l h r wrying from 0.4 to 1.0.).

tion method for the full range of composition with x varying lrom x = 0 tol. The formation of Col_,Zn,Fe20r was confirmed by the XRD. The lattice eonstant was found to increase with the increase in zinc concentration. The average crystallite i i (DlvaR) decreased when the partial substitution of zinc incregsed (x=O-1.0). The specific saturation magnetition was found to decrease with the increase in zinc substitution excepl for x = 0. Low wercivity was observed for the high Zn concentration for the Co-Zn ferrite nanoparticles. In the present work, the samples prrpared by co.precipitation method showed superparamagnetic behavior, which was documented by the hysteresis loop, measured at room temperature. Here the CoFezO, prepared by co-precipitation method shows that it is 8 not very hard magnetic material l i n e the hysteresis loop is very small and it is known that ZnFe204

1151 R.V. Upadhyay. RV. Mclha. K Parrkh. D. Snn~vla. R.P. Psnl, J. Magn Mngn Matcr X I (19991 129.

116) Y. Shi. I . Dmg. X. Liu. J Wang. J Mapn Magn. Muter. 205 ( I W ) 249,

1111 C.N. Chinnwm). M. Senoux. B Jryudsvm. 0. Pemks.R~r K Shincda. K Toh~j~. I Colloid lntcrfaa k i 261 ( 0 3 ) 80

118) P.C. Moraia. V.K Carg. A.C. Olive~m. L.P S~lva, R.B h v d o . A.M.L Silva. E.C.D. Llma. J Mrgn. Magn Mutcr 225 (2WI) I 1

1191 P.C. Fann~n. B.K.P. bu lk . AT.Gisnniuis.S W Chtrkr.1. Phys.D 35 (2W2) 1395

1201 R. Aru~muru~sn. G. Vaidjrnathan. S. Sendhilnathan. B Jeysdevan. Physh-a B 161 (2WSI 215

1211 8. Jayadevun. C.N Chinnanmy. K Sh~noda. K Tohj~. J Appl Phys. 93 (2W3) 80%.

122) B.D. Culllly. Eicmenls of X.ray d~firn~on. wond cd. Addison. Wcsley Publ~siung Company. Lnndon. USA, 1918.

1231 C. Dong. I . A@I Cryn~llogr. 32 (1591 818 1241 D.H. Hnn. 1.P Wmg. H.L Lou.) Mrgn Mugn Muter 136(19Yll

116 1251 C..K. Kim. J .H. k, S Krloh. R Munkami. M~ le r R a Bull 16

(2Bi)l) 2241 1261 G.M Kalc, T Arokun. Appi Phyl Im 62 (19931 2324 1271 D, Crwngu. Ch Culuprru. J. Mupl Mrgn M a n 289 ( N J I XI (281 S. hy, J thole. ~ u l c r BUII 18 0011) 1653

Sci 41 (19991 1661

Len 4 ( 2 ~ 4 1 381. h6,j

I I

Synthesis and magnetic properties of Co-Zn magnetic fluid

Abstract

rurfuctant Thc magnclllallon

I 2W7 Published by Elrvler B V

PACS 7550Mm 7515 * a 7575Ti 87M4

Magnetic nano particles are of grml @ct\pological importance because of their use in tb @ration of

- 'Compondinp author, hl. : r91411l65S281iM7: fax.

~914132655IOI. &.MU lddnws: gvnp(~yahw.wrn (G Vaidyannhnn), mdh~.

~9f~yahoo.w.in (S Scndhilnatb) 'UP> Tcl.: +914132M1151.

substilulion using co-precip~tation method. Rath et al. [3] reported the dependence on cation distribution of particle size, lattice parameter and magnetic properties in nanoslze Mn-Zn ferrite for direrent degrees of Zn substitution prepared by hydrothermal precipitation method. The use of Mn-Zn ferrite for the preparation of temperature sensitive magnetlc fluid by co-precipitation method has already been studied 14-61, C00 .~Z&.~Fe~O4 fine particles have been prepared by chemical w-precipitation method foliowed by sintering 171. Control of particle size in the nanometer range by the variation of synthesis condition is always a difficult task. This becomes mandatory in the case of preparation F F using w-precipitalion method. In order to prepare F F having such fine particles, spcific size restriction is impomi wncidcring the stability criteria. In this paper, we report the preparation of Col-,Zn,Fe204 fluid with x varying from 0.1 to 0.7. Though funher Increase in the zinc wncentration ( ~ 0 . 7 ) leads to the formation of ferrite nanopanicles, due lo low magnetic volume force when dispersed in a carrier liquid, it is not of any practical use. Hence particles with x>0.7 are not considered

T n k I Magnnmuon mrsrurcmmls. SANS. Xay dillnnlon 01 the Outd slmplrs

-

Samplm Msgnci~. musunmcna IBrroFiud. FFI SANS (IcrmRu~d. FFI Rnrk un from XRD. Dx~n

hi, IkA ml Log.narmul Panrk 81.. D, Polydtspmrfy. o, Punlda NU. D, ( p ~ r r ~ l a ) (om) pmmrlcr. n, lnm) lnml

