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

0304-8853/$

doi:10.1016

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R.Hermann

(G.J. Long

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A Mossbauer spectral study of some iron nitride-basednanocomposites prepared by ball milling

Fernande Grandjeana,�, Raphael P. Hermanna, Gary J. Longb, Sanjay R. Mishrac

aDepartment of Physics, University of Liege, B5, B-4000 Sart-Tilman, BelgiumbDepartment of Chemistry, University of Missouri-Rolla, Rolla, MO 65409-0010, USA

cDepartment of Physics, University of Memphis, Memphis, TN 38152, USA

Received 19 August 2004; received in revised form 30 September 2004

Available online 20 November 2004

Abstract

Powder mixtures of (FeyN)x and (Al2O3)1�x or (SiO2)1�x 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 Mossbauer spectra have been measured. The 5 and 295K

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 Mossbauer 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 Mossbauer 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

4T indicates the presence of small superparamagnetic particles, a presence which is also confirmed by the temperature

dependence of the Mossbauer 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; Mossbauer spectroscopy; Magnetic nanocomposites

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.jmmm.2004.10.114

onding author. Tel.: +32 4 3663632; fax:

516.

addresses: [email protected] (F. Grandjean),

@ulg.ac.be (R.P. Hermann), [email protected]

), [email protected] (S.R. Mishra).

1. Introduction

Magnetic nanocomposite materials consistingeither of ultrafine metal or alloy particles dispersedin an insulating oxide matrix have been preparedby the sol–gel technique [1–3] and more recently byhigh-energy ball milling [4–6]. The morphological,structural, and magnetic properties of the resulting

d.

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F. Grandjean et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 215–226216

iron nitride-alumina and iron nitride-silica com-posites have been studied [4–6] by X-ray diffrac-tion, transmission electron microscopy, thermalanalysis, and magnetization measurements. Trans-mission electron microscopy and X-ray diffractionmeasurements indicate that the nanocompositesconsist of agglomerates or collections of ratherweakly bonded nanoparticles of ca. 100 nm dia-meter containing iron-nitride crystallites whosediameters decrease from ca. 35 to 8 nm for millingtimes of 0 to 128 h, respectively. An example ofa transmission electron micrograph is shown inFig. 1. The magnetization of the composite isstrongly reduced by dilution in alumina or silicafrom the bulk magnetization of 193 emu/g forg0-Fe4N and 123 emu/g for e-Fe3N. Further, asexpected for a reduction in size of the magneticiron-nitride crystallites, the saturation magnetiza-tion decreases and the coercive field increases withmilling time. The evolution with milling time of theX-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 1mm.

thermal analysis of these nanocomposites suggeststhat a transformation of g0-Fe4N into e-Fe3N anda-iron occurs as a result of the milling process.This transformation has already been observed[7–9] in studies of mechanical grinding synthesis ofiron nitrides.

Usually, after the long-term milling of materialssubjected to mechanical work in a high-energeticmill, the final distribution of particle sizes andshapes, i.e., the microstructure, becomes ratherstable, due to the continuous processes of fractureand soldering. The particle size distribution andparticle morphology depend on the kind ofmaterial being processed as well as on the millingfrequency, the ball-to-powder ratio, and thetemperature. However, the homogenizationprocess varies, depending on the ductility orbrittleness of the milled powder. Thus typically,very different microstructures are obtainedwhen the precursor powder is ductile, brittle, ora two-component mixture of ductile and brittlematerials.

The FeyN–ceramic composite resembles theductile–brittle case in which, during milling, theductile component particles acquire elongatedshapes, i.e., lamella, whereas the brittle componentparticles are simply fractured. At intermediatemilling times, the ductile particles become em-bedded in the brittle matrix, forming agglomeratesseveral millimeters in diameter. This is the typicalmicrostructure developed when milling a ceramicwith a metallic material, i.e., the so-called cermets[10,11]. A further increase in the milling timebrings the thinner ductile lamellae into closecontact, whereas the brittle particles become evermore tinier and finally dissolve within the ductileparticles. Depending upon the relative solubility ofthe precursors, the final product usually iscompletely homogenized [12,13]. Therefore, inorder to fully homogenize ductile–brittle systems,both extensive fragmentation of the brittle com-ponent and a high solubility of the brittlecomponent in the ductile component are required[14]. These conditions are fulfilled for the compo-sites studied herein and, hence, we expect a fullyhomogenized 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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F. Grandjean et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 215–226 217

