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Journal of Magnetism and Magnetic Materials 316 (2007) 422–425

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Low-temperature magnetic properties of Fe3C/ironoxide nanocomposite

B. Davida,�, O. Schneeweissa, M. Mashlanb, E. Santavac, I. Morjand

aInstitute of Physics of Materials, AS CR, Zizkova 22, CZ-61662 Brno, Czech RepublicbDepartment of Experimental Physics, Palacky University, Svobody 26, 77146 Olomouc, Czech Republic

cInstitute of Physics, AS CR, Na Slovance 2, CZ-18221 Praha 8, Czech RepublicdNational Institute for Laser, Plasma, and Radiation Physics, P.O. Box MG-36, R-76900 Bucharest, Romania

Available online 15 March 2007

Abstract

The magnetic properties of the Fe3C-based nanopowder synthesized by laser pyrolysis of ethylene and iron pentacarbonyl gases are

investigated in the temperature range 4–293K by Mossbauer spectroscopy and magnetic measurements in DC and AC fields. Fe3C was

identified as the dominant phase in the sample (75wt%). The mean size of the Fe3C crystallites is estimated to be 20 nm. The residual

phase in the sample is probably a mixture of nanocrystalline Fe3O4, g�Fe2O3, and a�Fe2O3. Zero-field-cooled and field-cooled DC

measurements as well as real and imaginary parts of AC susceptibility curves are discussed. The nanopowder exhibits superferromagnetic

behavior.

r 2007 Elsevier B.V. All rights reserved.

PACS: 81.07.Wx; 75.50.Bb; 75.50.Ts

Keywords: Fe3C; Nanopowder; Mossbauer spectroscopy; AC susceptibility; Magnetic properties

1. Introduction

In our previous articles [1,2], we have described basicstructural, magnetic and high-temperature properties of ananocrystalline Fe3C-based sample. Nevertheless, furthermagnetic measurements are needed to understand better itsmagnetic behavior.

In the present article, we extend our study by employingMossbauer spectroscopy and magnetic measurements atlow temperatures.

2. Experimental procedure

The nanopowder has been synthesized by the CO2 laserpyrolysis of iron pentacarbonyl FeðCOÞ5 and ethyleneC2H4 gases [3]. The sample was passivated in situ by agradual increase of air in the surrounding atmosphere [1].

- see front matter r 2007 Elsevier B.V. All rights reserved.

/j.jmmm.2007.03.091

onding author. Tel.: +420532290436; fax: +420541218657.

ddress: david@ipm.cz (B. David).

Transmission electron microscopy (TEM), X-ray diffrac-tion (XRD), infrared (IR), Raman and Mossbauer spectro-scopies were used for structural studies.The physical properties measuring system PPMS 9 from

quantum design equipped with the P500 AC/DC magne-tometry system was used for magnetic measurements.

3. Results and discussion

The TEM analysis of the sample [1] revealed the twotypes of nanoparticles and their morphology: (a) aggre-gated Fe3C crystalline particles with core–shell structure(core: Fe3C; shell: pyrolytic carbon) with diameter around20 nm; (b) aggregated nanoparticles containing oxygen andiron with the diameter around 10 nm. The diffractions fromFe3O4 were found in the powder electron diffractogram.The XRD pattern of the synthesized sample was well fitted

with orthorhombic cementite Fe3C (75wt%, mean coherencedomain length dXRD ¼ 18 nm) and Fe3O4=g�Fe2O3 (25wt%,dXRD ¼ 5 nm) [2].

ARTICLE IN PRESS

50010001500

Wavenumber [cm-1]

Inte

nsity [a.u

]

5001000

Ramanshift [cm-1]In

tensity [a.u

]

Fig. 1. (a) Infrared spectrum and (b) Raman spectrum for the Fe3C-based

nanopowder.

-10 -5 0 5 10

Velocity [mm/s]

0.94

0.96

0.98

1.00

Rela

tive tra

nsm

issio

n

0.94

0.96

0.98

1.00

-10 -5 0 5 10

at 4 K

at 293 Ka

b

Fig. 2. Mossbauer spectra for the Fe3C-based nanopowder. (a) Fe3C

component (gray filler) is the superposition of the FeG and FeS sextets.

Other components: sextet S1 (full line), distribution DIS1 (dashed line),

and doublet D1 (full line). (b) Fe3C component (gray filler) is the

superposition of the FeG and FeS sextets. FexOy component (network

filler) is a Gaussian distribution. The third component (black filler) is the

superposition of the ST1 and ST2 sextets.

Table 1

Parameters of the spectral sextets/doublets used for the fitting of the

Mossbauer spectra in Fig. 1

Component BHF [T] DBHF [T] eQ [mm/s] d [mm/s] A [%]

At 293K

Fe3C : FeS 21.2 — 0.03 0.21 30

Fe3C : FeG 20.3 — 0.01 0.20 60

S1 17.9 — �0.01 0.20 3

DIS1 8.8 8.0 0.02 0.42 5

D1 — — 0.23 0.18 2

At 4K

Fe3C : FeS 25.6 — 0.01 0.35 29

Fe3C : FeG 24.2 — 0.01 0.35 58

ST1 21.0 — 0.09 0.25 3

ST2 15.9 — �0.15 0.24 2

FexOy 48.4 8.3 0.03 0.45 8

BHF hyperfine magnetic induction; DBHF width of hyperfine field

distribution; eQ quadrupole shift; d isomer shift with respect to a�Fe; A

relative spectrum area.

B. David et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 422–425 423

The IR spectrum of the sample (Fig. 1a) exhibitsminimum at 602 cm�1 which is characteristic for bothFe3O4 and g�Fe2O3 [4].

