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A Mössbauer spectral study of some iron nitride-based nanocomposites prepared by ball milling
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Journal of Magnetism and Magnetic Materials 292 (2005) 215–226 www.elsevier.com/locate/jmmm
A Mo¨ssbauer spectral study of some iron nitride-based nanocomposites prepared by ball milling Fernande Grandjeana,, Raphae¨l P. Hermanna, Gary J. Longb, Sanjay R. Mishrac a Department of Physics, University of Lie`ge, B5, B-4000 Sart-Tilman, Belgium Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65409-0010, USA c Department of Physics, University of Memphis, Memphis, TN 38152, USA
b
Received 19 August 2004; received in revised form 30 September 2004 Available online 20 November 2004
Abstract Powder mixtures of (FeyN)x and (Al2O3)1x or (SiO2)1x with x ¼ 0:2 and 0.6 and y ¼ 3:8 have been ball milled for 4, 8, 16, 32, 64, and 128 h and their magnetic properties and Mo¨ssbauer spectra have been measured. The 5 and 295 K saturation magnetizations decrease with increasing milling time as a result of decreasing particle sizes. Fits with the Langevin function of the field dependence of the magnetization yield particle sizes that are slightly smaller than those determined from a Scherrer analysis of the broadening of the X-ray diffraction peaks. The Mo¨ssbauer spectra have been fit with a distribution of hyperfine fields between 0 and 40 T and the peaks in the distribution assigned to the different iron nitride phases present in the nanocomposites. This analysis of the Mo¨ssbauer spectra indicates that the g0 -Fe4N phase present in FeyN is transformed into e-Fe3+xN and a-iron by the milling process. The weighted average field decreases and the weighted average isomer shift increases with milling time, as expected for the simultaneous effects of size reduction, the above phase transformation, and mixing with the oxides. A peak in the hyperfine field distribution at 4 T indicates the presence of small superparamagnetic particles, a presence which is also confirmed by the temperature dependence of the Mo¨ssbauer spectra and the failure of the magnetization to saturate even at applied fields of 5.5 T. r 2004 Elsevier B.V. All rights reserved. Keywords: Ball milling; Magnetic nanoparticles; Mo¨ssbauer spectroscopy; Magnetic nanocomposites
1. Introduction
Corresponding
author. Tel.: +32 4 3663632; fax: +32 4 3664516. E-mail addresses: [email protected] (F. Grandjean), [email protected] (R.P. Hermann), [email protected] (G.J. Long), [email protected] (S.R. Mishra).
Magnetic nanocomposite materials consisting either of ultrafine metal or alloy particles dispersed in an insulating oxide matrix have been prepared by the sol–gel technique [1–3] and more recently by high-energy ball milling [4–6]. The morphological, structural, and magnetic properties of the resulting
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.10.114
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iron nitride-alumina and iron nitride-silica composites have been studied [4–6] by X-ray diffraction, transmission electron microscopy, thermal analysis, and magnetization measurements. Transmission electron microscopy and X-ray diffraction measurements indicate that the nanocomposites consist of agglomerates or collections of rather weakly bonded nanoparticles of ca. 100 nm diameter containing iron-nitride crystallites whose diameters decrease from ca. 35 to 8 nm for milling times of 0 to 128 h, respectively. An example of a transmission electron micrograph is shown in Fig. 1. The magnetization of the composite is strongly reduced by dilution in alumina or silica from the bulk magnetization of 193 emu/g for g0 -Fe4N and 123 emu/g for e-Fe3N. Further, as expected for a reduction in size of the magnetic iron-nitride crystallites, the saturation magnetization decreases and the coercive field increases with milling time. The evolution with milling time of the X-ray diffraction patterns and the differential
Fig. 1. Transmission electron micrograph of (FeyN)0.2 (Al2O3)0.8 milled for 64 h. The horizontal bar is 1 mm.
thermal analysis of these nanocomposites suggests that a transformation of g0 -Fe4N into e-Fe3N and a-iron occurs as a result of the milling process. This transformation has already been observed [7–9] in studies of mechanical grinding synthesis of iron nitrides. Usually, after the long-term milling of materials subjected to mechanical work in a high-energetic mill, the final distribution of particle sizes and shapes, i.e., the microstructure, becomes rather stable, due to the continuous processes of fracture and soldering. The particle size distribution and particle morphology depend on the kind of material being processed as well as on the milling frequency, the ball-to-powder ratio, and the temperature. However, the homogenization process varies, depending on the ductility or brittleness of the milled powder. Thus typically, very different microstructures are obtained when the precursor powder is ductile, brittle, or a two-component mixture of ductile and brittle materials. The FeyN–ceramic composite resembles the ductile–brittle case in which, during milling, the ductile component particles acquire elongated shapes, i.e., lamella, whereas the brittle component particles are simply fractured. At intermediate milling times, the ductile particles become embedded in the brittle matrix, forming agglomerates several millimeters in diameter. This is the typical microstructure developed when milling a ceramic with a metallic material, i.e., the so-called cermets [10,11]. A further increase in the milling time brings the thinner ductile lamellae into close contact, whereas the brittle particles become ever more tinier and finally dissolve within the ductile particles. Depending upon the relative solubility of the precursors, the final product usually is completely homogenized [12,13]. Therefore, in order to fully homogenize ductile–brittle systems, both extensive fragmentation of the brittle component and a high solubility of the brittle component in the ductile component are required [14]. These conditions are fulfilled for the composites studied herein and, hence, we expect a fully homogenized material after 64 to 128 h of milling. There is extensive confusion in the literature [4–6,12] over the use of the words ‘‘grains,’’
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‘‘particles,’’ and ‘‘crystallites.’’ In metallurgy each separate crystal is called a grain [15]. In crystallography, the average size of the crystallites is given by the width of the diffraction peaks. In magnetism, superparamagnetic behavior is determined by the size of a single domain particle. In this paper, we have chosen not to use the word ‘‘grain’’ in its metallurgical meaning and to use only crystallite and particle with the above meaning. Further, it must be noted that these crystallites and/or particles can also form agglomerates. Mo¨ssbauer spectroscopy is an ideal technique both for differentiating between the different ironnitride phases [16] and for studying the relaxation of small superparamagnetic particles. As a consequence, we report herein a Mo¨ssbauer spectral study of several iron-nitride nanocomposites studied as a function of milling time and temperature, with the goal of investigating the above transformation. In addition, the magnetization curves of the nanocomposites have been analyzed in order to obtain an estimate of the size of the single domain particles.
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acceleration spectrometer, which utilized a room temperature rhodium matrix cobalt-57 source and was calibrated at room temperature with a-iron foil. The Mo¨ssbauer spectral absorbers contained 30 mg/cm2 of the composites. The spectra of the milled samples were fit with a 0 to 40 T distribution of hyperfine fields by using the method of Wivel and Mørup [17]. These fits used a distribution of 22 sextets in which the areas of the components of each sextet were constrained to be in the ratio of 3:2:1:1:2:3. The spectra of the unmilled FeyN have also been fit with the minimum number of hyperfine fields needed to reproduce the observed spectrum; the resulting average hyperfine parameters were essentially the same as those obtained with the distribution fits. In all cases, the accuracy of the average isomer shifts, quadrupole shifts, line widths, and hyperfine fields is estimated to be 70.005, 70.01, 70.01 mm/s, and 70.2 T, respectively; the absolute accuracy is perhaps two to three times larger.
