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Magnetic properties of NdMn6Ge6 and SmMn6Ge6 compounds from susceptibility measurements and neutron diffraction study
Journal of Alloys and Compounds, 215 (1994) 187-193 J A L C O M 1222
187
Magnetic properties of NdMn6Ge6 and SmMn6Ge6 compounds from susceptibility measurements and neutron diffraction study B. C h a f i k E1 Idrissi, G. V e n t u r i n i a n d B. M a l a m a n * Laboratoire de Chimie du Solide Mindral, Universitd Henri Poincard, Associ~ au CNRS, UA 158, BP 239, F-54506 Vandoeuvre les Nancy Cedex (France)
E. R e s s o u c h e CEA/D~partement de Recherche Fondamentale sur la Matidre Condens~e/SPSMS-MDN, 17 avenue des Martyrs, 38054 Grenoble Cedex 9 (France)
(Received March 2, 1994)
Abstract Investigations by susceptibility measurements and neutron diffraction experiments have been performed on the ternary germanides NdMn6Ge6 and SmMn6Ge6 of the YCo6Ge6-type structure (P6/mmm). This structure, closely related to the HfFe6Ge 6 type, can be described as a disordered filled derivative of the CoSn B35-type structure. The Mn atoms build Kagom6 layers stacked along the c axis, in between which the rare earth atoms are randomly distributed. Owing to the Mn atom coordination of the rare earth, this structure appears closely related to the CaCus- and ThMn12-type structures. NdMn6Ge6 and SmMn6Ge6 are ferromagnetic below 417 and 441 K respectively. At 35 K a second magnetic transition related to a spin reorientation process takes place in NdMn6Ge6. Neutron diffraction study of NdMn6Ge6 confirms that both the rare earth and Mn sublattices order simultaneously above room tempera, ture. In the whole temperature range 2-300 K NdMn6Ge6 is a collinear ferromagnet (i.e. positive Nd-Mn interaction). In the temperature range 300-35 K the moment direction deviates by q)=40 ° from the c axis, whereas an easy plane occurs at 2 K (/XNd=2.87(43) /Z~ and IzM,=2.26(14 ) /zB). The results are discussed and compared with those previously obtained for the parent YMn6Ge6, GdMn6Ge6 and RMn6Sn6 (R = Tb-Er, Lu) compounds.
I. Introduction
In previous papers [1, 2] we reported on the magnetic structures of the HfFe6Ge6-type structure compounds TbMn6Sn6, HoMn6Sn6, LuMn6Sn6 and YMn6Ge6. The Tb and Ho compounds are characterized by ferromagnetic R and Mn sublattices antiferromagnetically coupled along the c axis [1], whereas LuMn6Sn6 and YMn6Ge6 (i.e. diamagnetic rare earth) are antiferromagnetic with an easy plane and an easy axis anisotropic direction respectively [2]. Recently we reported on the crystallographic and magnetic properties of RMn6Ge6 ( R - S c , Y, Nd, Sm, Gd-Lu) compounds [3]. They are isotypic either to Y C o 6 G e 6 [4] (R = Nd, Sm) or to HfFe6Ge6 [5] (R = Sc, Y, Gd-Lu). Susceptibility measurements performed between 80 and 800 K have shown that the light rare earth and Gd compounds exhibit a spontaneous magnetization. More recently it was shown [6, 7] that GdMn6Ge6 gives rise to two ordering temperatures *Author to whom correspondence should be addressed.
associated with antiferromagnetic ordering of the Gd and Mn sublattices and an easy plane prevails. At room temperature this compound clearly behaves as a typical ferrimagnet, while a "bootstrap mechanism" related to the unit cell volume variation yields an antiferromagnetic behaviour at low temperature [7]. The aim of this work was to confirm clearly the positive R-Mn exchange in the case of the light rare earth element Nd. Furthermore, the knowledge of the orientation of the moments was also important, since in the RT6X 6 compounds it has been observed that the substitution of Sn by Ge yields a change in the anisotropy direction of the Mn sublattice [2]. In this paper we report on the magnetic properties of NdMn6Ge6 and SmMn6Ge6 by the use of bulk magnetization measurements between 300 and 4.2 K and on the magnetic structure of N d M n 6 G e 6 determined by neutron diffraction experiments. 2. Crystal structure
NdMn6Ge6 and SmMn6Ge6 crystallize in the disordered YCo6Ge6-type structure. In a previous paper we
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B. Chafik El lddssi et aL / Magnetic properties of NdMn6Ge6 and SmMn6Ge6
188
reported on the crystallographic properties of the RFe6Sn 6 series [8]. These compounds, previously reported as disordered YCo6Ge6-type structure representatives [9], were obtained as new ordered-type structures [8]. The HfFe6Ge6-, ScFe6Ga 6- [10] and YCo6Ge 6type structures are drawn in Fig. 1 together with the structure of.TbFe6Sn6 (as an example)and we have shown that the new structural types observed in the iron stannides may be regarded as various stackings of the "nfFe6Ge6 blocks" separated by "ScFe6Ga6 slabs". The R element has the same Mn neighbouring in both the HfFe6Ge 6 and ScFe6Ga 6 blocks, i.e. hexagonal dipyramid, whereas the transition metal lies in two types of X ( = Ge, Sn) octahedra. The first one, denoted A in Fig. 1, has an axial symmetry and occurs in HfFe6Ge6 and ScFe6Ga6 (one-third of the octahedra); the second one, C, has a symmetry centre and only occurs in the ScFe6Ga6-type structure. The YCo6Ge6type structure probably results from a disordered stacking of these two blocks. Furthermore, it is note worthy that both types of octahedra occur in NdMn6Ge6 and may introduce a new easy magnetization direction of the Mn moments in this compound.
3. Experimental procedures The compounds were prepared from commercially available high purity elements: manganese (powder,
A .~
C -,~
HfFe6Ge6
ScFe6Ga6
a~, o o , [-110]
H
S
H C
A "~,TbFe6Sn6 . ~
•
4. Experimental results
bj ,
YC°6Ge6
0
b
H
S
S
H
$
99.9%), rare earth (R) elements (ingots, 99.9%) and germanium (pieces, 99.999%). Pellets of stoichiometric mixture were compacted using a steel die and then introduced into silica tubes sealed under argon (100 mmHg). The samples were annealed for 2 weeks at 1073 K. The purity of the final product was checked by the powder X-ray diffraction technique (Guinier, Cu Ka). The magnetic measurements were carried out using a Faraday balance (above 300 K) and a Manies magnetosusceptometer (between 4.2 and 300 K) in fields up to 1.5 T. Neutron experiments have been carried out at the Siloe reactor of the Centre d'Etudes Nucl6aires de Grenoble (CENG). Several patterns have been recorded in the temperature range 2-300 K with the DN5 multidetector (A=2.487/k). In the Yfo6Ge6-type structure (space group P6/mmm) the Mn and some of the Ge atoms occupy the 3(g): (½, 0, ½) and 2(c): (½, ~, 0) sites respectively whereas the rare earth and the other Ge atoms occupy the l(a): (0, 0, 0) and 2(e): (0, 0, zce (=0.31)) sites respectively with an occupancy factor mj=0.5. Using the scattering lengths bee = 8.185, bMn= -- 3.73, bNd = 7.69 Fermi and the form factors for Mn and Nd 3+ given in refs. 11 and 12 respectively, the scaling factor, the zGo atomic position and the Mn and Nd magnetic moments were refined by the mixed crystallographic executive for diffraction (MXD) least-squares fit procedure [13]. The MXD programme allows one to fit simultaneously the intensities of the nuclear and magnetic reflections.
