J Supercond Nov Magn (2012) 25:2041–2045DOI 10.1007/s10948-012-1559-4

O R I G I NA L PA P E R

Magnetic Field Dependence of Blocking Temperaturein Oleic Acid Functionalized Iron Oxide Nanoparticles

Sanju Tanwar · V.P.S. Awana · Surinder P. Singh ·Renu Pasricha

Received: 23 March 2012 / Accepted: 27 March 2012 / Published online: 17 April 2012© Springer Science+Business Media, LLC 2012

Abstract We report the synthesis of phase pure, mono-dispersed Fe3O4 nanoparticles of size ∼10 nm via chem-ical co-precipitation of ferrous and ferric ions, under con-trolled pH and temperature. The nanoparticles are oleic acidfunctionalized and hence dispersible in organic medium.The structure and morphology of nanoparticles are deter-mined by analyzing XRD pattern and TEM micrographs,confirming the formation of phase pure Fe3O4 nanoparti-cles. The magnetization studies reveal the superparamag-netic behavior of the nanoparticles at room temperature. Thechanges in blocking temperatures (TB) of magnetic nanopar-ticles with applied magnetic fields (Hap), noted from thecusp of the zero-field-cooled magnetization, the indicate ef-fects of dipole interactions. A decrease in blocking temper-ature from 95 K to 15 K has been observed on varying themagnetic field from 50 Oe to 5000 Oe. TB versus H rela-tion follows the equation TB(H) = To(1 − (H/Ho))

m, i.e.the Néel–Brown model of magnetic relaxation in nanoparti-cles.

Keywords Superparamagnetic · ZFC-FC · Blockingtemperature · Néel–Brown model

S. TanwarCentre for Converging Technologies, University of Rajasthan,JLN Marg, Jaipur, Rajasthan 302004, India

S. Tanwar · V.P.S. Awana (�) · S.P. Singh · R. PasrichaNational Physical Laboratory, Council of Scientific and IndustrialResearch, Dr. K.S. Krishnan Marg, New Delhi 110012, Indiae-mail: awana@mail.nplindia.ernet.inurl: www.freewebs.com/vpsawana

R. Pasrichae-mail: pasrichar@mail.nplindia.ernet.in

1 Introduction

Unique physical properties of nanoparticles (NPs) are atopic of intensive research [1]. A special place belongs tothe magnetic properties in which the difference betweenthe bulk material and the nanophase is especially signifi-cant. In particular, it has been shown that magnetization (peratom) and the magnetic anisotropy of nanoparticles are sig-nificantly larger than those of the bulk specimen [1]. Thechange in magnetic properties at nanoscale has been widelyexplored to develop superparamagnetic biocompatible ironoxide nanoparticles. Such magnetic nanoparticles with neg-ligible coercivity find immense applications in the field ofdiagnostic imaging (MRI) [2, 3], drug delivery [4, 5], mag-netic storage media [6], ferrofluids [7] and electronics [8].

Numerous routes have been employed for the synthesis ofiron oxide nanoparticles; thermal decomposition [9–12], mi-croemulsion [13, 14], hydrothermal synthesis [15–17] andco-precipitation [18–20] are few to be named. But still thesynthesis of monodisperse nanoparticles with uniform sizeand homogeneous distribution is a great challenge to over-come. Co-precipitation with addition of a base under inertatmosphere is a facile and convenient way to synthesize ironoxides (either Fe3O4 or γ -Fe2O3) from aqueous Fe2+/Fe3+salt at room temperature or slightly elevated temperature.The size, shape, and composition of the magnetic nanopar-ticles synthesized through this method depends on the typesof salt used (e.g. chlorides, sulfates, nitrates), the Fe2+/Fe3+ratio, the reaction temperature, the pH value and the ionicstrength of the media. Once the synthesis conditions arefixed, the quality of the magnetite nanoparticles is mostly re-producible. The functionalization of nanoparticles preventstheir agglomeration.

