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Materials Chemistry and Physics 132 (2012) 196– 202

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

jo u rn al hom epage : www.elsev ier .com/ locate /matchemphys

e3O4 inverse spinal super paramagnetic nanoparticles

baid ur Rahman, Subash Chandra Mohapatra, Sharif Ahmad ∗

aterials Research Lab, Department Of Chemistry, Jamia Millia Islamia, New Delhi 110025, India

r t i c l e i n f o

rticle history:eceived 15 June 2011eceived in revised form 3 November 2011ccepted 14 November 2011

eywords:

a b s t r a c t

The present article reports an energy efficient method for the synthesis of superparamagnetic ferrite(Fe3O4) nanoparticles (10–40 nm) and their annealing effect on the morphology, size, curie temperatureand magnetic behavior at 50, 300, 400 and 500 ◦C. The synthesized nanoparticles were characterizedby various spectroscopic techniques like FT-IR and UV–visible. The crystalline structure and particlesize were estimated through solid phase as well as the liquid phase using XRD, TEM and DLS techniques.

errite nanoparticlesuperparamagnetismSM and EPR

Superparamagnetic behavior of nanoparticles was confirmed by VSM. The EPR study reveals that the mainfeature of X-Band solid state EPR spectrum has strong transition at geff ∼ 3.23 (2100G) and a relativelyweak transition at geff ∼ 2.05 (3300G). The later transition further confirms the super paramagnetic natureof these nano ferrites. The activation energy and order of weight losses of nano ferrites were found to be:39.6 KJ mol−1 and 0.21 orders (600–800 ◦C), respectively, analyze with the help of TGA while the specificsurface area (23.1 m2 g−1) and pore size (9 A) were determined by Quanta chrome BET instrument.

. Introduction

Among the various nanostructure materials, metal oxideanoparticles are the important class of materials as their optical,agnetic and electrical properties find a wide range of high tech

pplications [1]. Fe3O4 nanoparticles are common ferrite with annverse cubic spinal structure. These class of compounds exhibitnique electrical and magnetic properties due to the transfer oflectrons between Fe2+ and Fe3+ on octahedral sites [2]. Fe3O4anoparticles have been the subject of intense interest becausef their potential applications in several advance technologicalreas due to their promising physical and chemical properties.enerally, these properties depend on the size and structure ofarticles [3,4]. Fe3O4 nanoparticles find wide applications in theeld of biomedical, as anticancer agent [5,6], corrosion protectiveigments in paints and coatings [7]. The magnetic atoms or ions inuch solid materials are arranged in a periodic lattice and their mag-etic moments collectively interact through molecular exchangeelds, which give rise to a long-range magnetic ordering. Amongll iron oxide nanoparticles, Fe3O4 represent the most interest-ng properties due to of its unique structure i.e. the presence ofron cations in two valence states, Fe2+, Fe3+ on tetrahedral andctahedral sites with an inverse cubic spinel structure. The coerciv-

ty and remenance values for the super paramagnetic nano Fe3O4anoparticles have been found to be zero by the earlier reportedethods [8]. Presently, cell labelling strategies find application of

∗ Corresponding author. Tel.: +91 11 26827508x3268; fax: +91 11 2684 0229.E-mail address: sharifahmad jmi@yahoo.co.in (S. Ahmad).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.11.032

© 2011 Elsevier B.V. All rights reserved.

superparamagnetic ferrite either through conjugating the magneticnanoparticles to the cellular surface of the stem cell or by inter-nalization of the particles into the cell. Superparamagnetic ferritecan work in both of these ways, since the potential to manipu-late their surface chemistry is plentiful and their sizes along withother attributes promote their successful uptake into cells. Thesuperparamagnetic nano ferrites also interface well with MRI tech-nology. The use of superparamagnetic particles play a crucial rolein the diagnostic imaging modality technique finds application inthe study of stem cell [9].