‘‘particles,’’ and ‘‘crystallites.’’ In metallurgy eachseparate crystal is called a grain [15]. In crystal-lography, the average size of the crystallites isgiven by the width of the diffraction peaks. Inmagnetism, superparamagnetic behavior is deter-mined by the size of a single domain particle. Inthis paper, we have chosen not to use the word‘‘grain’’ in its metallurgical meaning and to useonly crystallite and particle with the above mean-ing. Further, it must be noted that these crystallitesand/or particles can also form agglomerates.

Mossbauer spectroscopy is an ideal techniqueboth for differentiating between the different iron-nitride phases [16] and for studying the relaxationof small superparamagnetic particles. As a con-sequence, we report herein a Mossbauer spectralstudy of several iron-nitride nanocompositesstudied as a function of milling time and tempera-ture, with the goal of investigating the abovetransformation. In addition, the magnetizationcurves of the nanocomposites have been analyzedin order to obtain an estimate of the size of thesingle domain particles.

2. Experimental

The samples studied herein have been preparedas described earlier [4,5]. The starting materialsconsisted of 99.99% pure 325 mesh powders ofFeyN and Al2O3 or SiO2. Approximately 10 g ofthe appropriate amounts of FeyN and Al2O3 orSiO2 were placed in a 45 ml tungsten carbide vialwith a sample-to-ball mass ratio of 1:14. The millcontained 12 tungsten carbide balls with adiameter of 10 mm and a mass of 7.85 g. Thematerials were milled for 4, 8, 16, 32, 64, and, insome cases, 128 h at 400 rpm in a FritschPulverisette 7 planetary ball mill in the presenceof absolute ethanol, which was used to preventoxidation.

Magnetic measurements were performed at 5and 300 K at fields up to 5.5 T with a QuantumDesign, Inc. superconducting quantum interfer-ence magnetometer. The magnetization is given inemu per gram of the nanocomposite.

The Mossbauer spectra were obtained at severaltemperatures between 90 and 295 K on a constant-

acceleration spectrometer, which utilized a roomtemperature rhodium matrix cobalt-57 source andwas calibrated at room temperature with a-ironfoil. The Mossbauer spectral absorbers contained30 mg/cm2 of the composites. The spectra of themilled samples were fit with a 0 to 40 T distributionof hyperfine fields by using the method of Wiveland Mørup [17]. These fits used a distribution of22 sextets in which the areas of the components ofeach sextet were constrained to be in the ratio of3:2:1:1:2:3. The spectra of the unmilled FeyN havealso been fit with the minimum number ofhyperfine fields needed to reproduce the observedspectrum; the resulting average hyperfine para-meters were essentially the same as those obtainedwith the distribution fits. In all cases, the accuracyof the average isomer shifts, quadrupole shifts, linewidths, and hyperfine fields is estimated to be70.005, 70.01, 70.01 mm/s, and 70.2 T, respec-tively; the absolute accuracy is perhaps two tothree times larger.

3. Magnetic studies

The field dependence of the magnetization of(FeyN)0.2(SiO2)0.8 obtained for the indicatedmilling times at 300 K is shown in Fig. 2. The300 K magnetization curves of the 64 h milled(FeyN)0.2(Al2O3)0.8 and (FeyN)0.6(Al2O3)0.4 nano-composites have been reported earlier [4,5]. The 5and 300 K magnetization curves for the (FeyN)0.6

(SiO2)0.4 and (FeyN)x(Al2O3)1�x nanocomposites aresimilar to those shown in Fig. 2. All of thesemagnetization curves, none of which are completelysaturated at 300K even in an applied field of 5.5T,indicate the presence of small superparamagneticparticles. In all cases, the magnetization has been fitwith the Langevin function given by Eq. (1).