[-110]
H 0 R o X T
Fig. 1. HfFe6Ge6-, ScFe6Ga6-, TbFe6Sn6- and YCo6Ge6-type structures. Structural relationships. A and C correspond to the two kinds of TX6 octahedra as labelled and defined in the text.
4.1. Susceptibility measurements The thermal variation in the susceptibility of NdMn6Ge6 and SmMn6Ge6 from 500 to 4.2 K is shown in Fig. 2. As previously observed [3], both compounds exhibit Curie points at Tc = 417 and 441 K respectively. The main characteristic magnetic data are collected in Tables 1 and 2. Moreover, the susceptibility of NdMn6Ge6 measured under a low field (about 1 kOe) decreases strongly just below room temperature and a sharp kink occurs at Tt=35 K. The shape of the thermomagnetic curve (around 300 K) is characteristic of the occurrence of strong anisotropic effects, while the second transition observed at low temperature is due to a spin reorientation process (see below). In contrast, the thermal variation in the susceptibility of SmMn6Ge6 is smoother: it decreases slowly below 200 K and does not really exhibit any transition down to 4.2 K.
B. Chafik El Idrissi et al.
sot
189
Magnetic properties of NdMn6Ge 6 and SmMn6Ge 6 15
X
(emu/g)
4.2
K
40
1o =
Nd
\
o !i/ ...........
10
-2000
TIK) 0
. . . .
0
i
100
. . . .
i
200
. . . .
i
300
. . . .
i
. . . .
400
i
600
Fig. 2. Temperature dependence of the susceptibility of NdMn6Ge6 and SmMn6Ge6 (Hapv=1 kOe). TABLE 1. RMn6Ge6 (R =-Nd, Sm): magnetic data from susceptibility measurements Compound
Tc (K)
Tt (K)
0p (K)
/xcer (/XB)
/-~M~ (/ZB)a
NdMn6Ge6 SmMn6Ge6
417 441
35 -
418 439
8.55 7.62
3.16 3.09
a
__1 /£Mn -~- (Jt£e2 -- ]~R2) I/2'
T A B L E 2. R M n 6 G e 6 (R-=Nd, Sin): magnetic data from magnetization measurements
Compound
T (K)
O'm~x (/ZB) (H,.~,= 12 kOe)
H~ (Oe)
tr, (~B)
NdMn6Ge6
50 4.2 4.2
13 13.6 10.7
770 380 150
4.8 3.4 1.2
SmMn6Ge6
, ( ol 6000
10000
14000
Fig. 3. Magnetization curves for NdMn6Ge~ at 50 K (O) and 4.2 K (0).
. . . .
500
2000
The field dependence of the magnetization measured at various temperatures (Fig. 3) confirms the ferro(ferri)magnetic behaviour of both compounds in the whole temperature range studied, i.e. from Tc to 4.2 K. Moreover, we observed an increase in the magnetic hardness of NdMn6Ge6 from 300 K to Tt = 35 K followed by a small decrease down to 4.2 K, while the anisotropy appears to be much lower in SmMn6Ge6 since the coercive fields always remain very weak (Table 2). These results are in fair agreement with the thermal variation in the susceptibilities (see above). Finally, it is note worthy that the moment value observed at 4.2 K under a field of 12 kG (about 13.6 /XB) does not allow us to conclude unambiguously between a ferri- or ferromagnetic arrangement of the Nd and Mn sublattices.
4.2. Neutron diffraction study o f NdMn6Ge6
The temperature dependence of the neutron diffraction patterns recorded step by step from room temperature to 2 K shows an intensity increase of the nuclear reflections with no additional lines (Fig. 4). This implies a ferro(ferri)magnetic ordering in the whole temperature range studied, in agreement with the magnetometric measurements. Furthermore, the occurrence of magnetic contributions under the (00l) reflections implies that the moment direction deviates from the c axis for the Nd, Mn or both sublattices. At 2 K, using 05 (the deviation angle from the c axis), /ZM, and /XNd as magnetic parameters, the best fit to the data yields unambiguously a collinear ferromagnetic arrangement of both the Mn and Nd sublattices (Table 3). The deviation angle 05 is 90° with /XM,= 2.26(14) /XB and /ZNd= 2.87(47) /XB- The total resulting moment (about 16.4 /xB) is slightly higher than that obtained by magnetometric measurements (about 13.6 IZB), thus confirming the anisotropic effects discussed previously (see Section 4.1). Thus the magnetic structure consists of ferromagnetic (001) sheets of Mn atoms stacked along the c axis, in between which the Nd atoms are randomly distributed with a positive N d - M n exchange. At 2 K the moments are in the (001) planes (Fig. 5). Above T, = 35 K the same model refines nicely but we observe that the moments rise up from the (001) plane (Table 3). The thermal variations in the angle 05 and in the Mn and Nd moment values are displayed in Figs. 6 and 7 respectively. It can be clearly seen that 05 is approximately constant at about 40(10) ° in the temperature range 50-300 K and that an abrupt spin reorientation process of both the Nd and Mn sublattices occurs at T,= 35 K. On the other hand, the Mn moment value is nearly constant in the whole temperature range 300-2 K (about 2.2 /zB), while the Nd moment value increases slowly between 300 and
B. Chafik El Idrissi et al. / Magnetic prope~ies of NdMn6Ge6 and SmMn6Ge6
190
[
NdMn6Ge6
o "
....
•
II
II
II
o
II
o
,-
,~
~
>
~ ~
,.
o
7 .........................
20
0
II
II
30
40
S0 20
60
70
80
(deg.)
