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Physics Procedia 00 (2008) 000–000

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Proceedings of the JMSM 2008 Conference

Magnetic and Structural Properties of Nanocrystalline Iron Oxides

A. Kihala,b*, B. Bouzabataa, G. Fillionb, D. Fruchartb

a Laboratoire de Magnétisme et Spectroscopie des Solides (LM2S), Université Badji Mokhtar, BP-12 Annaba, Algérie.b Institut Néel, CNRS, 25 Rue des Martyrs, BP-166, 38042 Grenoble-Cedex 9, France.

Elsevier use only: Received date here; revised date here; accepted date here

Abstract

Hematite (�-Fe2O3) and magnetite (Fe3O4) powders, milled for various times up to 10 hours by a high energy ball millingtechnique, were structurally and magnetically characterized by X-ray diffraction and magnetic measurements in magnetic fieldup to 10 T in the 2 K – 300 K temperature range. From X-ray diffraction line analyses, it is found that when the milling time isincreased, the grain size of �-Fe2O3 particles falls down to about 15 nm, and the induced strains and distortions increase up to 0.9% at 10 hours of milling. The ZFC-FC magnetization curves of the milled samples for 1 hour, with an applied field of 5 mT,show that superparamagnetism is present, but with a blocking temperature well above 300 K. The magnetization at saturationincreases to a maximum of 0.78 Am2/kg, while the coercive force falls rapidly to a constant value after a milling time of 5 hours.Magnetite powders, milled up to 10 hours, transform partially to hematite, leading to a decrease of the magnetization at 300 K. Acorrelation between the microstructural evolution of the milled sample can be made with its magnetic properties.© 2009 Elsevier B.V.

PACS: 61.46.+w ; 75.50.Tt ; 81.07.Wx ; 75.30.Cr ; 75.50.Ee ; 75.50.Gg

Keywords: nanoscale materials ; nanopowders ; ball-milling ; iron oxides ; hematite ; magnetite ; magnetic susceptibilities ; magnetic saturation

1. Introduction

Small magnetic particle systems are interesting due to their applications in the fields of nanoscience andnanotechnology. The iron oxides as �-Fe2O3 (hematite), Fe3O4 (magnetite), �-Fe2O3 (maghemite) and FeO (wurstite)are important as electrical and magnetic materials [1-3]. Recently, high energy ball milling (HEBM) technique hasemerged as a non-expensive route to produce non-equilibrium phases in several varied forms, such asnanostructured and amorphous materials, nanocomposites and extended solid solutions. As a consequence of that,there is a great dispersion of results concerning the properties of iron oxides nanoparticles. In particular, extensivestudies on magnetite and maghemite nanoparticles have been made, but only few on hematite based particles.

In this context, we have prepared two series of nanosized iron oxides particles by the HEBM technique, startingfrom �-Fe2O3 and Fe3O4 powders respectively. Their structural and magnetic properties were investigated.

* Corresponding author. Tel.: +213 774061278; fax: +213 38872770.E-mail address: amel.kihal@gmail.com.

Received 1 January 2009; received in revised form 31 July 2009; accepted 31 August 2009

Physics Procedia 2 (2009) 665–671

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doi:10.1016/j.phpro.2009.11.008

Open access under CC BY-NC-ND license.

2 A. Kihal et al./ Physics Procedia 00 (2009) 000–000

2. Experiments

Hematite (99.9 % pure �-Fe2O3) and magnetite powders, from Alfa-Aesar, were put in a planetary ball mill wherehardened steel vials of 45 cm3 volume containing 6 stainless steel balls of 13 mm diameter were set in rotation at400 rpm. The ball-to-powder mass ratio was kept at 30:1. All milling experiments were performed in air atmosphere(dry milling). The powders were milled for various times ranging from 1 h to 10 h.

