phys. stat. sol. (c) 3, No. 9, 3211–3215 (2006) / DOI 10.1002/pssc.200567108

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

The effect of nitrogen insertion on the magnetic and structural

properties of Sm2–xGdxFe17 alloys (0 ≤ x ≤ 2)

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 Sm2–xGdxFe17Ny compounds

(x = 0-2) has been investigated. X-ray diffraction patterns at room temperature show that all our synthe-

sized samples are single phase and crystallize in the rhombohedral Th2Zn17-type structure with very small

amount of impurity. Nitrogen insertion leads to a monotonic increase in the unit cell volume, it leads also

to an increase in the Curie temperature Tc from 425 K for SmGdFe17 to 760 K for SmGdFe17N3. More-

over, the nitrogen insertion increases the saturated magnetization Ms. Ms increases from 17.9 µB/mole for

SmGdFe17 to 32.7 µB/mole for SmGdFe17N3. X-ray diffraction measurements on magnetically aligned

powder samples of Sm2–xGdxFe17Ny reveal a change in the easy magnetization direction from planar be-

fore nitrogen insertion to the c-axis after nitrogen insertion.

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

1 Introduction

One of the methods for fabricating new magnetic materials is to introduce non-metallic atoms such as H, B, C or N with small atomic-radius into host compounds. In such modified compounds, the 2:17-type nitride Sm2Fe17N3 was discovered by Coey and Sun [1], in which significant improvement on the intrin-sic magnetic properties was achieved upon nitrogen insertion. The Curie temperature Tc increased dra-matically from 398K to 773 K upon nitrogen uptake. Since the discovery of Sm2Fe17N3, worldwide ef-forts have been devoted not only to clarify the origin of improvement of basic magnetism but also to develop it into high performance permanent magnets [2, 3]. The reason was that the quality of the nitride samples was not so good. Later, much attention has been paid to the study of gas-phase interstitial modi-fication process itself for obtaining high-quality nitrides. Most of gas-phase reactions have been con-ducted at 673 K to 773 K on fine powder under some N2-gas atmosphere up to 15 MPa [1, 4, 5], nitrogen containing gas NH3 or mixed gas NH3+H2 [6] or N2+H2 [7]. For producing both isotropic and anisotropic bonded magnets, various techniques have been applied to Sm2Fe17N3. Unfortunately, high-performance Sm2Fe17N3 powder could not be obtained at the earlier stage. The problem was either how to improve the thermal and mechanical stability or how to develop a stable-processing route. The coercive mechanism was examined in details by Kou et al. [8] using isotropic Sm2Fe17N3 powder prepared by mechanically alloying of Sm and Fe powder. In this paper, we report our investigations on the structure, magnetic and magneto-crystalline anisotropy of the Sm2–xGdxFe17Ny compounds.

2 Experimental techniques

Sm2–xGdxFe17Ny powder samples were prepared from starting materials up to 99.9% purity by induction melting. After melting, the polycrystalline specimens were sealed in silica tubes under argon atmosphere,

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

3212 M. S. Ben Kraiem and A. Cheikhrouhou: Magnetic and structural properties of Sm2–x

GdxFe

17 alloys

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

annealed at 1273 K for 10 days and then quenched to room temperature in water. Phase purity was checked by X-ray diffraction (XRD) using a SIEMENS diffractometer with iron radiation (λ = 1.936 Å). The saturated magnetization (Ms) has been determined using an extracting-sample magnetometer in a magnetic applied field up to 8 T. The Curie temperature values were obtained from thermomagnetic analysis (TMA) using a home-made Faraday-type balance. Nitrogen insertion has been carried out under a static nitrogen gas with 15 MPa pressure at 723 K for 22 hours. The nitrogen content was checked by the weight increase in our samples. It is found to be 2.9±0.1 atoms.

3 Results and discussion

X-ray powder diffraction studies at room temperature show that all our synthesized alloys and its nitrides are single phase. We plot in Fig. 1 the X-ray diffraction patterns at room temperature for Sm2–xGdxF17 alloys and its nitrides. The Sm2–xGdxF17 alloys and its nitrides crystallize in the rhombohedral Th2Zn17-type structure. We plot in Fig. 2 the unit cell volume evolution as a function of Gd content for Sm2–xGdxFe17 alloys and its nitrides. The nitrogen insertion leads to an expansion of the unit cell volume. Table 1 summarizes the a and c lattice parameters and the unit cell volume V.