Fig. 4. Neutron diffraction patterns of NdMn6Ge6 at 300 and 2 K. T A B L E 3. NdMn6Ge6: observed and calculated intensities and refined parameters at 2 and 300 K
hkl
2 K
300 K
/~
/o
/~
/o
100 001 101 110
25 122 176 434
22(1) 123(2) 175(3) 439(5)
26 120 135 432
26(1) 121(2) 135(3) 432(5)
200 111 002 201 102
70 150 31 28 12
73(5) 144(6) 31(8) 31(8) 14(7)
75 162 13 37 28
76(4) 162(5) 15(6) 30(7) 20(6)
a (A) c (A) zce ~M, (/xB) /zr~d (/zB) (°) R (%)
Table 3 gives the observed and calculated intensities together with the various adjustable parameters and the lattice constants at 300 and 2 K.
5.222(4) 4.077(3) 0.329(3) 2.26(14) 2.87(43) 90 1.9
5.245(3) 4.088(2) 0.323(3) 2.08(5) 1.47(10) 48(2) 2.1
50 K (from about 1.5 to 2/~B) and very sharply below about 40 K. Finally, it is note worthy that the Nd atoms carry a moment up to room temperature, implying that the Nd and Mn sublattices probably order simultaneously at Tc (=417 K). Nevertheless, a high temperature neutron diffraction study would be necessary to confirm this hypothesis.
5. Discussion
This study represents the first determination of the magnetic structure of a YCo6Ge6-type representative. According to the present neutron diffraction study, NdMn6Ge6 is a collinear ferromagnet from room temperature to 2 K. Furthermore, this compound exhibits an easy plane, at least at low temperature (below 40 K). The same conclusive evidence has also been obtained in GdMn6Ge6 by magnetic measurements on aligned powder [6, 7]. As generally observed in rare earth-3d intermetallics, the Nd-Mn couplings are positive. This exchange interaction is very strong, causing the Nd moments to order simultaneously with the Mn moments. As already remarked by several authors [14-17], the Nd-Mn magnetic interactions are among the highest interactions observed in such compounds. This effect has been attributed to the larger exchange interaction between the 4f and 5d electrons in the light rare earth, resulting in an increase in the spatial extent of the 4f shell with the rare earth atomic number. It is note worthy that YMn6Ge 6 is antiferromagnetic (TN=485 K) and that an easy axis prevails throughout the temperature range
B. Chafik El Idrissi et aL I Magnetic properties o f NdMn6Ge 6 and SmMn6Ge 6
191
C
i../,4=,.-i r____~l~_ I
~ :
I IlI
i j'~"~-q. ~7 Nd) I
"4fD" IM n i
ii I
c
I
I=... : ~4~" I - ~
l Mn(3/4)
:
t
. ~
i?.,,4~Mn
Y(1/2) i
i
Nd,L
~" . . . . . (a)
b
a
(h) Fig. 5. Magnetic structure of (a) YMn6Ge6 at 300 K and (b) NdMn6Ge6 at 2 K (Nd,L circles indicate random occupancy of the Nd atomic positions; see text).
t----
90
~(°)
60
..........
30
T 0
,
0
i
50
'
100
i
,
150
!
200
'
(K)
i
,
250
3 0
Fig. 6. T e m p e r a t u r e dependence of the angle ~ between the direction of the Nd and M n moments and the c axis (the dotted lines are a guide for the eye).
3Mn Nd
','T¢
!~ T(K) 0
.
0
.
.
.
i
100
.
.
.
.
i
200
.
.
.
.
i
300
,
'
400
500
Fig. 7. Temperature dependence of the Nd and Mn m o m e n t values.
from room temperature to about 2 K [2]. Moreover, the moment rotation between successive layers of Mn, observed below 80 K, corresponds to an exchange energy
apparently sufficient to sustain long-range order with a periodicity of about 105 /~ and certainly indicates that the basal plane magnetic anisotropy must be very small. The magnetic behaviour observed in this compound can be considered to be due only to the Mn sublattice. Therefore these results reinforce the occurrence of strong interactions between Nd and Mn moments in NdMn6Ge6. Finally, the strong R-Mn couplings are able to return and align the Mn moments, giving rise to the ferromagnetic structures of the Nd and Sm compounds and the ferrimagnetic room temperature structure of GdMn6Ge6. These conclusions are in fair agreement with those of Venturini et al. deduced from the threshold field variations observed in R, Y1 _,Mn6Sn6 (R - Ce-Nd, Sm, Gd-Ho) [14]. The @ angle value of about 40(10) ° (between the direction of the moments and the c axis) observed for both the Mn and Nd sublattices above 40 K is probably due to the competition between the easy axis magnetization of the Mn moments (as in YMn6Ge6) and the easy plane magnetization of the Nd sublattice. At 2 K the in-plane orientation of the moments is related to the self-anisotropy of the Nd, which occurs only at low temperature, as was previously seen in the terbium and holmium manganese stannides [1]. It is note worthy that the change in the easy direction of the Mn moments upon substituting Sn by Ge [2] seems also to occur with the rare earths, since at low temperature the Nd and Tb moments with the same Steven's factor have an easy axis anisotropy in the stannides and an easy plane in the germanides [18]. However, in both GdMn6Ge6 and GdMn6Sn6 the same easy plane prevails at 4.2 K [6, 7]. Under these conditions it is note worthy that the easy plane observed in
192
B. Chafik El Idrissi et al. / Magnetic properties of NdMn6Ge6 and SmMn6Ge6
GdMn6Ge 6 appears very surprising and, up to now, unexplained. The next remark relates to the sign of the various Mn-Mn exchange integrals and the resulting magnetic structure of NdMn6Ge6. The in-plane Mn-Mn interactions are obviously ferromagnetic as observed in all the RT6X 6 compounds studied and two different hypotheses could be proposed for the tridimensional arrangement. First let us consider the magnetic structure of VMn6Ge 6. It shows the following Mn-Mn interlayer interactions: the superexchange Mn-(R-X)-Mn interactions are antiferromagnetic while the Mn-(X)-Mn interactions are ferromagnetic (see Fig. 5 of ref. 2). It is note worthy that the sign of this latter interaction is always positive in all known RMn6X6 compounds. Assuming the same scheme of Mn-Mn interactions, the ferromagnetic structure observed for NdMn6Ge6 results from a manganese sublattice spin reorientation with the substitution Y ~ Nd (with a positive N d - M n coupling). In contrast, from magnetic measurements on GdMn6Ge 6 and some solid solutions (Nd, Y, Tb) Brabers et al. [7] have concluded that the antiferromagnetic ordering (i.e. the superexchange Mn-(R-X)-Mn interaction) can be broken by increasing the lattice constant (leading to larger Mn-Mn distances). According