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Modern Physics Letters BVol. 26, No. 22 (2012) 1250140 (11 pages)c© World Scientific Publishing Company

DOI: 10.1142/S0217984912501400

SYNTHESIS, MICROSTRUCTURAL AND MAGNETIC

CHARACTERIZATIONS OF SELF-ASSEMBLED

HEMATITE NANOPARTICLES

BHAVYA BHUSHAN

Centre for Applied Physics, Central University of Jharkhand, Brambe,Ranchi, Jharkhand 835205, India

bhavyaiuc@gmail.com

SAMRAT MUKHERJEE

Department of Applied Physics, Birla Institute of Technology, Mesra,Ranchi, Jharkhand 835215, India

DIPANKAR DAS

UGC-DAE Consortium for Scientific Research, Kolkata Centre,III/LB 8, Bidhannagar, Kolkata 700098, India

Received 5 April 2012Revised 3 May 2012

Accepted 23 May 2012Published 27 July 2012

Phase pure hematite nanoparticles coated with octyl ether and oleic acid were synthe-

sized by a facile chemical route. Nanoparticles in the size range 7–25 nm were obtainedby annealing the as-prepared samples at different temperatures. Transmission electronmicrographs revealed that the particles tend to organize themselves in chain-like struc-tures consisting of three to six nanoparticles along a common axis. The reason behindthis oriented attachment is traced to the exchange coupling between the nanoparticles.Effect of exchange coupling was also evidenced as suppression of superparamagnetism inlower dimension hematite nanoparticles. Morin transition was observed around 260 K bymagnetization measurement for hematite nanoparticles having average crystallite size of25 nm.

Keywords: Nanostuctures; chemical synthesis; Mossbauer spectroscopy; magneticproperties.

1. Introduction

Hematite (α-Fe2O3) is one of the most used metal oxides with various applications

in many scientific and industrial fields.1 In the bulk state, hematite shows interesting

magnetic characteristics. At low temperatures (T < 260 K) it is antiferromagnetic

with spins oriented along the rhombohedral [111] axis. Above 260 K, known as

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B. Bhushan, S. Mukherjee & D. Das

Morin transition temperature (TM), a reorientation of spins by about 90◦ takes

place2,3 and the spins become slightly canted to each other, causing the development

of weak ferromagnetism. The Morin transition is reported to be absent4 for hematite

nanoparticles with diameters less than ∼ 8 nm. Because of its chemical stability,

hematite nanoparticles are preferred over nanoparticles of pure transition metals

like Fe, Co, Ni etc. in various traditional and emerging industrial applications.1,5

Due to immense applications of hematite nanoparticles in scientific and indus-

trial fields, the study of its size as well as interparticle interaction dependent mag-

netic properties is crucial. Magnetic fluctuations in nanoparticles have been the

subject of numerous studies6–9 and one of the most important problems has been

understanding the superparamagnetic (SPM) relaxation. The SPM relaxation is

characterized by flipping of sublattice magnetization direction between easy axes

due to thermal energy.6,10 This results in disappearance of coercivity in the mag-

netization measurement and the collapse of the magnetic hyperfine splitting in

Mossbauer spectra.11

Out of several factors12,13 that influence the magnetic properties of hematite

nanoparticles, two very important factors, i.e. size of particles and their spatial

arrangement in an assembly, are the focuses of the present study. In an assembly

of magnetic nanoparticles, both dipole–dipole as well as exchange interactions are

important. Since, for antiferromagnetic particles the dipolar interaction is extremely

weak, these can be ideal systems for studying the effect of exchange interaction on

the self assembly of nanoparticles. Study on self assembled magnetic nanoparticles

are still at a nascent stage but offers many exciting prospect in significant reduction

in the size of electronic and optical devices and also in fabrications of new devices.14

In the present paper, we report the synthesis of hematite nanoparticles and their

microstructural, magnetic and hyperfine characterization using X-ray diffraction

(XRD), transmission electron microscopy (TEM), vibrating sample magnetometry

(VSM) and 57Fe Mossbauer spectroscopy. Hematite nanoparticles in the present

case were effectively coated with organic surfactant molecules to minimize the spin

disorder generally produced at the grain boundaries. We have shown in this article

that these coated hematite nanoparticles tend to form chain like structure instead

of forming clusters, which has been argued to be due to the exchange interactions

between the adjacent particles.