X-ray diffraction measurements were performed using a Siemens D5000 diffractometer with Co (K�) radiation inthe 2� range from 20° to 80° with counting time varying between 4 and 12 s for steps of 0.02°. We have consideredthe most important reflexions for X-ray profile analysis. The mean grain size was obtained from the Scherrerrelation. The particle morphology of the ball-milled materials was observed by scanning electron microscopy(SEM).

The magnetic characterization was performed in dc magnetic fields up to 10 T and in the temperature range of 2K – 300 K by the extraction method in the magnetometer of the Néel Institute at Grenoble. Zero-field-cooled (ZFC)and field-cooled (FC) curves were taken from 5 to 300 K, using a commercial Quantum Design SQUIDmagnetometer.

3. Results and discussion

3.1. Hematite �-Fe2O3

SEM micrographs (Fig. 1) of hematite milled for 1 h to 10 h showed that all samples consisted of agglomerates ofparticles varying from 200 to 700 nm in size.

The XRD patterns of the unmilled (0 h) and milled powders (Fig. 2) show the hematite peaks, with no apparenttransformation to magnetite [4]. Usually, this transformation is observed and depends on the milling atmosphere andimpurities induced by the deformation process.

Fig. 1. SEM micrographs of (a) unmilled hematite, (b) milled for 1 h, (c) milled for 5 h and (d) milled for 10 h.

666 A. Kihal et al. / Physics Procedia 2 (2009) 665–671

A. Kihal et al./ Physics Procedia 00 (2009) 000–000 3

A broadening of the reflection peaks is observed, due to the particle size reduction and to the increase in the rootmean square (rms) of the atomic-level strain induced from the fracture and welding processes. The deduced valuesof these two parameters are shown in Fig. 3. The grain size of the �-Fe2O3 particles after 10 h of milling falls toabout 15 nm and the average strain increases from 0.5 % at 1 h to 0.9 % at 10 h.

The relative deviations of the cell parameters �a/a=(a-a0)/a0, �c/c=(c-c0)/c0, and unit-cell volume�V/V=(V-V0)/V0, from the values a0, c0 and V0 of the bulk unmilled hematite are shown versus milling time in Fig.4. We observe an anisotropic expansion of the unit cell, with a significant dilatation at 1h of milling with an increaseof the cell volume of 0.59 %. This effect seems to be due to the rapid cold working as previously observed by others[5]. For higher milling times, a and c remain roughly constant.

The ZFC magnetization curve is obtained by cooling in a zero field from room temperature and then measuringmagnetization as temperature is increased in a small applied field. The FC magnetization curve is obtained bymeasuring at decreasing temperatures in the same small applied field. The magnetization versus temperature curves

20 40 60 80

0

200

400

600

(300

)

(214

)

(024

)

(116

)

(018

)

(012

)

(113

)(110

)

(104

)

5 h

10 h

3 h

1 h

0 h

I(a.

u)

2 θθθθ ( deg. )

Fig. 2. X-ray powder diffraction pattern (Co Kα radiation) of unmilled (0 h) and milled hematite �-Fe2O3 (milling time: 1 h, 3 h, 5 h and 10 h).

0 2 4 6 8 1010

20

30

40

50

60

70

80

Milling time ( h )

Cry

stal

lite

size

(nm

)

2

4

6

8

10

R.m

.s.strain.10

-3

Fig. 3. Evolution of the crystallite size and the microdeformationversus milling time of α-Fe2O3.

0 2 4 6 8 10

-0,1

0,0

0,1

0,2

0,3

0,4

0,5

0,6

ΔΔ ΔΔa/a

, ΔΔ ΔΔc/

c,ΔΔ ΔΔV

/V(%

)

Milling time ( h )

Fig. 4. Relative deviations of the cell parameters and cell volumeversus milling time. �a/a (open square), �c/c (circle), �V/V (triangle)(see definitions in text).