A magnetic nanoparticle generally is in a single domainstate with uniaxial anisotropy [21]. For a sample composed

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of identical particles, with the easy axis aligned in the samedirection, anisotropic energy, U = KV (K = anisotropicconstant, V = volume of the nanoparticles) can be extractedfrom the temperature-dependent magnetic measurements[22]. The blocking temperature or the energy barrier canalso be obtained from low field zero-field-cooled (ZFC) andfield-cooled (FC) magnetization curves [22, 23]. In a non-interacting particle sample, the peak temperature (TP) in azero-field-cooled magnetization curve is simply the averageblocking temperature (TB) of all the particles. Theoretically[24] it is known that the single particle energy barrier KV

is lowered by an applied field H , so that TB(H) for non-interacting NPs should follow the equation:

TB(H) = T (0)(1 − H/H0)m (1)

where the magnitude of exponent m has been predicatedto be between 1.5 to 2 [25, 26]. As the field increases thenanoparticles become interacting resulting in the gradual in-crease of exponent m from 1.5 to 2 or above.

In the present work, we have synthesized oleic acidcapped Fe3O4 nanoparticles having homogeneous size dis-tribution of 10 ± 1 nm. We have also measured the depen-dence of blocking temperatures (TB) with applied magneticfields (Happ) and fitted the experimental results accordingto the Néel–Brown relaxation model [24]. It was found thatoleic acid capped mono-dispersed Fe3O4 nanoparticles areideally non-interacting (m = 1.5) up to an applied field of500 Oe, and thereafter they become interacting for higherfields. Such non-interacting superparamagnetic nanoparti-cles can be the ideal candidates for biomedical applica-tions [27].

2 Experimental

2.1 Materials

Ferrous chloride tetrahydrate (FeCl2·4H2O), ferric chlo-ride hexahydrate (FeCl3·6H2O), ammonia (NH3), ethanol(C2H5OH), oleic acid (C17H34COOH), toluene (C7H8) areacquired from Alfa Aesar and used as received. Milliporewater has been used for all experiments.

2.2 Synthesis of Iron Oxide Nanoparticles

The size controlled iron oxide nanoparticles were synthe-sized via co-precipitation method. 60 mM ferrous chloridetetrahydrate (FeCl2·4H2O) and 120 mM ferric chloride hex-ahydrate (FeCl3·6H2O) were mixed at room temperaturein volume ratio 2:3 of Fe+2/Fe+3 in millipore water. Themixture was reduced by drop wise adding 25 % ammonia(NH3) solution under vigorous stirring at 85 °C under non-oxidizing nitrogen gas environment for about 2 hours main-taining pH ∼ 9.0 during the reaction. The precipitate was

washed five times with millipore water by centrifugation at8000 rpm. The aqueous solution of nanoparticles was thensubjected to surface functionalization.

2.3 Surface Functionalization of Synthesized Iron OxideNanoparticles

The nanoparticles synthesized above were surface modifiedusing oleic acid for making them completely dispersible inan organic solvent. 10 ml aqueous solution of iron oxidenanoparticles (1 mg/ml) was mixed with 5 × 10−4 M oleicacid solution in methanol and stirred for 6 hours till all thenanoparticles get precipitated at the bottom. After decantingwater-methanol solution, addition of 10 ml toluene and 15minutes sonication in the precipitate resulted in clearly dis-persed functionalized NPs in toluene as shown in inset ofFig. 1(c).

2.4 Characterization

X-ray diffraction (XRD) pattern of the synthesized nanopar-ticles were measured on Rigaku X-ray diffractometer. Trans-mission electron microscopy studies of the nanoparticleswere performed using a Tecnai G2 F30 S-Twin (FEI; Su-per Twin lens with Cs = 1.2 mm) instrument operating atan accelerating voltage at 300 kV, having a point resolutionof 0.2 nm, and with a lattice resolution of 0.14 nm. The IRspectrum (1000–2000 cm−1) was measured using a Nico-let 5700 FTIR spectrometer. Static and dynamic magneticmeasurements as a function of applied field and tempera-ture were performed on a physical property measurementsystem (PPMS-Quantum Design) at different temperatures.Both zero-field-cooled (ZFC) and field-cooled (FC) mag-netization plots were taken from 300 K to 5 K for differ-ent values of applied field Happ (50 Oe < H < 5000 Oe).This was done to mark the ZFC-FC branching and in a waythe blocking temperature (TB). Isothermal magnetization(M–H ) study was conducted at different temperatures inapplied fields of up to one Tesla to investigate the value ofpossible saturation moment of the nanoparticles studied.

