On the structural and magnetic properties of the new ternary nitride series R2Fe17Nx

J o u r n a l o f AUoys a n d C o m p o u n d s , 178 (1992) 15-22 J A L 5026

15

Review

On the structural and magnetic properties of the new ternary nitride series R2FelTNx O. Isnard, S. Miraglia, C. K o l b e c k , E. T o m e y , J. L. S o u b e y r o u x a n d D. F r u c h a r t L a b o r a t o i r e d e C r i s t a l l o g r a p h i e d u CNRS associd & l ' U n i v e r s i t d J. Fourier, B P 166X, F-38042 Grenoble ( F r a n c e )

M. Guillot S e r v i c e N a t i o n a l d e s C h a m p s I n t e n s e s d u CNRS, B P 166X, F-38042 Grenoble (France)

C. Rillo I n s t i t u t o d e C i e n c i a d e M a t e r i a l e s d e A r a g S n , U n i v e r s i d a d de Z a r a g o z a , 50009 Z a r a g o z a ( S p a i n )

(Received July 5, 1991)

Abstract A systematic analysis of the crystal and magnetic properties of the nitrides R2Fe~7Nx has been undertaken using X-ray and neutron diffraction, a.c. susceptibility and high-field measurements. The maximum composition corresponds to Xma~= 3 and to the filling of the distorted 2R-4Fe octahedral site. There is evidence of high anisotropy fields even for the easy-plane systems. Crystal data, as well as magnetic properties, are discussed.

1. I n t r o d u c t i o n T h e p o s s i b i l i t y o f i n s e r t i n g light e l e m e n t s s u c h as h y d r o g e n , c a r b o n or n i t r o g e n within t h e m e t a l s u b l a t t i c e o f R2Fe17 c o m p o u n d s h a s r e c e n t l y r e n e w e d t h e i n t e r e s t in t h e s e m a g n e t i c alloys, since b o t h t h e Curie t e m p e r a t u r e a n d t h e crystal-field a n i s o t r o p y c a n b e significantly i m p r o v e d [ 1 - 5 ] . T h e o r d e r i n g t e m p e r a t u r e is r a i s e d to v a l u e s h i g h e r t h a n f o r R2FeI4B a n d the a n i s o t r o p y fields a r e f o u n d to b e a b o u t 20 T o r e v e n higher. A b e t t e r k n o w l e d g e of t h e f u n d a m e n t a l p r o p e r t i e s ( e . g . crystal-field e n h a n c e m e n t , e a s y - a x i s orie n t a t i o n ) is r e q u i r e d . F u r t h e r m o r e , if l a r g e c o e r c i v e fields a r e to b e r e a c h e d , t h e s y n t h e s i s c o n d i t i o n s m u s t b e i m p r o v e d in o r d e r t o a v o i d t h e f o r m a t i o n o f f r e e iron p a r t i c l e s d u r i n g t h e s y n t h e s i s p r o c e s s . Soft m a g n e t i c m a t e r i a l s s u c h a s iron c a n a c t as n u c l e a t i o n s i t e s o f r e v e r s e m a g n e t i c d o m a i n s a n d i n d u c e a d r a m a t i c d e c r e a s e in t h e coercivity.

0925-8388/92/$5.00

© 1992- Elsevier Sequoia. All rights reserved

16

This paper deals with the crystal structure determination, a systematic analysis using a.c. susceptibility measurements and high-static-field measurements.

2. E x p e r i m e n t a l d e t a i l s

The alloys were prepared using the cold crucible technique and highfrequency melting. The purity of the starting metals (R, Fe) was at least 99.9%. Annealing of the ingots was applied for two weeks at temperatures between 900 and 1050 °C depending on the rare earth metal. After the phase purity was checked by X-ray diffraction, the alloys were finely crushed by ball milling to a mean grain size of about 5 /zm. The high-temperature reactivity with ammonia is very fast but does not prevent the insertion of a certain amount of hydrogen simultaneously with the nitrogen. The nitrogen gas was purified successively by passing it over P205, copper and magnesium beds. It was then introduced into the chamber of a thermogravimetric device (SETARAM model B70). By doing this, 5 g of nitride could be synthesized in a few hours. The progress of the reaction is directly monitored via the increase of mass of the sample. A good compromise between time and highquality sample is to operate at a temperature close to 460 °C. Large amounts of samples containing less than 5% free iron could be produced. Neutron diffraction experiments were performed using the medium- (D 1 B) and high-resolution (D2B) diffractometers of the Institut Laue-Langevin, Grenoble. The a.c. susceptibility measurements were performed between 77 and 300 K using an excitation field H of 10 Oe and a frequency of 120 Hz [6]. The complex susceptibility X~.c.=X'-ix" was measured in our set-up: the X' component is the initial susceptibility related to the variation of the magnetization of the sample, while X" is nonzero when there is magnetic energy absorption within the sample. For the high-field measurements ( H < 2 0 T) the extraction technique was used with a Bitter coil-type magnetometer of the Service National des Champs Intenses (Grenoble).