2. Experimental Methods

The hematite nanoparticles were synthesized by a soft chemical route in the pres-

ence of organic surfactants. Calculated amount of Fe(NO3)3 · 9H2O was dissolved

in water in the presence of a few drops of 1 M HCl. 6 mmol each of octyl ether and

oleic acid were added and the mixture was stirred vigorously for 15 min at 80◦C.

0.5 M NaOH was then added dropwise and the pH of the solution was adjusted to

9. A brownish red precipitate was obtained which was centrifuged and collected. It

was then washed thoroughly with water upto pH 7. It was also rinsed with absolute

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Characterizations of Self-Assembled Hematite Nanoparticles

alcohol to remove any unreacted organic surfactant present. The precipitate was

collected and heat-treated at 200◦C, 250◦C, 300◦C and 900◦C in air for 12 h each

to get hematite particles of different sizes. In the proceeding part of this article,

the samples are identified by their heat treatment temperature, i.e. B200 means

the sample prepared by treating the precursor at 200◦C. XRD studies were carried

out in a Philips X-ray diffractometer (model no. PW 1830) with a cobalt target

(λ = 1.78897 A). The morphological characterization of the nanoparticles was made

with a JEOL 2010 model high-resolution transmission electron microscope operat-

ing at 200 kV. Room temperature Mossbauer measurements were carried out in a

standard PC based set-up consisting of a 1024 channel MCA card, working in the

constant acceleration mode. A 10 mCi 57Co in Rh matrix was used as the source.

The system was calibrated with a high purity iron foil of thickness 12 µm. A least

squares fitting program LGFIT215 was used to fit the experimental data assuming

Lorentzian line shape. The low temperature Mossbauer measurement was carried

out using a closed cycle refrigerator from Janis, USA with a special anti-vibration

stand. Magnetization measurements were done using a Lakeshore 7400 series VSM.

3. Results and Discussions

3.1. XRD studies

Figure 1 shows the representative X-ray diffractograms obtained for the hematite

nanoparticles samples B200 and B900. From the diffraction peaks the single-phase

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Fig. 1. XRD patterns of B200 and B900.

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B. Bhushan, S. Mukherjee & D. Das

corundum structure of hematite was confirmed after comparing the peak positions

with that given in the standard JCPDS file. After making the correction for instru-

mental broadening, the average crystallite size in the samples was calculated from

the broadening of the (104) diffraction line using the Scherer formula

D =0.9λ

β1/2 cos θ, (1)

where β1/2 is the full width at half maximum of the X-ray line profile, λ is the

wavelength for Co Kα. The average crystallite size was found to increase steadily

with increase of annealing temperature. The mean crystallite sizes were seen to vary

from 7 nm to 25 nm as the heat-treatment temperature was increased from 200◦C

to 900◦C. The average crystallite size for the sample B200, B250, B300 and B900

were found to be 7 nm, 10 nm, 12 nm and 25 nm, respectively. The broadening

observed in the XRD line profile of B200 sample is attributed to the small size of

the hematite nanoparticles.

3.2. TEM studies

Figure 2(a) shows the TEM image of the sample B300. The particles are almost

spherical in shape without clear facets having an average particle size of 12± 2 nm

that is in good agreement with the results obtained from XRD data. It is interesting

to note that the nanoparticles are no longer forming clusters rather they form small

chains [see Fig. 2(a)] consisting of about three to six particles. High resolution TEM

image shown in Fig. 2(c) clearly exhibits that the particles tend to attach themselves

with their neighbors along a common axis and the lattice planes are continued from

one particle to its neighboring particle. The self-assembly of hematite nanoparticles

in chain like structures and its epitaxial growth might be due to the strong exchange

coupling among these nanoparticles.16 A high resolution TEM image from a portion

of the B300 nanoparticle is shown in Fig. 2(d). Clear lattice fringes are visible

indicating defect-free nature of the sample with high degree of crystallinity. The

lattice plane is identified to be [104] plane of hematite from its inter planer distance

as indicated in the figure. Hematite nanoparticles exhibited polycrystalline nature

that is clear from the selected area diffraction pattern of the sample B300 shown

in Fig. 2(b).