Karaoglu et al. report the poly ethylene glycol (PEG) assistedhydrothermal route to study the influence of the hydrolyzing agenton the properties of PEG-iron oxide (Fe3O4) nano composites andKöseoglu et al. reports the investigation on the structural and mag-netic properties of Mn0.2Ni0.8–Fe2O4 nanoparticles synthesizedby a PEG assisted hydrothermal rout [10,11]. Guobin Ding et al.reports the development and characterization of a magnetic micel-lar nanocarrier based on the amphiphilic copolymer [methoxypoly (ethylene glycol)-poly(d,l-lactideco-glycolide)] MPEG-PLGAand magnetite (Fe3O4) nanoparticles, and discuss its potential fordouble-targeted hydrophobic drug delivery [12]. Cheng-Hao Liuet al. reports the development of a reusable, single-step system forthe detection of specific substrates using oxidase-functionalizedFe3O4 nanoparticles (NPs) as a bienzyme system and using amplexultrared (AU) as a fluorogenic substrate [13]. Phadatare et al.,reports The PEG assisted NiFe2O4 nanoparticles for the possible

biomedical applications such as magnetic resonance imaging, drugdelivery, tissue repair, magnetic fluid hyperthermia, etc. [14].

Literatures available on the synthesis of nanoferrite using (PEG)based polyol methods as reported by Karaoglu et al., [10] the

mistry and Physics 132 (2012) 196– 202 197

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EG assisted hydrothermal route for PEG–Fe3O4 nanocompos-te, solvothermal method, thermal decomposition method, etc.11,15,16]. However, scant literature available on the synthesis oferrite nanoparticles using ethylene glycol at low temperature.

Here in, we have simply used the ethylene glycol (EG) of lowolecular weight (58 amu) which may be attributed to the syn-

hesis of ferrite nanoparticles at reasonably low temperature asompared to those of reported in literatures. While most of thearlier reported synthesis involve high temperature (>200 ◦C) andong time (6–48 h), we have developed a novel efficient method forhe synthesis of ferrite nanoparticles involving minimum possibleemperature and time (50 ◦C, 4 h) using EG as solvent, stabilizer andeducing agent. The formation of the nanoparticles in the polyol atH 13 occurs through the nucleation of tiny crystallites followedy the growth of stable nano crystalline particles. The utilization ofthylene glycol in the synthesis of the magnetic nanoparticles wasot only found to be easier and efficient but also has appreciableontrol over the composition and shape of the nanoparticles. Thextraction of nanoparticles from the reaction mixture was foundo be simpler owing to the greater miscibility of EG in water and

ethanol. The manuscript also discussed the effect of annealingemperature on the morphology and magnetism of nanoferrite.

. Experimental

.1. Materials

All the chemicals were used of analytical grade. Ferrous sulphateeptahydrate (Merck India), ethylene glycol (EG), ammoniumydroxide (SD fine chemicals Pvt. Ltd., India) and hydrogen per-xide (Fisher Scientific, India) were used as received.

.2. Synthesis of Fe3O4 nanoparticles

Ferrous sulphate heptahydrate (3 g, 1.08 × 10−2 M) and ethylenelycol (50 ml) were taken in a 250 ml three necked flask, fitted with

reflux condenser. The solution was stirred under nitrogen atmo-phere for 30 min, followed by drop wise addition of 20 ml 0.5%queous solution of 50% H2O2. The reaction mixture was heated at0 ◦C for 4 h under continuous stirring. The pH of the reaction wasaintained at pH 13 by occasional addition of 25% aqueous ammo-

ia solution. The progress of reaction was visually monitored. Theanoparticles formed were dialyzed for 24 h and purified repeat-dly by magnetic field separation, decantation and redispersion.inally, the ultra fine precipitate of Fe3O4 nanoparticles was filterednd washed several times with water and methanol. The ferriteowder was then dried under vacuum at 50 ◦C for 72 h, then Fe3O4anoparticles were annealed at 300, 400, and 500 ◦C.

.3. Characterization

XRD were obtained on Philips X-ray diffractometer (Modelhilips W3710) using copper K� radiation. TEM studies were car-ied out using electron microscope (Model Philips Morgagni 268)perating at 80 kV at AIIMS, New Delhi, India. The light scat-ering experiments were performed on a Photocor FC. All the

easurements were done at a scattering angle of 90◦ at tem-erature 20 ◦C. BET surface area experiment was carried out byuantachrome instrument (model NOVA 2000e USA) under liq-id nitrogen environment to determine the surface area and poreize of Fe3O4 nanoparticles. FT-IR spectra were measured onerkin-Elmer spectrometer (Model 1750). UV–visible spectra were

ecorded in aqueous medium on Perkin Elmer Lambda (ModelZ-221). The magnetic measurements were performed on vibrat-ng sample magnetometer (VSM) (model Lake Shore’s new 7400eries) at 27 ◦C by applying a field of 1000 A m−1 at a frequency of

Fig. 1. X-ray diffraction pattern of nanoferrite.

