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Magnetic properties of fully nitrogenated R2Fe17N3 (R = Ce, Nd, Pr)
~
ELSEVIER
Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
jmmltlel magnetic mMedals
Magnetic properties of fully nitrogenated R2Fe17N 3 (R Ce, Nd, Pr) O. Isnard
a,b,,,
S. M i r a g l i a b, D . F r u c h a r t
b, j . D e p o r t e s
c, p . L ' H e r i t i e r
d
a Institut Laue Langevin, BP 156X, 38042 Grenoble Cddex, France b Laboratoire de Cristallographie du CNRS, associ# ?t l'Universit~ J. Fourier, BP 166X, 38042 Grenoble Cedex 09, France c Laboratoire Louis Ndel du CNRS, associd ~ l'Universitd J. Fourier, BP 166X, 38042 Grenoble Cedex 09, France d Laboratoire de Mat#riaux et de Gdnie Physique, UA 1109 CNRS, ENSPG, BP 46, 38402 St Martin d'Hdres, France
(Received 30 April 1993; in revised form 30 August 1993)
Abstract
The magnetic properties of R2Fe17N3, with R = Ce, Pr, Nd, are reported from 5 K up to 725 K. The use of a high-pressure nitrogenation technique enables us to analyze the nitrogen-saturated compounds. Occupancy of the octahedral interstitial sites by N atoms is explained in terms of chemical and crystallographic considerations. The magnetic measurements have been performed in a continous field up to 7 T in the temperature range 4.2-725 K. The Curie temperatures and magnetic properties of these compounds are compared with those of other related compounds.
I. Introduction
Considerable progress has been made in the development of p e r m a n e n t magnetic materials in the last few decades. The discovery of Nd2FelaB energized the investigation of intermetallics containing rare earths and iron. Recently the possibility of inserting light elements such as hydrogen, carbon or nitrogen in the metal sublattice of R2Fel7 or 'RMn12'-type phases has increased interest in these compounds. A common feature of all these interstitial materials is the increased iron-iron bond length, in comparison with the related binary phases.
* Corresponding author. Tel:+ (33) 76 48 71 11; fax: +(33) 76 48 39 06.
This study deals with rare-earth iron nitrides of formula R2Fe17N 3 (R = Ce, Nd, Pr), retaining the rhombohedral (R-3m) Th2Zn17-type structure. High nitrogen content materials have been obtained by nitrogen gas pressure synthesis. T h e magnetic properties of these compounds have been measured from 4 K to the Curie temperature and will be discussed and compared with those of R2Fe17 or R 2 F e l 7 H x.
2. Experimental details 2.1. S a m p l e p r e p a r a t i o n a n d n i t r o g e n a t i o n
The purity of the starting elements was 99.95% for rare-earth metals and 99.99% for iron. The alloy samples were melted in a H F induction furnace using the so-called cold crucible method,
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(93)E0547-P
O. Isnard et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
then cooled down to room temperature (RT). The ingots were wrapped in a tantalum foil, then sealed in argon-filled silica tubes and annealed for about three Weeks at 1000°C. Standard X-ray diffractometry revealed that the samples were single phase after the annealing treatment. The X-ray patterns were recorded by using a Guinier focusing camera and indexed in a rhombohedral unit cell. Prior to nitrogenation, the starting R2Fe17 alloys were crushed and sieved down to a typical size of 20 I~m (in diameter). The nitrogenation reaction was performed in a stainless steel autoclave, under high pressure (e.g. 15 MPa) of high-purity nitrogen gas (typically 5N5), The process was activated by heating the powder at temperature ranging from 250 to 350°C. The first estimation of the nitrogenation uptake was done by a gravimetric method and then confirmed by neutron diffraction experiments. The resulting N-content was at least 2.9 atoms per formula unit. Then, X-ray diffractometry revealed the presence of few traces of a-iron after nitrogen uptake, the sample being mainly single phase. The low iron content was confirmed by neutron diffraction experiments.
2.2. Magnetization measurements The magnetic measurements were performed using the so-called 'axial extraction method', implemented in the facilities of the Laboratoire Louis N6el. From 4 K to room temperature, the experiments were carried out in a liquid-helium cryostat with superconducting coils, providing a continuous magnetic field up to 7 T. A complete description of this apparatus can be found in Ref. [1]. The Curie temperatures were determined using a Faraday-type torque. For this doing, the sample was sealed under vacuum in a small silica tube in order to prevent oxidation during heating.
3. Results and discussion
3.1. Structural features Among the three interstitial elements (H,C,N) that have been already inserted in the R2Fel7
77
Nd2Fe17
Q
•
Nd (6c) Fel (6c)
• Fe2 (9d)
Fe3 (18~)
~ Fe4 (18h)
1 (9e)
o
2(18g)
Fig. 1. Schematic representation of the Nd2Fe17 structure showing the two kinds of interstitial sites: 1 octahedral (site 9e) and 2 tetrahedral (site 18g). Nitrogen atoms exclusively occupy the octahedral sites (9e).
