- Email: [email protected]
Neutron diffraction study of magnetic ordering in NdNiAl and PrNiAl
ELSEVIER
Journal of Magnetism
and Magnetic Materials
164
(I 996) I83- 186
Neutron diffraction study of magnetic ordering in NdNiAl and PrNiAl P. Javorskf
a.b.*, V. Sechovsk$ b, R.R. Arons ‘, P. Burlet ‘, E. Ressouche ‘, P. Svoboda ‘, G. Lapertot a
Received 7 March 1996; revised
13May I996
Abstract Polycrystalline samples of NdNiAl and PrNiAl have been studied by powder neutron diffraction and magnetization measurements. Antiferromagnetic ordering below T, = 2.4 K (for NdNiAl) and 6.5 K (for PrNiAl) has been confirmed. The magnetic structure of both compounds can be described by a propagation vector k = (1/2,O,q) with q = 0.46 and 0.41 for NdNiAl and PrNiAl, respectively. Kewvr&: curves
Neutron diffraction;
Rare earth intermetallic compounds; Spin arrangements
1. Introduction RNiAl (R = rare earth except for La and Eu) intermetallic compounds crystallize in the ZrNiAltype hexagonal structure (space group P62m) [ 11. Ferromagnetic ordering at low temperatures has been reported in Ref. [2] for most compounds, but detailed specific heat, susceptibility, resistivity and magnetization studies [3-51 provided evidence of a more complex magnetic behaviour. Complex magnetic structures in some RNiAl (R = Tb, Er and Ho) compounds have been determined employing neutron diffraction experiments [6-81. Indications of antifer-
I Corresponding author. [email protected].
Fax:
+42-2-2491-5050;
email:
in magnetically
ordered
materials:
Magnetization
romagnetic ordering below 2.4 and 6.5 K in NdNiAl and PrNiAl, respectively, were revealed by susceptibility and specific heat measurements [3,4]. The aim of the present work is to study the type of magnetic ordering in NdNiAl and PrNiAl by magnetization and powder neutron diffraction measurements.
2. Experimental Polycrystalline samples (m = 3 g) of NdNiAl and PrNiAl used for magnetization measurements were prepared by arc-melting stoichiometric mixtures of the pure elements (3N for Nd and Pr, 5N for Ni and Al) in an argon atmosphere. The samples for neutron diffraction (m = 20 g each) were prepared in a water-cooled copper crucible using high frequency heat-
0304-8853/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SO304-8853(96)0038 l-2
ing. X-ray analysis showed NdNiAl (both samples) and PrNiAl (3 g sample) to be single phase. In the larger PrNiAl sample. a certain amount of Pr oxides was detected. Magnetization measurements were performed in fields up to 5 T at several temperatures using a SQUID magnetometer and in fields up to 35 T (at 4.2 K) at the High-Field Installation of the University of Amsterdam. Two types of fine powder samples were used: powder grains free to orient in an applied field (‘free powder’) and powder consisting of randomly oriented grains fixed by frozen ethanol (‘polycrystal’). Neutron diffraction experiments with powder samples were performed on the DN5 diffractometer at Siloed Grenoble. at a wavelength of A = 2.496 f 0.006 A with samples placed in a vanadium container. The measured data were analysed using the Rietveld refinement method, using the FULLPROF program [9] with the Fermi lengths and absorption coefficients from Ref. [lo]. To apply group theory [ 111 we used the KAREP and CS programs [ 121.
3. Results and discussion The magnetization curves for NdNiAl and PrNiAl are shown in Figs. 1 and 2. Below the ordering temperatures [4], both compounds exhibit a metamagnetic transition. The value of the critical field for PrNiAl (TN = 6.5 K) and NdNiAl (TN = 2.4 K) is
0.0 0
IO
20
30
40
B VI Fio 1. High-field magnetization curve for NdNiAl measured at 4.;. K (i.e. above T,): inset: low-field magnetization curves measured at 2 and 4 K (‘polycrystal’).
i
2.0 +
.
I “free powder”
“polycr~stal” . .v .
I
.
0.0
.
.
.