16 Hz. Thermo gravimetric analysis (TGA/DTA) of the synthesizednanoparticles was done on EXSTAR TG/DTA 6000 under nitrogenatmosphere at a heating rate of 20 ◦C min−1. Activation energy andorder of decomposition were determined using freeman Carrol andcoat’s plots methods [17,18], by plotting the graph.

� log(dw/dt)� log wr

Vs�T−1

� log wr

where dw, dt, wr and �T are the changes in mass, time, dw andtemperature in Kelvin, respectively. The plot was found to be lin-ear with intercept at x axis and the slope (−ve or +ve sign) are foundto be characteristic of physical and chemical reaction of nano fer-rites. Activation energy and order of decomposition of nanoferritewas determined on the basis of the calculated slope and inter-cept, respectively [17,18]. The EPR spectrum was recorded usingVarian E-109 spectrometer operating in the X-band (9.5 GHz) at25 ◦C.

3. Result and discussions

3.1. Synthesis

Super paramagnetic ferrite nanoparticles were synthesizedin situ through the precipitation of Fe3O4 nanoparticles using metalsalt precursor (FeSO4) in EG found to be very simple cost-effectiveand eco-friendly.

3.2. X-ray diffraction (XRD) studies

The XRD pattern of the prepared and annealed (300, 400 and500 ◦C) Fe3O4 nanoparticles samples are shown in Fig. 1. All thediffraction peaks can be indexed to an inverse spinel structure ofFe3O4 nanoparticles, which is in good agreement with the litera-ture value (JCPDS Card No. 19-0629). The strong and sharp peakssuggested that Fe3O4 crystals are highly crystalline. However, thebroadening in the reflection peaks was due to the particles size atnano domain. All XRD patterns show diffraction peak at 2� = 35.70◦

corresponds to the spinel phase of Fe3O4 nanoparticles. The XRDpeaks of magnetic nanoparticles at 2� = 30.22◦, 43.52◦, 57.43◦ and63.11◦ were found to be in good agreement with those of previ-ously reported 2� values of Fe3O4 nanoparticles and matches well

with the JCPDS Card No.19-0629. The average crystallite size ofnanoparticles was calculated from the lower full-width-at-half-maximum (FWHM) of (3 1 1) diffraction reflection using Scherrer’sequation: D = 0.9�/ cos �, where D is the particle size, � is the X-ray

198 O.u. Rahman et al. / Materials Chemistry and Physics 132 (2012) 196– 202

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at 400 and 500 C is due to the aggregation of the nanoparticles inwater. So aggregation in water results larger hydration sphere andcontributed to larger particle size [22].

Fig. 2. Transmission electron micrographs of nanoferrite

avelength (nm), � is Bragg’s angle; is the excess line broaden-ng (radiant), and the particle size of Fe3O4 nanoparticles was foundn the range of 20–25 nm using Scherer equation. The intensity ofll diffraction peaks at 2� = 35.70◦ are followed the decreasing ofntensity due to formulation more crystalline phase particles in allnnealed Fe3O4 nanoparticles. The estimated particle size at 300 ◦Cas estimated as ∼21 nm [19,20].

.3. Transmission electron microscopy (TEM) studies

The TEM specimens were prepared by dispersing the pow-ered compounds in double distilled water and sonicating themor 45 min. One drop of the sonicated solution was placed on tohe wax coated copper grid and then dried in air. Fig. 2 revealshat Fe3O4 nanoparticles are spherical in shape and particles sizes in accordance with the crystallite size as estimated in previ-us Section 3.2 by the Scherrer’s equation (20–25 nm). However,e3O4 nanoparticles are found to be well interconnected witheast agglomeration, this may perhaps be due to the high surfaceharge onto Fe3O4 nanoparticles and magneto dipole interaction.he increase in the annealing temperature enlarged the size of therimary particle, as expected from the crystallization phenomenonFig. 3.). These values were 20, 15, 30 and 35 nm for the annealedamples at 50, 300, 400 and 500 ◦C, respectively. Interestingly whenhe temperature is increasing, crystallite size also increases but at00 ◦C has some deviation which would be uniformly distributedanoparticles with almost no agglomeration that may be attributedo improve the electronic properties for desired application21].

