phys. stat. sol. (c) 3, No. 9, 3233–3238 (2006) / DOI 10.1002/pssc.200567104

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Crystallographic and magnetic properties of a new rare-earth

iron nitride Nd2–xSmxFe17Ny (y ~ 3)

M. S. Ben Kraiem*, 1

and A. Cheikhrouhou1, 2

1 Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B.P. 802, 3018 Sfax, Tunisie 2 Laboratoire de Magnétisme Louis Néel, B.P. 166, 38042 Grenoble Cedex 9, France

Received 5 September 2005, revised 8 January 2006, accepted 26 April 2006

Published online 1 August 2006

PACS 61.10.Nz, 61.66.Dk, 75.30.Cr, 75.30.Kz, 75.50.Bb

The effect of nitrogen insertion on the structural and magnetic properties of Nd2–xSmxFe17 metallic alloys

has been investigated. X-ray powder diffraction characterizations show that all the nitrides are single

phase and crystallize in the Th2Zn17-type structure. The lattice parameters of the Nd2–xSmxFe17Ny com-

pounds decrease with samarium content. Magnetization studies show that all the samples are ferromag-

netic at room temperature. Nitrogen insertion leads to an increase of the Curie temperature Tc. Tc is found

to increase from 370 K for NdSmFe17 to 760 K for NdSmFe17N2.8. It leads also to an increase of the satu-

ration magnetization Ms. s s

s

M (y) M (0)

M (0)

-

decreases with increasing samarium content from 58% for x = 0

to 45% for x = 2. X-ray diffraction characterizations on magnetically aligned powder samples of

Nd2–xSmxFe17Ny reveal a change in the easy magnetization direction from planar for x = 0 to c axis for

x = 2.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Recently, several research efforts have been made to improve the hard magnetic properties of the binary R2Fe17 (R=rare-earth) intermetallic alloys by interstitial and/or substitutional modifications of the 2:17 structure. These efforts were stimulated by the important effect of the nitrogen or carbon insertion on the physical properties of these intermetallic alloys. In fact this insertion leads to an increase in the Curie temperature Tc and in the saturation magnetization. Moreover this insertion changed the anisotropy from basal planar to uniaxial in Sm2Fe17Nx [1, 2]. A volume expansion, which can be as much as 6% com-pared to the binary compounds, is also observed in the interstitial compounds. It is noteworthy that Sm2Fe17Nx possesses better hard magnetic properties than Nd2Fe14B and is a promising candidate for per-manent magnet applications [3]. Nitrogen insertion of the R2Fe17 compounds involves a diffusion process which is initiated by exposing the parent material to a nitrogen-containing atmosphere at high temperature. Generally speaking, nitride samples prepared using N2 gas have compositions with x < 3 [4]. In this paper, we report our investigations on the structure, magnetic and magneto-crystalline anisotropy of the Nd2–xSmxFe17Ny compounds.

2 Experimental techniques

Nd2–xSmxFe17 powder samples were prepared from starting materials of at least 99.9% purity by induc-tion melting. After melting, the powder samples were sealed in silica tubes under argon atmosphere and annealed at 1273 K for 10 days and then quenched in water. Phase purity was checked by X-ray diffrac-

* Corresponding author: e-mail: bensalah@yahoo.fr

3234 M. S. Ben Kraiem and A. Cheikhrouhou: Crystallographic and magnetic properties Nd2–xSmxFe17Ny

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

tion (XRD) using a SIEMENS diffractometer with iron radiation (λ = 1.936 Å). The Curie temperatures were obtained from thermomagnetic analysis (TMA) using a home-made Faraday-type balance. The saturation magnetization Ms has been deduced from magnetization measurements versus magnetic ap-plied field up to 7 T at several temperatures performed using a vibrating sample magnetometer. Nitrogen insertion in our samples has been carried out under a static nitrogen gas at a pressure of 13 MPa at 733 K for 24 hours. The nitrogen content was checked by the weight increase in our samples. The nitrogen content is found to be 2.9±0.1 atoms.

3 Results and discussion

X-ray powder diffraction study at room temperature shows that all our synthesized alloys and its nitrides are single phase with very small amount of α-Fe as secondary phase in the nitrides. The Nd2–xSmxFe17 alloys and its nitrides crystallize in the rhombohedral Th2Zn17-type structure. We plot in Figs. 1 and 2 respectively the evolution of the unit cell parameters and the unit cell volume versus Sm content in both alloys and its nitrides.