1 I
. .vy
1
0
IO
20
30
40
n VI Fig. 2. High-field magnetization curve of PrNiAl measured at 4.2 K; inset: low-field magnetization curves measured at 4 and 8 K (‘polycrystal’).
about 6 T (at 4.2 K) and 1 T (at 2 K), respectively. The absence of any remanence corroborates the conclusion about the antiferromagnetic ordering. Both compounds exhibit quite similar magnetization behaviour in high fields. Beyond initial signs of saturation, seen from a strong curvature around B g 8 T, the magnetization still increases considerably with increasing field and no saturation is reached up to 35 T. The values of the magnetic moments observed at 35 T (2.0 and 2.lp_a/f.u. for PrNiAl and NdNiAl, respectively) are only about 63 and 65% of the free Pr3+ and Nd’+ ion values, respectively. The diffraction patterns for NdNiAl and PrNiAl were recorded above (T= 10 K) and below (T= I.8 K) the ordering temperatures (= 5 h each pattern). Additional measurements with small temperature steps between 1.8 K and the expected TN [4] were performed with a short exposure time ( = 15 min). The unit cell of the ZrNiAl-type hexagonal structure contains three rare earth atoms at (x,0,1 /2), (0,x, l/2) and ( --x, - .\-,I /2) positions, three Al atoms at ( y,O.O), (O,v,O) and c-y, - y,O) and three Ni atoms at (l/3,2/3,0), (2/3,1/3.0) and (O,O, l/2) positions. At T = IO K, only nuclear reflections of the studied compounds were detected. Refinement of these data for NdNiAl gives the lattice parameters a = 7.00 k 0.01 A and c = 4.07 k 0.01 A and the atomic position parameters xNd = 0.578 * 0.001 and J,, = 0.241 k 0.003. These values are comparable to those of other RNiAl compounds [2,6-g]. In the case of
the PrNiAl sample, several additional peaks of Pr oxides were observed. Thus we could not separate the nuclear peaks well and evaluate the structure parameters precisely. At T= I.8 K. numerous additional magnetic reflections appear in the neutron powder patterns of both compounds. The poor statistics due to the short measurement time prevented us analyzing details of the temperature dependence of the magnetic reflections. Nevertheless. we note that the intensities of all magnetic reflections continuously decrease with increasing temperature and vanish at the proposed T,. For the PrNiAl sample. all newly observed (magnetic) retlections at I .X K belong to the studied phase. and not to the oxides. This conclusion is also corroborated by the fact that all these peaks can be indexed by a single propagation vector (see following). In agreement with results of bulk measurements [3,4]. no clear indications of any additional magnetic phase transition below T, have been obtained. The lack of any additional intensity on top of the nuclear peaks down to I .8 K confirms the absence of a ferromagnetic component in the magnetic structure of NdNiAl and PrNiAl. All the magnetic reflections (see Figs. 3 and 4) can be described by a propagation vector k = (1/2,0.~/) with q = 0.46 (NdNiAl) or 0.41 (PrNiAl). In order to sort out all possible orientations of the magnetic moments. we used group theory [ 11.12].
110 -~-~~
Fig, 3. Neutron
NdNiAl
o 0.46) I
diffraction
patterns for NdNiAl
experimental
points, (-1
( .t = 0.57X.
1’= 0.241 ): mapetic
inset only.
k = (0.5
calculated
pattern\ reflection\
at T =
I .8 K:
(
.)
of nuclear retlectiom are indexed
in the
Sl
Following
’ -
7
Fig. 1. Differential
I ,OK): the
PrNiAl k = (0.5 0 0.41)
neutron difl’raction
most intensive
rctlections
the procedure
patterns for PrNiAI
( I, hh -
are indexed.
described in Ref. [I I] for or 0.41) we obtained the same final results as for k = (I /2.0. I /2). which are described in detail in Ref. [ 131. There are six possible solutions if all the rare earth magnetic moments (negligible Ni moments are considered) propagate with the same propagation vector. They are all oriented along the c-axis or they lie in the basal plane. Moments at (x.0.1/2) positions are coupled ferromagnetically or antiferromagnetically with the nearest neighbour moments at ( -x. - .Y,I /2) positions. Furthermore, one has to also take into account the possibility that the rare earth moments at different positions propagate with different propagation vectors k = (1/2.O.q), k’ = (O.l/2,q) or k” = (- I /2,1/2.4) which are equivalent in hexagonal symmetry. Due to the relatively weak intensities of the magnetic reflections in both compounds we could not make a final decision as to the magnetic moment directions and magnitudes. However, we can make certain qualitative conclusions. The intensities of the (07 0) + k and (0 I 1) + k reflections are relatively very intense for NdNiAl, but almost absent for PrNiAl. According to the representation theory results, and also taking into account the intensities of (000) + k. (OOT)+k, (OlO)+ k and (Oli)+k reflections (which are relatively strong), it follows that magnetic moments at (.~.0.1/2) and ( -x, - s. I /2) are, if they propagate with the same propagation vector, coupled antiferromagnetically in NdNiAl and ferromagnetically in PrNiAl.