3. Magnetic studies 2. Experimental The samples studied herein have been prepared as described earlier [4,5]. The starting materials consisted of 99.99% pure 325 mesh powders of FeyN and Al2O3 or SiO2. Approximately 10 g of the appropriate amounts of FeyN and Al2O3 or SiO2 were placed in a 45 ml tungsten carbide vial with a sample-to-ball mass ratio of 1:14. The mill contained 12 tungsten carbide balls with a diameter of 10 mm and a mass of 7.85 g. The materials were milled for 4, 8, 16, 32, 64, and, in some cases, 128 h at 400 rpm in a Fritsch Pulverisette 7 planetary ball mill in the presence of absolute ethanol, which was used to prevent oxidation. Magnetic measurements were performed at 5 and 300 K at fields up to 5.5 T with a Quantum Design, Inc. superconducting quantum interference magnetometer. The magnetization is given in emu per gram of the nanocomposite. The Mo¨ssbauer spectra were obtained at several temperatures between 90 and 295 K on a constant-
The field dependence of the magnetization of (FeyN)0.2(SiO2)0.8 obtained for the indicated milling times at 300 K is shown in Fig. 2. The 300 K magnetization curves of the 64 h milled (FeyN)0.2(Al2O3)0.8 and (FeyN)0.6(Al2O3)0.4 nanocomposites have been reported earlier [4,5]. The 5 and 300 K magnetization curves for the (FeyN)0.6 (SiO2)0.4 and (FeyN)x(Al2O3)1x nanocomposites are similar to those shown in Fig. 2. All of these magnetization curves, none of which are completely saturated at 300 K even in an applied field of 5.5 T, indicate the presence of small superparamagnetic particles. In all cases, the magnetization has been fit with the Langevin function given by Eq. (1). MðHÞ ¼ M s ½cothðV rM s H=kTÞ kT=V rM s H; (1) where V is the average volume of the monodomain particle, r is the density of the particle, Ms is the saturation magnetization, k is the Boltzmann constant, T is the temperature, and H is the applied field. A density [18] of 6.57 103 kg/m3 for g0 -Fe4N was used in these fits. The diameters of the
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F. Grandjean et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 215–226 Table 1 The saturation magnetization FeyN particle diameters Composite
Msb Diameterc Diameterd Msa (emu/g) (emu/g) (nm) (nm)
(FeyN)0.2(Al2O3)0.8 48.1 (FeyN)0.6(Al2O3)0.4 93.7 (FeyN)0.2(SiO2)0.8 57.6 (FeyN)0.6(SiO2)0.4 111.6
43.9 94.6 59.6 104.5
8.8 6.4 8.3 6.6
14 8.0 8.5 8.5
a The 300 K saturation magnetization measured at 5 T for the 4 h milled nanocomposite. b The calculated 300 K saturation magnetization corresponding to the amount of FeyN present in 1 g of composite by assuming a magnetization of 123 emu/g for Fe3N. c The diameter obtained from the Langevin fits. d The diameter obtained from the Scherrer analysis, see Refs. [4,5].
Fig. 2. Field dependence of the magnetization of (FeyN)0.2 (SiO2)0.8 obtained at 300 K and indicated milling times. The solid lines are the Langevin functions given by Eq. (1).
assumed spherical particles of FeyN obtained from these fits are given in Table 1 together with the crystallite diameter obtained from the Scherrer analysis [4,5] of the prominent Fe3N X-ray diffraction peak. No significant reduction in particle diameter is observed in the Langevin fits for milling times between 4 and 64 h. Only a significant reduction between 4 and 8 milling hours was observed in the Scherrer analysis [4,5]. As is shown in Table 1, the Langevin fits give magnetic monodomain diameters that are slightly smaller than the crystallite size obtained from the Scherrer analysis. Eq. (1) does not take into account any distribution in size of the particles and hence the diameter should be considered as an average diameter. It is difficult to assign an error bar to the diameter obtained from the Langevin fits, but it appears that the four diameters given in Table 1 may not be significantly different. Hence, the only significantly larger diameter was obtained [4] from the Scherrer analysis of the X-ray peaks in (FeyN)0.2(Al2O3)0.8. Hence, the monodomain magnetic particles seem to be monocrystalline. The saturation magnetization observed at 300 K for the 4 h milled samples of the four nanocomposites are in good agreement with the calculated values obtained from the dilution of Fe3N, with a saturation magnetization of 123 emu/g, in the nonmagnetic oxides; see Table 1. The additional small reduction in saturation magnetization with milling times between 8 and 64 h results from an additional
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small reduction in the FeyN particle size and, hence, increase in the superparamagnetic fraction. It is well known [19,20] that surface effects on small particles reduce the saturation magnetization.
4. Mo¨ssbauer spectroscopy The Mo¨ssbauer spectrum of the FeyN used in this study and measured at 295 K is shown at the top of Fig. 3a. The FeyN spectra have also been
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measured at 190 and 90 K and are very similar to that shown in Fig. 3a. This spectrum is similar but not identical to the spectrum of g0 -Fe4N shown in Fig. 2A of Ref. [7] Specifically, the peaks at 0.5 and 4 mm/s have a different shape, a difference that occurs because of the presence of both g0 -Fe4N and e-Fe3N in the sample studied herein. In the absence of published hyperfine parameters in the earlier work [7], no further comparison is possible. At all temperatures, the FeyN spectra show four components with fields of ca. 4, 11, 22, and 34 T.
Fig. 3. (a) Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields of (FeyN)0.6(Al2O3)0.4 obtained at 295 K as a function of milling time.
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Table 2 Iron-57 Mo¨ssbauer spectral parameters obtained for FeyN from an analysis with discrete sextets T (K)
/dSa (mm/s)
/HS (T)
/QSS (mm/s)
/GS (mm/s)
Area (%)
Assignment
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0.23 0.31 0.44 0.34
33.9 22.1 7.5 18.0
0.02 0.04 0.02 0.03
0.26 0.33 2.46 1.02
10 56 34 —
g0 -Fe4N e-Fe3N+ g0 -Fe4N e-Fe3+xN wt. avg.
190
0.30 0.38 0.52 0.40
35.6 23.6 11.4 22.0
0.02 0.01 0.03 0.01
0.24 0.35 1.07 0.50
11 67 22 —
g0 -Fe4N e-Fe3N+g0 -Fe4N e-Fe3+xN wt. avg.
90
0.37 0.43 0.57 0.45
36.4 24.5 13.4 23.0
0.01 0.00 0.08 0.02
0.25 0.31 1.15 0.50
11 65 24 —
g0 -Fe4N e-Fe3N+g0 -Fe4N e-Fe3+xN wt. avg.
a
The isomer shifts are given relative to room temperature a-iron foil.
