Journal of Magnetism and Magnetic Materials 257 (2003) 113–118

Size dependent magnetic properties of iron oxide nanoparticles

Jhunu Chatterjee, Yousef Haik*, Ching-Jen Chen

Biomagnetic Engineering Laboratory, Department of Mechanical Engineering, FAMU-FSU College of Engineering, 2525,

Pottsdamer Street, Tallahassee, FL 32310, USA

Received 19 March 2002; received in revised form 13 August 2002

Abstract

gFe2O3 nanoparticles has been synthesized by a combination of chemical and ultrasonication procedure and further

stabilized with surfactant. Their magnetic properties are compared with the different fractions (10–12, 20–30, 100–

150 nm) of commercially available iron oxide. The sizes obtained from the scanning transmission electron micrographs

are correlated with the magnetic properties of the particles.

r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Magnetic particles; Ultrasonication; Magnetic properties; Blocking temperature

1. Introduction

Magnetic particles made up with syntheticpolymers and biodegradable polymers are beingwidely used in biomedical applications such as cellseparations, protein purification, immunoassaysand drug delivery [1–6]. In most cases, theseparticles have maghemite (gFe2O3) or magnetite(Fe3O4) as the magnetic material in the core. Theseiron oxides behave differently in magnetic fielddepending on their sizes. It has been established byseveral groups that abrupt changes in propertiestake place when particles are in nanometer range[7,8]. Such as nanocrystalline iron oxide aresuperparamagnetic when the particle sizes aresufficiently small and they behave as ferrimagneticwhen the grain size is in micrometer range.

Depending on the magnetic properties and sizeof the core material, the applications of theencapsulated particles will change as well, suchas, particles in nanometer range are being used formRNA isolation and targeted drug deliverywhereas particles in micron range can be used incell separation. The relation between particle sizeand blocking temperature is established [9].Though literatures are available [10–12] forsynthesizing iron oxide particles with narrowdistribution by chemical route, commercial grades(mostly synthesized by dry methods) of iron oxideare more preferred for synthesizing encapsulatedmagnetic particles because of their availability. Itbecomes important to study the structural andmagnetic properties of the commercial iron oxidesand to compare them with the synthesizedparticles.Earlier [2] we used commercial gFe2O3 particles

with wide distribution of particle sizes. Thoseparticles were synthesized by plasma vaporization

*Corresponding author. Tel.: +1-850-410-6431; fax: +1-

850-410-6484.

E-mail address: haik@eng.fsu.edu (Y. Haik).

0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 0 6 6 - 1

process and had very wide size distribution (10–150 nm). Magnetic properties of albumin submi-cron particles synthesized using those iron oxideswere not very well defined. In this article we reportsynthesis of nanosized maghemite and comparedthe magnetic properties and sizes of differentfractions of a commercial grade maghemite withthe synthesized one. We adopted the coprecipita-tion technique [10] coupled with sonochemicalmethod in order to have particles with extremelysmall size and with narrow distribution of sizes.These nanoparticles have a tendency to agglomer-ate. Further modification of these particles wasdone by using a cationic surfactant (Cetyldimethylethyl ammonium bromide, CTAB) inorder to maintain discrete particles.These particles were encapsulated with albumin

[2], polyethylene [13], polypropylene, hydroxypropyl cellulose [10] for biomedical applications.Suslick et al. [14] have shown the effect of

ultrasound to produce nanostructured materials.No direct interaction of sound field with molecularspecies takes place during ultrasound application;moreover, the acoustic cavitation at the transienthigh temperature and high pressure causes somechemical effect, giving rise to nanostructuredmaterials. We observed similar phenomena whencoupled the chemical and sonochemical procedure.Particle sizes were determined by scanning trans-mission electron microscopy.In order to determine the magnetic property of

the commercial sample, magnetic moment vs.temperature data for both unfractionated and fordifferent fractions of commercial sample wereplotted. No blocking temperature (blocking tem-perature is the transition temperature between theferrimagnetic and superparamagnetic state) wasobtained for the unfractionated sample. Fractionswith smaller size distribution showed blockingtemperatures below room temperature. We com-pared the data obtained for different fractions ofcommercial maghemite with the maghemitesynthesized in our lab. Very small sized sampleexhibited superparamagnetism and high coerciv-ities. These particles have high potential in diverseareas such as information storage, ferrofluids [15]and magnetic gels [16] other than the previouslymentioned biomedical applications.

