A 57Fe Mössbauer study of nanostructured Sm2Fe17−xCoxC3

Journal of Alloys and Compounds 455 (2008) 35–41

A 57Fe M¨ossbauer study of nanostructured Sm2Fe17−xCoxC3 L. Bessais a,∗ , C. Dj´ega-Mariadassou a , D.K. Tung b , V.V. Hong b , N.X. Phuc b a

LCMTR, UPR209, CNRS, 2/8 rue Henri Dunant, B.P. 28 F-94320 Thiais, France b Institute of Materials Science, NCST, Hoang Quoc Viet Road, Vietnam

Received 18 December 2006; received in revised form 11 January 2007; accepted 12 January 2007 Available online 18 January 2007

Abstract Sm2 Fe17−x Cox C3 (x ≤ 2) were synthesized by means of reaction with solid hydrocarbon at 693 K on Sm2 Fe17−x Cox powders obtained by high energy milling and subsequent annealing at 1125 ◦ C. X-ray diffraction analysis by Rietveld method has shown that both series crystallize in the rhombohedral Th2 Zn17 -type structure. The Curie temperature TC increase of Sm2 Fe17−x Cox C3 versus Co content is only ruled by electronic effect less pronounced than for the non-carbonated series. The analysis of their M¨ossbauer spectra, have been analyzed on the basis of the binomial law. The hyperfine parameter sets were assigned according to the relationship between the Wigner–Seitz cell volume of each iron site and their isomer shift δ so that δ{6c} > δ{18h} > δ{18f} > δ{9d}. It results that Co preferentially occupies the 18h site with the recurrent sequence HHF {6c} > HHF {9d} > HHF {18f} > HHF {18h}. This sequence is in agreement with the number of iron near-neighbors. The increase with Co content of hyperfine field, is correlated to the increase of the core electron polarization field ruled by the asymmetrical filling of the 3d band by the additional 3d Co electron. © 2007 Elsevier B.V. All rights reserved. Keywords: Mechanical milling; Nanostructured materials; M¨ossbauer effect; Intermetallic compounds; Magnetic properties

1. Introduction Among the most promising materials able to present excellent hard magnetic properties, the Sm2 Fe17 alloys partially Fe substituted and containing interstitials element like N or C have received considerable attention. Extensive works have been devoted to low Si, Ga, Al substitution in the Sm2 Fe17−x Mx X (M = Si, Ga, Al, X = N, C) alloys [1–3]. However, only few works deal with low Co contents (x ≤ 3) [4–7] probably due to the fact that high Co concentration leads to the hard Sm–Co alloys which do not need any interstitials to turn uniaxial. Although nitrogen is found more effective than carbon for the enhancement of the intrinsic magnetic properties, the lower thermal stability of the nitrides makes the carbides extremely attractive. The advantage of Co is that even a low content increases significantly the Curie temperature and can still maintain a high anisotropy. Therefore, a systematic investigation of the intrinsic magnetic properties of the Sm2 Fe17−x Cox C3 with x ≤ 2 appears available to carry out.

∗

Corresponding author. Tel.: +33 1 4978 1197. E-mail address: [email protected] (L. Bessais).

0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.01.068

The purpose of the present work is to obtain a good knowledge of the structure and the intrinsic magnetic characteristics like Curie temperature and hyperfine parameters of the nanostructured Sm2 Fe17−x Cox C3 series, obtained by high energy milling, annealing and subsequent carbonation of a metallic Sm2 Fe17−x Cox lattice. It must be emphasized that the process of high energy milling with annealing, is implemented in the solid state and is particularly convenient for samarium-based alloys due to the extreme volatility of samarium. This process insures a better control of stoichiometry than the conventional melting techniques and provides large homogeneous batches of materials. We intent herein to analyze the combined effect of cobalt and carbon on the structure, Curie temperature and hyperfine parameters of nanostructured Sm2 Fe17 . We shall demonstrate, directly and experimentally for the first time, by means of M¨ossbauer spectrometry, the location of cobalt, at once, in the simplest not carbonated Th2 Zn17 unit cell. One must have in mind that M¨ossbauer spectrometry, which accesses to the electron density at the Fe nucleus, offers the most suitable local probe to the study of samarium based alloys owing to the huge samarium neutron cross-section absorption coefficient. Moreover, our preliminary careful X-ray structural analysis provides