Cot .Fa@< 23 31 0.125 I I I9 0 65 11 71 12.02 CCZ d b ibtA 20 19 0.390 7 19 0.69 11 20 Ia.S5 C%d*nFc,O, 1749 0 142 5.46 OM) 1169 8 NO C o o ~ Z s I~F~:O, 15 80 U.523 J 083 060 11.5! 8 OX C o o d b ~ F e @ ~ I3 I 0 W5 4 92 0 M IIU1 8.05 C a d % xFc20r 1 1 20 0495 4 70 0 69 l0W 711 iooruZnosoFerO~ 9 20 o 415 4 U OW IO.01 712 C%luZnu rnFrlO~ 7.32 0 457 4 25 0 J 6.6 7.25

*here D , IS the median maenetic diameter of the log- normal volume drstribution, Md IS the domain magnetiza. lion of the particles, M, is the saturation magnetizat~on o r the fluid and Ho IS the intercept obtained by extrapolation to M = 0 of the straight line oblained for high fields ~mhile plotting M = j ( l / H ) , a = (m- 3 ) / 2 x where X, is the initial susceptibility or the fluid. The lognormal parameter a, was determined from the magnetization measurements uslng the equation:

prepared by co.precipilation method showed superpara. magnetic behdvlor, whlch ~ , d o c u m c n t c d by hystcrcsls loop, measured dl r o w Q!mnture Particle size wns found to decrease with i n c r e w in rlnc substitution

Acknowledgments

The author! a18 thankrul to the rcreree's comments. whrch mabled In bringing the manuscrlp! to the present form Og6 of the authors Dr G Va~dyanathan graterully &owlc&s CSlR (Ref no 03(100I)/04~EMR.ll) ror the

4na l i aa l assistance rece~ved through the prolccl

Referewes

I l l E Atuana D Z~ns E Blums R Mualnn J Muer SCI Y (IWI The partlcle size and lognormdl parameter mku la t td 1253

ror the prepared Ruld samples are glven In Table I , The pi E Auna D Zina E Blum R Mvasln Magn O~drcd~namikn 36

P p ~ ~ 9 : ( ~ ~ h ; ; l ~ K"'ksrn' Duk.

panlcJes D~~~ [ I 2 ] and K Shlnnds K Toh) I Appl

partlcle sru 161 T Updhyly R V Upndhyay R V Mchta Phyl Rev 0 15 (1997) 55R< , . ....

, , 171 S. Day. J Ghou. Maer. Rsl. Bull. 38 i2W3) 1653. I81 S.D Llkhia. C. hdhakn~hnnmunhy. O.T. Munhy. R. Nlganjm.

Characterization of Col-,Zn,Fe204 nanoparticles synthesized by co- precipitation method

G. V a i d y a n a t h a n ' , S. S e n d h i l n a t h a n h . *

Flnc nanopanlcles oi Cot ,Zn,Fe104 W I I ~ stolch~omelnc proporttan (r) vurylng from b la i d were prepared by the chem!cul co. preclpltatlon method The sumplea were chdrdctenzed ul~l~zlng X-ra) d~llract~on (XRF)), vibrrtlng sample mu~ne!omcter (VSMI Fourler transform Infrared spectroscopy (FTIR) and electron pdramngncls relpnaw WR) tcchnlques The vpec~hc hplurallon magnetlzatlon IMs) of the parllcles has measured at room temperature The p~e(tpuldparl~cler wcrc coaled ulth olclc ac~d a the rurfactanl b) suhable method ior the prepdrdtlon of lerrofluld I 2W7 Published by Elsevler B V

I, lntroduetlon magnetlc property can be altered by Ihe a d d ~ t ~ o n of the zlnc A d d ~ t ~ o n of nnc also affects the lattice parameter (Q)

Magnet~c nanopart~cles have attrdctcd t!i# a(&1Upn of Varlous prcparatlon techn~ques. such as rcvcrse mlcelle researchers of varlous fields due to the~r c%@dve appllcp techn~que [12-141, sol-gel pyrolysis method [IS] hydro. uons m rnformat~on storage system, &~&&gnost~cs, thermal techn~que 1161 and mechan~cal alloying (171 arc lerroflu~d technology, etc fi-5j Th1s4 @ ~ l y baause the used to prepdre ferr~te nanopart~cles But co.preclp!tatton nano~drt~cles d ~ r e r from those &e&respond~ng bulk method ts cons~dercd to be economtcal means of produc~ng matenal [6,7] For the prepd@t&Wof mdgnetlc flu~ds. nanopanccles with a part@?a,m$ order of lOnm and

s (Co-Zn) arc chosen due to thelr

calortc energy wnverslon [10,11] It 1s well known that the

fine particles [18,19] for the preparation of temperature sensitive fcrrofluid. Hydrothermal method even though cheaper is much used in emuls~on prcparatlon for water based ferroiluid, lor heat transfer studies it is required to prepare fertofluid with higher boiling point and lower volatilily. For this, chemrcal co-precipitation techn~quc is much suitable for better surfactant adherence. Using co- precip~tation melhod we can prepare nano-sized transfor. mer oil based ferrofluids having high boiling point snd viscosity, which could be usod for the preparation 01 temperatun msit ive magnetic fluid. The phyeical proper. tics of nanoparticles are of current i n t e r ~ t due to h e s k - dependent behavior obsctvcd in the nanometer kngth r a l e and high crystallinity. In the present work, we have synthesized GI-,Zn,Fe104 with x varying from 0 to 1.0. The influence of zinc subrtitution on the ctynallite ri2c and