The relation between blocking temperature andthe particle volume can be given as [9]

TB ¼Ea

KB ln ft; ð1Þ

Where t is the experimental measuring time, theterm in the denominator in RHS can be treated asconstant. The term Ea is the anisotropy energybarrier has to be only considered for explaining thechange in blocking temperature.

Ea ¼ KV ; where K is the anisotropy energydensity constant and V is the volume of particle.

2. Experimental

2.1. Materials

Ferrous chloride, ferric chloride, sodium hydro-xide, nitric acid, CTAB are all obtained fromSigma Chemical Company and used as such.Commercial maghemite was obtained from Nano-phase Technologies Incorporation.

2.1.1. Synthesis

The preparation of iron oxide nanoparticles wasdone mainly in three steps.(1) Coprecipitation of ferrous chloride and ferric

chloride by sodium hydroxide: Ferrous chloride andferric chloride were mixed in a molar ratio of 1:2 indeionized water at a concentration of 0.1M ironions. The solution was used immediately afterpreparation. A highly concentrated solution ofSodium hydroxide (10M) was added to it forcoprecipitation with continuous stirring.(2) Heating and sonication: The solution with the

precipitate was stirred in high speed for 1 h at 201Cthen it was heated at 901C for 1 h with continuousstirring. The iron oxide dispersion was thensonicated for 10min at 50% amplitude using aCole Parmer Ultrasonic Homogenizer.(3) Peptization with nitric acid: The ultrafine

magnetic particles obtained were peptized by nitricacid (2M).The precipitate was then washed repeatedly with

deionized water and filtered and dried in vacuumto get fine iron oxide particles.

J. Chatterjee et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 113–118114

Modification of maghemite with CTAB: CTAB(30% by weight of iron oxide) was mixed with ironoxide suspension by vigorous stirring for 4 h. Thecoated particles were characterized by TEM.

2.1.2. Fractionation of the commercial maghemite

particles

The ultrafine commercial magnetic particles aredispersed in distilled water with sonication and a0.5 T magnet is placed below it for 1min. Largerparticles separated immediately and collected inthe bottom of the tube. The supernatant wastransferred in another tube and fractionated in thesame way for another two times. For the last twofractions time was time held over the magnet wasmuch higher. The particles were thus separatedinto three fractions.

2.1.3. Characterization

The X-ray diffraction pattern was obtained forthe iron oxide particles to determine its crystallinephase. (Philips Wide Angle X-ray Diffractogram.)The particle size and shape of iron oxide was

determined by transmission electron microscopy(TEM). 1ml of very dilute aqueous suspension ofeach fraction of particles was placed on carboncoated copper grids and observed under a scan-ning transmission electron microscope (JEOLSTEM) at 200 kV. Electron micrographs as wellas the X-ray diffraction pattern were obtained withthis microscope.Magnetization measurements were performed

using a superconducting quantum interferencedevice (SQUID) magnetometer with a helium flowcryostat. Weighed amount of samples are packedin gel capsules and placed tightly in the glass tubeensuring no movement in either direction.

3. Results and discussion

The wide angle X-ray diffractogram for thesynthesized iron oxide particles was almost similarto that of commercial maghemite (gFe2O3) asshown in Fig. 1. It shows that these particles arecrystalline in nature. Transmission electron micro-scopy for the unfractionated commercial particlesshows that particles are spherical in shape and they