3 Results and Discussion

The XRD data corroborate the purity and crystalline phaseof the nanoparticles. Figure 1(a) shows the XRD pattern ofthe as synthesized iron oxide nanoparticles. The presence ofsharp peaks in the diffraction pattern indicates the formationof good crystalline structure. The as-grown nanoparticles arefound to possess a preferential (311) orientation of the cubicphase thus confirming the inverse cubic spinel structure ofmagnetite (JCPDS Card No. 19-629) [28]. The particle size

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Fig. 1 (a) XRD pattern, (b) TEM micrograph and (c) FTIR spectra of pure oleic acid (curve 1) and Fe3O4 nanoparticles capped with oleic acid(curve 2). The inset shows dispersibility of Fe3O4 capped nanoparticles in aqueous and organic media

Fig. 2 (a) Hysteresis curves, (b) behavior at lower magnetic field region of Fe3O4 nanoparticles cooled from 300 K (curve 2) to 5 K (curve 1)

calculated from the (311) plane based on the Scherrer equa-tion is 11 nm, which is in agreement with the mean diame-ter of 10.5 nm obtained from the transmission electron mi-croscopy (TEM) micrographs as shown in Fig. 1(b). To findthe effect of capping on crystalline structure of nanoparti-cles, XRD investigations of both capped and un-capped NPshas been conducted but no difference was found in the po-sition and intensity of peaks, which confirms that cappingdoes not affect the crystalline structure of Fe3O4 nanoparti-cles. The surface functionalization of NPs also inhibits theagglomeration owing to steric charge repulsion between thecapped oleic acid moieties. This capping of oleic acid on thesurface of Fe3O4 nanoparticles was confirmed from a care-ful FTIR study. The –COOH stretch at 1710 cm−1 in pureoleic acid (curve 1) is shifted to 1560 cm−1 in organic dis-

persion of oleic acid capped Fe3O4 nanoparticles (curve 2)as shown in Fig. 1(c). This observation confirms the hypoth-esis that the oleic acid molecules are oriented on the sur-face with the carboxylic groups exposed towards the solventthereby resulting in a significant shift in the value of its fre-quency [29].

Figure 2(a) shows the dependence of magnetization onthe applied magnetic field (hysteresis loops) for the Fe3O4

nanoparticles at temperatures 300 K, 200 K, 100 K, 50 Kand 5 K. Interestingly, no saturation of moments is ob-served in the sample up to field strengths of 10 kOe. Thenon-saturation of the magnetization is a clear indication ofthe presence of superparamagnetic particles. Moreover, wewould like to point out that the saturation magnetization in-creases with a decrease in temperature.

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Fig. 3 (a) Magnetization (M)versus temperature (T )measured in the ZFC and FCmodes. (b) Behavior of blockingtemperature with applied field.(c) Dependence of the blockingtemperature TB on the appliedfield H (solid squares);H 2/3 = H0(1 − TB/T0)

Figure 2(b) shows the magnitified region of the magne-tization hysteresis loops in the range of −400 to +400 Oe.This indicates the coercive field of the pertaining hystere-sis loops. The presence of a small hysteresis loop revealsthe presence of ferromagnetism. At low temperatures thethermal energy is not able to overcome the magnetocrys-talline anisotropy energy, therefore the magnetic momentsof the particles do not rotate back completely to their easydirections. Thus we have nonzero magnetization in a zerofield and consequently, nonzero coercivity is observed. Onthe other hand, at higher temperatures, the thermal energyis dominant and the orientations of magnetic moments arerandom, so the value of coercivity (100 Oe � 10000 Oe)becomes less as shown in Fig. 2(b) for the temperature 5 K(curve 1, below blocking temperature) and 300 K (curve 2,above blocking temperature). The average magnetic particlesize can also be obtained by measuring the slope of magneti-zation curve (dM/dH)H=0 at 300 K. The average magneticparticle diameter calculated using Eq. (2):

D =(

18kBT (dM/dH)H≡0

πρM2s

)(2)

where ρ is density (4.9 g/cm3 for magnetite) [30] and Ms

is saturation magnetization at 300 K [30, 31] turns out to be8.4 nm. The difference between the particle size calculatedby TEM morphology and magnetization curve may be dueto the presence of magnetically inactive layer on the surface

of nanoparticles in the form of oleic acid which acts as ashielding between the applied field and the nanoparticles.