The substitution of samarium by gadolinium atoms occupying the 6c sites induces a small decrease of the unit cell volume V up to x = 1.5 then decreases until x = 2. We suggest that such behaviour is proba-bly due to the anomalous character of the R2Fe17 compounds as regards: thermal expansion, forced vol-ume magnetostriction,

Table 1 Structural data of Sm2–xGdxFe17Ny compounds.

samples a (Å) c (Å) V (Å3) TC (K) MS (µB/mole) y Sm2Fe17 8.526(0) 12.42(0) 785.8(0) 400 25.3(1) Sm2Fe17Ny 8.730(0) 12.65(1) 835.0(0) 775 33.3(8) 3 Sm1.5Gd0.5Fe17 8.537(2) 12.41(3) 783.5(3) 423 18.9(2) Sm1.5Gd0.5Fe17Ny 8.600(3) 12.62(1) 807.6(5) 777 29.8(8) 3 SmGdFe17 8.515(7) 12.40(5) 779.1(0) 428 18.0(0) SmGdFe17Ny 8.571(2) 12.60(1) 800.0(0) 779 29.1(8) 2.9 Sm0.5Gd1.5Fe17 8.513(0) 12.36(5) 776.1(8) 448 17.0(7) Sm0.5Gd1.5Fe17Ny 8.561(2) 12.58(4) 790.0(0) 782 28.1(4) 2.9 Gd2Fe17 8.525(7) 12.40(9) 781.0(0) 460 16.3(4) Gd2Fe17Ny 8.657(3) 12.76(0) 828.0(0) 785 26.4(0) 3

2 0 3 0 4 0 5 0 6 0 7 0 8 02 * (T h e t a )

S m2F e

1 7

S m2F e

1 7N

3

S m1 ,5G d

0 ,5F e

1 7

S m1 ,5G d

0 ,5F e

1 7N

2 ,9

S m0 ,5G d

1 ,5F e

1 7N

2 ,9

S m0 ,5G d

1 ,5F e

1 7

G d2

F e1 7

G d2F e

1 7N

3

770

780

790

800

810

820

830

840

0 0 .5 1 1.5 2

Sm2-x

G dxFe

17N

~3

Sm2-x

G dxFe

17

V (

Å3)

x (G d. a t )

Fig. 1 X-ray diffraction patterns for Nd2–xGdxFe17 alloys

and its nitrides with x=0, 0.5, 1.5 and 2.0.

Fig. 2 Unit cell volume evolution versus Gd

content in the Sm2–xGdxFe17 and Sm2–xGdxFe17Ny.

phys. stat. sol. (c) 3, No. 9 (2006) 3213

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

Thermomagnetic analysis of Sm2–xGdxFe17Ny compounds shows a typical ferromagnetic behaviour at room temperature. We plot in Fig. 3-a the temperature dependence of the magnetization for Sm2–xGdxFe17Ny (with y = 0.5; 1) and its nitrides. Nitrogen insertion leads to an increase of the Curie temperature Tc.

The nitrogen insertion leads to a strong increase in the Curie temperature Tc. The Curie temperature of R2Fe17 compounds is determined by the combination of three types of exchange interactions: R–R, Fe–Fe and R–Fe [9, 10]. The enhancement of the Curie temperature in the interstitial compounds has been pointed out to be mainly due to the volume effect [11, 12]. Coey et al. [10] report about Curie tempera-ture a large enhancement of the Fe–Fe interactions and a slight decrease of the R–Fe interactions in the interstitial compounds compared to their R2Fe17 counterparts. We plot in Fig. 3-b the evolution of the Curie temperature Tc versus Sm content in the alloys and its nitrides. The origin of the enhancement of the magnetic properties in the Sm2–xGdxFe17Nx (0 ≤ x ≤ 3) compounds with nitrogen uptake can be mainly attributed to the following two reasons: first, the magnetovolume effects by the expansion of the Th2Zn17 type lattice [13–16], and second, the formation of chemical bonds between Fe and N atoms as the cases of the Fe–N bonds in Sm2–xGdxFe17Ny [17, 18]. The analysis of the reasons of the enhancement of magnetic properties should be an important theme for understanding the magnetic properties of the rare earth-transition metal compounds including the third elements, such as N and C. The measured lat-tice expansion with the nitrogen uptake in the Sm2Fe17N3 is about 6–7 vol% compared with the Sm2Fe17 starting alloy [16, 19], and the introduced nitrogen atoms occupy the 9e and/or 18g sites in the structure. The four iron sites exist in the Th2Zn17 type structure of Sm2Fe17Nx, and they can be categorized into the following two groups: the first nearest neighbours for the N(9e) sites are Fe (18h) and Fe (18f) sites in which the direct Fe–N hybridizations can be formed, and the second nearest neighbours for the N(9e) sites are Fe (6c) and Fe (9d) sites; they have the direct chemical bonds only between the surrounding Fe (18f) and Fe (18 h) sites [16, 20, 21]. It was revealed by neutron diffraction studies that the lattice expan-sion in the Th2Zn17 type structure with the nitrogen uptake results in the expansion of Fe–Fe interatomic distances at the Fe sites, except the Fe (6c) site, at which the distances between the nearest neighbour Fe (18f) atoms show almost constant values before and after the nitrogen insertion into the structure [22]. The origin of the enhancement of the magnetic properties, therefore, can be explained either by the mag-netovolume effects and/or by the effects of the formation of the chemical bonds. The effects of the chemical bond formation typically appear in the differences in the magnetic properties of Sm2Fe17Xy when the third element (X) was either nitrogen (N) or carbon (C). The carbon atoms in the Sm2Fe17Cy compounds also occupy the 9e sites, and the carbon uptake results in the lattice expansion of 6 vol%,

