Magnetic and Structural Properties of Nanocrystalline Iron Oxides

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Physics Procedia 00 (2008) 000–000 Physics Procedia 2 (2009) 665–671 www.elsevier.com/locate/XXX www.elsevier.com/locate/procedia

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. Elsevier2009; use only: Received date here;form revised accepted date 31 hereAugust 2009 Received 1 January received in revised 31 date Julyhere; 2009; accepted

Abstract Hematite (Į-Fe2O3) and magnetite (Fe3O4) powders, milled for various times up to 10 hours by a high energy ball milling technique, were structurally and magnetically characterized by X-ray diffraction and magnetic measurements in magnetic field up 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 is increased, the grain size of Į-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 saturation increases 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. A correlation between the microstructural evolution of the milled sample can be made with its magnetic properties. © 2009 Elsevier B.V. All rights reserved 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 and nanotechnology. The iron oxides as Į-Fe2O3 (hematite), Fe3O4 (magnetite), Ȗ-Fe2O3 (maghemite) and FeO (wurstite) are important as electrical and magnetic materials [1-3]. Recently, high energy ball milling (HEBM) technique has emerged as a non-expensive route to produce non-equilibrium phases in several varied forms, such as nanostructured 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, extensive studies 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, starting from Į-Fe2O3 and Fe3O4 powders respectively. Their structural and magnetic properties were investigated.

* Corresponding author. Tel.: +213 774061278; fax: +213 38872770. E-mail address: [email protected].

doi:10.1016/j.phpro.2009.11.008

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2. Experiments Hematite (99.9 % pure Į-Fe2O3) and magnetite powders, from Alfa-Aesar, were put in a planetary ball mill where hardened steel vials of 45 cm3 volume containing 6 stainless steel balls of 13 mm diameter were set in rotation at 400 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 (KĮ) radiation in the 2ș range from 20° to 80° with counting time varying between 4 and 12 s for steps of 0.02°. We have considered the most important reflexions for X-ray profile analysis. The mean grain size was obtained from the Scherrer relation. 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 2 K – 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 SQUID magnetometer. 3. Results and discussion 3.1. Hematite Į-Fe2O3 SEM micrographs (Fig. 1) of hematite milled for 1 h to 10 h showed that all samples consisted of agglomerates of particles 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 apparent transformation to magnetite [4]. Usually, this transformation is observed and depends on the milling atmosphere and impurities 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.

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600 10 h 5h 3h

20

40

(300)

(214)

(116)

0h

0

(018)

(024)

(113)

(104)

(110)

1h

200

(012)

I ( a. u )

400

60

80

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

A broadening of the reflection peaks is observed, due to the particle size reduction and to the increase in the root mean square (rms) of the atomic-level strain induced from the fracture and welding processes. The deduced values of these two parameters are shown in Fig. 3. The grain size of the Į-Fe2O3 particles after 10 h of milling falls to about 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 ǻa/a=(a-a0)/a0, ǻc/c=(c-c0)/c0, and unit-cell volume ǻV/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 increase of 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. 10

80

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Δa/a, Δc/c, ΔV/V (%)

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R. m. s. strain .10-3

Crystallite size ( nm )

60

0,4 0,3 0,2 0,1 0,0

20

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10 0

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Milling time ( h )

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

-0,1 0

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4

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8

10

Milling time ( h )

Fig. 4. Relative deviations of the cell parameters and cell volume versus milling time. ǻa/a (open square), ǻc/c (circle), ǻV/V (triangle) (see definitions in text).

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

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measured under ZFC and FC conditions in a magnetic applied field of μ 0H = 5 mT are presented in Fig. 5. They show a large irreversibility on the magnetization, typical of mostly single domain particles characterized by a superparamagnetic (SPM) regime at high temperature [6], but with blocking temperatures above 300 K, and only few 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 field up 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 in the figures 6 and 7a, 7b respectively. For all the milled powders there is an hysteretic behaviour in low fields; the magnetization increases rapidly at fields up to about 0.3 T and then vary almost linearly without reaching any saturation in a field of 10 T. This typical weak ferromagnet like behaviour confirms the main antiferromagnetic interactions in hematite particles, responsible of a large magnetic susceptibility in high fields which is superimposed to a spontaneous ferromagnetic moment, coming either from true weak ferromagnetism for temperatures above the Morin transition (MT), either for uncompensated moments mainly at the surface of the particles [8]. α-Fe2O3

2

T = 20 K

α-Fe2O3 0,5

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M ( Am2 / kg )

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M ( Am / kg )

1,0 3

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3h 5h 10h

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0,0

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-5

0 μ0 H ( T )

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10

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

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0,2

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μ0 H ( T )