structure, nitrogen is the most promising candidate for permanent magnet applications. Many structural studies have been devoted to these compounds, including neutron diffraction experiments and EXAFS measurements to locate the nitrogen atoms within the host lattice. Nitrogen accommodation is a controversial matter [2-7]. Our neutron diffraction work [4,6,7] has shown the exclusive occupancy of 9e sites (Fig. 1). Refinements in which the magnetic contribution to diffraction is neglected claim occupancy of the 18g sites. We have shown that because of large correlations between some crystallographic and magnetic parameters that may lead to convergence instabilities, discarding 'the magnetic contribution would lead to consideration of
78
O. Isnard et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
the 18g site as possible for nitrogen accommodation. It is felt that this result is an experimental artefact arising from the refinement strategy, since occupancy of the tetrahedral sites would lead to a surprisingly short Fe~4)-N distance of 1.447 using the usual iron radius of 1.27/~, this Fe~4)-N bond seems quite unrealistic. Furthermore, in a recent review Christodoulou et al. [8] also coneluded and showed that no other interstitial site except the octahedral one is large enough to accommodate nitrogen. Besides, the interatomic Fe-N distances in the octahedral sites are between 1.85 and 1.95 ~-~' and agree very well with the atomic radii 1.27 A for Fe and = 0.7 A for N. The F e - N distances found in the iron nitrides Fe4N [9] or Fe16N 2 [10] are very close to those observed considering the octahedral sites in R2Fe17N x. Moreover, it is well known that N preferentially accommodates an octahedral environment, as observed in Fe4N, Fe16N 2 and other binaries or ternaries [11-14]. Finally, the exclusive filling of the octahedral sites by N leads to an upper nitrogen content of 3 atoms/f.u., which is the limit found by most authors. EXAFS measurements support the picture of an exclusive occupancy of the octahedral sites (9e) [15]. The interstitial elements have been shown to have a significant influence on the physical properties of these compounds: -increase of the net magnetization in comparison with the R2Fel7 alloys; -huge increase in the Curie temperature; -large influence on the magnetocrystalline anisotropy terms [4,16,17]; and -limitation of the local magnetic moments of the iron atoms having N nearest neighbours through d - p hybridization [18,19]. So far, no study of fully nitrogenated compounds has been reported; most previous studies indicate a nitrogen content of 2.2-2.6 N atoms/f.u. [21]. The purpose of this study is to improve measurements of the effects of N atoms on the magnetic properties of the ternary R2Fe17N x compounds, x reaching the saturation value. First, the effect of the interstitial elements on Curie temperature are analyzed and compared with some related compounds R2Fe17C x,
Table 1 Structural and magnetic parameters of the R2FelTN 3 compounds Ce2Fe17N~ 3 Pr2FelTN- 3 N d 2 F e l 7 N - 3 Space group a = b (~,)
R - 3m 8.739(2)
R - 3m 8.795(2)
R - 3m 8.786(2)
c (/k) V (,g3) Tc (K) M s at 4 K (/XB/f.u.) Y~/a.atomicat 4 K
12.738(2) 842 738 38.0
12,659(2) 848 731 43.2
12,667(2) 847 746 43.9
39.2(2.2)
46.0(2.0)
45.6(3.0)
36.0
39.6
40.0
34.2(2.6)
45.0(2.0)
38.2(2.2)
2.85(0.1)
2.95(0.1)
(t~B/f.u.) a M S at 300 K
(t~B/f.u.) ~'~4~atomic at
300 K (/z B/f.u.) a N atoms/f.u, a 2.95 (0.1) a According to Refs. [6] and [18].
R(FeM)12 and R(FeM)12N. Then magnetization measurements are analyzed and compared with the magnetic parameters deduced from neutron diffraction experiments. The results are compared to the features of the series of R2Fe17H x compounds. 3.2. Curie temperature
As a consequence of the higher nitrogen content, the compounds we studied have higher Curie temperatures than reported elsewhere [4,21]. In Table 1 are listed the lattice parameters as well as the cell volumes. In Fig. 2, Curie temperatures of several rare-earth-iron intermetaUies are plotted versus the average near-neighbour F e - F e distances. It is obvious that the Curie temperature is closely related to the F e - F e distances in Nd 2Fe17, Nd2Fe14B, NdFe11Mo and related compounds. It is worth noting that the hydrided compounds have slightly lower Curie temperatures than expected from the interatomic distances. This is consistent with the fact that hydrogen starts to desorb when heating before reaching the Curie temperature [22], but in a small ampoule, the gas pressure rapidly reaches equilibrium. The thermomagnetic curves provide an estimation of Curie temperatures of the fully hydrided compounds (= 550 K).
O. Isnard et aL /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82 600
. . . . . . . . . . . . . . . . . . . .
45
' ....
' ....
~ ....
•
•
•
•
•
t ....
' ....
79
t ....
, ....
,,
" L 4 5
•&A•
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35
6oo Tc
:
40
(K)
~,
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i
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•
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•
",,
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•
,,
25
30
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-
25
40o
20 3oo
2.57
2.58
2.59
2.60
2.61
15
2.62
. . . . . . . .
0
( A )
l
I . . . .
100
200
....
300
I ....
400
~ ....
500
] ....
600
I '
700
20
'
1 5
800
x (K)
Fig. 2. Variation of Curie temperature (K) as a function of the average F e - F e distances (A) for several iron-neodymium compounds. Nd2Fe~"~ Nd2Fe17N~, Nd2Fe17N2.7*, Nd2Fe14B, Nd(FeMo)~2, Nd(FeMo)12N* and Nd2Fe17D~. s (* [4], * [25], + [27], # this work.