to these authors, such an effect is observed in GdMn6Ge6, which behaves ferromagnetically at room temperature and antiferromagnetically at 4.2 K owing to the decrease in the lattice constants with decreasing temperature. They called this behaviour "bootstrap ferrimagnetism". Under these conditions, since the compound NdMn6Ge6 has a larger unit cell volume than GdMn6Ge6, the Mn sublattice would be ferromagnetic in this compound and the Y ~ Nd substitution simply implies a spin rotation of the Mn moments from the c axis to the (001) plane. It is rather difficult to conclude. Actually, in the YMn6Ge6 compound, within the standard deviations, a ferromagnetic coupling through the "X plane" occurs in the incommensurate basal plane component of the spiral structure which occurs at low temperature (i.e. when the lattice constants decrease). This low temperature behaviour is deduced from the refined spiral turn angles which yield spin rotations of 125(15) ° through the "R-X plane" and 13(15) ° through the "X plane". Furthermore, as also noted by Brabers et al. [7], the previously defined correlation between unit cell volume and magnetic properties is not applicable in the larger RMn6Sn 6 compounds [1, 2]. Finally, the solid solution LaxYl_xMn6Ge6 (x>0.25, i.e. a unit cell volume around that of GdMn6Ge 6 or larger) would be more convincing, since the Y and La atoms would not interfere. Unfortunately, up to now,
we have not succeeded in preparing these compounds and further experiments are in progress. The last remark concerns the magnitude of the Mn and Nd magnetic moments and their thermal variations. The Mn moment exhibits a classical thermal behaviour (Fig. 7). At 2 K its saturated value (2.26(14) /zB) is intermediate between those observed in YMn6Ge6 (1.95 /zB) and TbMn6Sn6 (2.39 /zB). This agrees well with the general trend of the evolution of the transition metal moment in binary Fe-X compounds (X-=Ge, Sn) [19]. In such phases the Fe moment decreases with the interatomic T-X distances and with the substitution of Sn by Ge. This is in good accordance with the calculations of Haydock and You [20]. In the present compound the substitution of Y by Nd yields an increase in the cell parameters and thus in the Mn-Ge distances and consequently the Mn moment increases from YMn6Ge 6 to NdMn6Ge6. Moreover, for the same size of the R element (Y, Tb) the Mn moment is larger in the stannide than in the corresponding germanide [1, 2]. For the Nd compound at 2 K the moment value is close to the free-ion value (about 2.9(4)/zB). Its unusual thermal variation (Fig. 7) can be regarded as resulting from two Brillouin curves: the low temperature part could be related to the "intrinsic" Nd moment ordering and the high temperature part to the polarization of the rare earth moment by the transition metal sublattice.
6. Conclusions
This study presents new information about the exchange interaction between light rare earth and Mn moments and on the anisotropy direction in RMn6Ge6 compounds as well as the magnetic behaviour of a YCo6Ge6-type structure compound. The magnetic structure of NdMn6Ge6 (easy plane) clearly confirms that the magnetocrystalline anisotropy of the RMn6X6 compounds arises from the competition between the magnetic contributions from the transition metal and rare earth sublattices. The Nd sublattice. anisotropy is apparently dominant at low temperature, while the partial reorientation of the Mn sublattice between 40 and 300 K gives evidence of the strong anisotropy of the easy magnetization axis of Mn from which the observed behaviour probably originates (Table 2). It will now be interesting to study light rare earth stannides such as NdMn6Sn6 on one hand and the solid solution Yl-xPrxMn6Sn6, crystallizing in the less complicated HfFe6Ge6-type structure, on the other hand. Such work is in progress. In addition, the main influence of the unit cell volume on the magnetic behaviour of the RT6Ge 6 compounds has to be precise. Therefore it clearly appeared necessary
B. Chafik El Idrissi et al. / Magnetic properties of NdMn6Ge 6 and SmMn6Ge 6
to check the magnetic structure of the TbMn6Ge6 parent compound by the use of neutron diffraction experiments. This study is also in progress and the conclusions will be reported elsewhere [18]. References 1 B. Chafik El Idrissi, G. Venturini and B. Malaman, J. LessCommon Met., 175 (1991) 143. 2 G. Venturini, R. Welter, B. Malaman and E. Ressouche, J. Alloys Comp., 200 (1993) 51. 3 G. Venturini, R. Welter and B. Malaman, J. Alloys Comp., 185 (1992) 99. 4 W. Bucholz, H.U. Schuster, Z. Anorg. Allg. Chem., 482 (1981) 40. 5 R.R. Olenitch, L.G. Akselrud and Ya.P. Yarmoliuk, Dopov. Akad. Nauk Ukr. RSR A (2) (1980) 84. 6 F.M. Mulder, R.C. Thiel, J.H.V.J. Brabers, F.R. de Boer and K.H.J. Buschow, J. Alloys Comp., 190 (1993) L29. 7 J.H.V.J. Brabers, V.H.M. Duijin, F.R. de Boer and K.H.J. Buschow, J. Alloys Comp., 198 (1993) 127. 8 B. Chafik E1 Idrissi, G. Venturini and B. Malaman, Mater. Res. Bull., 26 (1991) 1331.
193
9 0 . E . Koretskaia and R.V. Skolozdra, Inorg. Mater., Engl. Transl., 22 (1986) 606. 10 N.M. Belyavina and V.Ya. Markiv, Dopov. Akad. Nauk Ukr. RSR B (12) (1982) 30. 11 C.G. Shull and Y. Yamada, J. Phys. Soc. Jpn., 22 (1962) 1210. 12 C. Stassis, H.W. Deckman, B.N. Harmon, J.P. Desclaux and A.J. Freeman, Phys. Rev. B, 15 (1977) 369. 13 P. Wolfers, J. Appl. Crystallogr., 23 (1990) 554. 14 G. Venturini, R. Welter and B. Malaman, J. Alloys Comp., 197 (1993) 101. 15 K.H.J. Buschow, in E.P. Wohlfarth and K.H.J. Buschow (eds.), Ferromagnetic Materials, Vol. 6, Novel Permanent Materials, Elsevier, Amsterdam, 1992, p. 134. 16 J.J.M. Franse and R.J. Radwanski, Crystal-field and exchange interactions in hard magnetic materials, Proc. NA TOAdvanced Study Institute on Supermagnets, Hard Magnetic Materials, II Ciocco, June 1990, Kluwer Dordrecht, p. 199. 17 E. Belorisky, M.A. Fremy, J.P. Gavignan, D. Givord and H.S. Li, J. Appl. Phys., 61 (1987) 3971. 18 C. El Idrissi, G. Venturini, E. Ressouche and B. Malaman, J. Alloys Comp., in press. 19 D. Fruchart, B. Malaman, G. Le Caer and B. Roques, Phys. Status Solidi A, 78 (1983) 355. 20 R. Haydock and N.V. You, Solid State Commun., 33 (1980) 299.