[PHYSB : 3023621

magnetic properties depends on the preparation condi- tions. Preparation and properties of Co-Zn ferrites have been reported only for particular value of x or limited values. leyadevan et al. 1201 successfully prepared nearly monodispened single.domain cobalt ferrite particles, which could be used for the highdensity recording med~a. Morais et al. 1211 have reported the possibility of controlling the size of the nanoparticles using direrent ctirring speeds. Temperalure sensitive magnetic fluid having Coo lZno ++lo4 particles was used for the study of thermal convection [LO] and diestcr based ferrofluid having Coo ,Zno,,Fe204 nanoparticles have been reported [:?I Arulmurugan et al. 1231 have reported vanation of physical properties for Col.Jn,Fe204 nanoparticles w ~ t h i varying from 0.1 to 0.5. To the best of our knowledge. complete range of Col. ,Zn,Fe204 nanopartlclcs with x iarying from 0 to 1.0 have not ye1 been reported. Here we report the modified preparation condition for the produc- {Ion of fine particles of Col_,Zn,Fe204 with x varying from 0 to 1.0. The structural and magnetic properties. uhlch depend on the erect of zinc substitution, arc also i~ud~ed . In the present paper, we report a comparative dudy of the Fourier transform infrared spectroscopy IFTIR) and electron paramagnetic resonance (EPR) ,pectra for Col .Zn,Fe204 with x varying from 0 to 1.0 far the oleic acid uncoated (SI) and coated ($2) particles.

1. Experimental

I i Synrheris of Co,_,Zn,FeiO, uncoared nanqarrrcles SI J

Col.,Zn,Fe204 fine particle with x varying from8 to 1.0

10.63M dissolved in

Particles were collected at this stage by using magnetic pparation. These particles were washed several times with distilled water lollowed by acetone and dried at room emperature (RT). We shall denote this precipitate as S1.

' 2 Synthesis ofCo,-,Zn,Fe20c coafednanoparfieles (SZi

The pH of the solution was reduad to x 10.5 as coating o i s~ l fac t~n t takes p l a a only at pH in betwan 10 and I I.

Oleic acid ( C 1 ~ H ~ O 2 ) was used as the surfactant ancr heating it with NaOH solution at a pH of 10. for the conversion of oleic acid to sodium oleale. The sod~um oleate solution was transferred to the redctlon vessel and stirred continuously for nearly 3 h. Coating or surfactant was cnrrled out al a temperature of about 80°C and mainlamed at that temperature for 3Om1n. To coagulate the oleic acid coated particles, dilute HCI was added. Aner decantation, the product was washed a number of tlmrs wlth dtstilled water to remove soluble impurities. After removlng the excess water by washing il with acetone. the coated particles were collected. We shall denote this precipitate as S2.

The X-rdy dllfrdct~on (XR$ tterns of the samples were recorded on a Phi11 s d A L Y T 1 C A L X'PERT PRO X-ray. powder d y uwng Cu K, (; = I 54060A) rad~atto$ SI scans of the selected dllfractlon peaks w& out ~n step mode (step ~ I Z 0 OS', measurement tim* 5 s, measurement temperature 25 C standarb $I bm;iler) The crystall~te SIZ o r the nanocrystalbne~m&s was measured from the X-ray llne broddenlng malyvs using DebyeScherrer formula after accounttngbr the instrumental broddcning

w h c r r i ~ s the wavelength of X-ray used In A, fl the FWHM l a Adrans In the 20 scale, 0 the Bragg angle. D X ~ " the crystall~le size In nm 1251 The lattice constanl (00) was determined as a functlon of zinc content Cobalt ztnc ferrite has a spinel structure 1231

2.4. Magnerlc measurements

RT magnetic measuremenls with a maximum magnetic field of 1194.15kA/m were carried out using a Lkeshore vibrating sample magnetometer (VSM) (model 7404) and parameters like specific saturation magnetization (M,), coercive force (He) and remanence (M,) wcre evaluated for Col.,Zn,Fe204 uncoated nanoparticles (SI).

2.1 FTlR rneasuremenrs

FTlR spectra wcre recorded for dry samples (uncoated (SI) and coatcd (S2) with oleic acid) of Co1.,Zn,Fe2Or with x varying from 0 to 1.0 with an ABB BOMEM 104 FTlR (range 4@&#3~rn-~) spectrometer. The d q samples were in KBr matrix, and spectra were measrtc aceording lo transmittance method. The spectra wen resolved with a resolution of 4cm-I. For spinel structure it is allowed to identify some M e 4 vibrations as well a! the presence of the water adsorbed on the particle surfsce To understand the adsorption mechanism of the oleic acic on the surface of the cobalt nanoparlicles FTlR mcuure

ments were also carried oul on pure olelc acid and transformer oil.

2.6. EPR mcaturemenrs

EPR measurements were carried out using JEOL JES. TE 100 spectrometer having X h n d frequencies (9GHz). EPR spectra were recorded at RT at 300 K and at l~quid nitrogen temperature (LNT) at 77 K for the dry samples (uncoated (SI) and coated (S2) with oleic acid) of Col-Zn,FezO, with x varying from 0 lo 1.0. The powder samples containing in 3mm dtameter quartz tubes were cooled to 77 K using a liquid nitrogen dewar insert loaded in the cavity. and the cooling was done at direrent values of the applied magnetic fields. The spectrum is the first derivative microwave absorption with respect to field (dP; dH). For each sample. the H, resonant magnetic field 1261, the value of pcak.to.peak line.width (AH,,) was computed as the difference between the extreme values H I and Hz of the magnetic field (the maximum and minimum of the resonance curves, respectively). Thc resonant magnetic field (H,) was computed as (HI + H2), 2