3.3. Magnetization studies

Figures 3(a) and 3(b) show the hysteresis curves for the sample B200 and B900, re-

spectively, measured at 300 K. The insets show the corresponding curves measured

at 80 K. The M–H curve of the sample B200 recorded at 300 K is typical of sam-

ple consisting of single-domain particles undergoing SPM relaxation. The value of

coercivity, HC and remanent magnetization, MR, is negligibly small (see Table 1).

But when the sample is taken to a temperature of 80 K and the M–H response

is recorded, we get an enhancement, however small, in the values of HC and MR.

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Characterizations of Self-Assembled Hematite Nanoparticles

Fig. 2. (a) TEM image of B300, (b) selected area diffraction pattern of B300, (c) HRTEM imageof B300 showing that particles are attached to one another and (d) HRTEM image of an individualB300 nanoparticle showing the [104] crystal plane.

Table 1. Magnetic measurement data.

Sample ID Temperature (K) Remanent magnetization (emu/g) Coercive field (Oe)

B20080 0.0045 47

300 0.0011 11.8

B90080 0.011 169

300 0.038 300

This points to the slowing down of the spin relaxations due to reduction of thermal

energy. For the sample B900, spontaneous magnetization, MS, at 15 kOe and HC

measured at 300 K is substantially higher than that measured at 80 K. This in-

dicates a phase transition from a weakly ferromagnetic (WF) to antiferromagnetic

(AF) phase as the temperature is lowered from 300 K to 80 K.

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B. Bhushan, S. Mukherjee & D. Das

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Fig. 3. (a) M–H curve of B200 recorded at 300 K. The inset shows the corresponding curve at80 K and (b) M–H curve of B900 recorded at 300 K. The inset shows the corresponding curve at80 K.

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Characterizations of Self-Assembled Hematite Nanoparticles

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Fig. 4. ZFC–FC curves of (a) B200 and (b) B900 in an applied field of 50 Oe.

Figures 4(a) and 4(b) show the zero-field cooled and field cooled (ZFC-FC)

magnetization curves for the sample B200 and B900, respectively in an applied field

of 50 Oe. For the sample B900, when the temperature is increased, magnetization

changes from a lower value corresponding to a perfectly AF phase, to a higher value,

corresponding to the WF phase. The temperature at which the magnetization has

its inflection point, half way between the AF state and the WF state, gives the

value of the Morin transition temperature. The Morin transition temperature, TM

was obtained by differentiating the ZFC curve of the sample, which came out to

be ∼ 260 K. In the case of sample B200, the magnetization of both FC and ZFC

cycles is seen to decrease with increase of temperature. Although the branching

temperature of the two cycles is seen, the exact blocking temperature is below the

lowest temperature attainable in the present case. In the temperature region shown

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B. Bhushan, S. Mukherjee & D. Das

here, magnetization in the ZFC cycle shows thermodynamic equilibrium properties

typical of the unblocked SPM state. In this case, the spin relaxation time is much

smaller than the measurement time even for the largest particles present.

3.4. Mossbauer spectroscopy studies

Mossbauer spectroscopy is one of the best-suited techniques for studying mag-

netic relaxations in iron oxide nanoparticles. The Morin transition temperature in

hematite can readily be detected by Mossbauer spectroscopy. This particular tran-

sition is marked by a change in the sign of the quadrupole splitting parameter, ε,

above and below TM. The quadrupole splitting parameter depends on the canting

angle φ of the spins with respect to the c axis [111] and is given by

ε =ε′

2(3 cos2 φ− 1) , (2)

where ε′ is the quadrupole interaction strength. Thus, in the antiferromagnetic

state (φ = 0◦), ε is positive and in the weakly ferromagnetic state (φ = 90◦), ε is

negative.

Figure 5 shows Mossbauer spectra of the samples B200, B250, B300 and B900

recorded at room temperature. Mossbauer spectrum of the sample B200 shows a

doublet superposed on a six-finger pattern. The average crystallite size of B200

is 7 nm as calculated from XRD data. Room temperature Mossbauer spectrum of

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Fig. 5. Mossbauer spectra of (a) B200, (b) B250, (c) B300 and (d) B900 recorded at 300 K.