With increasing Sm content, the parameter a decreases in both alloys and its nitrides; while parameter c decreases in the alloys and increases in the nitrides. The decrease of a and c parameters in the alloys may be explained by the difference in the ionic radius of Nd and Sm (r(Nd3+) = 1.08 Å and r(Sm3+) = 1.04 Å). The increase of the c parameter in the nitrides can be explained by the effect of nitrogen insertion in the unit cell. The unit cell volume in both alloys and nitrides decreases linearly with increasing Sm content, it is found to decrease from 792 Å3 for x = 0 to 781 Å3 for x = 2 in the alloys and from 841 Å3 for x = 0 to 835 Å3 for x = 2 in the nitrides. Nitrogen insertion leads to an expansion of the unit cell volume. V(y) V(0)

V(0)

-

increases with increasing Sm content from 6.1% for x = 0 to 6.8% for x = 2. We report in

Fig. 3-a the temperature dependence of magnetization at 500 Oe of our alloys and it nitrides. All our samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The nitrogen insertion leads to an important increase of the Curie temperature Tc. We plot in Fig. 3-b the evolution of the Curie temperature Tc versus Sm content in the alloys and its nitrides and we report in Table 1 the crystallographic data of our synthesized alloys and its nitrides. The Tc increase observed upon nitrogen insertion is a common feature [5] in the R2Fe17 compounds, a feature which has been shown to be related at least in part to the unit cell expansion. This expansion corresponds to an overall increase in the iron-iron inter-atomic distances and favours the ferromagnetic

8,5

8,6

8,7

8,8

12,4

12,5

12,6

12,7

0 0,5 1 1,5 2

a - Nd2-x

SmxFe

17N

~3

a - Nd2-x

SmxFe

17

c - Nd2-x

SmxFe

17N

~3

c - Nd2-x

SmxFe

17

a (Å

) c (Å)

x (Sm. at)

780

800

820

840

860

0 0,5 1 1,5 2

Nd2-xSm

xFe

17N~3

Nd2-xSm

xFe

17V (

Å3)

x (Sm. at)

Fig. 1 Cell parameter evolution versus Sm content in

both Nd2–xSmxFe17 and Nd2–xSmxFe17Ny.

Fig. 2 Unit cell volume evolution versus Sm con-

tent in both Nd2–xSmxFe17 and Nd2–xSmxFe17Ny.

phys. stat. sol. (c) 3, No. 9 (2006) 3235

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exchange interactions. Furthermore, the observed compositional dependence of the ordering temperature is in agreement with the itinerant electron model for the magnetic exchange [5–7]. This result has been explained as being due to the magneto-volume effect and associated to changes in the 3d band structure. According to the spin fluctuation theory [8], the Curie temperature of R–Fe compound is proportional

to 0

0

M

χwhere M0 is the zero temperature magnetic moment and χ0 is the exchange susceptibility given by,

1

0 2

F F B

1 1 11

2N (E ) N (E ) 2χ

µ

-

È ˘= + -Í ˙≠ ØÎ ˚

where N↑(EF) and N↓(EF) are the densities of states (DOS) at the Fermi level. From band structure cal-culations Jaswal et al. [9] have shown that there is reduction in the DOS at the Fermi level after nitrogen insertion. This may explain the large increase in the Curie temperature.

Table 1 Structural data of Nd2–xSmxFe17Ny compounds.

Samples a (Å) c (Å) V (Å3) y ∆V/V (%) Nd2Fe17 8.572(8) 12.45(7) 792.0(0) Nd2Fe17Ny 8.782(0) 12.59(2) 841.0(0) 2.9 6.10 Nd1.5Sm0.5Fe17 8.566(0) 12.43(6) 790.2(4) Nd1.5Sm0.5Fe17Ny 8.760(8) 12.61(7) 838.6(4) 2.8 6.12 NdSmFe17 8.555(0) 12.42(8) 787.8(1) NdSmFe17Ny 8.749(7) 12.63(0) 837.3(8) 2.8 6.30 Nd0.5Sm1.5Fe17 8.538(0) 12.42(4) 784.4(8) Nd0.5Sm1.5Fe17Ny 8.741(6) 12.64(2) 836.6(2) 2.8 6.60 Sm2Fe17 8.526(0) 12.42(0) 781.9(0) Sm2Fe17Ny 8.730(0) 12.65(1) 835.0(0) 3 6.8

The relationship between the unit cell volume V and the Curie temperature Tc of R–Fe intermetallics has been investigated [10, 11]. A combined model of localized and itinerant d electrons was assumed and it was suggested that only a fraction of 5% or less of the 3d electrons are truly itinerant while the rest can be considered as localized. Since the interstitial nitrogen atom expands the unit cell of the parent com-pounds, the changes in Tc upon interstitial modification are associated with the volume changes. Studies on the Curie temperature of the structurally related 2:17 nitrides and carbides [10, 11] have shown a

300 400 500 600 700 800

Sm2Fe

17

NdSmFe17

Nd2Fe

17

Sm2Fe

17N

3

NdSmFe17

N2.8

Nd2Fe

17N

2.9

Temperature (K)

Magn

etiz

ati

on

(a.

u)

Fig. 3-a Magnetization evolution versus temperature of

Nd2–xSmxFe17 (with x = 0.0, 1.0 and 2.0) compounds

before and after nitrogen insertion.