3. Result and discussion

3.1. Physicul characrer1:artun

Generally XRD cdn be ured to characterize the crystal. I~nlty of nanopdrtlcles and ~t glves average d~ameters of aU the nanopartlcles The preclp~tated fine pdrucles w e n characterized by XRD for structurdl determlnauon and estrmatlon of crystalllte srze XRD patterns were analyad and rndexed uslng Powder X software [271 Die cxpri. mental peaks were matched w~th the theoplafly @er. ated one wrth no peak being left un-rndqe$ The lattice constant (00) was computed uslng t h k 4 W w m d w~th them respective ( h k o parameters q l w o f t h e d~ffrac- tlon pattern conforms the lo 14 of cubrc spinel

range The peaks of(220), (3 1 I), (400), (422), (5 1 I) and (440) have ban dcconvoluted to Lorenlz~an curves, uslng Peak Fit software for the dotemnation of FWHM value of the Indexed peaks 128) The lat tra constant was found to ~ncrease from 8 384 to 8 451 ti wlth the Increase In zinc concentration The lattice constant (OO) was found to lnaease w t h the ~ncrease e Zn content, whch suggested the formation of a compositionally homogeneovs s l r d solution. The latbce constant (OO) was found to be w t h n

the range o l the lat t la consunts olZnFcz0, and CoFcrO, This incmse could be also attributed to the substitut~on of the large sized Zn cation for the small sized Co cation. Thc

Fig I lndrxcd X.uy dillrvctcon punern Lr unwatd Co,.Jn,Fc204 nanapanlctcs vllh r s 0. 0.2, 04. 0.6. 0 8 and I 0 alter mprsipilat~on IS11

lattice constant obtained for CoFe204 (8.384i) is c lox to the known of bulk CoFclO, (8.395A) [I]. The lattice constant for Cc-Zn ferrites is very c lox to the values reported by Lee el al 1291. Fig. 2 shows thac the lattice constant increases with increav in zinc concentration. The crystallitc sire (DXRD) was estimated by the DebycScher. rer tormula [27] usin& the full wtdth a\ half maumum value of the respective indexed peaks. The average crystallite sire ( D r v r ~ ~ ) decreases from 12.02 to 6.92nm when the partial substitution of zinc increaxs (x = 0 to 1.0). This value 1s close to the crystallite size reported by Arulmurugan ec al

Fjg 1 Var~rtlon of svrragc c r y ~ t a i l ~ z ~ ~ SIZC ID,..~RI wllh Zn con* lion for rhe umasld Col.Ln.fcfl, nannpamcicb vtth x urryq,hamU

1231. Though sll the samples were prepared under ident~cul condition. the crystallite s h was not the same for all concentratrons of Zn. This is probably due to che preparation condition lollowed here, which gives n v to d ik ren t rates of lerritc formation lor dikrenr conantra. tions of unc, favoring the varralion of cryslallite sue. The v a r i a h of rverage crys\alli\e size wdh the zinc concen- tration IS given in Fig. 3. Fcrrofluids can be convcnientl) prepared by making use of particles in this sire range.

Fig I Vvr~it~on of apxih sturutlon mugncrmtmn w~th Zn conanIra. son for the umorrtd Co,-2nLnStP4 nanopnxlcx wwhh x vrnlnp fmm U to 1.0 (SI)

F I ~ 4 Room m p r s t u n mrgnn~ntlon cum for unwsld nsnopanl- SI (a) Co,$~00 , b) Coo& ,b&~ P1.c) C C ~ ~ & P Z S ~ C I O < (4

C%idryd&, (c) Cop&aFcP~ (0 CooZb&cP& (6) n) 11) C O O ~ ~ Z ~ ~ ~ F C P ~ 0)

4 l&#@1 Sd 0;) hl0FhD4

I , , . , . , . , , I 0.0 0.2 0.4 0.8 0.8 1.0

Zn owwnlnm

FI 6. Varirtlon of ran- wlb Zn w m v a u m for thr vncol!d fn Q,.Jn,FeLl, nmopnickr w~th x varying rmm0 UI I.O(Sl). (Inn: Vsriniim d m m wid L ummtntion for cO1..ZrXcP4 with x varying Irm 0.4 UI 1.0.)

Rg 7 Vanat~on 01 cocrc~v>l) wllh Zn conccntrauon for uncoated Co,.3n,R10, nanopan~cles uith r rary~ng lrum D 10 I D (Sl i ( I n w Vanatton of merc~vity wtth Zn concentration for C o , ,Zn.FrlO, wllh 8

varying lram 0 4 to I 0 )

In the cubic system of rcrrimagnctic spinels, the magnetic order is mainly due lo I super emhangt Inlernction

Tau I Comprnm or t h matn FllR Irmlmlamrr brndr br Fato, md B r u w t d Co,.Jn,Fc@, nanopmlc*aul~h r vaylna l r m O s, I olnlr wpmlp lut lon (SII

~ --

Samples I R rhrorpttan hnd, lcm 'I

,'. I',

Fe10, ?d4l 41 IS11 8 X U 215 1 9 2 0 Cot oR,O, 1S73M 1510 I8 911 56 181 97 Chisdliu~~Fe@, 1219.90 1518 65 9247h 511 J I ChixuZbruFc:O~ 313699 .J$UI.11 924 76 627 11 Coo~Lliul~tFc20~ 3355 12 " I#? 11 MI 09 a 8 7 7 ChiauZn~oFe~O~ 139202 'fJ0011 M I 0 3 SXI 97 C % ~ d n ~ y ~ k , O , l 1 4 6 ~ ~ " " 1 W 3 1 924 70 60R 77 COUUZSNIFQO, 31SS.b 1510 IX 941 69 63557 ChiuSnor~FcP~ 'I1(Lld"', ISIU 31 Y24 76 MY1 11 C%NZnooFcfJ, , 1328.2 1510 I8 914 89 581 97 Csi d l i u a F ~ > Q ~ ISI(1IX W(14! 5 ~ 1 4 ; Znl0Fr~O4 r : $2 i l l 0 I8 91312 W877