A. Kihal et al. / Physics Procedia 2 (2009) 665–671 667

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measured under ZFC and FC conditions in a magnetic applied field of μ0H = 5 mT are presented in Fig. 5. Theyshow a large irreversibility on the magnetization, typical of mostly single domain particles characterized by asuperparamagnetic (SPM) regime at high temperature [6], but with blocking temperatures above 300 K, and onlyfew percent being in the super paramagnetic relaxation regime at 300 K as it was seen in Mössbauer experiments[7].

Other measurements of the magnetization of all the samples were performed as a function of the magnetic fieldup to 10 T at different temperatures in the 2 K-300 K range. Typical results at T=300 K and T=20 K are shown inthe figures 6 and 7a, 7b respectively. For all the milled powders there is an hysteretic behaviour in low fields; themagnetization increases rapidly at fields up to about 0.3 T and then vary almost linearly without reaching anysaturation in a field of 10 T. This typical weak ferromagnet like behaviour confirms the main antiferromagneticinteractions in hematite particles, responsible of a large magnetic susceptibility in high fields which is superimposedto a spontaneous ferromagnetic moment, coming either from true weak ferromagnetism for temperatures above theMorin transition (MT), either for uncompensated moments mainly at the surface of the particles [8].

For each magnetization curve, one can consider the high field linear part as defining a high field magneticsusceptibility χhf by its slope and a saturation magnetization Ms by its extrapolation to zero field. For all thesamples, the saturated ferromagnetic contribution Ms depends on the milling time but is temperature independentwithin the experimental errors of its determination. For example, in the figure 8, the dependence with the millingtime of Ms is reported at 300 K and at 100 K. For each milling time, the values are almost identical and it indicatesthat the Morin transition is not seen in our experiments. Such suppression of the Morin transition has been alreadydescribed in the past for powders of nanoscale particles [9, 10] and recently related to some microstructural changesin ball-milled samples [11]. Ms begins to increase, reaching a maximum value after 3 h of milling (either at 100 K or300 K) and then decreases slightly. This can be explained by the raise of uncompensated moments, mainly at thesurface of the particles, which give a spontaneous specific moment aligned with the antiferromagnetic (AF)direction of each grain and related to its mean diameter d by a power law 1/dα with an exponent α in the range: 1 to2, [8]. For very small grains, this increase can be overcomed by the fact that the antiferromagnetic structure isbecoming less organized, without well defined AF direction and with surface spins having rather random directions,resulting in a smaller moment of the particle.

-0,4 -0,2 0,0 0,2 0,4-1,0

-0,5

0,0

0,5

1,0

αααα-Fe2O

3

T = 20 K

M(A

m2/k

g)

μμμμ0000H ( T )

Fig. 7b. Insight on the hysteresis loops, showing the coercive field Hc,for different milling times at T = 20 K.

Fig. 7a. Hysteresis loops for different milling times at T = 20 K.

-10 -5 0 5 10

-3

-2

-1

0

1

2

3 αααα-Fe2O

3

T = 20 K

M(A

m2

/kg

)

μμμμ0000H ( T )

3h5h10h

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The high field magnetic susceptibility χhf is also almost temperature independent and its value for the unmilledpowder (χhf = 0.19 ± 0.01 Am2/kg/T) is in good agreement with the previous determination on powders and crystals[12-14]. All the χhf values for the milled samples are slightly higher but within the range 0.20 – 0.21 Am2/kg/T; evenat low temperature.

On another way, the variations of the coercive force Hc with the milling time at the same temperatures (300 and100 K) are shown in the figure 9. μ0Hc decreases from 0.07 T (for T = 300 K) and 0.11 T (for T = 100 K) to about0.03 T above 5 h of milling. These values are in agreement with previous studies where μ0Hc was found to varybetween 0.03 T and 0.4 T depending on the hematite particles microstructure [11].