Further proof of the higher nitrogen content in our samples is the larger F e - F e distances measured in Nd2Fe17N 3 [18] than in Nd2Fe17N2.7 [4]. The Curie temperature of Nd2Fe17C0. 7 is reported in Fig. 2; it is observed to follow the same trend, thus confirming that whatever the interstitial element, the increase in Curie temperature is directly related to the increase in F e - F e distances. A fit to the data leads to the equation: Tc = - 25900 + 10200(dFe_Fe) ,
"
(I)
where Tc is in K and ( d F e _ F e ) in ~. An increase in the average F e - F e distance by 0.01 /~ raises the Curie temperature by 100 K. In a recent paper, Coey et al. [20] have proposed a linear variation of the Curie temperature versus the volume of the unit cell; here we show that the ( F e - F e ) distance is the relevant parameter, since it gives a better fit to more data. Indeed, it is interesting to note that relation (I) is verified by three families of R - F e compounds containing interstitial elements or not. Thus, relation (I) seems to be quite general for t h e rare earthiron-rich intermetallics. It is well known that in R2Fe17, as in the other compounds, due to very short F e - F e distances, the exchange interaction can be locally of negative character, i.e. on the dumbbell sites. The effect of the interstitials on the shortest distances
Fig. 3. Thermal variation of magnetization (measured under 70 kOe) for Nd2Fel7 II, Nd2Fet7N 3 • as compared to Nd2Fel7D4. 8 • from Ref. [27].
has been discussed elsewhere [4,22,23]; the insertion of either hydrogen or nitrogen leads to make positive all the F e - F e exchange interactions. 3.3. Magnetization properties
For the three nitrides here studied, magnetization measurements reveal large saturation values. According to the high Curie temperatures of these compounds, the magnetization is only slightly lower at 300 K than at 4 K (see Table 1). The variations of magnetization between 4 and 725 K are plotted in Figs. 3-5. Obviously, the cerium compound exhibits the lower magnetization if it is 40
....
35
I ....
I,,,,I
•
•
....
I ....
40 "35
30 =_
I ....
•
-30
25
-25
20,
-20
15-
15
10
.... 0
i ..... 50
i .... 100
i .... 150
i .... 200
i .... 250
10 300
T (K)
Fig. 4. Thermal variation of magnetization (measured under 70 k O e ) f o r C¢2Fe17 m, Ce2FezTN3 • as compared to Ce2FelTD4.s • from Ref. [27].
O. Isnard et aL /Journal of Magnetism and Magnetic MateriaLs 131 (1994) 76-82
80 45
ii
,,I
. . . .
I . . . .
I . . . .
I ....
I . . . .
I
. . . .
I . . . .
45
~O@OIIo0oll
40.
~ ,AAAAAAAA • ~ B ~ B "
35.
.40
,,,,,,-ee
=
".
+,si .
°
35 •
"30
.~'s
• • • @
20 ~
",, li o
15i
""
10 0
.20
~'Is
.... , .... , .... , .... , .... , .... , .... ,,,' 10 100 200 300 400 500 600 700 800 •r (~)
Fig. 5. Thermal variation of magnetization (measured under 70 kOe) for Nd2FelTN 3 • and Ce2FelTN 3 A, as compared to Pr2FelTN 3 o.
assumed that cerium is a nonmagnetic rare earth element. The Values measured on NdzFelTN 3 and PrzFe17N 3 are very similar at all temperatures, the magnetization being only slightly lower (about 0.5 /z B) in the praseodymium compound, indicating that the magnetic moment carried by the Pr atom is lower than the Nd moment. Assuming that the total magnetization shared by all the iron sublattices is kept constant whatever the R element, one can determine the magnetic moment on the rare earth atom. Comparison with the cerium compound leads to a neodymium moment of 3/~B in Nd2Fe17N 3 at 5 K and about 2/z B at 300 K. Applying the same arguments to Pr2Fet7N 3 gives 2.6/~B and 1.8/zB at 4 and 300 K, respectively. These crude evaluations appear to be slightly lower than the theoretical values of the Nd 3+ and pr3+; precise neutron diffraction work [18,6] has confirmed the latter ones with excellent accuracy. Such shifts as indicated by magnetization measurements should be interpreted as the possible contribution of a polarized magnetic moment on cerium atoms. This could be an indication of a mixed valence state in Ce2Fel7N 3 (Ce 4+, Ce3+), in very good agreement with XANES study [24] on the cerium compounds. It is worth noting (Table 1) that the values of magnetization measurements are in close agree-
ment with the sum of the local magnetic moments as determined by neutron diffraction [25]. An even better correspondence (Table 1) has been found in Ce and Nd nitrides in which the iron magnetic moments were refined independently [18] on each of the iron sites. In the neutron diffraction study of Pr2Fe17N 3 [6], only an average iron moment has been refined, and the agreement with the present magnetization measurements is not as good as for the other two compounds. This confirms that the iron magnetic moment varies significantly from one site to another, covering a wide range of about 1.8-3/z B. This effect can- be analyzed in terms of the local environment [26]. It is not only observed with the R2Fel7-type structure but is even more general. At this point, it is interesting to compare the results obtained on the R2Fe17 hydrides and nitrides. First, it has been established that nitrogenation induces higher magnetization than in the R2Fe17 starting alloys. Nitrogen insertion leads to higher magnetization and Curie temperature values than does the insertion of H [27]. This is clear for the series of neodymium compounds where at both 4 K and room temperature the largest magnetization is found for the nitrogenated alloy (see Table 1 and Ref. [27]). It is even more pronounced in the cerium series of compounds where at room temperature the satu-
40
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,,,.I,
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o
o
o
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.... o
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AI
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o
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f
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10
20
30
40
....
50
0
I ,+'+
60
70
H(KOe)
Fig. 6. Isothermal magnetization of C¢2Fe17N 3 measured in the range 0-70 kOe at 5 K (<>), 300 K ( ~, ), 500 K ( • ) , 600 K
(+) and 725 K (o).