~, .,

fit. 8. F l l R spktn lor FcG, and lor u r ro l t d Col.Jn,Fcr04 FI&. 9. FllR sptn lor mlrd CO~.&.F@~ ~ n 0 p f l b k I with I ~ ~ n o p n r k a with x = 0,O.S and 1.0 lftcr mpcipiution 6 1 ) x - 0. 0.1 ud l .O Jlcr maling with el& uld 62).

mechanism occumng between the metal ions in the A and possible lo vary magndlc properties of the fine pan~cles. B sublattices. The substitution of nonmagnetic ion such as FIB 4 shows the RT hysteresis loop of the p r e p a d Zn, which has a preferent~al A site occupancy results in the u n w n d powder samples for various dnc substitut~ons. reduction of the exchange interaction between A and 0 Fig. 5 shows the changes in the spa~f i c saturation sites. Hence by varying the degree o l u n c substitution it is magnetinlion with the d c g m of unc substitution. From

Tnbb 2 Comparim ofthe maln FTlR r ranmt lnna bands lor cortrd Co,.&n,FezO, nanopvruclca w t ~ h A vaylng imm 0 u 1.0 ancr mrt lng wnh alctr rcd 62)

Srmpln IR a t u q l b . *Ib (em-')

Cot oFez0, C % d h 1oFeP4 C s d s d c z o , C h d s lbetO4 CouhoZs~FeP, Coo x2k wFc204 L o o Z n u nrFc*Ol Cov dhroFciO4 Cov zZlla"oFez0, Co, ~ZswFczOa Zn, $e,O,

344030 292294 285147 111317 1422MI 2921 6R 2852.11 1113.11 3422.W 2927.80 2846.59 1713.11 1384.93 2921.68 2846 19 1113 17 3391 29 1922.34 2851 95 1112.02 33974U 292249 2851 95 1713 17 MU370 292291 281285 171198 1422 W 2923 ll 285303 110681 3311403 8 2 1 74 2852.32 1714W 3410 12 292190 2853.61 1706.81 ISM 10 2822.61 215293 1713 56

Fi& 10. EPR lpktra lor w t e d Co,.&Je@,vith x - 0 , 0.2. 04, 0.6,O.Baad 1.0U RT(IOOK)(SI).

Fig. 5 it can k seen that the variadon pattern of specific saturation magnetization (M,) as a function of Zn content shows an increase for small substitut~ons, goes through a maximum value ( 4 6 . S ~ ~ m ' i k g at 1194.15 kA/m for x = 0.1) and then decreases. It is clear from Fig. 4 that the panicles do not show any saturation for x = 0.9 and 1.0 even at 1194. IS kAlm. It almost behaves linear. Figs, 6 and 1 show the variation of remanence (M,) and coercivity (H,), which decrease with the increase in zinc substitution. Inset in Figs. 6 and 7 shows the vanation of remanence (M,) and coercivity ( H e ) for Col. J.n,FelO, with x varying from 0.4 to 1.0.

Zinc ferrite belongs to the category of normal spinels and its net magnellzation IS zero [!5]. The anomalies in the magnetic properties of unc ferrite may be due to that the zinc ferrite samples are not completely the normal spinel.

1 This anomalies also have b a n explained by Kamiyama el al. [3C-321. The occupancy of ~ e " ' ion lowrted at the A s ~ t e

is larger in smeller crystsll~le samples and the catron substitution increases with decrcssing crystullitc s l a

The changes in magnetic properties such as M,. H,. M, are due lo the lnflucna of the cationic stoich~omctry and their occupancy in the specific sites. In additlor,, formation of dead layer on the surface, existence of random canting of psnicle surface spins [28.331, non-saturation efTmts due to random distribution of p r l ~ c l e s t a , dcv~ation from the normal cation distribution, presence of adsorbcd water. etc. 11 I] might be the cause for the reduction of magnetic properties of nanopanicles. Renction temperature of 85'C and pH 12.0 were used fa synthesis CoovZna lFe204 fine particles (10.85nm in size) having highest specific saturn. t ~ o n magnetization of 4 6 . 5 5 ~ m ' i k ~ . The dcviat~on of cation distr~butlon in nanopartsles from the bulk. 1nAu. ences the temperature dependence of magnetization

Fig. I ! . EPR lpnn lor u m u d Co,.Jn.F& W~th x -0.0.2. OA. 0.6, 0.8 lad 1.0 11 LNl (77K) (50.

[PHYSB : 3023621

(1713.17-1706.81cm~') was derived from the exetence of the carbonyl nretch 141,421. The bands at v , (1562.99-1541 em-') and (1438.09-1406,46~m-~) exhi- bit the presence of the oleaa Ions, chem~cally bounded to the metal atoms from the ferrophase panicle surface at the level of the oxygen atoms 1421. The assignments of peaks are listed in Table. 2.