187
Magnetic properties of NdMn6Ge6 and SmMn6Ge6 compounds from susceptibility measurements and neutron diffraction study B. C h a f i k E1 Idrissi, G. V e n t u r i n i a n d B. M a l a m a n * Laboratoire de Chimie du Solide Mindral, Universitd Henri Poincard, Associ~ au CNRS, UA 158, BP 239, F-54506 Vandoeuvre les Nancy Cedex (France)
E. R e s s o u c h e CEA/D~partement de Recherche Fondamentale sur la Matidre Condens~e/SPSMS-MDN, 17 avenue des Martyrs, 38054 Grenoble Cedex 9 (France)
(Received March 2, 1994)
Abstract Investigations by susceptibility measurements and neutron diffraction experiments have been performed on the ternary germanides NdMn6Ge6 and SmMn6Ge6 of the YCo6Ge6-type structure (P6/mmm). This structure, closely related to the HfFe6Ge 6 type, can be described as a disordered filled derivative of the CoSn B35-type structure. The Mn atoms build Kagom6 layers stacked along the c axis, in between which the rare earth atoms are randomly distributed. Owing to the Mn atom coordination of the rare earth, this structure appears closely related to the CaCus- and ThMn12-type structures. NdMn6Ge6 and SmMn6Ge6 are ferromagnetic below 417 and 441 K respectively. At 35 K a second magnetic transition related to a spin reorientation process takes place in NdMn6Ge6. Neutron diffraction study of NdMn6Ge6 confirms that both the rare earth and Mn sublattices order simultaneously above room tempera, ture. In the whole temperature range 2-300 K NdMn6Ge6 is a collinear ferromagnet (i.e. positive Nd-Mn interaction). In the temperature range 300-35 K the moment direction deviates by q)=40 ° from the c axis, whereas an easy plane occurs at 2 K (/XNd=2.87(43) /Z~ and IzM,=2.26(14 ) /zB). The results are discussed and compared with those previously obtained for the parent YMn6Ge6, GdMn6Ge6 and RMn6Sn6 (R = Tb-Er, Lu) compounds.
I. Introduction
In previous papers [1, 2] we reported on the magnetic structures of the HfFe6Ge6-type structure compounds TbMn6Sn6, HoMn6Sn6, LuMn6Sn6 and YMn6Ge6. The Tb and Ho compounds are characterized by ferromagnetic R and Mn sublattices antiferromagnetically coupled along the c axis [1], whereas LuMn6Sn6 and YMn6Ge6 (i.e. diamagnetic rare earth) are antiferromagnetic with an easy plane and an easy axis anisotropic direction respectively [2]. Recently we reported on the crystallographic and magnetic properties of RMn6Ge6 ( R - S c , Y, Nd, Sm, Gd-Lu) compounds [3]. They are isotypic either to Y C o 6 G e 6 [4] (R = Nd, Sm) or to HfFe6Ge6 [5] (R = Sc, Y, Gd-Lu). Susceptibility measurements performed between 80 and 800 K have shown that the light rare earth and Gd compounds exhibit a spontaneous magnetization. More recently it was shown [6, 7] that GdMn6Ge6 gives rise to two ordering temperatures *Author to whom correspondence should be addressed.
associated with antiferromagnetic ordering of the Gd and Mn sublattices and an easy plane prevails. At room temperature this compound clearly behaves as a typical ferrimagnet, while a "bootstrap mechanism" related to the unit cell volume variation yields an antiferromagnetic behaviour at low temperature [7]. The aim of this work was to confirm clearly the positive R-Mn exchange in the case of the light rare earth element Nd. Furthermore, the knowledge of the orientation of the moments was also important, since in the RT6X 6 compounds it has been observed that the substitution of Sn by Ge yields a change in the anisotropy direction of the Mn sublattice [2]. In this paper we report on the magnetic properties of NdMn6Ge6 and SmMn6Ge6 by the use of bulk magnetization measurements between 300 and 4.2 K and on the magnetic structure of N d M n 6 G e 6 determined by neutron diffraction experiments. 2. Crystal structure
NdMn6Ge6 and SmMn6Ge6 crystallize in the disordered YCo6Ge6-type structure. In a previous paper we
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)01222-4
B. Chafik El lddssi et aL / Magnetic properties of NdMn6Ge6 and SmMn6Ge6
188
reported on the crystallographic properties of the RFe6Sn 6 series [8]. These compounds, previously reported as disordered YCo6Ge6-type structure representatives [9], were obtained as new ordered-type structures [8]. The HfFe6Ge6-, ScFe6Ga 6- [10] and YCo6Ge 6type structures are drawn in Fig. 1 together with the structure of.TbFe6Sn6 (as an example)and we have shown that the new structural types observed in the iron stannides may be regarded as various stackings of the "nfFe6Ge6 blocks" separated by "ScFe6Ga6 slabs". The R element has the same Mn neighbouring in both the HfFe6Ge 6 and ScFe6Ga 6 blocks, i.e. hexagonal dipyramid, whereas the transition metal lies in two types of X ( = Ge, Sn) octahedra. The first one, denoted A in Fig. 1, has an axial symmetry and occurs in HfFe6Ge6 and ScFe6Ga6 (one-third of the octahedra); the second one, C, has a symmetry centre and only occurs in the ScFe6Ga6-type structure. The YCo6Ge6type structure probably results from a disordered stacking of these two blocks. Furthermore, it is note worthy that both types of octahedra occur in NdMn6Ge6 and may introduce a new easy magnetization direction of the Mn moments in this compound.
3. Experimental procedures The compounds were prepared from commercially available high purity elements: manganese (powder,
A .~
C -,~
HfFe6Ge6
ScFe6Ga6
a~, o o , [-110]
H
S
H C
A "~,TbFe6Sn6 . ~
•
4. Experimental results
bj ,
YC°6Ge6
0
b
H
S
S
H
$
99.9%), rare earth (R) elements (ingots, 99.9%) and germanium (pieces, 99.999%). Pellets of stoichiometric mixture were compacted using a steel die and then introduced into silica tubes sealed under argon (100 mmHg). The samples were annealed for 2 weeks at 1073 K. The purity of the final product was checked by the powder X-ray diffraction technique (Guinier, Cu Ka). The magnetic measurements were carried out using a Faraday balance (above 300 K) and a Manies magnetosusceptometer (between 4.2 and 300 K) in fields up to 1.5 T. Neutron experiments have been carried out at the Siloe reactor of the Centre d'Etudes Nucl6aires de Grenoble (CENG). Several patterns have been recorded in the temperature range 2-300 K with the DN5 multidetector (A=2.487/k). In the Yfo6Ge6-type structure (space group P6/mmm) the Mn and some of the Ge atoms occupy the 3(g): (½, 0, ½) and 2(c): (½, ~, 0) sites respectively whereas the rare earth and the other Ge atoms occupy the l(a): (0, 0, 0) and 2(e): (0, 0, zce (=0.31)) sites respectively with an occupancy factor mj=0.5. Using the scattering lengths bee = 8.185, bMn= -- 3.73, bNd = 7.69 Fermi and the form factors for Mn and Nd 3+ given in refs. 11 and 12 respectively, the scaling factor, the zGo atomic position and the Mn and Nd magnetic moments were refined by the mixed crystallographic executive for diffraction (MXD) least-squares fit procedure [13]. The MXD programme allows one to fit simultaneously the intensities of the nuclear and magnetic reflections.