The hyperfine parameters derived from fits of the FeyN spectra with discrete magnetic sextets are given in Table 2 along with tentative assignments of the different components. Specifically, the sextets with fields of ca. 22 and 34 T may be assigned [16] to the e-Fe3N and g0 -Fe4N phases observed by X-ray diffraction [4–6]. In an alternative analysis with a hyperfine field distribution of the starting material, see the top of Fig. 3b, the broad peak between 11 and 15 T may be assigned [16] to some non-stoichiometric e-Fe3+xN. The peak at ca. 4 T may be assigned to small superparamagnetic particles of FeyN present in the starting material or may be an artifact resulting from the fitting procedure, which does not take into account any distribution of isomer shifts or quadrupole shifts. The Mo¨ssbauer spectra of (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(SiO2)0.8 obtained at 295 K for the indicated milling times are shown in Figs. 3a and 4a, respectively. All the spectra of the milled samples shown herein are quite different from the spectrum [7] of g0 -Fe4N milled for 2 h in a steel vial containing steel balls under nitrogen atmosphere. More specifically, the differences are found at 2.5 mm/s and between 2 and 3.5 mm/s. These differences may result either from the different milling conditions used in Ref. [7] or from the presence of the alumina or silica in the samples studied herein. The spectra shown in Figs. 3a and
4a are characteristic of samples composed of small particles with a distribution of size. Hence, they have been analyzed with a distribution [17] of hyperfine fields, distributions that are shown in Figs. 3b and 4b. In all of the spectra of the milled samples, the observed quadrupole shifts are less than 70.01 mm/s and in no case is there any indication of the presence of any iron(II) oxide or iron(III) oxide in the spectra. This absence indicates that the amount of these phases present in the milled samples is less than 2 vol%, in agreement with the X-ray diffraction results. The spectra observed after 4 h of milling are very similar to the spectra shown for 8 h of milling and indicate that, with a few hours of milling, there are dramatic changes in the spectra and thus in the distribution of particle sizes or of magnetic phases. A comparison of the spectra and of the hyperfine field distribution in (FeyN)0.2(Al2O3)0.8 and (FeyN)0.6(Al2O3)0.4, and in (FeyN)0.2(SiO2)0.8 and (FeyN)0.6(SiO2)0.4, shows that the x ¼ 0:2 nanocomposites contain a larger amount of small particles, which contribute to the small hyperfine field region of the spectra between 0 and 1 mm/s. Hence, it seems that the Mo¨ssbauer spectra are more sensitive to a change in the particle size distribution than are the magnetic measurements. Ideally, one would like to fit the Mo¨ssbauer spectra shown in Figs. 3 and 4 with a relaxation line shape profile, but such fits are virtually
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Fig. 4. (a) Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields of (FeyN)0.2(SiO2)0.8 obtained at 295 K as a function of milling time.
impossible to do because of the presence of the distribution of particle sizes, which yields a distribution of relaxation rates; fits of such a distribution are not feasible. The average isomer shifts and hyperfine fields of the four nanocomposites studied herein, derived
from the Wivel and Mørup [17] fit of the distribution of the hyperfine fields, are shown in Figs. 5a and b, respectively. The average isomer shift increases rather sharply between 4 and 36 h of milling and more slowly after 36 h of milling. Hence, the initial effect of milling on the isomer
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avg. isomer shift (mm/s)
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0.34
0.32
0.30 (a)
avg. hyperfine field (T)
0.28 20 18 16 14 12 10
(b)
8 largest field fraction
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
(c)
-0.02 0
20
40
60 80 100 milling time (hr)
120
Fig. 5. Dependence of (a) the 295 K average isomer shift, (b) hyperfine field, and (c) the area fraction of the largest hyperfine field component, upon milling time for (FeyN)0.2(Al2O3)0.8, open squares, and (FeyN)0.6(Al2O3)0.4, solid squares, (FeyN)0.2 (SiO2)0.8, open circles, and (FeyN)0.6(SiO2)0.4, solid circles. The lines shown in (c) are the result of a fit with Eq. (2) as is discussed in the text.
shift is important and the overall increase in isomer shift of ca. 0.05 mm/s results from an intimate mixing of the iron nitride and alumina or silica particles within the agglomerates. Further, this increase in isomer shift is also due, at least in part, to an increase in the e-Fe3N volume fraction, which has a higher isomer shift of 0.28 mm/s as
compared with the g0 -Fe4N isomer shift of 0.20 mm/s [21]. Within 4 to 16 h of milling the g0 -Fe4N phase in the four nanocomposites, characterized by the peak at 34 T in the field distributions in Figs. 3b and 4b, virtually disappears. In contrast, the eFe3N phase, characterized by the peak at 22 T in the field distributions, persists for 64 h of milling, see Fig. 4b. In the case of (FeyN)0.6(Al2O3)0.4, this sextet persists even after 128 h of milling, see Fig. 3b. Thus, in the four nanocomposites, the sextet with the largest field corresponding to g0 Fe4N disappears after ca. 8 h of milling and the sextet at 22 T corresponding to the e-Fe3N phase is partly replaced by sextets with fields of ca. 4 and 10 T, fields which are not characteristic [16] of stoichiometric iron-nitride phases. However, nonstoichiometric e-Fe3+xN show fields between 5.9 and 7.0 T and 15.7 and 17.0 T. Hence, the peaks in the hyperfine field distributions at 4 and 10 T could be assigned either to some non-stoichiometric eFe3+xN or are an artifact of the fitting procedure, which mimics the contribution of the small superparamagnetic particles, relaxing on the Mo¨ssbauer time scale, by sextets with small fields. These changes are revealed by the decrease in the average hyperfine field with milling time shown in Fig. 5b. The disappearance of the g0 -Fe4N phase with milling time in both (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(Al2O3)0.8 is shown in Fig. 5c, which reveals a decrease with milling time in the area of the peak centered at 34 T in the field distributions shown in Figs. 3b and 4b. This dependence has been fit with F ðtÞ ¼ 0:145 expðBtC Þ;
(2)
where F is the fractional area of the peak. Similarly slightly poorer fits, limited to 64 h of milling, have been obtained for (FeyN)0.2(SiO2)0.8 and (FeyN)0.6(SiO2)0.4. The dependence given by Eq. (2) has been proposed by Matteazzi et al. [22] for mechanical alloying of iron carbides. They have proposed that the parameters B and C are related with the impact energy determined by the rotational frequency and ‘‘work conditions’’ determined by the number of balls, respectively. The same equation has also been successfully applied [23] to the mechanical alloying of iron nitrides.
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Fig. 6. (a) Temperature dependence of the Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields, for (FeyN)0.6(Al2O3)0.4 after milling for 64 h.
Because the four nanocomposites studied herein are ball milled in the same conditions, same rotational frequency and same number of balls, the parameters B and C are expected to be similar in the four fits. Indeed, the values for B and C are between 0.27 and 0.32 and between 0.35 and 0.43, respectively. Similar values for B have been observed [23] for the mechanosynthesis of g0 Fe4N. Further, a value of C less than two indicates [23] an exothermic process for the transformation of the g0 -Fe4N phase. In conclusion, the dependence of the Mo¨ssbauer spectra upon milling time suggests both a transformation of the g0 -Fe4N phase into e-Fe3+xN and a-iron, as proposed [5,6] from the differential thermal analysis measurements, and the increasing presence of fine superparamagnetic particles, albeit particles with a distribution of sizes. In order to
shed some light on the role of these two effects, the temperature dependence of the Mo¨ssbauer spectra was investigated. The temperature dependence of the Mo¨ssbauer spectra of both (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(Al2O3)0.8, and the corresponding distributions of hyperfine fields are shown in Figs. 6 and 7. From the observed spectra and distribution of hyperfine fields, it is apparent that the field component found at ca. 4 T decreases dramatically upon cooling from 295 to 90 K, whereas the remaining three components at ca. 13, 24, and 32 T remain relatively unchanged. This change in the low-field component upon cooling is clearly a sign that some of the particles are superparamagnetic at the higher temperatures and blocked at the lower temperatures. It is remarkable that the peak at the largest field in the 90 K distribution, see
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Fig. 7. (a) Temperature dependence of the Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields, for (FeyN)0.2(Al2O3)0.8 after milling for 64 h.