3.4. EPR measuremenrs oJCo,. ,Zn,Fe*O, unroofed ( S l ) and roared ( S 2 ) nanuparrirb

The EPR spectra obtained at RT and at LNT ror uncoated Col.,Zn,Fe204 with x = 0. 0.2.0.4.0.6. 0.8 and 1.0 particles after co-prec~pitation (SI) are shown in Figs. 10 and 11, respectively. Figs. 12 and 13 give the EPR spectrum obtained at RT and at LNT for Col-,Zn,Fe104 with .r-0, 0.2. 0.4. 0.6. 0.8 and 1.0 part~cles aner coatlng with oleic a c ~ d (521. The peak-to.pcak line.wldth (AHrp) were round to decrease with increase In zinc concentration while the resonant magnetic field (H,) was found to

increase with the Increase In nnc conantration for b o ~ h at RT (302 K) and at LNT (77 K) or coated and uncoated particles. The line.width d~rcascd from 245630 to 14.636 kAlm on increasing Zn conantratlon for uncoated Cc-Zn ferr~tc (SI) at RT (3W K). The decrease in the line. width (AHpp) with Zn concentration may be due to dipdt-dipolc ~nteractions in Cc-Zn Drrite. At low temperalure. the 11nc-width was large due to the scatter ~n direction of anlsolropic field of pardcles. As the tempcru- lure was increased. the tendency to make magnetic momen! so tropic cnused Ilnc.width lo decrease 1431. Magnchc dipole ~nteractions among particles and super.crchangc ~nteractions between the magnetic ions though oxygen ions are two predom~nant faclorr that deterin~ne the EPR resonance parameters and peak-topeak 11nc.width (AHpp). Strong dipole ~nteractions gve i( largc peak.10-peak line. w~dth (AHpp) and strong , r.exchange Intcracuons produa a small pcak-to-pa ewidth (AHpp) 14-41 Figs. 10 and 12 show the c* d the broad line w~th the increase in Zn c o n ~ 9 a t i q . Clearly the broad s~gnal

.' '.,?>.*

Tabk 3 EPR data of unccalrd Coi.Zn,Fc?J, nanoprn~cks nth x wrytng Imm 0 to 1.0 d c r a+prs1pt8uen ISlI

Sample RT (WK) LNT (77Kl

Lla.w~dth t y p H, (kA1m) AH, IkA,mI L~wwldth l y p H, (kArml AHm, (kA!m)

Col oFe04 Nrrrov 245 I10 1821 Coo&!% ~OFCIO~ - Narrow 245 155 3172 C%dbmFezo, - Narrow 245 W 3 1x1 c S i ~ b ~ ~ u ~ e , o ~ - Narrow 245 498 3 Mb CooulnoaFe>O, Broad I6l.PW 245630 Narrow 245 1113 1076 Coa db loFc201 Broad 206.220 222 155 Nsrinu 245 UW 3004 CooruZb,Fe,O, Broad 231 259 125 372 Coo db aFc,O, Broud 242 985 95925 CoodoZS ~oFe:0, Broad 256 119 62 MY Brord l9 l49 l 3WBI1 COU ,&bwFr?O, Broad 262 558 41 2b2 Broad I95 552 288 180 Zn, $hO4 Broad 265.644 14.636 Broad 2 % 1 ~ II~.XUI

I I lndlcato that the rrctrum is not complctr In X.band frequency apnromrttr

.L;1 r! jLl

Tdhlr 4 ,!,ti. '

EPR dald of coatrd Col.,Zn,Fc:04 nanopniela wnh x varytng from 0 to I O alter co.pralpltuuon (S21 ; 1. S ~ ~ Q ~ C S 1

RT (IW K l " LiW (77KI - -

L~nr.wldth t y a H, ikA'mi AH, (IAtm) ~ 1 ~ 1 4 t h ~ H, (kAlml AH, (kA!ml

CII 0Fe20, t&W 241,366 5867 Co,,&b ,oFc,O, Broad I75.WI 295482 Manor 241 44 5 791 Co, ,&b 2nFc:04 Brnsd 177.742 289 537 Narrow 241.479 5 M2 Cn, ,&n+ sFe20, Broad 177919 27368 Fhrrow 241 Sly J 522 (oswZnopFc~O, Broad 220 591 197 845 Narrow 241 553 c 494 1 ~,~dn~loFe,O, Broad 233 463 137 IS7 Narrow 241.597 5 UlS ~ ~ ~ Q H I F C ~ O , Broud 240.452 %?I 5 ( 0,) &Ito ioFclO, Brodd 241.1Y8 Coo &Q .Fc20, Broad 259.205 :%, Broad IX4155 .W818 VnoloZb wFc~O, Broad 259 410 r.161 Broad IR 078 2% 745 Znl 0Fe2Or Broad 259.M9 2027R Broad 251 W 81 469

I I ~nd~cata that thc swtrum ta not eomplctr m X.band.hsqulloyRcnromctc~ " ,

,, L

replaced by the sharper one (for .i= 1.0) [46]. Tables 3 and 4 represent the EPR data of coated (S1) and uncoated (S2) C0~-,~Zn,Fe20~ with x baying from 0 to 1.0 particles after w-precipitation. All \he RT spctra at 300 K (Figs. 10 and 12) show a single broad line, indicating the non.existence of isolated @+and Zn2*. EPR spactra for uncoated (SI) particles OrCot-&pgO, with x varying from 0 to 1.0 at LNT ('7K) show a narrow line at the center in addition to a $ndgle broad line. It is known that in a randomly oriented

dispersed ferromagnetic part~cle the absorption linc.width turns oul lo be a non.monolonic function of temperature. At low temperature, the line-width is large due to scatter in direction of anisotropic field of particle (inhomogeneous broaden~ng) 1431. In Tables 3 and 4 the dash indicates that spatrum is no1 complete (both the values of HI and HI cannot be obtained). Using the X-band IEOL JES.TE 1W spectrometer the complete spectrum may not be possible for those concentrattons. Perhaps complele spectrum msy be obtained uring the Q-band frequency spectrometer.