.4. Dynamic light scattering (DLS) studies

Powdered sample of Fe3O4 nanoparticles were dispersed in dou-le distilled water with the help of sonicator and the light scatteringxperiments of sample were performed on a Photocor FC. DLSmages show that the average sizes of the Fe3O4 nanoparticles were

) 50 ◦C, (b) 300 ◦C, (c) 400 ◦C and (d) 500 ◦C, respectively.

obtained as 45–60, 20–30, 55–60, 60–70 nm as shown Fig. 4(a–d).DLS studies also confirmed the range and size of the particlesannealed at 50, 300, 400 and 500 ◦C analyzed by TEM experiment asdiscussed in Section 3.3. However, in case of DLS studies the hydra-tion sphere was found to be more for Fe3O4 nanoparticles annealedat 50, 400 and 500 ◦C than 300 ◦C by an average of 20–30 nm. Thehigher value of average size (compared to TEM) obtained in DLSoriginates from the fact that DLS measures the hydrodynamic radiiof the particles, which include the solvent layer at the interface. Asdiscussed in previous section size of annealed Fe3O4 particle sizeat 400 and 500 ◦C is 30 and 35 nm, respectively, the larger particle

◦

Fig. 3. Particle size variation with the annealing temperature.

O.u. Rahman et al. / Materials Chemistry and Physics 132 (2012) 196– 202 199

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magnetic nanoparticles, which is in accordance with the previouslyreported literature [27].

The absorption band at 330 nm indicates the formation of nanosized particles.

Fig. 4. Size distribution plot of Fe3O4 nanoparti

.5. BET

The average surface area of the Fe3O4 nanoparticles was foundo be 23.14 m2 g−1 and pore size of the nanoparticles comes out toe 9 A. Hence the synthesized nanoparticles is porous in nature.

.6. FT-IR spectroscopic studies

FT-IR spectra of annealed samples were recorded at room tem-erature depicted in Fig. 5(a–d). In Fig. 5a, the characteristic M–Oharp peak observed at 585 cm−1. This can be attributed to the highegree of crystalline nature of the Fe3O4 nanoparticles [23,24]. Inig. 5a, the peaks at 3413 cm−1 is ascribed to the stretching vibra-ions of hydrogen-bonded surface water molecules and hydroxylroups. The peak at 1619 cm−1 is assigned to the O–H bending23–26]. Moreover, the peak at 1000 cm−1 is attributed to vibra-ional band of C–O bond for pure ethylene glycol [10], the peak at50 cm−1 may be due to the deformation bending of mono substi-uted –C–C– of ethylene glycol, which may be present at the surfaces impurity. In case of samples annealed at 300, 400 and 500 ◦C theharp doublet is observed at 585 cm−1. And peaks at 3413 cm−1 and

619 cm−1 have been vanished this can be attributed for the com-lete removal of surface water molecule, forming highly crystallinee3O4 nanoparticles. This is in consistent with that of XRD resultss discussed in Section 3.2.

t: (a) 50 ◦C, (b) 300 ◦C, (c) 400 ◦C and (d) 500 ◦C.

3.7. UV–visible spectroscopic studies

The UV–visible spectrum of Fe3O4 nanoparticles (Fig. 6) showsan absorption band in the region of 330–450 nm, which originatesprimarily from the absorption and scattering of UV radiation by

Fig. 5. FT-IR spectra of nanoferrite.

200 O.u. Rahman et al. / Materials Chemistry and Physics 132 (2012) 196– 202

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Fig. 6. UV–visible spectrum of nanoferrite in water.