Figs. 6b and 7b, occurs at ca. 32 T, i.e., at a field which is smaller than the 34 T field characteristic of g0 -Fe4N. The milling time dependence of the position of three main peaks in the hyperfine field distributions of the (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2 (Al2O3)0.8 composites is shown in Fig. 8a and the temperature dependence of the position of the same peaks in the hyperfine field distributions of the 64 h milled (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2 (Al2O3)0.8 is shown in Fig. 8b. It is clear that increasing milling time and increasing temperature have the same effect of decreasing the peak position, i.e., decreasing the hyperfine field. This behavior is compatible with the presence of a superparamagnetic fraction in the samples. However, at 90 K the highest hyperfine field is observed at 32.3 T, a value which is closer to the hyperfine
field in a-iron than to the 90 K hyperfine field in g0 -Fe4N, see Table 2, whereas at 0 h of milling it is observed at 34.3 T, a value which is characteristic of g0 -Fe4N. Hence, the temperature dependence of the Mo¨ssbauer spectra is compatible with the transformation of the g0 -Fe4N phase into eFe3+xN and a-iron. This transformation has also been observed [7–9] in the ball-milling process of g0 -Fe4N and in the mechanosynthesis of iron nitrides. The e-Fe3N phase appears [9] to be very stable during milling procedures.
5. Discussion The goal of this work has been to investigate by iron-57 Mo¨ssbauer spectroscopy ball-milled nanocomposites of iron nitride and alumina or silica.
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spectra have been analyzed with a distribution of hyperfine fields. The dependences of these distributions upon milling time and temperature indicate that the g0 -Fe4N phase is eliminated from the mixture with increasing milling time and exothermically transformed into e-Fe3+xN and a-iron via the mechanical energy provided by the mechanical grinding. Such a transformation has been reported [24] at high temperatures and during mechanical grinding of the g0 -Fe4N phase [7] and is also supported by the differential thermal analysis [5,6] of the present samples. The 295 K Mo¨ssbauer spectra of the 64 h milled (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(Al2O3)0.8 nanocomposites contain approximately 50% of their absorption area between 0 and 10 T and thus the blocking temperature of the superparamagnetic particles is around 295 K. For particles with a radius of 4 nm of g0 -Fe4N with an anisotropy constant [25] of 1.5 104 J/m3, a blocking temperature of 287 K is calculated, a value which is in fair agreement with the estimate obtained from the Mo¨ssbauer spectra.
Acknowledgments
Fig. 8. (a) Milling time dependence of the three peak positions in the hyperfine distributions of (FeyN)0.6(Al2O3)0.4, closed symbols, and (FeyN)0.2(Al2O3)0.8, open symbols, composites, and (b) temperature dependence of the position of the same peaks in the hyperfine field distributions of the 64 h milled (FeyN)0.6(Al2O3)0.4, closed symbols, and (FeyN)0.2(Al2O3)0.8, open symbols.
The spectra indicate that no oxidation of the iron nitride occurs with ball milling. Further, the shape of the Mo¨ssbauer spectra and their dependence upon milling time and temperature suggests that the samples consist of small particles showing a size distribution with an average size of 6 to 8 nm after 8 h of milling, some of them showing a superparamagnetic behavior in agreement with the absence of saturation in the magnetization curves under an applied field of 5.5 T. The Mo¨ssbauer
The authors thank Dr. N. Ali of Southern Illinois University, Carbondale, IL, for providing the magnetic measurements. GJL thanks the Francqui Foundation of Belgium for his appointment as a ‘‘Chaire Francqui Interuniversitaire au titre e´tranger’’ during the 2002–2003 academic year. This work was supported in part by the ‘‘Fonds National de la Recherche Scientifique,’’ Brussels, Belgium, Grant 9.456595 and the ‘‘Fonds de la Recherche Fondamentale Collective’’, Grant 2.4522.01. The authors also wish to thank the Department of Physics at the University of Memphis for their support and encouragement during the pursuit of this study and CARS at the University of Chicago for their support of the magnetization studies. References [1] L.C. Klein, in: A.S. Edelstein, R.C. Cammarata (Eds.), Nanomaterials: Synthesis, Properties and Applications, Institute of Physics, Bristol, 1996.
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[2] V. Pilai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243. [3] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, New York, 1990. [4] S.R. Mishra, G.J. Long, F. Grandjean, R.P. Hermann, S. Roy, N. Ali, A. Viano, Eur. Phys. J. D 24 (2003) 93. [5] A.M. Viano, S.R. Mishra, Mat. Res. Soc. Symp. Proc. 703 (2002) V9. [6] S.R. Mishra, A. Viano, S. Roy, N. Ali, J. Losby, J. Nanosci. Nanotechnol. 3 (2003) 227. [7] J. Foct, R.S. de Figueiredo, NanoStruct. Mater. 4 (1994) 685. [8] W.A. Kaczmarek, Scripta Metall. Mater. 33 (1995) 1687. [9] Y. Chen, T. Halstead, J.S. Williams, Mater. Sci. Eng. A 206 (1996) 24. [10] T. Klassen, R. Gunther, B. Dickau, A. Bartels, R. Bormann, H. Mecking, Mater. Sci. Forum 269–272 (1998) 37. [11] J. Sort, J. Nogues, S. Surinach, J.S. Munoz, E. Chappel, F. Dupont, G. Chouteau, M.D. Baro, Mater. Sci. Forum 368–388 (2002) 465. [12] S. Surinach, M.D. Baro, J. Segura, M.T. ClavagueraMora, N. Clavaguera, J. Am. Ceram. Soc. 81 (1998) 2504.
[13] S. Surin˜ach, M.D. Baro´, J. Segura, M.T. ClavagueraMora, Mater. Sci. Eng. A 134 (1991) 1368. [14] B.S. Murty, S. Ranganathan, Int. Mater. Rev. 43 (1998) 101. [15] L.H. Van Vlack, in: Materials Science for Engineers, Addison-Wesley Pub., Reading, MA, 1970, p. 112. [16] Y. Jiraskova, S. Havlicek, O. Schneeweiss, V. Perina, C. Blawert, J. Magn. Magn. Mater. 234 (2001) 477. [17] C. Wivel, S. Mørup, J. Phys. E: Sci. Instr. 14 (1981) 605. [18] Handbook of Chemistry and Physics, 68th ed., CRC Press, Boca Raton, 1987–1988, B p. 68. [19] M. Guzman, J.L. Delplancke, G.J. Long, J. Delwiche, M.-J. Hubin-Franskin, F. Grandjean, J. Appl. Phys. 92 (2002) 2634. [20] V. Mancier, J.-L. Delplancke, J. Delwiche, M.-J. HubinFranskin, C. Piquer, L. Rebbouh, F. Grandjean, J. Magn. Magn. Mater. 281 (2004) 27. [21] P. Schaaf, Hyp. Int. 111 (1998) 113. [22] P. Matteazzi, D. Basset, F. Miani, G. Le Cae¨r, NanoStruct. Mater. 2 (1993) 14. [23] I.F. Vasconcelos, R.S. de Figueiredo, J. Phys. Chem. 107 (2003) 3761. [24] K.H. Jack, J. Appl. Phys. 76 (1994) 6620. [25] L. Varga, W.D. Doyle, J. Appl. Phys. 79 (1996) 4995.