MðHÞ ¼ Ms½cothðVrMsH=kTÞ � kT=VrMsH�;

(1)

where V is the average volume of the monodomainparticle, r is the density of the particle, Ms is thesaturation magnetization, k is the Boltzmannconstant, T is the temperature, and H is theapplied field. A density [18] of 6.57� 103 kg/m3 forg0-Fe4N was used in these fits. The diameters of the

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Fig. 2. Field dependence of the magnetization of (FeyN)0.2

(SiO2)0.8 obtained at 300K and indicated milling times. The

solid lines are the Langevin functions given by Eq. (1).

Table 1

The saturation magnetization FeyN particle diameters

Composite Msa

(emu/g)

Msb

(emu/g)

Diameterc

(nm)

Diameterd

(nm)

(FeyN)0.2(Al2O3)0.8 48.1 43.9 8.8 14

(FeyN)0.6(Al2O3)0.4 93.7 94.6 6.4 8.0

(FeyN)0.2(SiO2)0.8 57.6 59.6 8.3 8.5

(FeyN)0.6(SiO2)0.4 111.6 104.5 6.6 8.5

aThe 300K saturation magnetization measured at 5T for the

4 h milled nanocomposite.bThe calculated 300K saturation magnetization correspond-

ing to the amount of FeyN present in 1 g of composite by

assuming a magnetization of 123 emu/g for Fe3N.cThe diameter obtained from the Langevin fits.dThe diameter obtained from the Scherrer analysis, see

Refs. [4,5].


assumed spherical particles of FeyN obtained fromthese fits are given in Table 1 together with thecrystallite diameter obtained from the Scherreranalysis [4,5] of the prominent Fe3N X-raydiffraction peak. No significant reduction inparticle diameter is observed in the Langevin fitsfor milling times between 4 and 64 h. Only asignificant reduction between 4 and 8 milling hourswas observed in the Scherrer analysis [4,5]. As isshown in Table 1, the Langevin fits give magneticmonodomain diameters that are slightly smallerthan the crystallite size obtained from the Scherreranalysis. Eq. (1) does not take into account anydistribution in size of the particles and hence thediameter should be considered as an averagediameter. It is difficult to assign an error bar tothe diameter obtained from the Langevin fits, butit appears that the four diameters given in Table 1may not be significantly different. Hence, the onlysignificantly larger diameter was obtained [4] fromthe Scherrer analysis of the X-ray peaks in(FeyN)0.2(Al2O3)0.8. Hence, the monodomain mag-netic particles seem to be monocrystalline.

The saturation magnetization observed at 300Kfor the 4h milled samples of the four nanocompo-sites are in good agreement with the calculatedvalues obtained from the dilution of Fe3N, with asaturation magnetization of 123 emu/g, in the non-magnetic oxides; see Table 1. The additional smallreduction in saturation magnetization with millingtimes between 8 and 64h results from an additional

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small reduction in the FeyN particle size and, hence,increase in the superparamagnetic fraction. It is wellknown [19,20] that surface effects on small particlesreduce the saturation magnetization.

4. Mossbauer spectroscopy

The Mossbauer spectrum of the FeyN used inthis study and measured at 295 K is shown at thetop of Fig. 3a. The FeyN spectra have also been

Fig. 3. (a) Mossbauer spectra, and (b) the corresponding distribution

function of milling time.

measured at 190 and 90 K and are very similar tothat shown in Fig. 3a. This spectrum is similar butnot identical to the spectrum of g0-Fe4N shown inFig. 2A of Ref. [7] Specifically, the peaks at 0.5 and4mm/s have a different shape, a difference thatoccurs because of the presence of both g0-Fe4Nand e-Fe3N in the sample studied herein. In theabsence of published hyperfine parameters in theearlier work [7], no further comparison is possible.

At all temperatures, the FeyN spectra show fourcomponents with fields of ca. 4, 11, 22, and 34 T.

of hyperfine fields of (FeyN)0.6(Al2O3)0.4 obtained at 295K as a

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Table 2

Iron-57 Mossbauer 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 33.9 0.02 0.26 10 g0-Fe4N

0.31 22.1 �0.04 0.33 56 e-Fe3N+ g0-Fe4N

0.44 7.5 �0.02 2.46 34 e-Fe3+xN

0.34 18.0 �0.03 1.02 — wt. avg.