Fig. 3-b Curie temperature evolution versus

Sm content of Nd2–xSmxFe17 compounds

before and after nitrogen insertion.

300

400

500

600

700

800

0 0.5 1 1.5 2

Nd2-x

SmxFe

17

Nd2-x

SmxFe

17N

~3

Cu

rie

Tem

pera

ture

(K

)

x (at. Sm)

3236 M. S. Ben Kraiem and A. Cheikhrouhou: Crystallographic and magnetic properties Nd2–xSmxFe17Ny

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

linear dependence of Grüneisen coefficient Γ versus Tc, where Γ is given by cy c0 c0c

y 0 0

(T T ) /TdlnT

dlnV (V V ) /VΓ

-

= =

-

where Tcy and Vy are respectively the Curie temperature and the unit-cell volume of the nitrides, while y is the interstitial atom content. Tc0 and V0 are the corresponding values for the parent compounds. Valleanu et al. [12] have studied the dependence of the Grüneisen coefficient Γ on the 2:17 interstitial carbides and nitrides and found that for low carbon concentrations it has a Tc

-2 dependence while for high carbon/nitrogen concentrations (≥ 2), it has a linear dependence on Tc [12, 13]. The variation of Γ with Curie temperature in Nd2–xSmxFe17 compounds is shown in Fig. 4. It is observed that Г exhibits a linear dependence of Tc of the form Γ = 92.41–0.098Tc which is in good agreement with the combined model of localized and itinerant d electrons.

Fig. 4 Dependence of Cln(T )

ln(V)

∂

∂ versus TC

of Nd2–xSmxFe17Ny.

The magnetization curves as a function of magnetic applied field M (H) at 300 K of Nd2–xSmxFe17Ny compounds with different Sm content before and after nitrogen insertion are plotted in Fig. 5-a. The saturated magnetization (Ms) was obtained by fitting the experimental data of M versus 1/H2 using the polynomial law [13]. The values of the saturated magnetization before and after nitrogen insertion are plotted as a function of Sm content in Fig. 5-b. As we can observe, the saturated magnetization in both alloys and nitrides decreases with increasing Sm content. The saturated magnetization is larger in the

nitrides than the alloys. s s

s

( ) (0)

(0)

M y M

M

-

decreases with increasing samarium content. It is found to de-

crease from 58% for x = 0 to 45% for x =2. In R2Fe17 alloys, nitrogen insertion modifies the neighbourhood of the rare-earth sites, the number of nearest atoms is increased: three nitrogen atoms become the nearest neighbours of the rare-earth element (at 2.53 Å), the nearest neighbours of iron atoms at about 3 Å, as in the pure alloy [14]. Furthermore, nitrogen atoms, which were the nearest neighbours before nitrogenation, are aligned along the c axis. Such a change in the rare-earth element environment (distance, number, nature, ...) might have a large influence on the crystal-field properties. Recently Coehoorn and Buschow [15] have shown that, according to Zhong and Ching work [16], the crystalline electric field A20 at the rare-earth site is mainly due to the aspherical charge-density distribu-tion of the valence electrons of the R atoms (6p and 5d shells) [17], and they proposed the contribution from the change density on the neighbour atoms to be of secondary importance in iron-rich rare-earth-transition metal alloys such as Nd2Fe14B. A change in the environment of the rare earth from two irons to three atoms in the plane may have an important effect on the asphericity of the electronic shells of the R

14

15

16

17

18

19

20

21

740 750 760 770 780 790 800 810

Γ =

d l

n T

c

/d ln

V

Tc

(K)

x = 2

x = 1.5

x = 1

x = 0.5

x = 0

phys. stat. sol. (c) 3, No. 9 (2006) 3237

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metal. This explain of band structure corresponds to the change of the magnetization ∆M for Nd2–xSmxFe17Ny compounds.