Journal of Magnetism and Magnetic Materials 292 (2005) 215–226 www.elsevier.com/locate/jmmm
A Mo¨ssbauer spectral study of some iron nitride-based nanocomposites prepared by ball milling Fernande Grandjeana,, Raphae¨l P. Hermanna, Gary J. Longb, Sanjay R. Mishrac a Department of Physics, University of Lie`ge, B5, B-4000 Sart-Tilman, Belgium Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65409-0010, USA c Department of Physics, University of Memphis, Memphis, TN 38152, USA
b
Received 19 August 2004; received in revised form 30 September 2004 Available online 20 November 2004
Abstract Powder mixtures of (FeyN)x and (Al2O3)1x or (SiO2)1x with x ¼ 0:2 and 0.6 and y ¼ 3:8 have been ball milled for 4, 8, 16, 32, 64, and 128 h and their magnetic properties and Mo¨ssbauer spectra have been measured. The 5 and 295 K saturation magnetizations decrease with increasing milling time as a result of decreasing particle sizes. Fits with the Langevin function of the field dependence of the magnetization yield particle sizes that are slightly smaller than those determined from a Scherrer analysis of the broadening of the X-ray diffraction peaks. The Mo¨ssbauer spectra have been fit with a distribution of hyperfine fields between 0 and 40 T and the peaks in the distribution assigned to the different iron nitride phases present in the nanocomposites. This analysis of the Mo¨ssbauer spectra indicates that the g0 -Fe4N phase present in FeyN is transformed into e-Fe3+xN and a-iron by the milling process. The weighted average field decreases and the weighted average isomer shift increases with milling time, as expected for the simultaneous effects of size reduction, the above phase transformation, and mixing with the oxides. A peak in the hyperfine field distribution at 4 T indicates the presence of small superparamagnetic particles, a presence which is also confirmed by the temperature dependence of the Mo¨ssbauer spectra and the failure of the magnetization to saturate even at applied fields of 5.5 T. r 2004 Elsevier B.V. All rights reserved. Keywords: Ball milling; Magnetic nanoparticles; Mo¨ssbauer spectroscopy; Magnetic nanocomposites
1. Introduction
Corresponding
author. Tel.: +32 4 3663632; fax: +32 4 3664516. E-mail addresses: [email protected] (F. Grandjean), [email protected] (R.P. Hermann), [email protected] (G.J. Long), [email protected] (S.R. Mishra).
Magnetic nanocomposite materials consisting either of ultrafine metal or alloy particles dispersed in an insulating oxide matrix have been prepared by the sol–gel technique [1–3] and more recently by high-energy ball milling [4–6]. The morphological, structural, and magnetic properties of the resulting
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.10.114
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iron nitride-alumina and iron nitride-silica composites have been studied [4–6] by X-ray diffraction, transmission electron microscopy, thermal analysis, and magnetization measurements. Transmission electron microscopy and X-ray diffraction measurements indicate that the nanocomposites consist of agglomerates or collections of rather weakly bonded nanoparticles of ca. 100 nm diameter containing iron-nitride crystallites whose diameters decrease from ca. 35 to 8 nm for milling times of 0 to 128 h, respectively. An example of a transmission electron micrograph is shown in Fig. 1. The magnetization of the composite is strongly reduced by dilution in alumina or silica from the bulk magnetization of 193 emu/g for g0 -Fe4N and 123 emu/g for e-Fe3N. Further, as expected for a reduction in size of the magnetic iron-nitride crystallites, the saturation magnetization decreases and the coercive field increases with milling time. The evolution with milling time of the X-ray diffraction patterns and the differential
Fig. 1. Transmission electron micrograph of (FeyN)0.2 (Al2O3)0.8 milled for 64 h. The horizontal bar is 1 mm.
thermal analysis of these nanocomposites suggests that a transformation of g0 -Fe4N into e-Fe3N and a-iron occurs as a result of the milling process. This transformation has already been observed [7–9] in studies of mechanical grinding synthesis of iron nitrides. Usually, after the long-term milling of materials subjected to mechanical work in a high-energetic mill, the final distribution of particle sizes and shapes, i.e., the microstructure, becomes rather stable, due to the continuous processes of fracture and soldering. The particle size distribution and particle morphology depend on the kind of material being processed as well as on the milling frequency, the ball-to-powder ratio, and the temperature. However, the homogenization process varies, depending on the ductility or brittleness of the milled powder. Thus typically, very different microstructures are obtained when the precursor powder is ductile, brittle, or a two-component mixture of ductile and brittle materials. The FeyN–ceramic composite resembles the ductile–brittle case in which, during milling, the ductile component particles acquire elongated shapes, i.e., lamella, whereas the brittle component particles are simply fractured. At intermediate milling times, the ductile particles become embedded in the brittle matrix, forming agglomerates several millimeters in diameter. This is the typical microstructure developed when milling a ceramic with a metallic material, i.e., the so-called cermets [10,11]. A further increase in the milling time brings the thinner ductile lamellae into close contact, whereas the brittle particles become ever more tinier and finally dissolve within the ductile particles. Depending upon the relative solubility of the precursors, the final product usually is completely homogenized [12,13]. Therefore, in order to fully homogenize ductile–brittle systems, both extensive fragmentation of the brittle component and a high solubility of the brittle component in the ductile component are required [14]. These conditions are fulfilled for the composites studied herein and, hence, we expect a fully homogenized material after 64 to 128 h of milling. There is extensive confusion in the literature [4–6,12] over the use of the words ‘‘grains,’’
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‘‘particles,’’ and ‘‘crystallites.’’ In metallurgy each separate crystal is called a grain [15]. In crystallography, the average size of the crystallites is given by the width of the diffraction peaks. In magnetism, superparamagnetic behavior is determined by the size of a single domain particle. In this paper, we have chosen not to use the word ‘‘grain’’ in its metallurgical meaning and to use only crystallite and particle with the above meaning. Further, it must be noted that these crystallites and/or particles can also form agglomerates. Mo¨ssbauer spectroscopy is an ideal technique both for differentiating between the different ironnitride phases [16] and for studying the relaxation of small superparamagnetic particles. As a consequence, we report herein a Mo¨ssbauer spectral study of several iron-nitride nanocomposites studied as a function of milling time and temperature, with the goal of investigating the above transformation. In addition, the magnetization curves of the nanocomposites have been analyzed in order to obtain an estimate of the size of the single domain particles.
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acceleration spectrometer, which utilized a room temperature rhodium matrix cobalt-57 source and was calibrated at room temperature with a-iron foil. The Mo¨ssbauer spectral absorbers contained 30 mg/cm2 of the composites. The spectra of the milled samples were fit with a 0 to 40 T distribution of hyperfine fields by using the method of Wivel and Mørup [17]. These fits used a distribution of 22 sextets in which the areas of the components of each sextet were constrained to be in the ratio of 3:2:1:1:2:3. The spectra of the unmilled FeyN have also been fit with the minimum number of hyperfine fields needed to reproduce the observed spectrum; the resulting average hyperfine parameters were essentially the same as those obtained with the distribution fits. In all cases, the accuracy of the average isomer shifts, quadrupole shifts, line widths, and hyperfine fields is estimated to be 70.005, 70.01, 70.01 mm/s, and 70.2 T, respectively; the absolute accuracy is perhaps two to three times larger.