190 0.30 35.6 0.02 0.24 11 g0-Fe4N

0.38 23.6 �0.01 0.35 67 e-Fe3N+g0-Fe4N

0.52 11.4 �0.03 1.07 22 e-Fe3+xN

0.40 22.0 �0.01 0.50 — wt. avg.

90 0.37 36.4 0.01 0.25 11 g0-Fe4N

0.43 24.5 0.00 0.31 65 e-Fe3N+g0-Fe4N

0.57 13.4 0.08 1.15 24 e-Fe3+xN

0.45 23.0 0.02 0.50 — wt. avg.

aThe isomer shifts are given relative to room temperature a-iron foil.


The hyperfine parameters derived from fits of theFeyN spectra with discrete magnetic sextets aregiven in Table 2 along with tentative assignmentsof the different components. Specifically, thesextets with fields of ca. 22 and 34 T may beassigned [16] to the e-Fe3N and g0-Fe4N phasesobserved by X-ray diffraction [4–6]. In an alter-native analysis with a hyperfine field distributionof the starting material, see the top of Fig. 3b, thebroad peak between 11 and 15 T may be assigned[16] to some non-stoichiometric e-Fe3+xN. Thepeak at ca. 4 T may be assigned to small super-paramagnetic particles of FeyN present in thestarting material or may be an artifact resultingfrom the fitting procedure, which does not takeinto account any distribution of isomer shifts orquadrupole shifts.

The Mossbauer spectra of (FeyN)0.6(Al2O3)0.4

and (FeyN)0.2(SiO2)0.8 obtained at 295 K for theindicated milling times are shown in Figs. 3a and4a, respectively. All the spectra of the milledsamples shown herein are quite different from thespectrum [7] of g0-Fe4N milled for 2 h in a steel vialcontaining steel balls under nitrogen atmosphere.More specifically, the differences are found at�2.5 mm/s and between 2 and 3.5 mm/s. Thesedifferences may result either from the differentmilling conditions used in Ref. [7] or from thepresence of the alumina or silica in the samplesstudied herein. The spectra shown in Figs. 3a and

4a are characteristic of samples composed of smallparticles with a distribution of size. Hence, theyhave been analyzed with a distribution [17] ofhyperfine fields, distributions that are shown inFigs. 3b and 4b. In all of the spectra of the milledsamples, the observed quadrupole shifts are lessthan 70.01 mm/s and in no case is there anyindication of the presence of any iron(II) oxide oriron(III) oxide in the spectra. This absenceindicates that the amount of these phases presentin the milled samples is less than 2 vol%, inagreement with the X-ray diffraction results. Thespectra observed after 4 h of milling are verysimilar to the spectra shown for 8 h of milling andindicate that, with a few hours of milling, there aredramatic changes in the spectra and thus in thedistribution of particle sizes or of magnetic phases.A comparison of the spectra and of the hyperfinefield 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 nano-composites contain a larger amount of smallparticles, which contribute to the small hyperfinefield region of the spectra between 0 and 1mm/s.Hence, it seems that the Mossbauer spectra aremore sensitive to a change in the particle sizedistribution than are the magnetic measurements.Ideally, one would like to fit the Mossbauerspectra shown in Figs. 3 and 4 with a relaxationline shape profile, but such fits are virtually

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Fig. 4. (a) Mossbauer spectra, and (b) the corresponding distribution of hyperfine fields of (FeyN)0.2(SiO2)0.8 obtained at 295K as a

function of milling time.


impossible to do because of the presence of thedistribution of particle sizes, which yields adistribution of relaxation rates; fits of such adistribution are not feasible.