3. R e s u l t s a n d d i s c u s s i o n

Most of the samples over the whole series of rare earths were synthesized and nitrided. The values of the cell parameters as well as the corresponding nitrogen content x are reported in Table 1. The cell parameters were measured using an X-ray standard diffractometer (hCu, backscattering (002) graphite monochromator). Some of them were studied using a Guinier focusing chamber (AFe). Both rhombohedral (Th2Znl 7) and hexagonal (Wh2Ni17) types of structure are observed, the first with light rare earths, the second with heavy ones.

17 TABLE 1 Structural parameters of R2Fe~ and R2Fe~TN~ compounds Compound

a (/~)

c (/~)

Y (/~a)

Ce2Fe17 Ce2Fel~N2.~ Nd2Fej 7 Nd2Fe~TN2.4 Pr2Fe~v Pr2FelvN2.a6 SmzFe17 Sm2Fel 7N2.2 Gd2Fe17 Gd2Fe 17N2.~ Ho2Fe17 Ho2FelvN2.1 Er2Fe17 Er2Fe~TN2.45 Y2Fe17 Y2Fel 7N2.s

8.489 8.743 8.589 8.776 8.581 8.795 8.548 8.730 8.539 8.715 8.435 8.609 8.430 8.584 8.459 8.646

12.413 12.688 12.488 12.661 12.463 12.659 12.435 12.630 12.433 12.653 8.313 8.480 8.291 8.469 8.306 8.484

774.8 847.9 797.8 844.5 794.8 847.9 786.8 834.1 785.1 832.3 512.2 544.3 510.7 540.5 514.7 549.2

AV/N (/~3)

Aa/a

Ac/c

(%)

(%)

8.6

3.0

2.2

6.4

2.2

1.4

6.2

2.5

1.6

7.2

2.2

1.6

6.3

2.0

1.8

6.0

2.1

2.0

6.1

2.1

1.8

6.1

2.2

2.1

F o r the y t t r i u m c o m p o u n d , the stabilized Th2Ni17 t y p e r e m a i n s a f t e r u p t a k e of nitrogen. After n i t r o g e n a t i o n , t r a c e s of s o m e p o o r l y crystallized iron w e r e f o u n d in all t h e s a m p l e s . This i m p u r i t y is c h a r a c t e r i z e d b y a b r o a d X-ray diffraction peak. F r o m t h e b e h a v i o u r o f the lattice p a r a m e t e r s (Table 1), it c a n b e d e d u c e d t h a t t h e cell e x p a n s i o n m o s t l y c o n c e r n s t h e b a s a l p l a n e of the s t r u c t u r e . T h i s p h e n o m e n o n is n o t s u r p r i s i n g if o n e t a k e s into a c c o u n t the f a c t t h a t n i t r o g e n is localized in t h e b a s a l p l a n e o f t h e s t r u c t u r e [7]. It s h o u l d b e n o t e d t h a t the i n c r e a s e in cell v o l u m e p e r n i t r o g e n a t o m is quite c o n s t a n t (AV/N - 6.2 +_0 . 2 / ~ a t o m ) - 1 a c r o s s the s e r i e s e x c e p t f o r t h e a n o m a l o u s r a r e e a r t h m e t a l c e r i u m w h i c h e x h i b i t s a h i g h e r cell e x p a n s i o n . This b e h a v i o u r m i g h t c o r r e s p o n d to t h e t e n d e n c y o f c e r i u m to c h a n g e its v a l e n c y (Ce 3÷ - ~ C e 4 + ) . This p h e n o m e n o n h a s a l r e a d y b e e n o b s e r v e d o n the 2 - 1 7 h y d r i d e [4]. By u s i n g n e u t r o n diffraction, the l o c a t i o n o f n i t r o g e n w a s r e c e n t l y d e t e r m i n e d in t h e r h o m b o h e d r a l t y p e o f s t r u c t u r e o f Nd2FelTNx a n d Pr2FelTNx [ 7l. In c o n t r a s t to h y d r o g e n [4], t h e n i t r o g e n a t o m s o c c u p y o n l y o n e interstitial t y p e o f site, a d i s t o r t e d o c t a h e d r o n f o r m e d b y t w o R a t o m s a n d f o u r s u r r o u n d i n g F e a t o m s [7] (Figs. 1 a n d 2). H o w e v e r , the o c c u p a t i o n o f b o t h the o c t a h e d r a l site a n d t h e t e t r a h e d r a l site w a s r e c e n t l y r e p o r t e d [8] to o c c u r in t h e h e x a g o n a l t y p e o f s t r u c t u r e . H e r e , n i t r o g e n is f o u n d to o c c u p y the t e t r a h e d r a l interstitial sites in a h e x a g o n a l Th2Ni17 s y s t e m , w h e r e a s n o n e o f t h o s e sites a r e o b s e r v e d to b e filled in t h e c o m p o u n d s o f t h e light r a r e e a r t h s with l a r g e r t e t r a h e d r a l sites ( r h o m b o h e d r a l ThzZn17). At the s a m e time, t h e t e t r a h e d r a l site o c c u p a t i o n