Preparation tahnique suitable for preparation or Co-Zn substituted ferrite nanoparticlm is reported. Unco~tcd Cot-,Zn,Fe204 nanoparticlea can be prepared by the M. precipitation method for the full range of composition with x varying from x = 0 to 1.0. The formation of Col-Jn,. Fe204 was confirmed by the XRD. The lattice constant waa found to increase with the increase in zinc conantration.

STUDY OF SMALL ANGLE NEUTRON SCATTERlNG (SANS) ON Co-Zn MAGNETIC FLUID

C, ~aid~anathan" ' ,S. ~endhl lna than~, V, K. AswalP a-Depmmcnt of Physics, Pondichery Engineering College, Pondicherry.605014. India.

b-Department of Physics, Sri Manskula Vinayagar Engineering College. Pondichery . f10S 107, India. C-Solid State Physics Division, Bhabha Atomic Rcsearch&tre, Trombay.Mumbai4M) 085. India.

Abstract Co-Zn ferrofluid (Col.,Zn,FelOl) were synthesized by chemical co-precipitation

method. The fine particles were suitably dispersed in transformer oil using oleic acid as the surfactant. The transformer oil feaofluid after dilution was characterized by SANS. The size of the particles were estimated from SANS and compared with the size of the particle measured from magnetization and X-ray dimaction.

PACS : 75.50.Mm; 75.75.ta; 75.75.Tt; R7.64.pj Keywords: co-precipitation; ferrofluid, femte

*corresponding author Tel.: 191 413 2655281x647 Fax: +91 413 2655101 E-mail addresses: gvn nec@vahoo.com (G. Vaidyanathan),

sendhil29~vahoo.co.in (S. Sendhilnathan).

1. Introduction There is a great interest in the preparation and application of nanometer size

materials since they can exhibit novel properties of industrial interest. Magnetic properties of nano sized particles of both ferromagnetic and femmagnetic materials have attracted considerable attention in recent yean because of their unique properties, which makes them very appealing both from their scientific point of view and the technological significance enhancing the performance of the existing materials [I]. For this reason, research on the synthesis and characterization of magnetic nanoparticles has received increasing interest in recent years. Metal oxides such as femtes are of particular interest for therapeutic and diagnostic medical applications due to their properties that can be tailored by changing input parameters of synthesis [2]. In recent years, a number of chemical and physical methods have been attempted to produce nano size ferrites. Various preparation techniques, such as sol-gel pyrolysis method [3] hydrothermal technique [4] and mechanical alloying [5] are used to prepare ferrite nanoparticles. But co-precipitation method is considered to be an economical way of producing fine particles [6,7]. The physical properties of nanoparticles are of current interest due to the size-dependent behavior observe in the nanometer length scale and high crystallinity. Controlling the particle size in the nanometer range by the varying the synthesis condition is always a difficult task. This becomes mandatory in the case of preparation fernfluid using co-precipitation method. In order to prepare ferrofluid having such fine particles, specific size restriction is imposed considering the stability criteria. In this paper, we report the preparation and characterization of Col.,Zn,Fe2Or fluid with x

varying from 0.1 to 0.7. Though further increase in the zinc concentration (x > 0.7) leads to the formation of ferrofluid, because of the low magnetic volume force when dispersed in a carrier liquid, particles with x > 0.7 are not of much interest.

Here we also report the size of the femfl particles of transformer oil based Y! f m f l u i d (FF) for Co1.,Zn,Fez0~ with x varying om 0.1 to 0.7 except x 4 . 3 using SANS. Small angle neutron, small -angle X-ray and wide-angle light-scattering are powerful tools to investigate the structure of colloids in details [8,9]. The scattering data contain information on the structure of the samples in the nanometer scale. This rang is interesting, if the atoms or molecules of the sample form colloid particles, or more generally, inhomogeneous parts of this size can be found in the medium. In this paper, magnetic diameter (D,), Physical diameter (D,d) are compared with the particle size obtained from the SANS (D,). By this comparison the thickness of surfactant coating (t) on the magnetic nano particles are can be obtained. The techniques used for the above are magnetization measurements, X-ray diffraction and small angle neutron scattering.

2. Experimental Procedure Nanocrystalline cobalt-zinc ferrites were synthesized using chemical co-

precipitation method. The aqueous solution containing femc chloride (FeC13), cobalt(1l) chloride (CoClz,6H~O), zinc chloride (ZnCIz) in their respective stoichiometry were mixed and added to the boiling solution of sodium hydroxidc (NaOH) (0.63 M dissolved in 1200 ml, of distilled water) after adjusting the pH to be around 12, within 10 seconds under constant stirring. The solutions were maintained at 85' C for I h. Precipitation and formation of nano ferrites take place by the conversion of metal salts into hydroxides, which occur immediately, followed by transformation of hydroxides into ferrites [lo]. Fine particles were collected at this stage by using magnetic separation. The collected particles were washed several times with distilled water. The synthesis procedure and the associated principles are described in detail elsewhere [I I]. Oleic acid (CIWH3402) was used as the surfactant (heating it with NaOH solution at a pH of lo), for the conversion of oleic acid to sodium oleate. The sodium oleate solution was transferred to the reaction vessel and stirred for nearly 3 hours. Coating of surfactant was carried out at 8O0C and maintained at that temperature for 30 minutes. To coagulate the oleic acid coated particles, dilute HCI was added. After decantation, the product was washed a number of times with distilled water to remove soluble impurities. Afler removing the excess water by washing the product with acetone, the coated particles were collected. These coated particles were dispersed in transformer oil and centrifuged at 5000 rpm for 2 hours. The ferrofluid of Col.,Zn,Fe204 fine particles were prepared only for x varying from 0 to 0.7.