The average isomer shifts and hyperfine fields ofthe four nanocomposites studied herein, derived

from the Wivel and Mørup [17] fit of thedistribution of the hyperfine fields, are shown inFigs. 5a and b, respectively. The average isomershift increases rather sharply between 4 and 36 h ofmilling and more slowly after 36 h of milling.Hence, the initial effect of milling on the isomer

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0.28

0.30

0.32

0.34

avg.

isom

er s

hift

(mm

/s)

8

10

12

14

16

18

20

avg.

hyp

erfin

e fie

ld (

T)

-0.02

0.00

0 20 40 60 80 100 120

0.02

0.04

0.06

0.08

0.10

0.12

0.14

larg

est f

ield

frac

tion

milling time (hr)

(a)

(b)

(c)

Fig. 5. Dependence of (a) the 295K 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 inisomer shift of ca. 0.05 mm/s results from anintimate mixing of the iron nitride and alumina orsilica particles within the agglomerates. Further,this increase in isomer shift is also due, at least inpart, 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 of0.20 mm/s [21].

Within 4 to 16 h of milling the g0-Fe4N phase inthe four nanocomposites, characterized by thepeak at 34 T in the field distributions in Figs. 3band 4b, virtually disappears. In contrast, the e-Fe3N phase, characterized by the peak at 22 T inthe field distributions, persists for 64 h of milling,see Fig. 4b. In the case of (FeyN)0.6(Al2O3)0.4, thissextet persists even after 128 h of milling, seeFig. 3b. Thus, in the four nanocomposites, thesextet with the largest field corresponding to g0-Fe4N disappears after ca. 8 h of milling and thesextet at 22 T corresponding to the e-Fe3N phase ispartly replaced by sextets with fields of ca. 4 and10 T, fields which are not characteristic [16] ofstoichiometric iron-nitride phases. However, non-stoichiometric e-Fe3+xN show fields between 5.9and 7.0 T and 15.7 and 17.0 T. Hence, the peaks inthe hyperfine field distributions at 4 and 10 T couldbe assigned either to some non-stoichiometric e-Fe3+xN or are an artifact of the fitting procedure,which mimics the contribution of the small super-paramagnetic particles, relaxing on the Mossbauertime scale, by sextets with small fields. Thesechanges are revealed by the decrease in the averagehyperfine field with milling time shown in Fig. 5b.

The disappearance of the g0-Fe4N phase withmilling time in both (FeyN)0.6(Al2O3)0.4 and(FeyN)0.2(Al2O3)0.8 is shown in Fig. 5c, whichreveals a decrease with milling time in the area ofthe peak centered at 34 T in the field distributionsshown in Figs. 3b and 4b. This dependence hasbeen fit with

F ðtÞ ¼ 0:145 expð�BtCÞ; (2)

where F is the fractional area of the peak. Similarlyslightly 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 byEq. (2) has been proposed by Matteazzi et al.[22] for mechanical alloying of iron carbides. Theyhave proposed that the parameters B and C arerelated with the impact energy determined by therotational frequency and ‘‘work conditions’’ de-termined by the number of balls, respectively. Thesame 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 Mossbauer 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 hereinare ball milled in the same conditions, samerotational frequency and same number of balls,the parameters B and C are expected to be similarin the four fits. Indeed, the values for B and C arebetween 0.27 and 0.32 and between 0.35 and 0.43,respectively. Similar values for B have beenobserved [23] for the mechanosynthesis of g0-Fe4N. Further, a value of C less than two indicates[23] an exothermic process for the transformationof the g0-Fe4N phase.

In conclusion, the dependence of the Mossbauerspectra upon milling time suggests both a trans-formation of the g0-Fe4N phase into e-Fe3+xN anda-iron, as proposed [5,6] from the differentialthermal analysis measurements, and the increasingpresence of fine superparamagnetic particles, albeitparticles with a distribution of sizes. In order to

shed some light on the role of these two effects, thetemperature dependence of the Mossbauer spectrawas investigated.

The temperature dependence of the Mossbauerspectra of both (FeyN)0.6(Al2O3)0.4 and(FeyN)0.2(Al2O3)0.8, and the corresponding distri-butions of hyperfine fields are shown in Figs. 6 and7. From the observed spectra and distribution ofhyperfine fields, it is apparent that the fieldcomponent found at ca. 4 T decreases dramaticallyupon cooling from 295 to 90 K, whereas theremaining three components at ca. 13, 24, and32 T remain relatively unchanged. This change inthe low-field component upon cooling is clearly asign that some of the particles are superparamag-netic at the higher temperatures and blocked at thelower temperatures. It is remarkable that the peakat the largest field in the 90 K distribution, see

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Fig. 7. (a) Temperature dependence of the Mossbauer 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 fieldwhich is smaller than the 34 T field characteristicof g0-Fe4N.