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Characterizations of Self-Assembled Hematite Nanoparticles

Table 2. Hyperfine parameters of all hematite nanoparticles.

ISa (mm/s) QSb (mm/s) Hyperfine field (T) Intensity

Sample ID (± 0.01 mm/s) (± 0.01 mm/s) (± 0.3 T) (%)

B200Dc 0.40 0.73 — 45

Sd 0.30 −0.22 49.3 55

B250D 0.45 0.66 — 12

S 0.40 −0.13 50 88

B300 S 0.33 −0.20 50.2 100

B900 S 0.34 −0.17 51.3 100

B200 LTe S 0.30 −0.20 54.6 100

aIS = Isomer shift.bQS = Quadrupole splitting.cD = Doublet.dS = Sextet.eLT = Low temperature (20 K).

such fine particles should consist of a doublet, i.e. the particles should undergo SPM

relaxation.10 Wide particle size distribution may be a possible cause for the presence

of sextet in sample B200. Another reason for the existence of sextet is traced to

be the presence of exchange coupling among the hematite nanoparticles. Exchange

coupling is known to beat the SPM limit in magnetic nanoparticles.17,18 Decreased

contribution of doublet is a direct evidence of exchange interaction present in our

hematite nanoparticles. This fact is also supported by the TEM data as discussed

in earlier section. The SPM fraction is seen to be less in the sample B250 as the

intensity of the doublet is much less than that in the sample B200, whereas the

lines due to magnetically ordered state are much stronger. The SPM doublet was

absent in the samples B300 and B900, which show clear sextets indicating that the

particles are magnetically ordered. The hyperfine parameters, i.e. isomer shift (IS),

quadrupole splitting (QS) and the internal magnetic field (Hint) computed from the

fitting are given in Table 2. The IS values confirms the presence of only 3+ oxidation

state of Fe in the samples. The value of QS was seen to decrease in the samples heat

treated at higher temperatures. As the particle sizes gradually increase with heat

treatment, the charge ordering distortion around the iron nucleus reduces which

results in lower QS values.

To confirm that the doublets observed at room temperatures were indeed due

to superparamagnetism, Mossbauer spectrum of B200 was recorded at 20 K. The

doublet seen at room temperature disappeared and a clear six-finger pattern (Fig. 6)

was observed. The appearance of a sextet pattern at the cost of the doublet is a

clear signature of SPM relaxation in the sample and it is also evident that at 20 K

even the smallest particles are in the blocked state. Another point to be noted is

that the QS of the sextet still remains negative. This indicates that the sample

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B. Bhushan, S. Mukherjee & D. Das

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Fig. 6. Mossbauer spectrum of B200 recorded at 20 K.

has still not undergone the Morin transition. It is known that Morin transition is

severely suppressed with decreasing particle size, and below a certain critical size,

Morin transition disappears altogether. The increase in Hint at 20 K as compared

to the room temperature value follows from the relation

Hobs = H0

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)

, (3)

where, Hobs is the observed hyperfine field at a temperature T and H0 is the field

extrapolated to absolute zero.

4. Conclusion

The present study establishes the role of surfactants in obtaining pure phase

hematite nanocrystals. The hematite nanoparticles synthesized by coating of oleic

acid and octyl ether showed a tendency of forming chains along a common axis

as observed in the electron micrographs. The origin of the behavior was traced

to the presence of exchange interaction. Presence of exchange interaction was also

evidenced in Mossbauer spectra of the fine hematite nanoparticles where super-

paramagnetism was suppressed due to exchange interaction. Within a small range

of particle size (7 to 25 nm) drastic changes in magnetic properties have been ob-

served which is clearly reflected in the absence and presence of Morin transition in

the hematite nanoparticles heat treated at 200◦C and 900◦C, respectively.

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Characterizations of Self-Assembled Hematite Nanoparticles

Acknowledgments

The authors thank Prof. A. Basumallick, Bengal Engineering and Science Univer-

sity, Shibpur, India, for carrying out XRD measurements.

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