3. 2. Magnetite Fe3O4

Figure 10 shows the X-ray diffraction patterns of magnetite powders in both unmilled (0 h) and milled (10 h)states. After 10 hours of milling, the Fe3O4 phase diffraction lines are broad and shifted to the higher angles,revealing a decrease of the lattice parameter, i.e., from a = 8.3928 Å in the initial sample to a = 8.3853 Å in themilled sample. The mean crystallite size of the Fe3O4 particles attains 17 nm. It can be seen that some diffractionpeaks are due to the presence of hematite. We estimate the mean grain size of the formed �-Fe2O3 particles to 16 nmwith a unit cell axis of a = 5.0398 Å and c = 13.7317 Å. In fact, according to recent reports [4, 15, 16], it is foundthat phase changes of iron oxides depend on the milling atmosphere, the presence of impurities, milling materialsand milling time. It is also observed [17] that the milled Fe3O4 is oxidized to �-Fe2O3 after 70 h of ball milling.However, when it is milled in an argon atmosphere [18], the �-Fe2O3 phase appears after 12 h and disappears after48 h. In our case, it seems that about 40% of the initial magnetite powder is transformed to hematite after 10 hoursof dry milling in air.

0 2 4 6 8 10

0,2

0,4

0,6

0,8αααα-Fe

2O

3

100 K300 K

Ms

(A

m2

/kg

)

t ( h )

Fig. 8. Saturation magnetization dependence with milling time at100 K and 300 K.

0 2 4 6 8 100,02

0,04

0,06

0,08

0,10

0,12

αααα-Fe2O

3

100 K300 K

μμ μμ 00 00H

c(T

)

t ( h )

Fig. 9. Coercive force dependence with milling time at 100 Kand 300 K.

A. Kihal et al. / Physics Procedia 2 (2009) 665–671 669

6 A. Kihal et al./ Physics Procedia 00 (2009) 000–000

The changes in ZFC and FC magnetization with temperature of our milled Fe3O4 sample are shown in Fig. 11.The ZFC magnetization of the milled sample for 10 h increases almost linearly up to 130 K with a little bumparound 35 K. Then, it smoothly decreases until 300 K. This behaviour has been previously observed in annealednanocrystalline Fe3O4 powders [19]. Furthermore, the FC and ZFC curves are distinct over all the temperature range.It reveals that the blocking temperature, at which the superparamagnetic relaxation is complete with regard to thetime of measurement, is still above 300 K for the concerned particles. This feature can be attributed for a large partto the hematite nanoparticles present in that powder. The Verwey transition, which is observed still rather sharply inthe non milled sample at about 120 K, seems to spread out over the whole range from 4 to 130 K for the milledsample. It is known that this transition temperature is shifted to lower temperatures for small particles [20], anddisappears completely for particle sizes smaller than 10nm. So, in our case, the observed curve may result of thedistribution in Fe3O4 grain size centered at about 17 nm with a related distribution of Verwey temperatures with acorresponding center at 35 K.

20 40 60 800

200

400

600

800

(440

)

(511

)

(422

)

(400

)

(222

)

(311

)

(220

)

(111

)

MagnetiteHem atite

10 h

0 h

I(a.

u.)

2 θθθθ ( deg. )

Fig. 10. X-ray powder diffraction pattern (Co Kα radiation) of unmilled and milled Fe3O4 powder to 10 h.

0 50 100 150 200 250 300

6

8

10

12

14

16

0h

T ( K )

M(A

m2

/kg

)

0,6

0,8

1,0

1,2

1,4

1,6

1,8

Tv

Fe3O

4

10h

Fig. 11. Zero-field-cooled (ZFC) and field-cooled (FC) magnetizationversus temperature (�0H = 10 mT) for Fe3O4 sample milled 10 h.

0 2 4 6 8 100

20

40

60

80

100

M(

Am

2/k

g)

μμμμ0000H ( T )

4K (0h)300K (0h)4K (10h)300K (10h)

Fig. 12. Magnetization curves of the unmilled and milled Fe3O4

sample at 4K and 300K.