O. Isnard et aL /Journal of Magnetism and Magnetic Materials 131 (1994) ?6-82 50
50
i
•
•
o
o o
oo
A&&6&
•
•
•
•
dLA
+++++
+
+
+ + + ++ "30
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•
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paper [26], the local magnetic moment aspects will be discussed in more detail.
.40
40
~. 30 ~
81
f
20
°°°°
•
•
•
•
•
•
•
lo-
oo -20 "10
o
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o
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. . . .
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. . . .
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. . . .
I
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. . . .
I
60
5. Acknowledgements This work has been undertaken under the auspices of the EC, granted by the Brite-Euram contract no. BREU-CT91-0405.
0
. . . .
70
H(KOe)
Fig. 7. Isothermal magnetization measured in the range 0-70 kOe of Pr2Fe17N3 at 5 K (~), 300 K (z~), 500 K (A), 600 K ( + ) and 725 K (e).
ration magnetization is 36 and 29.2/z a for the nitride and hydride, respectively (see Figs. 6 and 7). Although the R2Fel7 structure can accommodate a higher hydrogen content (5/f.u.) than nitrogen (3/f.u.), nitrogen insertion is more efficient in raising the magnetization. One has to account for the localized electron density involved in the F e - X (X = H~ N . . . . ) and F e - F e bonds when interstitials are inserted in the metal sublattices.
4. Conclusions
The use of the high-pressure nitrogenation technique has enabled us to synthesize fully nitrogenated compounds. The magnetic properties of R2Fe17N 3 with R = Ce, Pr, Nd have been measured from 5 K up to 725 K, and compared with previous neutron diffraction investigations. The results confirm the wide range of magnetic moment as observed on the different crystal sites. The magnetic properties of these nitrides are compared with those of the corresponding hydrides, as well as with pure R2Fe17 alloys. Nitrogen insertion acts favourably on the Curie temperature and iron sublattice magnetization through modifications of some electron correlations that increase local exchange interactions and local magnetic moments. In a forthcoming
6. References [1] A. Barlet, J.C. Genna and P. Lethuillier, Cryogenics 31 (1991) 801. [2] R.M. Ibberson, O. Moze, T.H. Jacobs and K.HJ. Buschow, J. Phys.: Condensed Matter. 3 (1991) 1219. [3] S.S. Jaswal, W.B. Yelon, G.C. Hadjipanayis, Y.Z. Wang and D.J. Sellmyer, Phys. Rev. Lett. 67 (1991) 644. [4] S. Miraglia, J.L. Soubeyroux, C. Kolbeek, O. Isnard, D. Fruchart and M. Guillot, J. Less-Common Metals 171 (1991) 51. [5l G.J. Long, O.A. Pringle, F. Grandjean, W.B. Yelon and K.H.J. Busehow, M.M.M. Conf. (1992), J. Appl. Phys. to appear. [6] O. Isnard, S. Miraglia, J.L. Soubeyroux, J. Panuetier and D. Fruehart, Phys. Rev. B 45 (1992) 2920. [7] O. Isnard, J.L. Soubeyroux, S. Miraglia, D. Fruchart, L.M. Garcia and J. Bartolome, Int. Conf. on Neutron Scattering, Physica B 180-181 (1992) 624. [8] C.N. Christodoulou and T. Takeshita, J. A1. Comp. 198 (1993) 1. [9] K.H. Jack, Proc. R. Soc. London, Ser. A 208 (1951) 200. [10] K.H. Jack, Proc. R. Soc. London, Ser. A 208 (1951) 275. [11] B.C. Frazer, Phys. Rev. B 129 (1958) 751. [12] G. Shirane, W.J. Kakei and S.L. Ruby, Phys. Rev. B 126 (1962) 49. [13] S. Ishida, K. Kitawatase, S. Fujii and S. Asano, J. Phys.: Condens. Matter 4 (1992) 756. [14] A. Sakuma, J. Magn. Magn. Mater. 102 (1991) 127. [15] T.W. Capehart, R.K. Mishra and F.E. Pinkerton, Appl. Phys. Lett. 58 (1991) 1395. [16] M. Katter, J. Wecker, L. Schultz and R. Gr6ssinger, J. Magn. Magn. Mater. 92 (1990) L14. [17] R. Coehoorn and G.H.O. Daalderop, J. Magn. Magn. Mater. 104-107 (1992) 1081. [18] O. Isnard, S. Miraglia, J.L. Soubeyroux and D. Fruchart, J. A1. Comp. 190 (1992) 129. [19] L.P. Li, H.-S. Li and J.M.D. Coey, Phys. Stat. Solidi (b) 166 (1991) K107. [20] Q. Qi, H. Sun, R. Skomski and J.M.D. Coey, Phys. Rev. B. 45 (1992) 12278. [21] H. Sun, J.M.D. Coey, Y. Otani and D.P.F. Hurley, J. Phys.: Condensed Matter 2 (1990) 6465.
82
O. Isnard et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
[22] O. Isnard, J.L. Soubeyroux, S. Miraglia, D. Fruchart, L.M. Garcia and J. Bartolome, Int. Conf. on Neutron Scattering, Physiea B 180-181 (1992) 629. [23] D. Fruchart and S. Miraglia, J. Appl. Phys. 69 (1991) 5578. [24] O. Isnard et al., Phys. Rev. B (1994) to appear.