[-110]
H 0 R o X T
Fig. 1. HfFe6Ge6-, ScFe6Ga6-, TbFe6Sn6- and YCo6Ge6-type structures. Structural relationships. A and C correspond to the two kinds of TX6 octahedra as labelled and defined in the text.
4.1. Susceptibility measurements The thermal variation in the susceptibility of NdMn6Ge6 and SmMn6Ge6 from 500 to 4.2 K is shown in Fig. 2. As previously observed [3], both compounds exhibit Curie points at Tc = 417 and 441 K respectively. The main characteristic magnetic data are collected in Tables 1 and 2. Moreover, the susceptibility of NdMn6Ge6 measured under a low field (about 1 kOe) decreases strongly just below room temperature and a sharp kink occurs at Tt=35 K. The shape of the thermomagnetic curve (around 300 K) is characteristic of the occurrence of strong anisotropic effects, while the second transition observed at low temperature is due to a spin reorientation process (see below). In contrast, the thermal variation in the susceptibility of SmMn6Ge6 is smoother: it decreases slowly below 200 K and does not really exhibit any transition down to 4.2 K.
B. Chafik El Idrissi et al.
sot
189
Magnetic properties of NdMn6Ge 6 and SmMn6Ge 6 15
X
(emu/g)
4.2
K
40
1o =
Nd
\
o !i/ ...........
10
-2000
TIK) 0
. . . .
0
i
100
. . . .
i
200
. . . .
i
300
. . . .
i
. . . .
400
i
600
Fig. 2. Temperature dependence of the susceptibility of NdMn6Ge6 and SmMn6Ge6 (Hapv=1 kOe). TABLE 1. RMn6Ge6 (R =-Nd, Sm): magnetic data from susceptibility measurements Compound
Tc (K)
Tt (K)
0p (K)
/xcer (/XB)
/-~M~ (/ZB)a
NdMn6Ge6 SmMn6Ge6
417 441
35 -
418 439
8.55 7.62
3.16 3.09
a
__1 /£Mn -~- (Jt£e2 -- ]~R2) I/2'
T A B L E 2. R M n 6 G e 6 (R-=Nd, Sin): magnetic data from magnetization measurements
Compound
T (K)
O'm~x (/ZB) (H,.~,= 12 kOe)
H~ (Oe)
tr, (~B)
NdMn6Ge6
50 4.2 4.2
13 13.6 10.7
770 380 150
4.8 3.4 1.2
SmMn6Ge6
, ( ol 6000
10000
14000
Fig. 3. Magnetization curves for NdMn6Ge~ at 50 K (O) and 4.2 K (0).
. . . .
500
2000
The field dependence of the magnetization measured at various temperatures (Fig. 3) confirms the ferro(ferri)magnetic behaviour of both compounds in the whole temperature range studied, i.e. from Tc to 4.2 K. Moreover, we observed an increase in the magnetic hardness of NdMn6Ge6 from 300 K to Tt = 35 K followed by a small decrease down to 4.2 K, while the anisotropy appears to be much lower in SmMn6Ge6 since the coercive fields always remain very weak (Table 2). These results are in fair agreement with the thermal variation in the susceptibilities (see above). Finally, it is note worthy that the moment value observed at 4.2 K under a field of 12 kG (about 13.6 /XB) does not allow us to conclude unambiguously between a ferri- or ferromagnetic arrangement of the Nd and Mn sublattices.
4.2. Neutron diffraction study o f NdMn6Ge6
The temperature dependence of the neutron diffraction patterns recorded step by step from room temperature to 2 K shows an intensity increase of the nuclear reflections with no additional lines (Fig. 4). This implies a ferro(ferri)magnetic ordering in the whole temperature range studied, in agreement with the magnetometric measurements. Furthermore, the occurrence of magnetic contributions under the (00l) reflections implies that the moment direction deviates from the c axis for the Nd, Mn or both sublattices. At 2 K, using 05 (the deviation angle from the c axis), /ZM, and /XNd as magnetic parameters, the best fit to the data yields unambiguously a collinear ferromagnetic arrangement of both the Mn and Nd sublattices (Table 3). The deviation angle 05 is 90° with /XM,= 2.26(14) /XB and /ZNd= 2.87(47) /XB- The total resulting moment (about 16.4 /xB) is slightly higher than that obtained by magnetometric measurements (about 13.6 IZB), thus confirming the anisotropic effects discussed previously (see Section 4.1). Thus the magnetic structure consists of ferromagnetic (001) sheets of Mn atoms stacked along the c axis, in between which the Nd atoms are randomly distributed with a positive N d - M n exchange. At 2 K the moments are in the (001) planes (Fig. 5). Above T, = 35 K the same model refines nicely but we observe that the moments rise up from the (001) plane (Table 3). The thermal variations in the angle 05 and in the Mn and Nd moment values are displayed in Figs. 6 and 7 respectively. It can be clearly seen that 05 is approximately constant at about 40(10) ° in the temperature range 50-300 K and that an abrupt spin reorientation process of both the Nd and Mn sublattices occurs at T,= 35 K. On the other hand, the Mn moment value is nearly constant in the whole temperature range 300-2 K (about 2.2 /zB), while the Nd moment value increases slowly between 300 and
B. Chafik El Idrissi et al. / Magnetic prope~ies of NdMn6Ge6 and SmMn6Ge6
190
[
NdMn6Ge6
o "
....
•
II
II
II
o
II
o
,-
,~
~
>
~ ~
,.
o
7 .........................
20
0
II
II
30
40
S0 20
60
70
80
(deg.)