3. Result and discussion Small angle neutron scattering (SANS) measurements were carried out using

SANS diffractometer at DHRWA reactor, Bhabha Atomic Research Centre, Trombay, Mumbai [lo]. The accessible wave vector transfer, Q is (4n / A ) Sin(0 / 2) ,9 is the

scattering angle, range of instrument is 0.018- 0.31 A', The scattering length of oleic acid, magnetite and D20 is 0 .4040~ 10-'~cm ,5.1562 x 10- '~cm and 1.9145 x 10-'*cm and the scattering length density for oleic acid, magnetite and D20 is 0.08 x 10'~cm",6.95 x 1010cm'2 and 6 . 4 0 ~ l0'%m" . SANS measurements were carried

out for the Col.,ZnXFe2O4 transformer oil based ferrofluid (FF) fluid samples with x varying from 0.1 to 0.7 except x=0.3 and D20 (benzened) based Col.,Zn,Fe20r transformer oil based ferrofluid based for x= 0,0.2,0.5, 0.7. The volume hction for all the samples was kept as 25% by diluting it. Patterns arc recorded at room tempraturc and experimental data were corrected for the kkground and for the empty cell contribution. Fig.1 shows the plot of &/dl2 -t Q for the transformer oil based ferrofluid. The plot generated using equation (I) is fitted to the experimental data and are shown in fig. I . For dilute polydispersed dispersion it is shown that [I21

d l , Where - 1s the coherent differential scattering cross section, N particle number

dR density, V is the volume of the particle and p, and p, are the scattering length densities of the particle and the solvent, respectively. P(Q) is the particle form factor and f (R) is the size distribution function. For fermfluid, the form factor is calculated assuming spherical particles and log-normal size distribution. For unpolarised neutron beam and in zero field (eqn. I) can be written as

=(&I&), t 2/3(&1&), (2)

where,

(&IdR)" = n(p, - P,")' ~V'(R,)I(R")P(Q~R",~R" (3) and

(&I&), = n(p,I2 Iv2(RM ) ~ ( R ~ ) P ( Q , R ~ ~ w ~ (4) where the suffixes n and M stand for nuclear and magnetic contributions, respectively. By incorporating appropriate values of the parameters the theoretical curve was generated. To fit the scattering curve we use the concept of a shell model consisting a sphere with an inner core radius Rp surrounded by a concentric shell of radius R, [12- 151. From the SANS measurement the particle size was determined for the C~l.~Zn,FezOd transformer oil based ferrofluid (FF) fluid samples with x= 0, 0.1, 0.2, 0.4, 0.6. Fig.1 shows the fitted SANS data for the C O ~ , ~ Z ~ ~ , , Fe20a at room temperature(300K).

The particle size was found to decrease with the increase in zinc substitution. The particle size (D,) of the fluid was found to vary from 12.75 - 6.6 nm decreasing with the increase in zinc substitution. The particle size obtained from X-ray d i h t i o n (Dd) [I I] agrees with SANS well. There is a very good contrast between the magnetic particle (Fefi) and the solvent (transformer oil). However, there is a poor contrast between a surfactant coating and the solvent. In view of the above, the SANS disaibution arises h m the core of the particles. Fig.2 shows the SANS pattern for a D20 (benzened) based ferrofluid. The particle size and log normal parameter calculated for the prepared fluid samples are given in Table.l. The particle size (Dm) obtained from the magnetization measurements of the fluid is less than the crystallite size of the particles

Did [I 11 This is due to the presence of a magnetic dead layer on the surface of the particles, which is seen From Table,l. In Co,.,Zn,Fe20, except x 4 and 0.7 coating thickness agrees with normal thickness of surfactant ie., nearly of 3nm. SANS results confirm that the surface modification increases the thickness of the surface layer, which in turn results in the decrease of magnetic radius.

4. Conclusion Preparation technique suitable for the preparation of Co-Zn ferrofluid is reported.

The fine particles were suitably dispersed in transformer oil based carrier liquid. Particle size was found to decrease with increase in Zn substitution. The particle size (D,) of thc fluid was found to vary from 12.75 - 6.6nm decreasing with the increase in zinc substitution.

Acknowledgements

One of the authors Dr. G. Vaidyanathan gratefully acknowledges CSlR (Ref. No, 03(1001)1041EMR-11) for the financial assistance received through the project.

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__ I_ _ - -_ _ 35

-Theaetical nt 0 E x p e t i M data

10

O!Xl 0.05 OdO 0.15 O B I I I I 4

Fig. 1. Sans pattern of ferrofluid at 300K

Fig, 2. Sans pattern for a D20 (benzene&) base C O ~ J F Q O ~ ferrofluid at 300K

Table,l. Magnetizntion measurements, SANS, X-rrl diffraction of the fluid samples