18

(~ Iron

¢

Fig. 1. Schematic representation of the R2FelvN3compounds of the Th2Zn~7 structure type. b y hydrogen is clearly more marked for light (big) than for heavy (small) rare earth 2 - 1 7 materials [4]. Furthermore the filling of only the octahedral sites 9e (6h) in the c o m p o u n d s of rhombohedral (hexagonal) symmetry agrees well with the maximum amount of nitrogen uptake (three atoms p e r formula unit) found for the whole R2Fel~ series which corresponds to a complete filling of the 9e (6h) site. The effect of nitrogen uptake can be regarded from two different points of view: (1) a steric consideration: nitrogen insertion is accompanied by anisotropic cell expansion and it influences all the interatomic distances and consequently modifies the F e - F e exchange interactions. Nitrogen insertion modifies markedly the neighbourhood of the rare earth site b e c a u s e three nitrogen atoms b e c o m e the nearest atoms to the rare earth metal at short distances (about 2.5/~), all being in the basal plane of the structure; (2) a crystal-field consideration: the more electronegative element modifies the charge and the electron density, thus affecting the crystal-field strength.

19 Ma netization

P- B /f.u.) 50

Q O°

• ::: ::: ::::::

::::

O@°

25

T=300K

0

10

5

15

20 H (tesla)

Magnetization

(

btB /f.u.)

; 04~4P 4b

O o

,~"

l,

+'' $':

4P4~

0 0° 0

T=300K l

0

10

20 H (tesla)

Fig. 2. S c h e m a t i c r e p r e s e n t a t i o n o f t h e R2Fe~vN3 c o m p o u n d s o f t h e Th2Ni]v s t r u c t u r e type. Fig. 3. Magnetization a s a f u n c t i o n o f t h e a p p l i e d field at 3 0 0 K for Nd2FeITN2. 4 ( u p p e r g r a p h ) a n d Sm2Fe]7N2. 2 (lower graph). The filled s q u a r e s refer to parallel m a g n e t i z a t i o n , o p e n s q u a r e s to p e r p e n d i c u l a r magnetization.

Here the effect of nitrogen must be more important for its nearest neighbours. In this case nitrogen insertion leads to an increase in the charge density and to a screening of the effects of the other atoms. The physical properties related to the rare earth atoms such as the magnetocrystalline anisotropy and the rare earth valence state have already been observed to be very sensitive to the presence of the surrounding nitrogen atoms [7]. A systematic study of the magnetic properties of these nitrides has been performed under fields up to 20 T at 4.2, 100, 200 and 300 K for each compound by using oriented powdered samples, the field being applied parallel or perpendicular to the alignment direction. Such magnetization measurements have enabled us to determine the anisotropy field of Sm2Fe 17N2.2, which is 15 T at room temperature. The results are in good agreement with those of Katter et aI. [9] determined in a higher temperature range but for