.8. Vibrating sample magnetometer (VSM) studies

The magnetic properties of Fe3O4 nanoparticles were character-zed by vibrating sample magnetometer (VSM, Model Lake Shore’sew 7400 series) at 300 K, using magnetization M and applied field

can be described by Langevin equation [28]:

= Ms(Cothy − 1), and Y = mH

kT

here Ms is the saturation magnetization of nanoparticles, ‘m’ is theverage magnetic moment of individual nano particle in the sam-le and k, the Boltzmann constant [29]. Fig. 7 shows the plots of theagnetization ‘M’ versus applied field ‘H’ (between −2000 Oe and

2000 Oe) of these prepared Fe3O4 samples were obtained after dif-erent annealed temperature. The VSM curve reveals the formationf a hysteresis loop for the Fe3O4 nanoparticles, with zero coer-ivity and remanance values, which exhibits superparamagneticehavior of Fe3O4 nanoparticles. On increasing the applied fieldrom 0 to 1000 Oe, the magnetization ‘M’ increases sharply; andecomes nearly saturated at about 1000 Oe. It was found that allhe samples have strong magnetic responses to a varying magneticeld. The hysteresis loops showed smooth change of magnetizationith applied field. Fig. 8 depicts the saturation of magnetization

alues of Fe3O4 samples increased with the annealing temperaturend further decreases with the increase in annealing temperature.

ig. 9 indicates that the prepared Fe3O4 samples posse’s mag-etic properties which are dependent on size and morphology ofhe nanoparticles. As we change the annealing temperature, parti-le size of nanoparticles also changes and surface area to volume

M(e

mu/

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-150 0 -100 0

50oC 300oC 400oC 500oC

-500 0 5 H (O e)

500 100 0 15500 200 0

ig. 7. vibrating sample magnetometer curve of Fe3O4 nanoparticles at room tem-erature.

Fig. 8. Variation of magnetization with the annealing temperature.

ratio changes and hence the magnetization changes. The Fe3O4nanoparticles obtained from non annealed sample possessed a sat-uration magnetization (Ms) of 65.4 emu g−1, while the annealedFe3O4 superstructures obtained after 300 ◦C, 400 ◦C and 500 ◦C hadMs values of merely 76.8 emu g−1, 61.2 emu g−1 and 43.3 emu g−1,respectively. While the reported Ms Value, is 84 emu g−1 for thebulk Fe3O4 particles and 80.7 emu g−1 for Fe3O4 nanoparticles[27,28,30]. As we increase the sintering temperature, the begin-ning magnetization increases from 65.4 emu g−1 to 76.8 emu g−1.However, the magnetization decreases as we increase the sinteringtemperature 300, 400 and 500 ◦C from 76.8 emu g−1 to 61.2 emu g−1

to 43.3 emu g−1, respectively. It is very interesting to note that themagnetization of ferrite nanoparticles follow the curie law of mag-netization, in the case of annealing curie temperature increase inbetween 300 and 400 ◦C after that curie temperature decrease viceversa with the saturation of magnetization value as it is evidentfrom Fig. 8. In present work, the value for the saturation of magneti-zation (Ms) is found to be higher than those of other earlier reportedwork [29,31]. Very significant and promising effect observes on theMs due to the annealing of Fe3O4 nanoparticles at optimum temper-ature 300 ◦C. At this temperature, the nanoparticles are uniformlydistributed with almost no agglomeration and further correlatedwith XRD, TEM and DLS studies as discussed in Sections 3.2–3.4,respectively.

3.9. Thermo gravimetric analysis

The thermal analysis of nanoferrite shows initially a negli-gible weight loss (0.4%) in the region of 90–150 ◦C, supportedby DTA endothermic peak, which confirms the loss of hydrogen

bonded water molecule present at the surface of nano ferrite. Asecond small weight loss (2%) occurred in the temperature range150–300 ◦C along the second DTA endothermic peak, which is

65.4

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20

30

40

50

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70

80

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50 30 0 40 0 50 0

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Av. Par �cle size(nm)

Temperature(oC)

M(emu/g)

Fig. 9. Variation of magnetization with annealing temperature and particle size.