3. Magnetic studies 2. Experimental The samples studied herein have been prepared as described earlier [4,5]. The starting materials consisted of 99.99% pure 325 mesh powders of FeyN and Al2O3 or SiO2. Approximately 10 g of the appropriate amounts of FeyN and Al2O3 or SiO2 were placed in a 45 ml tungsten carbide vial with a sample-to-ball mass ratio of 1:14. The mill contained 12 tungsten carbide balls with a diameter of 10 mm and a mass of 7.85 g. The materials were milled for 4, 8, 16, 32, 64, and, in some cases, 128 h at 400 rpm in a Fritsch Pulverisette 7 planetary ball mill in the presence of absolute ethanol, which was used to prevent oxidation. Magnetic measurements were performed at 5 and 300 K at fields up to 5.5 T with a Quantum Design, Inc. superconducting quantum interference magnetometer. The magnetization is given in emu per gram of the nanocomposite. The Mo¨ssbauer spectra were obtained at several temperatures between 90 and 295 K on a constant-
The field dependence of the magnetization of (FeyN)0.2(SiO2)0.8 obtained for the indicated milling times at 300 K is shown in Fig. 2. The 300 K magnetization curves of the 64 h milled (FeyN)0.2(Al2O3)0.8 and (FeyN)0.6(Al2O3)0.4 nanocomposites have been reported earlier [4,5]. The 5 and 300 K magnetization curves for the (FeyN)0.6 (SiO2)0.4 and (FeyN)x(Al2O3)1x nanocomposites are similar to those shown in Fig. 2. All of these magnetization curves, none of which are completely saturated at 300 K even in an applied field of 5.5 T, indicate the presence of small superparamagnetic particles. In all cases, the magnetization has been fit with the Langevin function given by Eq. (1). MðHÞ ¼ M s ½cothðV rM s H=kTÞ kT=V rM s H; (1) where V is the average volume of the monodomain particle, r is the density of the particle, Ms is the saturation magnetization, k is the Boltzmann constant, T is the temperature, and H is the applied field. A density [18] of 6.57 103 kg/m3 for g0 -Fe4N was used in these fits. The diameters of the
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F. Grandjean et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 215–226 Table 1 The saturation magnetization FeyN particle diameters Composite
Msb Diameterc Diameterd Msa (emu/g) (emu/g) (nm) (nm)
(FeyN)0.2(Al2O3)0.8 48.1 (FeyN)0.6(Al2O3)0.4 93.7 (FeyN)0.2(SiO2)0.8 57.6 (FeyN)0.6(SiO2)0.4 111.6
43.9 94.6 59.6 104.5
8.8 6.4 8.3 6.6
14 8.0 8.5 8.5
a The 300 K saturation magnetization measured at 5 T for the 4 h milled nanocomposite. b The calculated 300 K saturation magnetization corresponding to the amount of FeyN present in 1 g of composite by assuming a magnetization of 123 emu/g for Fe3N. c The diameter obtained from the Langevin fits. d The diameter obtained from the Scherrer analysis, see Refs. [4,5].
Fig. 2. Field dependence of the magnetization of (FeyN)0.2 (SiO2)0.8 obtained at 300 K and indicated milling times. The solid lines are the Langevin functions given by Eq. (1).
assumed spherical particles of FeyN obtained from these fits are given in Table 1 together with the crystallite diameter obtained from the Scherrer analysis [4,5] of the prominent Fe3N X-ray diffraction peak. No significant reduction in particle diameter is observed in the Langevin fits for milling times between 4 and 64 h. Only a significant reduction between 4 and 8 milling hours was observed in the Scherrer analysis [4,5]. As is shown in Table 1, the Langevin fits give magnetic monodomain diameters that are slightly smaller than the crystallite size obtained from the Scherrer analysis. Eq. (1) does not take into account any distribution in size of the particles and hence the diameter should be considered as an average diameter. It is difficult to assign an error bar to the diameter obtained from the Langevin fits, but it appears that the four diameters given in Table 1 may not be significantly different. Hence, the only significantly larger diameter was obtained [4] from the Scherrer analysis of the X-ray peaks in (FeyN)0.2(Al2O3)0.8. Hence, the monodomain magnetic particles seem to be monocrystalline. The saturation magnetization observed at 300 K for the 4 h milled samples of the four nanocomposites are in good agreement with the calculated values obtained from the dilution of Fe3N, with a saturation magnetization of 123 emu/g, in the nonmagnetic oxides; see Table 1. The additional small reduction in saturation magnetization with milling times between 8 and 64 h results from an additional
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small reduction in the FeyN particle size and, hence, increase in the superparamagnetic fraction. It is well known [19,20] that surface effects on small particles reduce the saturation magnetization.
4. Mo¨ssbauer spectroscopy The Mo¨ssbauer spectrum of the FeyN used in this study and measured at 295 K is shown at the top of Fig. 3a. The FeyN spectra have also been
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measured at 190 and 90 K and are very similar to that shown in Fig. 3a. This spectrum is similar but not identical to the spectrum of g0 -Fe4N shown in Fig. 2A of Ref. [7] Specifically, the peaks at 0.5 and 4 mm/s have a different shape, a difference that occurs because of the presence of both g0 -Fe4N and e-Fe3N in the sample studied herein. In the absence of published hyperfine parameters in the earlier work [7], no further comparison is possible. At all temperatures, the FeyN spectra show four components with fields of ca. 4, 11, 22, and 34 T.
Fig. 3. (a) Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields of (FeyN)0.6(Al2O3)0.4 obtained at 295 K as a function of milling time.
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Table 2 Iron-57 Mo¨ssbauer spectral parameters obtained for FeyN from an analysis with discrete sextets T (K)
/dSa (mm/s)
/HS (T)
/QSS (mm/s)
/GS (mm/s)
Area (%)
Assignment
295
0.23 0.31 0.44 0.34
33.9 22.1 7.5 18.0
0.02 0.04 0.02 0.03
0.26 0.33 2.46 1.02
10 56 34 —
g0 -Fe4N e-Fe3N+ g0 -Fe4N e-Fe3+xN wt. avg.
190
0.30 0.38 0.52 0.40
35.6 23.6 11.4 22.0
0.02 0.01 0.03 0.01
0.24 0.35 1.07 0.50
11 67 22 —
g0 -Fe4N e-Fe3N+g0 -Fe4N e-Fe3+xN wt. avg.
90
0.37 0.43 0.57 0.45
36.4 24.5 13.4 23.0
0.01 0.00 0.08 0.02
0.25 0.31 1.15 0.50
11 65 24 —
g0 -Fe4N e-Fe3N+g0 -Fe4N e-Fe3+xN wt. avg.
a
The isomer shifts are given relative to room temperature a-iron foil.