20

the same composition. The crossover of parallel and perpendicular curves indicates a more complex behaviour, i.e. a change of easy axis that is along neither the parallel nor the perpendicular direction (Fig. 3). For all the other compounds studied the rare earth metals (Nd, Gd, Ce, Ho) have negative second-order Stevens coefficients. Accordingly, the magnetization remains in the basal plane of the hexagonal structure and no crossing of the parallel and perpendicular magnetic signals has been observed up to 20 T. This indicates that the planar anisotropy of the Nd compound is higher than 20 T, which is not at all surprising for an additive effect of the planar anisotropies of the rare earth metal and the iron. The easy-plane behaviour of most of the R~Fe17 compounds and their corresponding nitrides is well characterized by the very close proximity of the MtI(H) and M±(H) curves (Fig. 3). First of all it should be noted that the magnetic hardening of the R2Fez7 compounds upon hydrogenation induces a magnification of the X':X" ratio. This is mainly due to a decrease in the absorption of magnetic energy within the sample (decrease in X") in the nitrides compared with that in R2Fe17. Susceptibility measurements performed on Sm2Fe17 reveal an anomalous behaviour at T - 165 K and 270 K (Fig. 4). Such a feature has already been observed by GrSssinger et al. [ 10], but up to now has not been well understood. The corresponding nitride does not exhibit such a phenomenon. This anomaly cannot be explained as a spin reorientation phenomenon from easy plane to easy axis because it is well known that Sm2Fe17 is easy plane even at room temperature. Only the singularity characteristic of the Curie temperature was detected by Xa.¢. on Er2Fe17 at Tc = 300 K, in agreement with the previous determination. The measurements performed in the range 7 7 - 3 2 0 K on the corresponding nitride do not provide evidence of the presence of any spin reorientation (SR) p h e n o m e n o n as reported by Hu et al. [11] in Er2Fe17N2.7 (Fig. 5). Since our sample is less rich in nitrogen, such a spin reorientation transition could occur at lower temperatures or else its occurrence really depends on the nitrogen content itself. Further studies such as magnetization measure. . . .

i

. . . .

r

. . . .

i

. . . .

i

. . . .

i

. . . .

. . . .

i

. . . .

i

. . . .

i

. . . .

i

. . . .

~

. . . .

S m2Fe17N2.2

Sm2Fe17

4

~2

A

1= c) v

"

. 1 0

2

~0

(a)

50

100

150

200 250 T (K)

300

350

50

CO)

t00

150

200 250 T (K)

300

350

Fig. 4. XLc. susceptibility measurements of (a) Sm2Felv and Co) Sm2Fez,zN2.2 as functions of temperature. The circles and triangles refer to the in-phase and out-of-phase signals respectively.

21 7

.........

i ....

,

i

....

i

7

Er2Fe17

6

. . . .

I

¸'

I

. . . .

i

. . . .

I

.

.

.

.

.

Er2Fe 17N2.4s

6

~5 01

~4

%3

~3 ~--"2

i¸

1

,,.

~

I 02

1

o

(a)

-1 5o

i

,

i

i

,

i

100

i

i

i

150

i

I

i

i

200 T (K)

,

,

,

,

250

,I

-1

,

300

350

. . . . . . . .

50 (b)

I , , j l l l , l l l , , , I h , , ,

t00

150

200 250 T (K)

300

350

o f (a) Er2Fe~7 a n d ( b ) Er2FelTN2.4s a s f u n c t i o n s o f the temperature. The circles and triangles refer to the in-phase and o u t - o f - p h a s e s i g n a l s , respectively. F i g . 5. X~.~. s u s c e p t i b i l i t y m e a s u r e m e n t s

'

I

'

'

'

I

'

'

'

I

'

'

'

I

'

'

'

6

f"

5

i!•

4

3

L

~'~

x

[ ' '

lo.51

4.5

~

~

165

3

I

210

255

300

T '~

' T '''

(b) l ~

'~=

~ ,

75

I

120

,

,

,

L

~

~

,

I

165 210 T (K)

,

,

k

I

255

,

,

,

300

, ~

Y2F

" ~

7

75

(e)

120

T (K) 14

E oJ b

~.................... J f f

75

(a)

~d2Fe17N2~

6

120

165

210

255

300

T (K)