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L. Bessais et al. / Journal of Alloys and Compounds 455 (2008) 35–41

the information required for the Wigner–Seitz cell volume calculation versus Co content, essential for a pertinent analysis of the M¨ossbauer spectra. On the basis of the results obtained on the Sm2 (Fe, Co)17 host lattice, the effect of carbon will be afterwards logically cleared up. 2. Experimental Polycrystalline Sm2 Fe17−x Cox alloys (x = 0.5, 1, 1.5, 2) were prepared by high-energy ball milling an appropriate stoichiometric mix of Sm2 Fe17 , Co (99.99%), Sm (99.99%) powders. An excess of Sm powder up to 12% was used in order to maintain an overpressure of Sm on the sample. The quality of the Sm2 Fe17 prealloy was checked by inductively coupled plasma atomic emission spectroscopy. After milling 5 h under high purity Ar atmosphere, with ball to powder ratio of 15/1 and kinetic shock energy, shock frequency and injected shock power values respectively equal to 0.81 J/hit, 62 Hz, and 19.5 W/g. Theses values have been estimated from a mathematical treatment of the process taking place in a planetary ball mill by means of kinematics equation giving the velocity of ball in the vial [8,9]. The powders, wrapped in tantalum foil were annealed for 30 min in sealed silica tube under 10−6 Torr at 1125 ◦ C. Carbonation to the nominal composition Sm2 Fe17−x Cox C3 was performed by reacting the Sm2 Fe17−x Cox powders ground to 50 ␮m with an appropriate amount of C14 H10 powders at 420 ◦ C for 24 h to insure a good homogeneity of the carbon distribution. Mg chips inside the reacting tube adsorbed the hydrogen overpressure resulting from the cracking of C14 H10 . X-ray diffraction (XRD) patterns were registered on a Brucker diffractometer with Cu K␣ radiation. The counting rate was 22 s per scanning step and the step size was 0.04◦ . An internal Si standard (NBS, SRM 640) was used to ˚ The spectrum measure the unit cell parameter with an accuracy of ±1 × 10−3 A. refinement was performed with the FULLPROF computing code based on the Rietveld technique, in the assumption of Thompson–Cox–Hastings line profile [10]. Isotropic Lorentzian and Gaussian contribution of size and micro-strains were taken into account. The full-width-at-half-maximum of the Gaussian HG and Lorentzian HL component of the profile function is given by: HG2 = u tan2 θ + v tan θ + w,

HL = ζ tan θ +

mated errors are ±0.1 T for hyperfine field HHF ± 0.005 mm/s. for isomer shift δ and quadrupole interaction 2ε.

3. Results and discussion 3.1. Structure analysis The XRD patterns of the Sm2 Fe17−x Cox reveal the presence of the main phase (∼95 wt%) with the rhombohedral ¯ Th2 Zn17 -type structure and additional lines of Sm2 O3 R3m and SmO–N, logically present owing the samarium excess which reacts with traces of oxygen or air during handling. No ␣(Fe–Co) is detected neither by XRD nor by M¨ossbauer spectrometry as shown below. The structure characteristics (unit cell parameters, atomic positions, line-width inducing the crystallite size) resulting from the Rietveld analysis are reported in Table 1. Although the XRD technique is not sensitive to the effect of cobalt compared to that of iron, in the last step of the Rietveld fit, we have located cobalt in the 18h, sites according to the following M¨ossbauer analysis, while iron occupies sites 6c, 9d, 18f and 18h. As an example, the Rietveld analysis of Sm2 Fe15.5 Co1.5 is presented in Fig. 1. It comes that up to x = 2, the effect of Co on the unit cell parameter is quite weak: a shows a trend for decrease, (a/a  −5.3 × 10−4 per Co atom), c increases clearly (c/c  +1.1 × 10−3 per Co atom) but the volume remains constant. The atomic positions are not affected by the substitution.