The hyperfine parameters derived from fits of the FeyN spectra with discrete magnetic sextets are given in Table 2 along with tentative assignments of the different components. Specifically, the sextets with fields of ca. 22 and 34 T may be assigned [16] to the e-Fe3N and g0 -Fe4N phases observed by X-ray diffraction [4–6]. In an alternative analysis with a hyperfine field distribution of the starting material, see the top of Fig. 3b, the broad peak between 11 and 15 T may be assigned [16] to some non-stoichiometric e-Fe3+xN. The peak at ca. 4 T may be assigned to small superparamagnetic particles of FeyN present in the starting material or may be an artifact resulting from the fitting procedure, which does not take into account any distribution of isomer shifts or quadrupole shifts. The Mo¨ssbauer spectra of (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(SiO2)0.8 obtained at 295 K for the indicated milling times are shown in Figs. 3a and 4a, respectively. All the spectra of the milled samples shown herein are quite different from the spectrum [7] of g0 -Fe4N milled for 2 h in a steel vial containing steel balls under nitrogen atmosphere. More specifically, the differences are found at 2.5 mm/s and between 2 and 3.5 mm/s. These differences may result either from the different milling conditions used in Ref. [7] or from the presence of the alumina or silica in the samples studied herein. The spectra shown in Figs. 3a and
4a are characteristic of samples composed of small particles with a distribution of size. Hence, they have been analyzed with a distribution [17] of hyperfine fields, distributions that are shown in Figs. 3b and 4b. In all of the spectra of the milled samples, the observed quadrupole shifts are less than 70.01 mm/s and in no case is there any indication of the presence of any iron(II) oxide or iron(III) oxide in the spectra. This absence indicates that the amount of these phases present in the milled samples is less than 2 vol%, in agreement with the X-ray diffraction results. The spectra observed after 4 h of milling are very similar to the spectra shown for 8 h of milling and indicate that, with a few hours of milling, there are dramatic changes in the spectra and thus in the distribution of particle sizes or of magnetic phases. A comparison of the spectra and of the hyperfine field distribution in (FeyN)0.2(Al2O3)0.8 and (FeyN)0.6(Al2O3)0.4, and in (FeyN)0.2(SiO2)0.8 and (FeyN)0.6(SiO2)0.4, shows that the x ¼ 0:2 nanocomposites contain a larger amount of small particles, which contribute to the small hyperfine field region of the spectra between 0 and 1 mm/s. Hence, it seems that the Mo¨ssbauer spectra are more sensitive to a change in the particle size distribution than are the magnetic measurements. Ideally, one would like to fit the Mo¨ssbauer spectra shown in Figs. 3 and 4 with a relaxation line shape profile, but such fits are virtually
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Fig. 4. (a) Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields of (FeyN)0.2(SiO2)0.8 obtained at 295 K as a function of milling time.
impossible to do because of the presence of the distribution of particle sizes, which yields a distribution of relaxation rates; fits of such a distribution are not feasible. The average isomer shifts and hyperfine fields of the four nanocomposites studied herein, derived
from the Wivel and Mørup [17] fit of the distribution of the hyperfine fields, are shown in Figs. 5a and b, respectively. The average isomer shift increases rather sharply between 4 and 36 h of milling and more slowly after 36 h of milling. Hence, the initial effect of milling on the isomer
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avg. isomer shift (mm/s)
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0.34
0.32
0.30 (a)
avg. hyperfine field (T)
0.28 20 18 16 14 12 10
(b)
8 largest field fraction
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
(c)
-0.02 0
20
40
60 80 100 milling time (hr)
120
Fig. 5. Dependence of (a) the 295 K average isomer shift, (b) hyperfine field, and (c) the area fraction of the largest hyperfine field component, upon milling time for (FeyN)0.2(Al2O3)0.8, open squares, and (FeyN)0.6(Al2O3)0.4, solid squares, (FeyN)0.2 (SiO2)0.8, open circles, and (FeyN)0.6(SiO2)0.4, solid circles. The lines shown in (c) are the result of a fit with Eq. (2) as is discussed in the text.
shift is important and the overall increase in isomer shift of ca. 0.05 mm/s results from an intimate mixing of the iron nitride and alumina or silica particles within the agglomerates. Further, this increase in isomer shift is also due, at least in part, to an increase in the e-Fe3N volume fraction, which has a higher isomer shift of 0.28 mm/s as
compared with the g0 -Fe4N isomer shift of 0.20 mm/s [21]. Within 4 to 16 h of milling the g0 -Fe4N phase in the four nanocomposites, characterized by the peak at 34 T in the field distributions in Figs. 3b and 4b, virtually disappears. In contrast, the eFe3N phase, characterized by the peak at 22 T in the field distributions, persists for 64 h of milling, see Fig. 4b. In the case of (FeyN)0.6(Al2O3)0.4, this sextet persists even after 128 h of milling, see Fig. 3b. Thus, in the four nanocomposites, the sextet with the largest field corresponding to g0 Fe4N disappears after ca. 8 h of milling and the sextet at 22 T corresponding to the e-Fe3N phase is partly replaced by sextets with fields of ca. 4 and 10 T, fields which are not characteristic [16] of stoichiometric iron-nitride phases. However, nonstoichiometric e-Fe3+xN show fields between 5.9 and 7.0 T and 15.7 and 17.0 T. Hence, the peaks in the hyperfine field distributions at 4 and 10 T could be assigned either to some non-stoichiometric eFe3+xN or are an artifact of the fitting procedure, which mimics the contribution of the small superparamagnetic particles, relaxing on the Mo¨ssbauer time scale, by sextets with small fields. These changes are revealed by the decrease in the average hyperfine field with milling time shown in Fig. 5b. The disappearance of the g0 -Fe4N phase with milling time in both (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(Al2O3)0.8 is shown in Fig. 5c, which reveals a decrease with milling time in the area of the peak centered at 34 T in the field distributions shown in Figs. 3b and 4b. This dependence has been fit with F ðtÞ ¼ 0:145 expðBtC Þ;
(2)
where F is the fractional area of the peak. Similarly slightly poorer fits, limited to 64 h of milling, have been obtained for (FeyN)0.2(SiO2)0.8 and (FeyN)0.6(SiO2)0.4. The dependence given by Eq. (2) has been proposed by Matteazzi et al. [22] for mechanical alloying of iron carbides. They have proposed that the parameters B and C are related with the impact energy determined by the rotational frequency and ‘‘work conditions’’ determined by the number of balls, respectively. The same equation has also been successfully applied [23] to the mechanical alloying of iron nitrides.
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Fig. 6. (a) Temperature dependence of the Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields, for (FeyN)0.6(Al2O3)0.4 after milling for 64 h.
Because the four nanocomposites studied herein are ball milled in the same conditions, same rotational frequency and same number of balls, the parameters B and C are expected to be similar in the four fits. Indeed, the values for B and C are between 0.27 and 0.32 and between 0.35 and 0.43, respectively. Similar values for B have been observed [23] for the mechanosynthesis of g0 Fe4N. Further, a value of C less than two indicates [23] an exothermic process for the transformation of the g0 -Fe4N phase. In conclusion, the dependence of the Mo¨ssbauer spectra upon milling time suggests both a transformation of the g0 -Fe4N phase into e-Fe3+xN and a-iron, as proposed [5,6] from the differential thermal analysis measurements, and the increasing presence of fine superparamagnetic particles, albeit particles with a distribution of sizes. In order to
shed some light on the role of these two effects, the temperature dependence of the Mo¨ssbauer spectra was investigated. The temperature dependence of the Mo¨ssbauer spectra of both (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(Al2O3)0.8, and the corresponding distributions of hyperfine fields are shown in Figs. 6 and 7. From the observed spectra and distribution of hyperfine fields, it is apparent that the field component found at ca. 4 T decreases dramatically upon cooling from 295 to 90 K, whereas the remaining three components at ca. 13, 24, and 32 T remain relatively unchanged. This change in the low-field component upon cooling is clearly a sign that some of the particles are superparamagnetic at the higher temperatures and blocked at the lower temperatures. It is remarkable that the peak at the largest field in the 90 K distribution, see
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Fig. 7. (a) Temperature dependence of the Mo¨ssbauer spectra, and (b) the corresponding distribution of hyperfine fields, for (FeyN)0.2(Al2O3)0.8 after milling for 64 h.