The Raman spectrum of the sample (Fig. 1b) exhibitssignificant maximum at 670 cm�1 which is typical forFe3O4 [4] and corresponds to its most active A1g band [5].The maxima at 220, 280 cm�1 are assigned to a�Fe2O3

traces.In the Mossbauer spectrum of our sample measured at

293K (Fig. 2a, Table 1) the Fe3C sextets dominate but thespectrum cannot be satisfactorily fitted with them only.

The y-iron carbide Fe3C (cementite) is orthorhombicwith space group Pnma [6]. It has the iron atoms locatedat the general site (FeG atoms) and at the special site(FeS atoms). Two times more iron atoms occupy thegeneral site than the special site. Previous studies ofbulk cementite samples led to the identification of twosimilar sextets measured at room temperature [7,8] and at4K [8]. Mossbauer spectra of nanoparticle cementitemeasured at room temperature and at 12K were alsoreported [9].

Our parameters of the Fe3C sextets in the spectrummeasured at 293K agree well with those cited [8,9]. The S1sextet can be ascribed to Fe3C surface/interface. The DIS1distribution is assigned to superparamagnetic FexOy

particles interacting with Fe3C phase. The D1 doubletbelongs to Fe3þ or superparamagnetic Fe3C particles.

In the low-temperature Mossbauer spectrum (Fig. 2b,Table 1) the FexOy distribution needed to fit the spectrumconfirmed the presence of iron oxide(s) in our sample [4].This iron oxide component could not be fitted with Fe3O4

sextets only [10]. In the spectra measured at various lowtemperatures, the iron oxide sextets/distribution withBHF4BHFðFe3CÞ started to appear below 100K. Theparameters of the Fe3C sextets correspond again well tothose cited [8,9]. The ST1, ST2 sextets can be assigned toFe3C surface/interface.

The hysteresis loop of our sample measured at 293Kprovided the values of coercivity HC ¼ 52 kA=m, remanentmagnetization sR ¼ 16Am2=kg, and saturation magneti-zation sS ¼ 102Am2=kg (at 6400 kA/m). The bulk valuefor Fe3C is sS(bulk Fe3CÞ ¼ 130Am2=kg [11]. Corre-sponding values for the hysteresis loop of the sample

measured at 4K are: HC ¼ 144 kA=m, sR ¼ 34Am2=kg,and sS ¼ 122Am2=kg (at 6400 kA/m).The hysteresis loop measured at 4K after the sample was

field-cooled (FC) in 800 kA/m was not shifted relative to

ARTICLE IN PRESSB. David et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 422–425424

the loop measured at 4K after zero-field cooling. It meansthat our sample does not exhibit glassy dynamics [12].

The zero-field-cooled (ZFC) and FC s (8 kA/m) curvesindicate a superferromagnetic state of the sample attemperatures lower than 300K [13,14]. A collective freezingof Fe3C particles magnetic moments (superspins) takesplace during zero-field cooling.

A noticeable feature of the high field cooling data(Fig. 3) is the increase of s(800 kA/m) starting at 40Kduring the cooling of the sample.

The measurement of the in-phase AC susceptibility (w0)and out-of-phase AC susceptibility (w00) after cooling thesample in zero magnetic field (Fig. 4) was repeatedlyperformed with the following results: (a) the blockingtemperature TB of superparamagnetic particles [15]taken as the absolute maximum of the w00 curve is 95K;(b) an abrupt decrease in w00 takes place at 40K (local

0 100 200 300

T [K]

0.6

0.8

1.0

1.2

1.4

1.6

(8 k

A/m

) [A

m2/k

g]

96

97

98

99

100

101

102

(800 k

A/m

) [A

m2/k

g]

ZFCin 8 kA/m

FC in 8 kA/m

FC in 800 kA /m

ZFCin 800 kA/m

Fig. 3. DC magnetization for the Fe3C-based nanopowder.

0 100 200 300

T [K]

120

140

160

180

200

'[c

m3/k

g]

0.0

0.4

0.8

1.2

''[c

m3/k

g]

' '

ZFC

'

ZFC

HAC= 800 A /m

f = 1 kHzHDC= 0 A/m

Fig. 4. AC susceptibility for the Fe3C-based nanopowder.

minimum at �35K). The value of TB was observed to beAC frequency dependent—it shifted to higher temperatureswith increasing frequency. The fall of w00 at 40K wasnot AC frequency dependent. It seems that the minimumof w00 at �35K can be attributed to the structural/magneticchanges in Fe3O4 at low temperatures [16–20]. Thedecrease of w00 at 40K coincides with the increase insð800 kA=mÞ at 40K in Fig. 3. It should be noted that inour case FexOy nanoparticles are aggregated into largerstructures [1].

4. Conclusions

The low-temperature magnetic properties of the Fe3C/iron oxide nanocomposite were described. Fe3C nanopar-ticles were found to be ferromagnetic and aggregated intolarger structures and covered by pyrolytic carbon. TheFe3C nanoparticles and their aggregates interact magneti-cally through dipole–dipole interactions or directly viainterfaces. From ZFC/FC curves and hysteresis loops,Fe3C phase was inferred to be superferromagnetic.FexOy nanoparticles and their aggregates with fluctuat-

ing magnetic moments were identified as the second phasein the sample. This phase undergoes transformation intomagnetically ordered state during cooling of the samplebelow 100K.

Acknowledgments

This work was supported by the Grant Agency of theCzech Republic (No. 202/04/0221), the Academy ofSciences of the Czech Republic (No. AV0Z20410507),and Ministry of Education, Youth and Sport of the CzechRepublic (No. 1M619895g201).

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