ξ cos θ

ξ provides a size value representing the volume averaged diameter of crystallites in all directions. The u parameter leads to an estimate of the isotropic broadening due to strain effects. The various structural parameters: X, Z atomic position, Debye–Waller factor and occupancy parameter s, cell parameter and the u and ξ profile parameter were least square fitted. The ‘goodness-of-fit’ indicators are calculated as follows:



K

RB =

|IK (o) − IK (c)|



I (o) K K

IK (o) is the observed Bragg intensity and IK (c) is the calculated one.



χ2 =

i

wi |yi (o) − yi (c)|2 N −P +C

where yi (o) is the intensity observed at the ith step in the step scanned powder diffraction pattern, yi (c) is that calculated and wi is the weight of the observation. N total number of points used in the refinement. P the number of refined parameters and C the number of strict constraint function. The Curie temperature TC were measured on a differential sample magnetometer MANICS in a field of 1000 Oe with around 10 mg sample sealed under vacuum in silica tube in order to prevent oxidation under heating. The 57 Fe M¨ ossbauer spectra were collected at room temperature using a constant acceleration 512-channel spectrometer working in the mirror image mode with a 50 mCi 57 Co in Rh source and absorbers containing 12 mg of natural iron per cm2 . The ␣-iron reference had a full-width-at-half-maximum of 0.25 mm/s. for the external peaks. The experimental spectra were least-square-fitted with Lorentzian lines without thickness broadening of the absorption lines. The esti-

Fig. 1. Rietveld analysis for (a) Sm2 Fe15.5 Co1.5 and (b) Sm2 Fe16.5 Co0.5 C3 . The ¯ SmO–N, and Sm2 O3 sets of ticks refer, respectively, from top to bottom, to R3m, ¯ SmO–N, Sm2 O3 , and ␣-Fe for (b). for (a) and to R3m,

L. Bessais et al. / Journal of Alloys and Compounds 455 (2008) 35–41

37

Table 1 a, c cell parameters, and RB , χ2 , Wyckoff positions, D (autocoherent diffraction domain size) from the Rietveld fit for Sm2 Fe17−x Cox and their carbides Compound

x = 0.5

x=1

x = 1.5

x=2

Sm2 Fe17−x Cox

˚ a (A) ˚ c (A) ˚ 3 V (A) RB ␹2 X {18f} X {18h} Z {18h} Z {6c} Z {6c(Sm)} ˚ D (A) TC (K)

8.550 12.450 788 4.56 1.61 0.289 0.501 0.156 0.096 0.343 670 470

8.548 12.456 788 4.73 1.58 0.290 0.502 0.157 0.096 0.343 675 528

8.545 12.463 788 4.05 1.40 0.291 0.501 0.156 0.096 0.343 650 580

8.544 12.467 788 3.58 1.76 0.291 0.502 0.156 0.095 0.343 625 629

Sm2 Fe17−x Cox C3

˚ a (A) ˚ c (A) ˚ 3 V (A) RB ␹2 X {18f} X {18h} Z {18h} Z {6c} Z {6c(Sm)} ˚ D (A) TC (K)

8.724 12.545 826.9 4.11 2.59 0.283 0.506 0.153 0.095 0.344 350 696

8.720 12.545 826.1 4.83 2.25 0.283 0.506 0.153 0.094 0.344 340 707

8.714 12.526 823.6 4.34 3.03 0.284 0.505 0.153 0.094 0.344 290 717

8.713 12.517 823.0 3.86 2.56 0.283 0.506 0.153 0.092 0.346 300 732

˚ ±0.001, ±5 A, ˚ and ±1 K, for respectively unit cell parameters, atomic position, D, and TC . Uncertainties are ±0.001 A,