[25] W.B. Yelon and G.C. Hadjipanayis, Proc. INTERMAG Conf., St Louis, MO (1992). [26] O. Isnard and D. Fruehart, J. AI. Comp. (in press). [27] O. Isnard, S. Miraglia, D. Fruchart and J. Deportes, J. Magn. Magn. Mater. 103 (1991) 157.
ELSEVIER
Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
jmmltlel magnetic mMedals
Magnetic properties of fully nitrogenated R2Fe17N 3 (R Ce, Nd, Pr) O. Isnard
a,b,,,
S. M i r a g l i a b, D . F r u c h a r t
b, j . D e p o r t e s
c, p . L ' H e r i t i e r
d
a Institut Laue Langevin, BP 156X, 38042 Grenoble Cddex, France b Laboratoire de Cristallographie du CNRS, associ# ?t l'Universit~ J. Fourier, BP 166X, 38042 Grenoble Cedex 09, France c Laboratoire Louis Ndel du CNRS, associd ~ l'Universitd J. Fourier, BP 166X, 38042 Grenoble Cedex 09, France d Laboratoire de Mat#riaux et de Gdnie Physique, UA 1109 CNRS, ENSPG, BP 46, 38402 St Martin d'Hdres, France
(Received 30 April 1993; in revised form 30 August 1993)
Abstract
The magnetic properties of R2Fe17N3, with R = Ce, Pr, Nd, are reported from 5 K up to 725 K. The use of a high-pressure nitrogenation technique enables us to analyze the nitrogen-saturated compounds. Occupancy of the octahedral interstitial sites by N atoms is explained in terms of chemical and crystallographic considerations. The magnetic measurements have been performed in a continous field up to 7 T in the temperature range 4.2-725 K. The Curie temperatures and magnetic properties of these compounds are compared with those of other related compounds.
I. Introduction
Considerable progress has been made in the development of p e r m a n e n t magnetic materials in the last few decades. The discovery of Nd2FelaB energized the investigation of intermetallics containing rare earths and iron. Recently the possibility of inserting light elements such as hydrogen, carbon or nitrogen in the metal sublattice of R2Fel7 or 'RMn12'-type phases has increased interest in these compounds. A common feature of all these interstitial materials is the increased iron-iron bond length, in comparison with the related binary phases.
* Corresponding author. Tel:+ (33) 76 48 71 11; fax: +(33) 76 48 39 06.
This study deals with rare-earth iron nitrides of formula R2Fe17N 3 (R = Ce, Nd, Pr), retaining the rhombohedral (R-3m) Th2Zn17-type structure. High nitrogen content materials have been obtained by nitrogen gas pressure synthesis. T h e magnetic properties of these compounds have been measured from 4 K to the Curie temperature and will be discussed and compared with those of R2Fe17 or R 2 F e l 7 H x.
2. Experimental details 2.1. S a m p l e p r e p a r a t i o n a n d n i t r o g e n a t i o n
The purity of the starting elements was 99.95% for rare-earth metals and 99.99% for iron. The alloy samples were melted in a H F induction furnace using the so-called cold crucible method,
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(93)E0547-P
O. Isnard et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
then cooled down to room temperature (RT). The ingots were wrapped in a tantalum foil, then sealed in argon-filled silica tubes and annealed for about three Weeks at 1000°C. Standard X-ray diffractometry revealed that the samples were single phase after the annealing treatment. The X-ray patterns were recorded by using a Guinier focusing camera and indexed in a rhombohedral unit cell. Prior to nitrogenation, the starting R2Fe17 alloys were crushed and sieved down to a typical size of 20 I~m (in diameter). The nitrogenation reaction was performed in a stainless steel autoclave, under high pressure (e.g. 15 MPa) of high-purity nitrogen gas (typically 5N5), The process was activated by heating the powder at temperature ranging from 250 to 350°C. The first estimation of the nitrogenation uptake was done by a gravimetric method and then confirmed by neutron diffraction experiments. The resulting N-content was at least 2.9 atoms per formula unit. Then, X-ray diffractometry revealed the presence of few traces of a-iron after nitrogen uptake, the sample being mainly single phase. The low iron content was confirmed by neutron diffraction experiments.
2.2. Magnetization measurements The magnetic measurements were performed using the so-called 'axial extraction method', implemented in the facilities of the Laboratoire Louis N6el. From 4 K to room temperature, the experiments were carried out in a liquid-helium cryostat with superconducting coils, providing a continuous magnetic field up to 7 T. A complete description of this apparatus can be found in Ref. [1]. The Curie temperatures were determined using a Faraday-type torque. For this doing, the sample was sealed under vacuum in a small silica tube in order to prevent oxidation during heating.
3. Results and discussion
3.1. Structural features Among the three interstitial elements (H,C,N) that have been already inserted in the R2Fel7
77
Nd2Fe17
Q
•
Nd (6c) Fel (6c)
• Fe2 (9d)
Fe3 (18~)
~ Fe4 (18h)
1 (9e)
o
2(18g)
Fig. 1. Schematic representation of the Nd2Fe17 structure showing the two kinds of interstitial sites: 1 octahedral (site 9e) and 2 tetrahedral (site 18g). Nitrogen atoms exclusively occupy the octahedral sites (9e).