Dynamic magnetic measurements as a function of ap-plied field and temperature were performed using PPMS tostudy the change in blocking temperature (TB) with appliedfield H (50 Oe < H < 5000 Oe). TB has been determinedfrom the peak position of the magnetization M versus H

data under the usual ZFC condition. Figure 3(a) shows theZFC and FC magnetization curve measured at different ap-plied magnetic fields from 50 Oe to 5000 Oe. For the ZFCmagnetization, the sample is cooled to 5 K in the absenceof magnetic field. A magnetic field is then applied and themagnetization is measured as the sample is being heated upto 300 K. The FC magnetization is measured by cooling thesample from 300 K to 5 K in presence of field. It is clearfrom the figure that the ZFC and FC magnetization bifur-cates at different points for different applied fields for thesame sample. The sharpness of ZFC peaks reflects the qual-ity of monodispersity of nanoparticles thus signifying thefact that functionalization of nanoparticles circumvent theagglomeration [32].

The maxima of the zero field cooled cusp in the curvecorresponds to the blocking temperature (TB) of the respec-tive sample. It is very well known that above TB the mag-netic moments of the particles in the material are randomlyoriented showing superparamagnetic behavior on applying

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external magnetic field, while below TB the magnetic mo-ments are frozen in random field direction.

The graph shown in Fig. 3(b) clearly indicates the de-crease in the blocking temperature (TB) with increasing ap-plied magnetic field (Happ). The experimental data (solidsquares) plotted have been fitted in Eq. (1) for m = 3/2 vari-ation for lower fields (H < 500 Oe) changing over to m = 2variation for higher fields. The value of m varies dependingupon the angle θ between applied field and anisotropy axisof the nanoparticles. The disagreement between the exper-imental points (curved line) and the theoretical predictionfor non-interacting particles (linear line) is usually imputedto the presence of magnetic dipolar interactions between theparticles. The existence of particle interactions i.e., contribu-tions from the neighborhood magnetic dipoles and surfaceeffects are large sources of magnetic anisotropy. However,from symmetry arguments it has been shown that a perfectspherical particle should have a zero net contribution fromsurface anisotropy [33]. For the present Fe3O4 nanoparti-cles with no significant anisotropy, no major contributionfrom the surface should be expected. Hence, the observedenhancement of the particle anisotropy should be mainly re-lated to the effect of dipolar interactions rather than to a sur-face effect. The larger blocking temperature observed at lowfields further agrees with the presence of dipolar interactionswhich might be due to incomplete coverage of the oleic acid,indicating that the iron oxide nanoparticles are quite closerto each other [34].

It can thus be inferred that the oleic acid capped Fe3O4

nanoparticles shows spin glass like freezing with the powerlaw dependence of blocking temperature as H 2/3 = H0

(1 − TB/T0) as shown in Fig. 3(c) where T0 representsthe blocking temperature at zero applied field. This depen-dence is similar to Almeida–Thouless line observed in spinglasses [35]. The two different regimes are demonstrated inthe figure, separated by H ∼ 150 Oe. This abrupt change inthe trend of blocking temperatures indicates that there is acertain crossover field where the applied field energy μHapp

(μ = magnetic moment) is comparable to the average singleparticle barrier, U . In a field where μHapp > U , the particlesare more susceptible to being unblocked; consequently, theblocking temperature decreases more rapidly with increas-ing applied fields.

4 Conclusion

Phase pure, mono-dispersed Fe3O4 nanoparticles of size∼10 nm were synthesized via chemical co-precipitation offerrous and ferric ions. The static and dynamic magneticmeasurements study of the synthesized Fe3O4 nanoparticlesshowed the existence of dipolar magnetic interactions, fol-lowing the Almeida–Thouless linear behavior below a cer-tain applied field, Happ ∼ 500 Oe.

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