have wide distribution of sizes. Fig. 2(a) is themicrograph for the unfractionated sample andFig. 2(b) is the size distribution plot for theparticles in Fig. 2(a). Fig. 3(a) is the micrographfor the synthesized maghemite surface modified byCTAB obtained at a magnification of 500K.Fig. 3(b) shows the distribution of particle sizesin the synthesized iron oxide sample. Particles sizesare determined from the micrographs (obtainedunder different magnifications) using an imageprocessing software, Digital Micrograph. Fromthe distribution plots of the commercial andsynthesized maghemite samples it is obvious thatthe commercial sample has very wide distribution(10–150 nm) and it has no particles less than10 nm, whereas, the synthesized maghemite has asize range ofB6–11 nm and most of the particles aabout 9 nm. The standard deviation in terms ofparticle size in the commercial sample as obtainedfrom Fig. 2(b) is about 54, whereas, the standarddeviation for synthesized sample is 1.58 asobtained from Fig. 3(b). Fig. 4(a) shows tempera-ture dependence of susceptibility for the wholeunfractionated sample on warming after: (a) cool-ing in zero field (ZFC), and (b) cooling in anapplied field of 50G. We did not observe anyblocking temperature within 300K and it isdetermined by measuring the peak position inZFC and FC curve. Due to presence of widedistribution in sizes as observed in TEM micro-graph, there might be an overlap of blocking

Fig. 1. Wide angle X-ray diffractogram for commercial and

synthesized maghemite.

J. Chatterjee et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 113–118 115

temperatures for particles with different sizes andso it could not be determined. But when the sameexperiment is repeated for different fractions weobserved the blocking temperatures for eachfraction in Fig. 4(b). This figure shows themagnetic moment (emu/gm) vs. temperature plotfor all the three fractions of maghemite along withthe synthesized one. Even though fractions 2 and 3contain smaller particles but still there aresignificant distribution of particle sizes.

There is not much difference in the absolutevalue of moments for the different fractions asobserved in the plot. The variation of temperatureat which ZFC and FC separates (Tsep) and themaxima (TB) in each plot with the sizes (asobtained from TEM micrograph) has been shownin Table 1.Relation between particle volume and blocking

temperature is given in Eq. (1). The decrease inblocking temperature with decreasing particle sizeas observed in Table 1 is supported by the fact thatthe blocking temperature is directly proportionalwith the anisotropy energy barrier (Ea ¼ KV ).

Fig. 2. (a) Transmission electron micrograph for unfractio-

nated commercial maghemite, and (b) particle size distribution

for commercial maghemite.

Fig. 3. (a) TEM of synthesized maghemite (6–11nm) at a

magnification of 500,000. (b) Particle size distribution for

synthesized maghemite.

J. Chatterjee et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 113–118116

Magnetization plots as a function of magneticfield is shown in Fig. 5 for sample C (fraction withsmallest size in the commercial sample) at both 5Kand at 300K. The blocking temperature of thisfraction was about 100K, which was below roomtemperature. Below the blocking temperature,

particle showed ferrimagnetic behavior with anincrease in Mr and Hc: A field is required to bringthe total sample moment to zero since the thermal

0.00E+00

1.00E+00

2.00E+00

3.00E+00

4.00E+00

5.00E+00

6.00E+00

7.00E+00

0.00E+00 5.00E+01 1.00E+02 1.50E+02 2.00E+02 2.50E+02 3.00E+02 3.50E+02

T (k)

Su

scep

tib

ility

, χ

γ Fe2O3

2.50E+01

3.00E+01

3.50E+01

4.00E+01

4.50E+01

5.00E+01

0.00E+00 5.00E+01 1.00E+02 1.50E+02 2.00E+02 2.50E+02 3.00E+02

Temp (οK)

Mo

men

t (e

mu

/gm

)

Com.Frac 1

Com.Frac 2

Com Frac. 3

Syn. Fe2O3

(a)

(b)

Fig. 4. (a) Temperature–susceptibility plot for unfractionated commercial maghemite, and (b) temperature–moment plot for

synthesized and for different fractions of commercial maghemite.

-5.0E+01-4.0E+01-3.0E+01-2.0E+01-1.0E+010.0E+001.0E+012.0E+013.0E+014.0E+015.0E+01

-1.0E+03 -5.0E+02 0.0E+00 5.0E+02 1.0E+03

Applied Field (Oe)

Mo

men

t (E

MU

/g)

5K 300K

Fig. 5. Applied field vs. magnetization plot for the third

fraction (smallest size) of the commercial maghemite.