structure, nitrogen is the most promising candidate for permanent magnet applications. Many structural studies have been devoted to these compounds, including neutron diffraction experiments and EXAFS measurements to locate the nitrogen atoms within the host lattice. Nitrogen accommodation is a controversial matter [2-7]. Our neutron diffraction work [4,6,7] has shown the exclusive occupancy of 9e sites (Fig. 1). Refinements in which the magnetic contribution to diffraction is neglected claim occupancy of the 18g sites. We have shown that because of large correlations between some crystallographic and magnetic parameters that may lead to convergence instabilities, discarding 'the magnetic contribution would lead to consideration of
78
O. Isnard et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
the 18g site as possible for nitrogen accommodation. It is felt that this result is an experimental artefact arising from the refinement strategy, since occupancy of the tetrahedral sites would lead to a surprisingly short Fe~4)-N distance of 1.447 using the usual iron radius of 1.27/~, this Fe~4)-N bond seems quite unrealistic. Furthermore, in a recent review Christodoulou et al. [8] also coneluded and showed that no other interstitial site except the octahedral one is large enough to accommodate nitrogen. Besides, the interatomic Fe-N distances in the octahedral sites are between 1.85 and 1.95 ~-~' and agree very well with the atomic radii 1.27 A for Fe and = 0.7 A for N. The F e - N distances found in the iron nitrides Fe4N [9] or Fe16N 2 [10] are very close to those observed considering the octahedral sites in R2Fe17N x. Moreover, it is well known that N preferentially accommodates an octahedral environment, as observed in Fe4N, Fe16N 2 and other binaries or ternaries [11-14]. Finally, the exclusive filling of the octahedral sites by N leads to an upper nitrogen content of 3 atoms/f.u., which is the limit found by most authors. EXAFS measurements support the picture of an exclusive occupancy of the octahedral sites (9e) [15]. The interstitial elements have been shown to have a significant influence on the physical properties of these compounds: -increase of the net magnetization in comparison with the R2Fel7 alloys; -huge increase in the Curie temperature; -large influence on the magnetocrystalline anisotropy terms [4,16,17]; and -limitation of the local magnetic moments of the iron atoms having N nearest neighbours through d - p hybridization [18,19]. So far, no study of fully nitrogenated compounds has been reported; most previous studies indicate a nitrogen content of 2.2-2.6 N atoms/f.u. [21]. The purpose of this study is to improve measurements of the effects of N atoms on the magnetic properties of the ternary R2Fe17N x compounds, x reaching the saturation value. First, the effect of the interstitial elements on Curie temperature are analyzed and compared with some related compounds R2Fe17C x,
Table 1 Structural and magnetic parameters of the R2FelTN 3 compounds Ce2Fe17N~ 3 Pr2FelTN- 3 N d 2 F e l 7 N - 3 Space group a = b (~,)
R - 3m 8.739(2)
R - 3m 8.795(2)
R - 3m 8.786(2)
c (/k) V (,g3) Tc (K) M s at 4 K (/XB/f.u.) Y~/a.atomicat 4 K
12.738(2) 842 738 38.0
12,659(2) 848 731 43.2
12,667(2) 847 746 43.9
39.2(2.2)
46.0(2.0)
45.6(3.0)
36.0
39.6
40.0
34.2(2.6)
45.0(2.0)
38.2(2.2)
2.85(0.1)
2.95(0.1)
(t~B/f.u.) a M S at 300 K
(t~B/f.u.) ~'~4~atomic at
300 K (/z B/f.u.) a N atoms/f.u, a 2.95 (0.1) a According to Refs. [6] and [18].
R(FeM)12 and R(FeM)12N. Then magnetization measurements are analyzed and compared with the magnetic parameters deduced from neutron diffraction experiments. The results are compared to the features of the series of R2Fe17H x compounds. 3.2. Curie temperature
As a consequence of the higher nitrogen content, the compounds we studied have higher Curie temperatures than reported elsewhere [4,21]. In Table 1 are listed the lattice parameters as well as the cell volumes. In Fig. 2, Curie temperatures of several rare-earth-iron intermetaUies are plotted versus the average near-neighbour F e - F e distances. It is obvious that the Curie temperature is closely related to the F e - F e distances in Nd 2Fe17, Nd2Fe14B, NdFe11Mo and related compounds. It is worth noting that the hydrided compounds have slightly lower Curie temperatures than expected from the interatomic distances. This is consistent with the fact that hydrogen starts to desorb when heating before reaching the Curie temperature [22], but in a small ampoule, the gas pressure rapidly reaches equilibrium. The thermomagnetic curves provide an estimation of Curie temperatures of the fully hydrided compounds (= 550 K).
O. Isnard et aL /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82 600
. . . . . . . . . . . . . . . . . . . .
45
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79
t ....
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,,
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,,
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-
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2.57
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2.59
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15
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. . . . . . . .
0
l
I . . . .
100
200
....
300
I ....
400
~ ....
500
] ....
600
I '
700
20
'
1 5
800
x (K)
Fig. 2. Variation of Curie temperature (K) as a function of the average F e - F e distances (A) for several iron-neodymium compounds. Nd2Fe~"~ Nd2Fe17N~, Nd2Fe17N2.7*, Nd2Fe14B, Nd(FeMo)~2, Nd(FeMo)12N* and Nd2Fe17D~. s (* [4], * [25], + [27], # this work.