¯ The carbonated compounds Sm2 Fe17−x Cox C3 are still R3m. Besides Sm2 O3 and SmO–N lines, a small amount of ␣Fe–Co (around 2 wt%) is detected. This weak contribution results from a small decomposition during carbonation of the host lattice built before carbonation. Consequently, we can consider that the Co composition of the 2/17 phase is not affected. The Rietveld analysis (Fig. 1 and Table 1) shows that a decreases with x, like the non-carbonated alloys, but more markedly (a/a  −1.00 × 10−3 per Co atom). On the contrary c decreases (c/c  −1.4 × 10−3 per Co atom). It results that the unit cell volume is poorly affected by the substitution. The Wyckoff atomic positions slightly modified compared to those before carbonation, induce a reduction of the 6c–6c, 6c–18f, and 18f–18f distances after carbonation (Table 2). Nevertheless, under carbonation, the large increase of the distances 6c(Sm)–18f corresponding to the basis of the C 9e octahedral site, contributes to the expansion of the unit cell volume. Under carbonation, the relative volume increase V/V, reaches 4.6% for x = 0.5 and 1, 4.4% for x = 1.5 and 2 (Table 1). It must be emphasized that carbonation has reduced drastically the autocoherent diffraction domain size from around ˚ Concomitantly a small precipitation of 650 down to 350 A. iron (<5%) is observed with autocoherent diffraction domain ˚ (Table 1). This precipitation is in agreesize around 100 A ment with Altounian et al. work [6]. At this level we can notice that this small amount of secondary phase does not affect the magnetic properties. However, this carbonation method opens the route to the elaboration of nanocomposite spring magnets, after monitoring up to 30% the ratio hard/soft phases.

3.2. Curie temperature The Curie temperature of the Sm2 Fe17−x Cox alloys increases from 470 K for x = 0.5 to 629 K for x = 2. As explained above, the unit cell volume remains constant with cobalt content. Having in mind that the Curie temperature is the result of two competitive Table 2 ¯ (a) non-carbonated Sm2 Fe17−x Cox and (b) Interatomic distances for the R3m carbonated Sm2 Fe17−x Cox C3 for x = 0.5, and 2 ˚ x = 0.5 Distance (A)

˚ x=2 Distance (A)

(a)

(b)

(a)

(b)

6c

2.38 2.62 2.75 2.65

2.37 2.68 2.74 2.35

2.37 2.62 2.75 2.64

2.35 2.68 2.73 2.66

6c(Fe) 9d 18f 18h (Fe/Co)

9d

2.44 2.45 2.62

2.46 2.47 2.68

2.44 2.46 2.62

2.45 2.47 2.68

18f 18h 6c

18f

2.48 2.55 2.75 2.43 –

2.47 2.59 2.74 2.46 1.89

2.49 2.55 2.75 2.44 –

2.46 2.59 2.73 2.45 1.89

18f 18h 6c 9d 9e

18h

2.52 2.45 2.65 2.55 –

2.56 2.47 2.65 2.59 1.92

2.53 2.66 2.48 2.57 –

2.56 2.71 2.50 2.57 1.92

18h 9d 6c 18f 9e

Site

˚ The uncertainty is ±0.01 A.