Figs. 6b and 7b, occurs at ca. 32 T, i.e., at a field which is smaller than the 34 T field characteristic of g0 -Fe4N. The milling time dependence of the position of three main peaks in the hyperfine field distributions of the (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2 (Al2O3)0.8 composites is shown in Fig. 8a and the temperature dependence of the position of the same peaks in the hyperfine field distributions of the 64 h milled (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2 (Al2O3)0.8 is shown in Fig. 8b. It is clear that increasing milling time and increasing temperature have the same effect of decreasing the peak position, i.e., decreasing the hyperfine field. This behavior is compatible with the presence of a superparamagnetic fraction in the samples. However, at 90 K the highest hyperfine field is observed at 32.3 T, a value which is closer to the hyperfine
field in a-iron than to the 90 K hyperfine field in g0 -Fe4N, see Table 2, whereas at 0 h of milling it is observed at 34.3 T, a value which is characteristic of g0 -Fe4N. Hence, the temperature dependence of the Mo¨ssbauer spectra is compatible with the transformation of the g0 -Fe4N phase into eFe3+xN and a-iron. This transformation has also been observed [7–9] in the ball-milling process of g0 -Fe4N and in the mechanosynthesis of iron nitrides. The e-Fe3N phase appears [9] to be very stable during milling procedures.
5. Discussion The goal of this work has been to investigate by iron-57 Mo¨ssbauer spectroscopy ball-milled nanocomposites of iron nitride and alumina or silica.
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spectra have been analyzed with a distribution of hyperfine fields. The dependences of these distributions upon milling time and temperature indicate that the g0 -Fe4N phase is eliminated from the mixture with increasing milling time and exothermically transformed into e-Fe3+xN and a-iron via the mechanical energy provided by the mechanical grinding. Such a transformation has been reported [24] at high temperatures and during mechanical grinding of the g0 -Fe4N phase [7] and is also supported by the differential thermal analysis [5,6] of the present samples. The 295 K Mo¨ssbauer spectra of the 64 h milled (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2(Al2O3)0.8 nanocomposites contain approximately 50% of their absorption area between 0 and 10 T and thus the blocking temperature of the superparamagnetic particles is around 295 K. For particles with a radius of 4 nm of g0 -Fe4N with an anisotropy constant [25] of 1.5 104 J/m3, a blocking temperature of 287 K is calculated, a value which is in fair agreement with the estimate obtained from the Mo¨ssbauer spectra.
Acknowledgments
Fig. 8. (a) Milling time dependence of the three peak positions in the hyperfine distributions of (FeyN)0.6(Al2O3)0.4, closed symbols, and (FeyN)0.2(Al2O3)0.8, open symbols, composites, and (b) temperature dependence of the position of the same peaks in the hyperfine field distributions of the 64 h milled (FeyN)0.6(Al2O3)0.4, closed symbols, and (FeyN)0.2(Al2O3)0.8, open symbols.
The spectra indicate that no oxidation of the iron nitride occurs with ball milling. Further, the shape of the Mo¨ssbauer spectra and their dependence upon milling time and temperature suggests that the samples consist of small particles showing a size distribution with an average size of 6 to 8 nm after 8 h of milling, some of them showing a superparamagnetic behavior in agreement with the absence of saturation in the magnetization curves under an applied field of 5.5 T. The Mo¨ssbauer
The authors thank Dr. N. Ali of Southern Illinois University, Carbondale, IL, for providing the magnetic measurements. GJL thanks the Francqui Foundation of Belgium for his appointment as a ‘‘Chaire Francqui Interuniversitaire au titre e´tranger’’ during the 2002–2003 academic year. This work was supported in part by the ‘‘Fonds National de la Recherche Scientifique,’’ Brussels, Belgium, Grant 9.456595 and the ‘‘Fonds de la Recherche Fondamentale Collective’’, Grant 2.4522.01. The authors also wish to thank the Department of Physics at the University of Memphis for their support and encouragement during the pursuit of this study and CARS at the University of Chicago for their support of the magnetization studies. References [1] L.C. Klein, in: A.S. Edelstein, R.C. Cammarata (Eds.), Nanomaterials: Synthesis, Properties and Applications, Institute of Physics, Bristol, 1996.
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[2] V. Pilai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243. [3] C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, New York, 1990. [4] S.R. Mishra, G.J. Long, F. Grandjean, R.P. Hermann, S. Roy, N. Ali, A. Viano, Eur. Phys. J. D 24 (2003) 93. [5] A.M. Viano, S.R. Mishra, Mat. Res. Soc. Symp. Proc. 703 (2002) V9. [6] S.R. Mishra, A. Viano, S. Roy, N. Ali, J. Losby, J. Nanosci. Nanotechnol. 3 (2003) 227. [7] J. Foct, R.S. de Figueiredo, NanoStruct. Mater. 4 (1994) 685. [8] W.A. Kaczmarek, Scripta Metall. Mater. 33 (1995) 1687. [9] Y. Chen, T. Halstead, J.S. Williams, Mater. Sci. Eng. A 206 (1996) 24. [10] T. Klassen, R. Gunther, B. Dickau, A. Bartels, R. Bormann, H. Mecking, Mater. Sci. Forum 269–272 (1998) 37. [11] J. Sort, J. Nogues, S. Surinach, J.S. Munoz, E. Chappel, F. Dupont, G. Chouteau, M.D. Baro, Mater. Sci. Forum 368–388 (2002) 465. [12] S. Surinach, M.D. Baro, J. Segura, M.T. ClavagueraMora, N. Clavaguera, J. Am. Ceram. Soc. 81 (1998) 2504.
[13] S. Surin˜ach, M.D. Baro´, J. Segura, M.T. ClavagueraMora, Mater. Sci. Eng. A 134 (1991) 1368. [14] B.S. Murty, S. Ranganathan, Int. Mater. Rev. 43 (1998) 101. [15] L.H. Van Vlack, in: Materials Science for Engineers, Addison-Wesley Pub., Reading, MA, 1970, p. 112. [16] Y. Jiraskova, S. Havlicek, O. Schneeweiss, V. Perina, C. Blawert, J. Magn. Magn. Mater. 234 (2001) 477. [17] C. Wivel, S. Mørup, J. Phys. E: Sci. Instr. 14 (1981) 605. [18] Handbook of Chemistry and Physics, 68th ed., CRC Press, Boca Raton, 1987–1988, B p. 68. [19] M. Guzman, J.L. Delplancke, G.J. Long, J. Delwiche, M.-J. Hubin-Franskin, F. Grandjean, J. Appl. Phys. 92 (2002) 2634. [20] V. Mancier, J.-L. Delplancke, J. Delwiche, M.-J. HubinFranskin, C. Piquer, L. Rebbouh, F. Grandjean, J. Magn. Magn. Mater. 281 (2004) 27. [21] P. Schaaf, Hyp. Int. 111 (1998) 113. [22] P. Matteazzi, D. Basset, F. Miani, G. Le Cae¨r, NanoStruct. Mater. 2 (1993) 14. [23] I.F. Vasconcelos, R.S. de Figueiredo, J. Phys. Chem. 107 (2003) 3761. [24] K.H. Jack, J. Appl. Phys. 76 (1994) 6620. [25] L. Varga, W.D. Doyle, J. Appl. Phys. 79 (1996) 4995.