Further proof of the higher nitrogen content in our samples is the larger F e - F e distances measured in Nd2Fe17N 3 [18] than in Nd2Fe17N2.7 [4]. The Curie temperature of Nd2Fe17C0. 7 is reported in Fig. 2; it is observed to follow the same trend, thus confirming that whatever the interstitial element, the increase in Curie temperature is directly related to the increase in F e - F e distances. A fit to the data leads to the equation: Tc = - 25900 + 10200(dFe_Fe) ,
"
(I)
where Tc is in K and ( d F e _ F e ) in ~. An increase in the average F e - F e distance by 0.01 /~ raises the Curie temperature by 100 K. In a recent paper, Coey et al. [20] have proposed a linear variation of the Curie temperature versus the volume of the unit cell; here we show that the ( F e - F e ) distance is the relevant parameter, since it gives a better fit to more data. Indeed, it is interesting to note that relation (I) is verified by three families of R - F e compounds containing interstitial elements or not. Thus, relation (I) seems to be quite general for t h e rare earthiron-rich intermetallics. It is well known that in R2Fe17, as in the other compounds, due to very short F e - F e distances, the exchange interaction can be locally of negative character, i.e. on the dumbbell sites. The effect of the interstitials on the shortest distances
Fig. 3. Thermal variation of magnetization (measured under 70 kOe) for Nd2Fel7 II, Nd2Fet7N 3 • as compared to Nd2Fel7D4. 8 • from Ref. [27].
has been discussed elsewhere [4,22,23]; the insertion of either hydrogen or nitrogen leads to make positive all the F e - F e exchange interactions. 3.3. Magnetization properties
For the three nitrides here studied, magnetization measurements reveal large saturation values. According to the high Curie temperatures of these compounds, the magnetization is only slightly lower at 300 K than at 4 K (see Table 1). The variations of magnetization between 4 and 725 K are plotted in Figs. 3-5. Obviously, the cerium compound exhibits the lower magnetization if it is 40
....
35
I ....
I,,,,I
•
•
....
I ....
40 "35
30 =_
I ....
•
-30
25
-25
20,
-20
15-
15
10
.... 0
i ..... 50
i .... 100
i .... 150
i .... 200
i .... 250
10 300
T (K)
Fig. 4. Thermal variation of magnetization (measured under 70 k O e ) f o r C¢2Fe17 m, Ce2FezTN3 • as compared to Ce2FelTD4.s • from Ref. [27].
O. Isnard et aL /Journal of Magnetism and Magnetic MateriaLs 131 (1994) 76-82
80 45
ii
,,I
. . . .
I . . . .
I . . . .
I ....
I . . . .
I
. . . .
I . . . .
45
~O@OIIo0oll
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~ ,AAAAAAAA • ~ B ~ B "
35.
.40
,,,,,,-ee
=
".
+,si .
°
35 •
"30
.~'s
• • • @
20 ~
",, li o
15i
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10 0
.20
~'Is
.... , .... , .... , .... , .... , .... , .... ,,,' 10 100 200 300 400 500 600 700 800 •r (~)
Fig. 5. Thermal variation of magnetization (measured under 70 kOe) for Nd2FelTN 3 • and Ce2FelTN 3 A, as compared to Pr2FelTN 3 o.
assumed that cerium is a nonmagnetic rare earth element. The Values measured on NdzFelTN 3 and PrzFe17N 3 are very similar at all temperatures, the magnetization being only slightly lower (about 0.5 /z B) in the praseodymium compound, indicating that the magnetic moment carried by the Pr atom is lower than the Nd moment. Assuming that the total magnetization shared by all the iron sublattices is kept constant whatever the R element, one can determine the magnetic moment on the rare earth atom. Comparison with the cerium compound leads to a neodymium moment of 3/~B in Nd2Fe17N 3 at 5 K and about 2/z B at 300 K. Applying the same arguments to Pr2Fet7N 3 gives 2.6/~B and 1.8/zB at 4 and 300 K, respectively. These crude evaluations appear to be slightly lower than the theoretical values of the Nd 3+ and pr3+; precise neutron diffraction work [18,6] has confirmed the latter ones with excellent accuracy. Such shifts as indicated by magnetization measurements should be interpreted as the possible contribution of a polarized magnetic moment on cerium atoms. This could be an indication of a mixed valence state in Ce2Fel7N 3 (Ce 4+, Ce3+), in very good agreement with XANES study [24] on the cerium compounds. It is worth noting (Table 1) that the values of magnetization measurements are in close agree-
ment with the sum of the local magnetic moments as determined by neutron diffraction [25]. An even better correspondence (Table 1) has been found in Ce and Nd nitrides in which the iron magnetic moments were refined independently [18] on each of the iron sites. In the neutron diffraction study of Pr2Fe17N 3 [6], only an average iron moment has been refined, and the agreement with the present magnetization measurements is not as good as for the other two compounds. This confirms that the iron magnetic moment varies significantly from one site to another, covering a wide range of about 1.8-3/z B. This effect can- be analyzed in terms of the local environment [26]. It is not only observed with the R2Fel7-type structure but is even more general. At this point, it is interesting to compare the results obtained on the R2Fe17 hydrides and nitrides. First, it has been established that nitrogenation induces higher magnetization than in the R2Fe17 starting alloys. Nitrogen insertion leads to higher magnetization and Curie temperature values than does the insertion of H [27]. This is clear for the series of neodymium compounds where at both 4 K and room temperature the largest magnetization is found for the nitrogenated alloy (see Table 1 and Ref. [27]). It is even more pronounced in the cerium series of compounds where at room temperature the satu-
40
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10
20
30
40
....
50
0
I ,+'+
60
70
H(KOe)
Fig. 6. Isothermal magnetization of C¢2Fe17N 3 measured in the range 0-70 kOe at 5 K (<>), 300 K ( ~, ), 500 K ( • ) , 600 K
(+) and 725 K (o).