320 400 480 560 640 720 800 880

Sm1.5

Gd0.5

Fe17

and Sm1.5

Gd0.5

Fe17

N3

SmGdFe17

and SmGdFe17

N2.9

Mag

neti

za

tio

n (

a.

u)

Temperature (K)

3 102

4 102

5 102

6 102

7 102

8 102

0 0.5 1 1.5 2

Sm2-x

GdxFe

17

Sm2-x

GdxFe

17N

~3

Cu

rie t

em

peratu

re (

K)

x (Gd. at)

Fig. 3-a Magnetization evolution versus tempera-

ture of Sm2–xGdxFe17 (with x = 0.5 and 1.0) com-

pounds before and after nitrogen insertion.

Fig. 3-b Curie temperature evolution versus Sm

content of Sm2–xGdxFe17 compounds before and

after nitrogen insertion.

3214 M. S. Ben Kraiem and A. Cheikhrouhou: Magnetic and structural properties of Sm2–x

GdxFe

17 alloys

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

though the Fe–C bonds differ in nature from the Fe–N bonds in the structure [23–25]; i.e. the enhance-

ment of the magnetic properties in the Sm2Fe17Nx compounds is clearly larger than the cases of the

Sm2Fe17Cy compounds [26]. Since the lattice expansion and the number of hybridizations are both the

intermediate values between these in the Sm2Fe17 starting alloy and in the Sm2Fe17N3, the study of the

magnetic properties of the Sm2Fe17Nx of intermediate nitrogen concentrations have also generated inter-

est for studying the origin of the enhancement of magnetic properties [27, 28].

The magnetization curves as a function of applied magnetic field M(H) of Sm2–xGdxFe17Ny compounds

with different Gd contents are plotted in Fig. 4. The saturated magnetization (Ms) was obtained by fitting

the experimental data of M versus 1/H2 using the polynomial law. The saturated magnetization at 300 K

for Sm2–xGdxFe17 and its nitrides are listed in Table 1. The values of the saturated magnetization before

and after nitrogen insertion are plotted as a function of Gd content in Fig. 5. It can be seen that, as well as

for the alloys, the saturated magnetization decreases with increasing Gd content, but the values for the

nitrides are higher than those obtained for the alloys (Sm2–xGdxFe17). For example, at 300 K, Ms = 18

µB/mole for SmGdFe17 and Ms = 29.18 µB/mole for SmGdFe17N2.9.

The X-ray diffraction patterns at room temperature of magnetically aligned powder samples of

Sm2–xGdxFe17Ny with 0 ≤ x ≤ 2 are shown in Fig. 6. The X-ray diffraction studies on the aligned powder

samples provide information concerning the magnetic anisotropy. For Sm2–xGdxFe17Ny compounds, the

easy magnetization direction changes from the basal plane to conical.

Fig. 6 X-ray diffraction patterns of magnetically aligned powder of Sm0.5Gd1.5Fe17N2.9.

15

20

25

30

35

0 0.5 1 1.5 2

Sm2-x

GdxFe

17

Sm2-x

GdxFe

17N

~3

Ms (

µB/m

ole

)

x (Gd. at)

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7

Gd2Fe17N3

Sm 1,5Gd0,5Fe17N3

Gd2Fe17

Sm 1,5Gd0,5Fe17

SmGdFe 17N3

Sm 0,5Gd1,5Fe17

Sm 0,5Gd1,5Fe17N3

Sm 2Fe17

Sm 2Fe17N3

SmGdFe 17

Ms (

µB/m

ole

)

µ0H (Tesla)

Fig. 4 Field dependence of the magnetization at room

temperature of Sm2–xGdxFe17Ny compounds before and

after nitrogen insertion.

Fig. 5 Magnetization evolution versus Sm

content in Sm2–xGdxFe17 at 300 K before and

after nitrogen insertion.

40 45 50 55 60 65 70 75 80

Sm0,5

Gd1.5

Fe17

N2.9

Inte

nsi

ty (

a.

u)

2θ (deg)

(01

5)

(21

4)

(23

1)

phys. stat. sol. (c) 3, No. 9 (2006) 3215

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4 Conclusion

We investigate the effect of the Gd substitution and the nitrogen insertion on the structural and magnetic

properties of Sm2–xGdxFe17Ny intermetallic alloys. The Th2Zn17-type structure of Sm2Fe17 is retained.

The nitrogen insertion leads to an increase of the unit-cell volume. The Curie temperature and the satu-

rated magnetisation increase after nitrogen insertion. Finally, the easy direction of magnetization for

nitrides samples with 0 < x < 2 is conical.

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

and Development of Competences.

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