k = (I /2.O.q) (q = 0.46
The antiferromagnetic coupling of moments at (x,0,1/2) and (-x,-x,1/2) is a common behaviour of all previously studied RNiAl (R = Tb. Ho, Er) compounds [6-8,131. From this point of view, PrNiAl is an exception. Here, we would like to point out that PrNiAl orders magnetically at higher temperature than NdNiAl, although, according to the de Gennes factor (0.8 and 1.841 for Pr3+ and Nd3+ ions, respectively), it should be the other way around. For other RNiAl, the ordering temperatures roughly follow the de Gennes factor [3]. Comparing models of magnetic structure with the rare earth magnetic moments lying in the basal plane or along the c-axis, better agreement between calculated and observed intensities can be achieved, for both compounds, with models of magnetic structure where Nd (Pr) magnetic moments lie in the basal plane. However, the obtained refinement factors of Rln = 20% are not satisfactory. This conclusion about the direction of the magnetic moments is corroborated by the observed magnetization curves: magnetization of the ‘polycrystal’ is almost the same as magnetization of the ‘free powder’. In the case of uniaxial anisotropy (magnetic moments along the c-axis), magnetization of the ‘polycrystal’, above the metamagnetic transition as long as the field is not too large, should be significantly lower and could be reduced to nearly 50% of the values obtained with the ‘free powder’. However, in order to come to a final decision about the direction and magnitude of the magnetic moments in PrNiAl and NdNiAl, single crystal experiments are necessary.
4. Conclusions NdNiAl and PrNiAl exhibit antiferromagnetic ordering below TN = 2.4 and 6.5 K, respectively. The magnetic structure of these compounds between 1.8 K and TN is incommensurate, described by the prop-
agation vector k = ( l/2,0,4) with q = 0.46 and 0.4 1 for NdNiAl and PrNiAl, respectively. The antiferromagnetic structure is destroyed at metamagnetic transitions at B = 1 and 6 T, respectively. A single crystal experiment is suggested in order to determine further details of the magnetic structures of NdNiAl and PrNiAl.
Acknowledgements One of the authors (P.J.) acknowledges the CENG and MRES for hospitality and financial support. Part of this work was supported by the Grant Agency of the Czech Republic (grant No. 202/96/0207) and the Grant Agency of Charles University (grant No. 145/95).
References [II A.E. Dwight et al.. Trans. Metall. Sot. AIME 242 (1968) 2075.
[21 H. Oeaterreicher,
J. Less-Common Metals 30 (1973) 225. [31 N.C. Tuan. V. Sechovski, M. DiviS. P. Svoboda, H. Nakotte, F.R. de Boer and N.H. Kim-Ngan, J. Appl. Phys. 73 (1993) 5677. [41 N.C. Tuan, Ph.D. thesis, Charles University, Prague, 1992. [51 P. JavorskG. N.C. Tuan, M. Divi’s, L. Havela, P. Svoboda, V. Sechovsk? and G. Hilscher, J. Magn. Magn. Mater. 140-144 (1995) 1139. [61 H. Maletta and V. Sechovskk, J. Alloys Compounds 207/208 (1994) 254. [71 P. Javorski, P. Burlet, E. Ressouche, V. Sechovskk and G. Lapertot. J. Magn. Ma&n. Mater. 159 ( 1996) 324. [81 P. Javorskk, P. Burlet, E. Ressouche, V. SechovskG. H. Michor and G. Lapertot, Physica B 225 (1996) 230. FULLPROF, version 2.4 (1993). ILL, 191 J. Rodriguez-Carvajal, unpublished. [lOI V.F. Sears, Neutron News 3 (I 992) 26. [I 11 E.F. Bertaut, J. Phys. (Paris) 32 (I 97 I) C I-462; E.F. Bertaut, Acta Crystallogr. A 24 (1968) 217. [121 E.R. Hovestreydt, M.I. Aroyo and H. Wondratschek, J. Appl. Crystallogr. 25 (1992) 544. A.V. Andreev, J. [I31 P. Javorsk9, P. Burlet. V. Sechovsk?. Brown and P. Svoboda, to be published.