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A. Kihal et al./ Physics Procedia 00 (2009) 000–000 7

Magnetization measurements versus applied field performed at T = 4 K and 300 K are presented in figure 12. It isseen that milling process induces a decrease of magnetization due mainly to the formation of superparamagnetic �-Fe2O3 particles. The change in the saturation magnetization Ms from 88 to 50.6 Am2/kg at 300 K confirms theproportion of hematite estimated from X-ray diffraction. Nevertheless, the observed decrease in Ms can also be dueto spin disorder at the surface of the Fe3O4 nanograins but this effect can be neglected in first approximation withregard to the effect of the transformation in hematite. In the meanwhile, the coercive force increases from 0.038 to0.063 T (at 4 K) and from 0.022 to 0.048 T (at 300 K).

4. Conclusion

We have shown the effect of ball milling on the structural and magnetic properties of iron oxides. By HEBM upto 10 hours of �-Fe2O3 powders, nanometer-sized particles were obtained without any change of phase. The Fe3O4

milled particles exhibited a partial transformation from cubic magnetite phase to hexagonal hematite phase that canexplain the decrease of its saturation magnetization and the complex behaviour of ZFC-FC curves at low field.Further investigations would be needed in order to precise the behaviour of such fine-particles iron oxide systems.

References

[1] P. Matteazi, G. Le Cäer, Mater Sci. Eng. A 149 (1991) 135.[2] T. Kosmac, T.H. Courtney, J. Mater. Res. 7 (1992) 1519.[3] W.A. Kaczmarek, B.W. Ninham, IEEE Trans. Magn. 30 (1994) 732.[4] M. Zduji�, C. Jovalekic, Lj. Karanovic, M. Mitric, Mater. Sci. Eng. A262 (1999) 204.[5] P. Sahu, M. De, M. Zduji�, Mater. Chem. & Phys. 82 (2003) 864.[6] R. Zysler, D. Fiorani, J.L. Dormann, A.M. Testa, J. Magn.Magn. Mater. 133 (1994) 71.[7] W. Kündig, H. Bömmel, G. Constabaris, R.H. Lindquist, Phys. Rev. 142 (1966) 327.[8] L Néel, C. R. Acad. Sci. 252 (1961) 4075.[9] Y. Bando, M. Kiyama, N. Yamamoto, T. Takada, T. Shinjo, H. Takaki, J. Phys. Soc. Japan 20 (1965) 2086.[10] N. Yamamoto, J. Phys. Soc. Japan 24 (1968) 23.[11] C. Cuadrado-Laborde, L.C. Damonte, L. Mendoza-Zelis, L.M. Socolovsky, I.L. Torriani, Physica B 354 (2004) 125.[12] R. Chevallier, J. Phys. Rad. 12 (1951) 172.[13] L. Néel, Rev. Mod. Phys. 25 (1953) 58.[14] A. Tasaki, K. Siratori, S. Iida, J. Phys. Soc. Japan 15 (1960) 1535[15] R. A. Borzi, S. J. Stewart, G. Punte, R. C. Mercader, M. Vasquez-Mansilla, R. D. Zysler, E. D. Cabanillas, J. Magn. Magn. Mater. 205

(1999) 234.[16] R. D. Zysler, M. Vasquez-Mansilla, C. Arciprete, M. Dimitrijewits, D. Rodriguez-Sierra, C. Saragovi, J. Magn. Magn. Mater. 224 (2001) 39.[17] M. Sorescu, J. Mater. Sci. Lett. 17 (1998) 1059.[18] E. Yamasue, H. Yamaguchi, H. Okumura, K.N. Ishihara, J. Alloys Comp. 434-435 (2007) 803.[19] C. R. Lin, Y. M. Chu, S. C. Wang, Materials Letters 60 (2006) 447.[20] G. F. Goya, Sol. Stat. Comm. 130 (2004) 783.

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