O. Isnard et aL /Journal of Magnetism and Magnetic Materials 131 (1994) ?6-82 50
50
i
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•
o
o o
oo
A&&6&
•
•
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dLA
+++++
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paper [26], the local magnetic moment aspects will be discussed in more detail.
.40
40
~. 30 ~
81
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20
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5. Acknowledgements This work has been undertaken under the auspices of the EC, granted by the Brite-Euram contract no. BREU-CT91-0405.
0
. . . .
70
H(KOe)
Fig. 7. Isothermal magnetization measured in the range 0-70 kOe of Pr2Fe17N3 at 5 K (~), 300 K (z~), 500 K (A), 600 K ( + ) and 725 K (e).
ration magnetization is 36 and 29.2/z a for the nitride and hydride, respectively (see Figs. 6 and 7). Although the R2Fel7 structure can accommodate a higher hydrogen content (5/f.u.) than nitrogen (3/f.u.), nitrogen insertion is more efficient in raising the magnetization. One has to account for the localized electron density involved in the F e - X (X = H~ N . . . . ) and F e - F e bonds when interstitials are inserted in the metal sublattices.
4. Conclusions
The use of the high-pressure nitrogenation technique has enabled us to synthesize fully nitrogenated compounds. The magnetic properties of R2Fe17N 3 with R = Ce, Pr, Nd have been measured from 5 K up to 725 K, and compared with previous neutron diffraction investigations. The results confirm the wide range of magnetic moment as observed on the different crystal sites. The magnetic properties of these nitrides are compared with those of the corresponding hydrides, as well as with pure R2Fe17 alloys. Nitrogen insertion acts favourably on the Curie temperature and iron sublattice magnetization through modifications of some electron correlations that increase local exchange interactions and local magnetic moments. In a forthcoming
6. References [1] A. Barlet, J.C. Genna and P. Lethuillier, Cryogenics 31 (1991) 801. [2] R.M. Ibberson, O. Moze, T.H. Jacobs and K.HJ. Buschow, J. Phys.: Condensed Matter. 3 (1991) 1219. [3] S.S. Jaswal, W.B. Yelon, G.C. Hadjipanayis, Y.Z. Wang and D.J. Sellmyer, Phys. Rev. Lett. 67 (1991) 644. [4] S. Miraglia, J.L. Soubeyroux, C. Kolbeek, O. Isnard, D. Fruchart and M. Guillot, J. Less-Common Metals 171 (1991) 51. [5l G.J. Long, O.A. Pringle, F. Grandjean, W.B. Yelon and K.H.J. Busehow, M.M.M. Conf. (1992), J. Appl. Phys. to appear. [6] O. Isnard, S. Miraglia, J.L. Soubeyroux, J. Panuetier and D. Fruehart, Phys. Rev. B 45 (1992) 2920. [7] O. Isnard, J.L. Soubeyroux, S. Miraglia, D. Fruchart, L.M. Garcia and J. Bartolome, Int. Conf. on Neutron Scattering, Physica B 180-181 (1992) 624. [8] C.N. Christodoulou and T. Takeshita, J. A1. Comp. 198 (1993) 1. [9] K.H. Jack, Proc. R. Soc. London, Ser. A 208 (1951) 200. [10] K.H. Jack, Proc. R. Soc. London, Ser. A 208 (1951) 275. [11] B.C. Frazer, Phys. Rev. B 129 (1958) 751. [12] G. Shirane, W.J. Kakei and S.L. Ruby, Phys. Rev. B 126 (1962) 49. [13] S. Ishida, K. Kitawatase, S. Fujii and S. Asano, J. Phys.: Condens. Matter 4 (1992) 756. [14] A. Sakuma, J. Magn. Magn. Mater. 102 (1991) 127. [15] T.W. Capehart, R.K. Mishra and F.E. Pinkerton, Appl. Phys. Lett. 58 (1991) 1395. [16] M. Katter, J. Wecker, L. Schultz and R. Gr6ssinger, J. Magn. Magn. Mater. 92 (1990) L14. [17] R. Coehoorn and G.H.O. Daalderop, J. Magn. Magn. Mater. 104-107 (1992) 1081. [18] O. Isnard, S. Miraglia, J.L. Soubeyroux and D. Fruchart, J. A1. Comp. 190 (1992) 129. [19] L.P. Li, H.-S. Li and J.M.D. Coey, Phys. Stat. Solidi (b) 166 (1991) K107. [20] Q. Qi, H. Sun, R. Skomski and J.M.D. Coey, Phys. Rev. B. 45 (1992) 12278. [21] H. Sun, J.M.D. Coey, Y. Otani and D.P.F. Hurley, J. Phys.: Condensed Matter 2 (1990) 6465.
82
O. Isnard et al. /Journal of Magnetism and Magnetic Materials 131 (1994) 76-82
[22] O. Isnard, J.L. Soubeyroux, S. Miraglia, D. Fruchart, L.M. Garcia and J. Bartolome, Int. Conf. on Neutron Scattering, Physiea B 180-181 (1992) 629. [23] D. Fruchart and S. Miraglia, J. Appl. Phys. 69 (1991) 5578. [24] O. Isnard et al., Phys. Rev. B (1994) to appear.
[25] W.B. Yelon and G.C. Hadjipanayis, Proc. INTERMAG Conf., St Louis, MO (1992). [26] O. Isnard and D. Fruehart, J. AI. Comp. (in press). [27] O. Isnard, S. Miraglia, D. Fruchart and J. Deportes, J. Magn. Magn. Mater. 103 (1991) 157.