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

For each magnetization curve, one can consider the high field linear part as defining a high field magnetic susceptibility χhf by its slope and a saturation magnetization Ms by its extrapolation to zero field. For all the samples, the saturated ferromagnetic contribution Ms depends on the milling time but is temperature independent within the experimental errors of its determination. For example, in the figure 8, the dependence with the milling time of Ms is reported at 300 K and at 100 K. For each milling time, the values are almost identical and it indicates that the Morin transition is not seen in our experiments. Such suppression of the Morin transition has been already described in the past for powders of nanoscale particles [9, 10] and recently related to some microstructural changes in ball-milled samples [11]. Ms begins to increase, reaching a maximum value after 3 h of milling (either at 100 K or 300 K) and then decreases slightly. This can be explained by the raise of uncompensated moments, mainly at the surface 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 to 2, [8]. For very small grains, this increase can be overcomed by the fact that the antiferromagnetic structure is becoming less organized, without well defined AF direction and with surface spins having rather random directions, resulting in a smaller moment of the particle.

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0,12

α-Fe2O3

0,8

α-Fe2O3

0,6

0,08

μ0 Hc ( T )

Ms ( Am2 / kg )

0,10

0,4

100 K 300 K

100 K 300 K

0,06

0,04

0,2 0,02

0

2

4

6

8

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t(h) Fig. 8. Saturation magnetization dependence with milling time at 100 K and 300 K.

0

2

4

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8

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t(h) Fig. 9. Coercive force dependence with milling time at 100 K and 300 K.

The high field magnetic susceptibility χhf is also almost temperature independent and its value for the unmilled powder (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; even at low temperature. On another way, the variations of the coercive force Hc with the milling time at the same temperatures (300 and 100 K) are shown in the figure 9. μ 0Hc decreases from 0.07 T (for T = 300 K) and 0.11 T (for T = 100 K) to about 0.03 T above 5 h of milling. These values are in agreement with previous studies where μ 0Hc was found to vary between 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 the milled sample. The mean crystallite size of the Fe3O4 particles attains 17 nm. It can be seen that some diffraction peaks are due to the presence of hematite. We estimate the mean grain size of the formed Į-Fe2O3 particles to 16 nm with 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 found that phase changes of iron oxides depend on the milling atmosphere, the presence of impurities, milling materials and milling time. It is also observed [17] that the milled Fe3O4 is oxidized to Į-Fe2O3 after 70 h of ball milling. However, when it is milled in an argon atmosphere [18], the Į-Fe2O3 phase appears after 12 h and disappears after 48 h. In our case, it seems that about 40% of the initial magnetite powder is transformed to hematite after 10 hours of dry milling in air.

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800 (311)

M ag n etite H em atite

400

(511)

(422)

(400)

(111)

(222)

200

(440)

(220)

I ( a. u. )

600

10 h 0h

0 20

40

60

80

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

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 bump around 35 K. Then, it smoothly decreases until 300 K. This behaviour has been previously observed in annealed nanocrystalline 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 the time of measurement, is still above 300 K for the concerned particles. This feature can be attributed for a large part to the hematite nanoparticles present in that powder. The Verwey transition, which is observed still rather sharply in the non milled sample at about 120 K, seems to spread out over the whole range from 4 to 130 K for the milled sample. It is known that this transition temperature is shifted to lower temperatures for small particles [20], and disappears completely for particle sizes smaller than 10nm. So, in our case, the observed curve may result of the distribution in Fe3O4 grain size centered at about 17 nm with a related distribution of Verwey temperatures with a corresponding center at 35 K. 100

16

1,8

Fe3O4

1,2

10

1,0

M ( Am2 / kg )

1,4

12

2

M ( Am / kg )

80

1,6

14

60

40

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

8 0h 10h

Tv

6

0,8 0,6

0

50

100

150

200

250

300

T(K)

Fig. 11. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature (ȝ0H = 10 mT) for Fe3O4 sample milled 10 h.

20

0 0

2

4

6

8

10

μ0 H ( T )

Fig. 12. Magnetization curves of the unmilled and milled Fe3O4 sample at 4K and 300K.

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Magnetization measurements versus applied field performed at T = 4 K and 300 K are presented in figure 12. It is seen that milling process induces a decrease of magnetization due mainly to the formation of superparamagnetic ĮFe2O3 particles. The change in the saturation magnetization Ms from 88 to 50.6 Am2/kg at 300 K confirms the proportion of hematite estimated from X-ray diffraction. Nevertheless, the observed decrease in Ms can also be due to spin disorder at the surface of the Fe3O4 nanograins but this effect can be neglected in first approximation with regard to the effect of the transformation in hematite. In the meanwhile, the coercive force increases from 0.038 to 0.063 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 up to 10 hours of Į-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 can explain 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. Zdujiü, C. Jovalekic, Lj. Karanovic, M. Mitric, Mater. Sci. Eng. A262 (1999) 204. [5] P. Sahu, M. De, M. Zdujiü, 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.