Fig. 4. Neutron diffraction patterns of NdMn6Ge6 at 300 and 2 K. T A B L E 3. NdMn6Ge6: observed and calculated intensities and refined parameters at 2 and 300 K
hkl
2 K
300 K
/~
/o
/~
/o
100 001 101 110
25 122 176 434
22(1) 123(2) 175(3) 439(5)
26 120 135 432
26(1) 121(2) 135(3) 432(5)
200 111 002 201 102
70 150 31 28 12
73(5) 144(6) 31(8) 31(8) 14(7)
75 162 13 37 28
76(4) 162(5) 15(6) 30(7) 20(6)
a (A) c (A) zce ~M, (/xB) /zr~d (/zB) (°) R (%)
Table 3 gives the observed and calculated intensities together with the various adjustable parameters and the lattice constants at 300 and 2 K.
5.222(4) 4.077(3) 0.329(3) 2.26(14) 2.87(43) 90 1.9
5.245(3) 4.088(2) 0.323(3) 2.08(5) 1.47(10) 48(2) 2.1
50 K (from about 1.5 to 2/~B) and very sharply below about 40 K. Finally, it is note worthy that the Nd atoms carry a moment up to room temperature, implying that the Nd and Mn sublattices probably order simultaneously at Tc (=417 K). Nevertheless, a high temperature neutron diffraction study would be necessary to confirm this hypothesis.
5. Discussion
This study represents the first determination of the magnetic structure of a YCo6Ge6-type representative. According to the present neutron diffraction study, NdMn6Ge6 is a collinear ferromagnet from room temperature to 2 K. Furthermore, this compound exhibits an easy plane, at least at low temperature (below 40 K). The same conclusive evidence has also been obtained in GdMn6Ge6 by magnetic measurements on aligned powder [6, 7]. As generally observed in rare earth-3d intermetallics, the Nd-Mn couplings are positive. This exchange interaction is very strong, causing the Nd moments to order simultaneously with the Mn moments. As already remarked by several authors [14-17], the Nd-Mn magnetic interactions are among the highest interactions observed in such compounds. This effect has been attributed to the larger exchange interaction between the 4f and 5d electrons in the light rare earth, resulting in an increase in the spatial extent of the 4f shell with the rare earth atomic number. It is note worthy that YMn6Ge 6 is antiferromagnetic (TN=485 K) and that an easy axis prevails throughout the temperature range
B. Chafik El Idrissi et aL I Magnetic properties o f NdMn6Ge 6 and SmMn6Ge 6
191
C
i../,4=,.-i r____~l~_ I
~ :
I IlI
i j'~"~-q. ~7 Nd) I
"4fD" IM n i
ii I
c
I
I=... : ~4~" I - ~
l Mn(3/4)
:
t
. ~
i?.,,4~Mn
Y(1/2) i
i
Nd,L
~" . . . . . (a)
b
a
(h) Fig. 5. Magnetic structure of (a) YMn6Ge6 at 300 K and (b) NdMn6Ge6 at 2 K (Nd,L circles indicate random occupancy of the Nd atomic positions; see text).
t----
90
~(°)
60
..........
30
T 0
,
0
i
50
'
100
i
,
150
!
200
'
(K)
i
,
250
3 0
Fig. 6. T e m p e r a t u r e dependence of the angle ~ between the direction of the Nd and M n moments and the c axis (the dotted lines are a guide for the eye).
3Mn Nd
','T¢
!~ T(K) 0
.
0
.
.
.
i
100
.
.
.
.
i
200
.
.
.
.
i
300
,
'
400
500
Fig. 7. Temperature dependence of the Nd and Mn m o m e n t values.
from room temperature to about 2 K [2]. Moreover, the moment rotation between successive layers of Mn, observed below 80 K, corresponds to an exchange energy
apparently sufficient to sustain long-range order with a periodicity of about 105 /~ and certainly indicates that the basal plane magnetic anisotropy must be very small. The magnetic behaviour observed in this compound can be considered to be due only to the Mn sublattice. Therefore these results reinforce the occurrence of strong interactions between Nd and Mn moments in NdMn6Ge6. Finally, the strong R-Mn couplings are able to return and align the Mn moments, giving rise to the ferromagnetic structures of the Nd and Sm compounds and the ferrimagnetic room temperature structure of GdMn6Ge6. These conclusions are in fair agreement with those of Venturini et al. deduced from the threshold field variations observed in R, Y1 _,Mn6Sn6 (R - Ce-Nd, Sm, Gd-Ho) [14]. The @ angle value of about 40(10) ° (between the direction of the moments and the c axis) observed for both the Mn and Nd sublattices above 40 K is probably due to the competition between the easy axis magnetization of the Mn moments (as in YMn6Ge6) and the easy plane magnetization of the Nd sublattice. At 2 K the in-plane orientation of the moments is related to the self-anisotropy of the Nd, which occurs only at low temperature, as was previously seen in the terbium and holmium manganese stannides [1]. It is note worthy that the change in the easy direction of the Mn moments upon substituting Sn by Ge [2] seems also to occur with the rare earths, since at low temperature the Nd and Tb moments with the same Steven's factor have an easy axis anisotropy in the stannides and an easy plane in the germanides [18]. However, in both GdMn6Ge6 and GdMn6Sn6 the same easy plane prevails at 4.2 K [6, 7]. Under these conditions it is note worthy that the easy plane observed in
192
B. Chafik El Idrissi et al. / Magnetic properties of NdMn6Ge6 and SmMn6Ge6
GdMn6Ge 6 appears very surprising and, up to now, unexplained. The next remark relates to the sign of the various Mn-Mn exchange integrals and the resulting magnetic structure of NdMn6Ge6. The in-plane Mn-Mn interactions are obviously ferromagnetic as observed in all the RT6X 6 compounds studied and two different hypotheses could be proposed for the tridimensional arrangement. First let us consider the magnetic structure of VMn6Ge 6. It shows the following Mn-Mn interlayer interactions: the superexchange Mn-(R-X)-Mn interactions are antiferromagnetic while the Mn-(X)-Mn interactions are ferromagnetic (see Fig. 5 of ref. 2). It is note worthy that the sign of this latter interaction is always positive in all known RMn6X6 compounds. Assuming the same scheme of Mn-Mn interactions, the ferromagnetic structure observed for NdMn6Ge6 results from a manganese sublattice spin reorientation with the substitution Y ~ Nd (with a positive N d - M n coupling). In contrast, from magnetic measurements on GdMn6Ge 6 and some solid solutions (Nd, Y, Tb) Brabers et al. [7] have concluded that the antiferromagnetic ordering (i.e. the superexchange Mn-(R-X)-Mn interaction) can be broken by increasing the lattice constant (leading to larger Mn-Mn distances). According