Fig. 6. Xa.c. s u s c e p t i b i l i t y m e a s u r e m e n t s o f (a) Nd2F%TN2.4, ( b ) Gd2FeI~N2.5, (c) Y2F%TN2. 4 as

functions of temperature. T h e u p p e r and lower curves refer to the i n - p h a s e s u s c e p t i b i l i t y measured parallel and perpendicular to the easy a x i s o f m a g n e t i z a t i o n .

m e n t s v s . t e m p e r a t u r e are n e e d e d in order to u n d e r s t a n d better the w a y in w h i c h t h e s e m a t e r i a l s b e h a v e u p o n nitridation. The a n o m a l i e s o b s e r v e d in t h e t w o R2Fe17 c o m p o u n d s in w h i c h the rare earth h a s a j > 0 (Er and S m ) are a l m o s t s m e a r e d o u t in t h e c o r r e s p o n d i n g nitrides. It s h o u l d b e n o t e d that neither R2Fel ~ (R - Y, Nd, Gd) n o r the c o r r e s p o n d i n g R2FelTN= h a v e r e v e a l e d a n y c u s p in t h e Xa.c. susceptibility curves. A s c a n be s e e n f r o m Fig. 6, this o c c u r s n e i t h e r in t h e parallel n o r in the p e r p e n d i c u l a r

22 X'~¢. susceptibility curves. Their r e g u l a r s h a p e d o e s n o t p r o v i d e e v i d e n c e o f

a n y spin reorientation. This is in g o o d a g r e e m e n t with the in-plane m a g n e t i c a n i s o t r o p y e x p e c t e d as a result o f the sign o f the s e c o n d - o r d e r Stevens coefficient a j < 0 for t h o s e t h r e e rare e a r t h e l e m e n t s a n d is c o n f i r m e d by a high-field m a g n e t i z a t i o n study.

4. C o n c l u s i o n s In the light o f the v e r y large e n h a n c e m e n t o f b o t h the Curie t e m p e r a t u r e a n d the m a g n e t i c a n i s o t r o p y (mainly d u e to the rare e a r t h element) i n d u c e d b y insertion o f interstitial nitrogen, s t r u c t u r a l a n d m a g n e t i c m e a s u r e m e n t s h a v e b e e n p e r f o r m e d . It is p o s s i b l e t o p r o d u c e a large q u a n t i t y o f nitrides b y d i r e c t s o l i d - g a s (N2 reaction). The modified a n d e x p a n d e d lattice exhibits a v e r y high level o f crystalline quality. M e a s u r e m e n t s o f the a.c. susceptibility as well as high-field m a g n e t i c m e a s u r e m e n t s s h o w that the nitrides c a n b e c h a r a c t e r i z e d as h a r d m a g n e t i c materials.

References 1 J. M. D. Coey and Hong Sun, J. Magn. Magn. Mater., 87 (1990) L251-L254. 2 B. Chevalier, J. Etourneau and J. M. D. Coey, in I. V. Mitchell et al. (eds.), Concerted European Action on Magnets report, Elsevier, London, 1988, p. 124. 3 K. H. J. Buschow, R. Coehoorn, D. B. de Mooij, K. de Waard and T. H. Jacobs, J. Magn. Magn. Mater., 92 (1990) L35. 4 0 . Isnard, S. Miraglia, J. L. Soubeyroux, D. Fruchart and A. Stergiou, J. Less-Common Met., 162 (1990) 273. 5 R. B. Helmholdt and K. H. J. Buschow, J. Less-Common Met., 155 (1989) 15. 6 C. Rillo, F. Lera, J. Bartolome and A. J. Van Duyneveldt, personal communication, 1991. 7 S. MiragUa, J. L. Soubeyroux, C. Kolbeck, O. Isnard, D. Fruchart and M. Guillot, J. LessCommon Met., 171 (1991) 51. 8 R. M. Ibberson, O. Moze, T. H. Jacobs and K. H. J. Buschow, J. Phys.: Condens. Mater., 3 (1991) 1219-1226. 9 M. Katter, J. Wecker and L. Schultz, J. Magn. Magn. Mater., P2 (1990) L14-L18. 10 R. GrSssinger, X. C. Kou, T. H. Jacobs and K. H. J. Buschow, J. AppL Phys., 59 (1991) 5596-5598. 11 Bo-Ping Hu, Hong-Shuo Li, Hong Sun, J. F. Lawler and J. M. D. Coey, Solid State Commu~, 76 (1990) 587-590.