The X-ray-diffraction studies on the aligned powder samples provide information concerning the mag-netic anisotropy. The room-temperature X-ray-diffraction patterns of magnetically aligned powder sam-ples of Nd2–xSmxFe17Ny with 0 ≤ x ≤ 2 are shown in Fig. 6. The easy magnetization direction of Nd2–xSmxFe17Ny compounds at room temperature changes from the basal plane to the c-axis with increas-ing Sm content. The presence of (h00) and (hk0) reflections together with the absence of all other reflec-tions for x = 0 and x = 0.5 indicate that the easy magnetization direction EMD lies in the basal plane (planar anisotropy). On the other hand, the presence of (00l) reflection together with the absence of (h00) and (hk0) reflections reveal that the easy magnetization direction is parallel to the c-axis (uniaxial anisot-ropy) for x = 2. However, the presence of (h00), (hk0) and (00l) or any other (hkl) reflections indicates that the EMD lies between “a” and “c” axis (cone anisotropy) for x = 1.0 and x = 1.5. The easy-magnetization direction in the R-Fe compounds reflects the competition between the Fe-sublattice anisot-ropy (favoring an easy magnetization direction perpendicular to the c axis) and the R-sublattice anisot-ropy (favoring an easy c-axis magnetization). In first approximation, the total anisotropy constant is the sum of the Fe–sublattice anisotropy constant K1(Fe) and the R–sublattice anisotropy constant K1(R) [18]: K1 = K1(Fe)+K1(R). It has been shown that K1(Fe) is negative for the R2Fe17 compounds. The R-sublattice anisotropy is de-termined by the product of the second-order crystal-field parameter A20 and the second-order Stevens coefficient αJ and can be expressed as:

2 2

1 J z 20

3K (R) ( ) r 3J J(J 1) A

2α= - - +

where <r2> is the mean of the second power of the 4f radius and the quantities in brackets are the expec-tation values. Previous investigations have shown that the second-order crystal-field parameter A20 is negative in the R2Fe17 compounds and depends on the crystal field associated with the surrounding of a given R element. The interstitial sites for nitrogen atoms are nearest neighbors of the rare earth sites and, because of the strong electronegativity of the N atoms, the leading term of the crystal-field coefficients A20 is changed strongly by negative values, which indicates a much stronger uniaxial anisotropy contri-bution to the total magnetic anisotropy from those rare earths elements of which the second order Ste-vens coefficient αJ is positive and the anisotropy constant K1 is positive [19].

0

50

100

150

200

0 1 2 3 4 5 6 7

Sm2Fe

17

Nd0.5

Sm1.5

Fe17

Nd2Fe

17N

2.9

Sm2Fe

17N

3

NdSmFe17

N2.8

Nd2Fe

17

Nd1.5

Sm0.5

Fe17

NdSmFe17

Nd1.5

Sm0.5

Fe17

N2.8

Nd0.5

Sm1.5

Fe17

N2.8

Ma

gn

eti

zati

on

(em

u/g

)

H (T)

(T = 300 K)

Fig. 5-a Field dependence of the magnetization at room

temperature of Nd2–xSmxFe17 compounds before and

after nitrogen insertion.

100

120

140

160

180

0 0.5 1 1.5 2

Nd2-x

SmxFe

17

Nd2-x

SmxFe

17N

~3

Ma

gn

eti

zati

on

(em

u/g

)

x (at. Sm)

(T = 300 K)

Fig. 5-b Magnetization evolution versus Sm

content in Nd2–xSmxFe17 at 300 K before and

after nitrogen insertion.

3238 M. S. Ben Kraiem and A. Cheikhrouhou: Crystallographic and magnetic properties Nd2–xSmxFe17Ny

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

Fig. 6 X-ray diffraction patterns of magnetically aligned powder of Nd2–xSmxFe17Ny.

4 Conclusion

The structural and magnetic properties of Nd2–xSmxFe17Ny have been studied. All our samples crystallize in the Th2Zn17-type structure. Nitrogen insertion in Nd2–xSmxFe17 leads to an increase in the unit-cell volume. The Curie temperature of Nd2–xSmxFe17Ny compounds increases linearly with increasing Sm content. The saturated magnetization Ms increases after nitrogen insertion. Ms in both alloys and nitrides decreases monotonically with increasing Sm content. Grüneisen coefficient values of these compounds show a linear dependence on the Curie temperature. Finally, the easy magnetization direction of Nd2–

xSmxFe17Ny compounds at room temperature changes from the basal plane to the c-axis with increasing Sm content.

Acknowledgements This work has been supported by the Tunisian Ministry of Scientific Research, Technology

and Development of Competences.

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4 5 5 0 5 5 6 0 6 5 7 0 7 5 8 0

Inte

nsi

ty (

u.a

)

2 θ ( d e g )

x = 0

x = 0 . 5

x = 1

x = 1 . 5

x = 2

( 0 0 6 )

( 2 2 0 )( 3 0 0 )

( 1 2 2 ) ( 0 2 4 )

( 0 3 3 )

( 2 2 0 )( 3 0 0 )

( 0 0 6 )