Notation

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L. Bessais et al. / Journal of Alloys and Compounds 455 (2008) 35–41

Fig. 2. The 293 K M¨ossbauer spectra of SmFe17−x Cox . Fig. 3. The 293 K M¨ossbauer spectra of SmFe17−x Cox C3 .

effects magnetovolumic and electronic, it can be assessed that with Co content the TC increase can only be explained by the electronic effect. The Fe weak ferromagnetic character is progressively modified into strong ferromagnetism upon the filling of the d band with the additional Co electron. Upon carbonation, the TC values are enhanced, compared to those of Sm2 Fe17−x Cox alloys (Table 1) from 75 to 34 ◦ C per carbon atom, for respectively x = 0.5 and 2. The resulting unit cell expansion induces a strengthening of the Fe–Fe interactions due to the increase of the Fe–Fe distances. With Co increasing, the electronic effect rules solely the TC evolution. 3.3. Hyperfine parameters The experimental spectra of Sm2 Fe17−x Cox and their carbides shown on Figs. 2 and 3 are complex spectra. Their line-widths are in the range of 0.25–0.27 mm/s. As a result of these narrow line-widths we observe more highly resolved

spectra, such conditions permit an accurate spectral analysis. The area ratio of the absorption lines was assumed to be 3:2:1 according to randomly oriented particles, as corroborated by X-ray analysis. The abundance of the various magnetic sub-sites relative to the different crystallographic sites is calculated with the binomial law. The assignment of the hyperfine parameter set of the individual sextet to each individual crystallographic site results from the correlation between isomer shifts and Wigner–Seitz cell volumes [11,12]. The larger the Wigner–Seitz cell volume of a site, the larger its isomer shift. The Wigner–Seitz cell volumes have been calculated by means of Dirichlet domains and coordination polyhedra for each crystallographic family. We have used the procedure of radical planes which results in a space partition without gaps between the polyhedra [13]. The radius values [14] of 1.81, ˚ have been taken for respectively Sm, Fe, 1.26, 1.25, and 0.77 A

Table 3 ˚ 3 ) in Sn12 Fe16 Co and in Sm2 Fe16 CoC3 as an example Near-neighbor environments and WSC volumes (A Site

Fe {6c}

Fe {9d}

Fe/Co {18h}

Fe {18f}

C {9e}

WSC volume Sm2 Fe16 Co

WSC volume Sm2 Fe16 CoC3

Fe{6c} Fe{9d} Fe/Co{18h} Fe{18f} C{9e}

1 2 1 2 2

3 0 2 2 0

3 4 2 4 2

6 4 4 2 2

0 0 1 1 0

12.39 11.23 11.96 11.74 –

12.69 11.69 12.42 11.81 2.87

˚ 3. The uncertainty is ±0.01 A

L. Bessais et al. / Journal of Alloys and Compounds 455 (2008) 35–41

Fig. 4. The compositional dependence of the hyperfine fields and the isomer shifts, from top to bottom, for Sm2 Fe17−x Cox .

Co, and C. The obtained WSC sequence is 6c > 18h > 18f > 9d (Table 3). All along the fitting process, the calculated abundances were maintained as fixed parameters. They were normalized after neglecting those lower than 1.4%. All other parameters viz. δ, HHF , and 2ε were free. In the second step of the refinement, the deduced averaged isomer shift values have been assigned to each site of the 6c, 18f, 18h, 9d families according to the WSC volume relationship. Finally, in the last step of the fit all hyperfine parameters were free. Figs. 4 and 5(a) show the compositional dependence of the isomer shift, at room temperature, for Sm2 Fe17−x Cox and their carbides. In the Sm2 Fe17−x Cox series, the isomer shift of the 6c, 18f, and 9d atom shows a noticeable increase with Co substitution, while the 18h-atom isomer shift remains quasi-constant. This trend can be understood in terms of the preferential Co atom occupation. The 6c, 9d, and 18f have respectively 3, 4 and 4 adjacent 18h neighbors and the 18h site atoms have only two. The isomer shift behavior of the 6c, 9d, and 18f site Fe atom is attributed to the additional 3d electrons brought by 18h Co neighbors. They enhance the shielding of the 4s electrons, reduce the s charge density at the nucleus, and result in an increasing isomer shift. In contrast, if 18h,-atom surrounding is poorly affected by the Co substitution, the s charge density is then not modified. No change is observed for the 18h isomer shift. The difference for the 18h-site behavior then corroborates the preferential occupa-