O.u. Rahman et al. / Materials Chemistry

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Fig. 10. Thermo gravimetric analysis of Fe3O4 nanoparticles.

robably due to the removal of trapped water molecules from theattice. The order of decomposition reaction and activation energys found to be 0.56 and 17.34 kJ mol−1, respectively. Further weightoss of 2.1% is observed in the range 650–800 ◦C with a DTA exother-

ic peak, the decomposition event can be explained by Eq. (I).

e3O4�,−1/2O2−→

(650−800 ◦C3FeO (I)

For Eq. (I), the order of decomposition reaction and activationnergy are found to be 0.2 and 39.5 kJ mol−1, respectively. Totaleight loss is about 4.5%, which is due to the phase transition from

e3O4 to FeO and found to be equal to 1/2 O2 [32], because FeOs thermodynamically stable above 570 ◦C in phase diagram of thee–O system [33]. So the TGA and DTA analysis can be corroboratedith each other in terms of kinetics and activation energy of nano

errite as shown in Fig. 10.

.10. The EPR spectrum

The EPR spectrum of nanoferrite was taken at room temperature298 K). The Fe3+ (3d5) are 6S ground state with long spin-latticeelaxation times characteristics of S-state ion, their EPR is gen-rally observed at room temperature. Fe3+ has geff ∼ 2. However,he Fe3+ ions often experience relatively large crystal field effectsuch that the separation between the three kramer’s doublet can

−1

xceed 0.3 cm in the energy of the X-band microwave. In suchne structure more often EPR lines are observed at low fields cor-esponding to geff � 2. Fig. 11 shows peak at geff ∼ 3.23 (2100G)ndicating that Fe3+ transition from −1/2 → +1/2. The lower valance

1000 200 0 300 0 400 0 500 0 60 00

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nsity

(a.u

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Magnetic Field [G]

ig. 11. EPR spectrum of Fe3O4 nanoparticles in solid state at microwave frequency.5 GHz.

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and Physics 132 (2012) 196– 202 201

state of iron, Fe2+, is also paramagnetic due to full spin latticerelaxation characteristic of orbitally degenerate. EPR of Fe2+ canbe observed at very low temperatures (∼4 K) [34]. While the lowintensity peak observed at geff ∼ 2.05 (3300G) can be attributed tothe super paramagnetic nature of nano ferrite. The room tempera-ture EPR spectrum is found to be corroborated with literature valueof super paramagnetic ferrite nanoparticles [35].

4. Conclusion

A single step synergistically energy conserved facile techniquehas been adopted to synthesize the super paramagnetic Fe3O4nanoparticles using ethylene glycol as solvent, stabilizer and reduc-ing agent. The broad absorption band at 330 nm in UV–visiblespectra and a sharp band at 585 cm−1 in FT-IR spectra in addi-tion to XRD, TEM and DLS studies further confirm the formation ofFe3O4 nanoparticles. The VSM analysis reveals that the saturationmagnetization values of Fe3O4 samples increased with the increasein annealing temperature to 300 ◦C and further decreases beyondthis temperature. In other words, prepared Fe3O4 samples possesssize and morphology dependent magnetic property. The magneti-zation follows a unique pattern that as we change the annealingtemperature particle size of nanoparticles changes and surfacearea to volume ratio also changes, henceforth the magnetizationchanges. Very significant and promising effect observes on the Ms

due to the annealing of Fe3O4 nanoparticles at optimum tempera-ture 300 ◦C. At this temperature, the nanoparticles are uniformlydistributed with almost no agglomeration. It is very interestingto note that the magnetization of ferrite nanoparticles follow thecurie law of magnetization. The EPR peak at geff ∼ 2.05 (3300G) alsoconfirms the super paramagnetic nature of the nanoferrite alongwith higher super paramagnetic property in comparison to earlierreported nano ferrites. The structure and mechanism of nano fer-rite is being established by TGA analysis. Due to high demand andapplication of nanoferrite in medical sciences, it would be a newchallenge for our super paramagnetic nanoferrite, so further studiesof nano ferrite from medical point of view will be performed in nearfuture.

Acknowledgements

Authors are thankful to UGC Delhi, India for financial supportUGC-BSR Meritorious fellowship and to UGC – Dr. D.S. Kothari postdoctoral fellowship (No. F.4-2/2006(BSR)/13-215/2008(BSR) andfor SAP program in department of chemistry, Jamia Millia Islamiafor providing TGA/DTA facility. Authors are also thankful to Dr.Tokeer Ahmad, Department of Chemistry, Jamia Millia Islamia, NewDelhi for providing the BET facility for surface area study.

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