Table 1

Comparison between the commercially available and synthe-

sized maghemite

Sample Tmax (K) Tsep (K) Dia (nm)

Frac 1 (com) 205 280 130

Frac 2 (com) 130 270 20–30

Frac 3 (com) 100 125, 270 10–12

Syn. Maghemite75 75 235 5–11

J. Chatterjee et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 113–118 117

energy of the system (KBT) as given in Eq. (1) isless than the energy barrier and there is no othermechanism than the external field to rotate theparticle/domain orientations and randomize thesystem. In the paramagnetic case, KBT is greaterthan the energy barrier, and thus thermal energycan ‘‘randomize’’ the system and bring themoment to zero. It is totally expected thatcoercivity (Hc) and remnant magnetization (Mr)are greater than zero for ferrimagnetic and zero forsuperparamagnetic systems [17]. The result showedthat at 5K, Hc is about 300Oe, while at 300K it isabout 30Oe. Similarly, Mr has changed. The areaunder the M2H curve, which is the work done bythe magnetic field has also been changed. In thesuperparamagnetic case, given the fact that KBT isgreater than the energy barrier, only thermalenergy is required to reorient the domains/particlesand diminishing hysterisis is observed as expectedin the superparamagnetic behavior.

4. Conclusion

Nanocrystalline maghemite (gFe2O3) has beensynthesized by a combination of chemical andultrasonication method with much narrower dis-tribution of sizes compared to that of thecommercial samples. These nanoparticles arebeing encapsulated with protein and other poly-mers. Magnetic properties for the different frac-tions of commercially available samples aremeasured and compared with the synthesizedmaghemite. Sharp blocking temperatures are notobserved for the fractionated commercial andsynthesized samples because of their distribution

in sizes. The fractionated samples of commercialmaghemite and the synthesized maghemite showedsuperparamagnetic characteristics.

References

[1] K. Sugibayashi, Y. Morimoto, T. Nadai, Y. Kato, Chem.

Pharm. Bull. 25 (1977) 3433.

[2] J. Chatterjee, Y. Haik, C.-J. Chen, J. Magn. Magn. Mater.

225 (2001) 21.

[3] E. Hornes, L. Korsness, Genet. Analy. Tech. Appl. 7

(1990) 145.

[4] A.E. Senyei, K.J. Widder, G. Czerlinski, J. Appl. Phys. 49

(1978) 3578.

[5] J.P. Iannotti, T.C. Baradet, M. Tobin, A. Alavi, M.J.

Staum, Orthop. Res. 9 (1991) 432.

[6] P.K. Gupta, C.T. Hung, D.G. Perrier, J. Pharm. Sci. 76

(1987) 141.

[7] S. Lefebure, V. Dubois, V. Cabuil, S. Neveu, R. Massart,

J. Mater. Res. 10 (1998) 2975.

[8] C.P. Bean, J.D. Livingston, J. Appl. Phys. 30 (1959) 120.

[9] L.M. Malkinski, J.Q. Wang, J.B. Dai, Tang, J. Appl. Phys.

Lett.75 (1999) 844.

[10] F.A. Tourinho, R. Franck, R. Massart, J. Mater. Sci. 25

(1990) 3249.

[11] A. Bee, R. Massert, S. Neveu, J. Magn. Magn. Mater. 149

(1995) 149.

[12] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H.B. Na, J. Am.

Chem. Soc. 123 (2001) 12798.

[13] J. Chatterjee, Y. Haik, C.J. Chen, J. Magn. Magn. Mater.

246 (2002) 382.

[14] K.S. Suslick, T. Hyeon, M. Fang, Chem. Mater. 8 (2002)

2172.

[15] R. Massart, E. Dubois, V. Cabuil, E. Hasmony, J. Magn.

Magn. Mater. 149 (1995) 1.

[16] M. Zrinyi, L. Barsi, A. Buki, Polym. Gels. Networks 5

(1997) 415.

[17] J.K. Vassiliou, V. Mehrotra, M.W. Russell, E.P. Giannelis,

R.D. McMichael, R.D. Shull, R.F. Ziolo, J. Appl. Phys. 73

(1993) 5109.

J. Chatterjee et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 113–118118