39

Fig. 5. The compositional dependence of the hyperfine fields and the isomer shifts, from top to bottom, for Sm2 Fe17−x Cox C3 .

tion of Co atoms on this site. Meanwhile, the dependence with composition of the average isomer shift can be considered as linear within a good approximation. Upon carbonation, there is an increase in the isomer shift for all Fe sites in all compounds. The increase is more pronounced in the case of 9d, 18f, and 18h sites. The increase in the isomer shift is due to the increase in the unit-cell volume upon carbonation. The increase in the unit-cell volume gives rise to an increase in the Wigner–Seitz cell volume and hence a reduction in the s-electron density, which in turn increases the isomer shift. The changes in the individual isomer shifts upon carbonation of Sm2 Fe17−x Cox may be understood both on the basis of the unit cell volume expansion and on the presence of a carbon nearneighbor. Although neither the 6c nor the 9d site has carbon near-neighbors in the carbide, they do exhibit increases in both their Wigner–Seitz cell volumes and their isomer shifts. Fe atoms at the 6c site have no carbon near neighbors, which may be the reason for the smaller increase in the isomer shift, on carbonation at that site. Though the 9d site also does not possess any carbon near neighbors, the increase in the Wigner–Seitz cell volume is larger and this accounts for the increase in the isomer shift at this site. The Fe atoms at 18f and 18h sites have one carbon near neighbor each. But the larger increase observed in the latter site is as a result of the combined effect of carbon near neighbors and the Wigner–Seitz cell expansion. The increase in the isomer shift at the 18f site is due to the proximity of carbon.

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L. Bessais et al. / Journal of Alloys and Compounds 455 (2008) 35–41

All four isomer shifts increase upon carbonation, the 6c and 9d because of the lattice and Wigner–Seitz cell expansion, the 18f because of the presence of the carbon near neighbor, and the 18ft because of both the Wigner–Seitz cell expansion and the presence of a carbon near neighbor. The increases in isomer shift, Delta(delta), and the Wigner–Seitz cell volume, Vws , may be compared as the ratio, (δ)/(ln Vws ). The 6c (2.22 mm/s) and 9d (2.37 mm/s) sites show similar values of this ratio. Although both the 18f and 18h sites have one carbon near-neighbor, only the 18h site experiences a substantial increase of its Wigner–Seitz cell volume. Hence, the 0.114 mm/s isomer shift increase for the 18f site is entirely due to the presence of carbon near-neighbor. In contrast, the larger 0.121 mm/s isomer shift increase for the 18h site is due to both volume expansion and the presence of a carbon near-neighbor. The value of (δ)/(ln Vws ) for the 18f site is very large (19.18 mm/s) because of the very small increase in ˚ 3 ) for this site. Wigner–Seitz cell volume (0.07 A Upon carbonation, the weighted average isomer shifts increase value is equal to 0.108 mm/s. This value leads to a ratio (δ)/(ln V), where V is the unit cell volume, of 2.25 mm/s, this value is larger than the value for R2 Fe17 Nx (1.70 mm/s) [15]. The hyperfine fields at various Fe sites follow the order H6c > H9d > H18f > H18h. The number of Fe near neighbors for 6c, 9d, 18f, and 18h are 13, 10, 10, and 9, respectively. There is no doubt that 6c dumbbell site should have the largest hyperfine field due to its highest iron near neighbors and the fewest rareearth neighbors. Similarly, the 18f site has a larger hyperfine field than the 18h site. The 9d site can be distinguished by its abundance from the 18f and 18h sites. It can be seen from Figs. 4 and 5(b) that there is an increase in hyperfine fields at all Fe sites on carbonation. In particular, the increase is pronounced in the case of 9d and 18h sites. The Fe-C distances are on the order of dFe–C (18h) < dFe–C (18f) < dFe–C (9d) < dFe–C (6c) Table 2. Two effects are occurring in these compounds on carbonation: the magnetovolume effect and the chemical one. The magnetovolume effect will cause an increase in the magnetic moment as a result of lattice expansion and a consequent 3d band narrowing. The increase in the hyperfine field implies an increase in the Fe magnetic moment. This increase is due to the magnetovolume effect present in Sm2 Fe17 compounds. Iron-rich compounds are classified as weak ferromagnets as, both the spin-up and spindown subbands are unsaturated. It comes that the density of states at the Fermi level is large and the volume dependence on the magnetic moment is also high [16]. Carbonation of these compounds is followed by lattice expansion. Consequently of this, most of the Fe–Fe distances increase and hence the 3d band gets narrowed the width of the 3d band is inversely proportional to the fifth power of the lattice parameter [17]. The chemical effect is due to the differences in the electronegativities of Fe and C and causes transfer of charges from the s and the p bands to the 3d bands of nearest Fe. This gives rise to a reduction in the magnetic moment. However; the later effect is significant only at 18f and 18h sites, where the Fe–C distances are the least. But the magnetovolume effect is predom-