The milling time dependence of the position ofthree main peaks in the hyperfine field distribu-tions of the (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2

(Al2O3)0.8 composites is shown in Fig. 8a and thetemperature dependence of the position of thesame peaks in the hyperfine field distributions ofthe 64 h milled (FeyN)0.6(Al2O3)0.4 and (FeyN)0.2

(Al2O3)0.8 is shown in Fig. 8b. It is clear thatincreasing milling time and increasing temperaturehave the same effect of decreasing the peakposition, i.e., decreasing the hyperfine field. Thisbehavior is compatible with the presence of asuperparamagnetic fraction in the samples. How-ever, at 90 K the highest hyperfine field is observedat 32.3 T, a value which is closer to the hyperfine

field in a-iron than to the 90 K hyperfine field ing0-Fe4N, see Table 2, whereas at 0 h of milling it isobserved at 34.3 T, a value which is characteristicof g0-Fe4N. Hence, the temperature dependence ofthe Mossbauer spectra is compatible with thetransformation of the g0-Fe4N phase into e-Fe3+xN and a-iron. This transformation has alsobeen observed [7–9] in the ball-milling process ofg0-Fe4N and in the mechanosynthesis of ironnitrides. The e-Fe3N phase appears [9] to be verystable during milling procedures.

5. Discussion

The goal of this work has been to investigate byiron-57 Mossbauer spectroscopy ball-milled nano-composites of iron nitride and alumina or silica.

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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 ironnitride occurs with ball milling. Further, the shapeof the Mossbauer spectra and their dependenceupon milling time and temperature suggests thatthe samples consist of small particles showing asize distribution with an average size of 6 to 8 nmafter 8 h of milling, some of them showing asuperparamagnetic behavior in agreement with theabsence of saturation in the magnetization curvesunder an applied field of 5.5 T. The Mossbauer

spectra have been analyzed with a distribution ofhyperfine fields. The dependences of these dis-tributions upon milling time and temperatureindicate that the g0-Fe4N phase is eliminated fromthe mixture with increasing milling time andexothermically transformed into e-Fe3+xN anda-iron via the mechanical energy provided by themechanical grinding. Such a transformation hasbeen reported [24] at high temperatures and duringmechanical grinding of the g0-Fe4N phase [7] and isalso supported by the differential thermal analysis[5,6] of the present samples. The 295 K Mossbauerspectra of the 64 h milled (FeyN)0.6(Al2O3)0.4 and(FeyN)0.2(Al2O3)0.8 nanocomposites contain ap-proximately 50% of their absorption area between0 and 10 T and thus the blocking temperature ofthe superparamagnetic particles is around 295 K.For particles with a radius of 4 nm of g0-Fe4N withan anisotropy constant [25] of 1.5� 104 J/m3, ablocking temperature of 287 K is calculated, avalue which is in fair agreement with the estimateobtained from the Mossbauer spectra.

Acknowledgments

The authors thank Dr. N. Ali of SouthernIllinois University, Carbondale, IL, for providingthe magnetic measurements. GJL thanks theFrancqui Foundation of Belgium for his appoint-ment as a ‘‘Chaire Francqui Interuniversitaire autitre etranger’’ during the 2002–2003 academicyear. This work was supported in part by the‘‘Fonds National de la Recherche Scientifique,’’Brussels, Belgium, Grant 9.456595 and the ‘‘Fondsde la Recherche Fondamentale Collective’’, Grant2.4522.01. The authors also wish to thank theDepartment of Physics at the University ofMemphis for their support and encouragementduring the pursuit of this study and CARS at theUniversity of Chicago for their support of themagnetization 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. Clavaguera-

Mora, N. Clavaguera, J. Am. Ceram. Soc. 81 (1998) 2504.

[13] S. Surinach, M.D. Baro, J. Segura, M.T. Clavaguera-

Mora, 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. Hubin-

Franskin, 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 Caer,

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.

a mössbauer spectral study of some iron nitride-based nanocomposites prepared by ball milling

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