to these authors, such an effect is observed in GdMn6Ge6, which behaves ferromagnetically at room temperature and antiferromagnetically at 4.2 K owing to the decrease in the lattice constants with decreasing temperature. They called this behaviour "bootstrap ferrimagnetism". Under these conditions, since the compound NdMn6Ge6 has a larger unit cell volume than GdMn6Ge6, the Mn sublattice would be ferromagnetic in this compound and the Y ~ Nd substitution simply implies a spin rotation of the Mn moments from the c axis to the (001) plane. It is rather difficult to conclude. Actually, in the YMn6Ge6 compound, within the standard deviations, a ferromagnetic coupling through the "X plane" occurs in the incommensurate basal plane component of the spiral structure which occurs at low temperature (i.e. when the lattice constants decrease). This low temperature behaviour is deduced from the refined spiral turn angles which yield spin rotations of 125(15) ° through the "R-X plane" and 13(15) ° through the "X plane". Furthermore, as also noted by Brabers et al. [7], the previously defined correlation between unit cell volume and magnetic properties is not applicable in the larger RMn6Sn 6 compounds [1, 2]. Finally, the solid solution LaxYl_xMn6Ge6 (x>0.25, i.e. a unit cell volume around that of GdMn6Ge 6 or larger) would be more convincing, since the Y and La atoms would not interfere. Unfortunately, up to now,
we have not succeeded in preparing these compounds and further experiments are in progress. The last remark concerns the magnitude of the Mn and Nd magnetic moments and their thermal variations. The Mn moment exhibits a classical thermal behaviour (Fig. 7). At 2 K its saturated value (2.26(14) /zB) is intermediate between those observed in YMn6Ge6 (1.95 /zB) and TbMn6Sn6 (2.39 /zB). This agrees well with the general trend of the evolution of the transition metal moment in binary Fe-X compounds (X-=Ge, Sn) [19]. In such phases the Fe moment decreases with the interatomic T-X distances and with the substitution of Sn by Ge. This is in good accordance with the calculations of Haydock and You [20]. In the present compound the substitution of Y by Nd yields an increase in the cell parameters and thus in the Mn-Ge distances and consequently the Mn moment increases from YMn6Ge 6 to NdMn6Ge6. Moreover, for the same size of the R element (Y, Tb) the Mn moment is larger in the stannide than in the corresponding germanide [1, 2]. For the Nd compound at 2 K the moment value is close to the free-ion value (about 2.9(4)/zB). Its unusual thermal variation (Fig. 7) can be regarded as resulting from two Brillouin curves: the low temperature part could be related to the "intrinsic" Nd moment ordering and the high temperature part to the polarization of the rare earth moment by the transition metal sublattice.
6. Conclusions
This study presents new information about the exchange interaction between light rare earth and Mn moments and on the anisotropy direction in RMn6Ge6 compounds as well as the magnetic behaviour of a YCo6Ge6-type structure compound. The magnetic structure of NdMn6Ge6 (easy plane) clearly confirms that the magnetocrystalline anisotropy of the RMn6X6 compounds arises from the competition between the magnetic contributions from the transition metal and rare earth sublattices. The Nd sublattice. anisotropy is apparently dominant at low temperature, while the partial reorientation of the Mn sublattice between 40 and 300 K gives evidence of the strong anisotropy of the easy magnetization axis of Mn from which the observed behaviour probably originates (Table 2). It will now be interesting to study light rare earth stannides such as NdMn6Sn6 on one hand and the solid solution Yl-xPrxMn6Sn6, crystallizing in the less complicated HfFe6Ge6-type structure, on the other hand. Such work is in progress. In addition, the main influence of the unit cell volume on the magnetic behaviour of the RT6Ge 6 compounds has to be precise. Therefore it clearly appeared necessary
B. Chafik El Idrissi et al. / Magnetic properties of NdMn6Ge 6 and SmMn6Ge 6
to check the magnetic structure of the TbMn6Ge6 parent compound by the use of neutron diffraction experiments. This study is also in progress and the conclusions will be reported elsewhere [18]. References 1 B. Chafik El Idrissi, G. Venturini and B. Malaman, J. LessCommon Met., 175 (1991) 143. 2 G. Venturini, R. Welter, B. Malaman and E. Ressouche, J. Alloys Comp., 200 (1993) 51. 3 G. Venturini, R. Welter and B. Malaman, J. Alloys Comp., 185 (1992) 99. 4 W. Bucholz, H.U. Schuster, Z. Anorg. Allg. Chem., 482 (1981) 40. 5 R.R. Olenitch, L.G. Akselrud and Ya.P. Yarmoliuk, Dopov. Akad. Nauk Ukr. RSR A (2) (1980) 84. 6 F.M. Mulder, R.C. Thiel, J.H.V.J. Brabers, F.R. de Boer and K.H.J. Buschow, J. Alloys Comp., 190 (1993) L29. 7 J.H.V.J. Brabers, V.H.M. Duijin, F.R. de Boer and K.H.J. Buschow, J. Alloys Comp., 198 (1993) 127. 8 B. Chafik E1 Idrissi, G. Venturini and B. Malaman, Mater. Res. Bull., 26 (1991) 1331.
193
9 0 . E . Koretskaia and R.V. Skolozdra, Inorg. Mater., Engl. Transl., 22 (1986) 606. 10 N.M. Belyavina and V.Ya. Markiv, Dopov. Akad. Nauk Ukr. RSR B (12) (1982) 30. 11 C.G. Shull and Y. Yamada, J. Phys. Soc. Jpn., 22 (1962) 1210. 12 C. Stassis, H.W. Deckman, B.N. Harmon, J.P. Desclaux and A.J. Freeman, Phys. Rev. B, 15 (1977) 369. 13 P. Wolfers, J. Appl. Crystallogr., 23 (1990) 554. 14 G. Venturini, R. Welter and B. Malaman, J. Alloys Comp., 197 (1993) 101. 15 K.H.J. Buschow, in E.P. Wohlfarth and K.H.J. Buschow (eds.), Ferromagnetic Materials, Vol. 6, Novel Permanent Materials, Elsevier, Amsterdam, 1992, p. 134. 16 J.J.M. Franse and R.J. Radwanski, Crystal-field and exchange interactions in hard magnetic materials, Proc. NA TOAdvanced Study Institute on Supermagnets, Hard Magnetic Materials, II Ciocco, June 1990, Kluwer Dordrecht, p. 199. 17 E. Belorisky, M.A. Fremy, J.P. Gavignan, D. Givord and H.S. Li, J. Appl. Phys., 61 (1987) 3971. 18 C. El Idrissi, G. Venturini, E. Ressouche and B. Malaman, J. Alloys Comp., in press. 19 D. Fruchart, B. Malaman, G. Le Caer and B. Roques, Phys. Status Solidi A, 78 (1983) 355. 20 R. Haydock and N.V. You, Solid State Commun., 33 (1980) 299.