inant at all sites and hence there is an increase in the magnetic moment at all sites. The increase in the hyperfine field is the smallest at the 6c site. This is because the Fe–C distance is the largest at this site and therefore, the effect of C is correspondingly smaller. The expansion of the crystallographic lattice is mainly centered on the 9d and 18h iron sites as indicated by the increase of their Wigner–Seitz cell volumes upon carbonation. In agreement with the predictions of band calculations [18] the 9d and 18h sites show a larger enhancement of their hyperfine fields and magnetic moments as compared to the 6c and 18f sites because of improved ferromagnetic exchange between these sites and their near neighbors. Indeed, all the distances between 9d and 18h sites and their iron near neighbors increase upon carbonation (Table 2). This increase induces a reduction of the iron 3d < iron 3d overlap and hence improves ferromagnetic coupling as observed from the Curie temperature enhancement. In contrast, the distances between the 6c and 18h sites and their near neighbors either increase or decrease upon carbonation. These changes yield a smaller improvement in the ferromagnetic exchange coupling for these two sites. 4. Conclusion The nanostructured carbides Sm2 Fe17−x Cox C3 (x ≤ 2) have been prepared via a two-steps method. The nanostructured host-lattice was built by high-energy milling and subsequent annealing at 1400 K. Carbonation was performed via a solid–solid reaction with hydrocarbon at 673 K. The Rietveld analysis shows that carbonation induces a rel¯ unit cell, higher than 4%. ative volume increase of the R3m The auto-coherent diffraction domain size is drastically reduced ˚ from 650 to 350 A. The Curie temperatures are enhanced by the Co substitution for Sm2 Fe17−x Cox from 470 to 629 K, and after carbonation from 696 to 732 K. They are only ruled by electronic effects versus Co content due to the relative constancy of the unit cell parameters with Co substitution, while for a given cobalt content, the magneto-volume effect is responsible for the Curie temperature increase under carbonation. The homogeneous carbon distribution induces small linewidth of the M¨ossbauer spectra. It has been possible to carry a thorough analysis with a model based on the Wigner–Seitz cell volume correlation with isomer shift: the greater the Wigner–Seitz cell volume, the greater the isomer shift. The isomer shift behavior of the 6c, 9d and 18f site Fe atom is attributed to the additional 3d electrons brought by 18h Co neighbors. On the contrary no change is observed for the 18h-isomer shift. The difference for the 18h-site behavior demonstrates the preferential occupation of Co atoms on this site in the alloys. Acknowledgments This work has been supported by grants from the Centre National de la Recherche Scientifique and the Vietnamese Academy of Science and Technology.

L. Bessais et al. / Journal of Alloys and Compounds 455 (2008) 35–41

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