Journal of Magnetism
and Magnetic Materials
164
(I 996) I83- 186
Neutron diffraction study of magnetic ordering in NdNiAl and PrNiAl P. Javorskf
a.b.*, V. Sechovsk$ b, R.R. Arons ‘, P. Burlet ‘, E. Ressouche ‘, P. Svoboda ‘, G. Lapertot a
Received 7 March 1996; revised
13May I996
Abstract Polycrystalline samples of NdNiAl and PrNiAl have been studied by powder neutron diffraction and magnetization measurements. Antiferromagnetic ordering below T, = 2.4 K (for NdNiAl) and 6.5 K (for PrNiAl) has been confirmed. The magnetic structure of both compounds can be described by a propagation vector k = (1/2,O,q) with q = 0.46 and 0.41 for NdNiAl and PrNiAl, respectively. Kewvr&: curves
Neutron diffraction;
Rare earth intermetallic compounds; Spin arrangements
1. Introduction RNiAl (R = rare earth except for La and Eu) intermetallic compounds crystallize in the ZrNiAltype hexagonal structure (space group P62m) [ 11. Ferromagnetic ordering at low temperatures has been reported in Ref. [2] for most compounds, but detailed specific heat, susceptibility, resistivity and magnetization studies [3-51 provided evidence of a more complex magnetic behaviour. Complex magnetic structures in some RNiAl (R = Tb, Er and Ho) compounds have been determined employing neutron diffraction experiments [6-81. Indications of antifer-
I Corresponding author. [email protected].
Fax:
+42-2-2491-5050;
email:
in magnetically
ordered
materials:
Magnetization
romagnetic ordering below 2.4 and 6.5 K in NdNiAl and PrNiAl, respectively, were revealed by susceptibility and specific heat measurements [3,4]. The aim of the present work is to study the type of magnetic ordering in NdNiAl and PrNiAl by magnetization and powder neutron diffraction measurements.
2. Experimental Polycrystalline samples (m = 3 g) of NdNiAl and PrNiAl used for magnetization measurements were prepared by arc-melting stoichiometric mixtures of the pure elements (3N for Nd and Pr, 5N for Ni and Al) in an argon atmosphere. The samples for neutron diffraction (m = 20 g each) were prepared in a water-cooled copper crucible using high frequency heat-
0304-8853/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SO304-8853(96)0038 l-2
ing. X-ray analysis showed NdNiAl (both samples) and PrNiAl (3 g sample) to be single phase. In the larger PrNiAl sample. a certain amount of Pr oxides was detected. Magnetization measurements were performed in fields up to 5 T at several temperatures using a SQUID magnetometer and in fields up to 35 T (at 4.2 K) at the High-Field Installation of the University of Amsterdam. Two types of fine powder samples were used: powder grains free to orient in an applied field (‘free powder’) and powder consisting of randomly oriented grains fixed by frozen ethanol (‘polycrystal’). Neutron diffraction experiments with powder samples were performed on the DN5 diffractometer at Siloed Grenoble. at a wavelength of A = 2.496 f 0.006 A with samples placed in a vanadium container. The measured data were analysed using the Rietveld refinement method, using the FULLPROF program [9] with the Fermi lengths and absorption coefficients from Ref. [lo]. To apply group theory [ 111 we used the KAREP and CS programs [ 121.
3. Results and discussion The magnetization curves for NdNiAl and PrNiAl are shown in Figs. 1 and 2. Below the ordering temperatures [4], both compounds exhibit a metamagnetic transition. The value of the critical field for PrNiAl (TN = 6.5 K) and NdNiAl (TN = 2.4 K) is
0.0 0
IO
20
30
40
B VI Fio 1. High-field magnetization curve for NdNiAl measured at 4.;. K (i.e. above T,): inset: low-field magnetization curves measured at 2 and 4 K (‘polycrystal’).
i
2.0 +
.
I “free powder”
“polycr~stal” . .v .
I
.
0.0
.
.
.
1 I
. .vy
1
0
IO
20
30
40
n VI Fig. 2. High-field magnetization curve of PrNiAl measured at 4.2 K; inset: low-field magnetization curves measured at 4 and 8 K (‘polycrystal’).
about 6 T (at 4.2 K) and 1 T (at 2 K), respectively. The absence of any remanence corroborates the conclusion about the antiferromagnetic ordering. Both compounds exhibit quite similar magnetization behaviour in high fields. Beyond initial signs of saturation, seen from a strong curvature around B g 8 T, the magnetization still increases considerably with increasing field and no saturation is reached up to 35 T. The values of the magnetic moments observed at 35 T (2.0 and 2.lp_a/f.u. for PrNiAl and NdNiAl, respectively) are only about 63 and 65% of the free Pr3+ and Nd’+ ion values, respectively. The diffraction patterns for NdNiAl and PrNiAl were recorded above (T= 10 K) and below (T= I.8 K) the ordering temperatures (= 5 h each pattern). Additional measurements with small temperature steps between 1.8 K and the expected TN [4] were performed with a short exposure time ( = 15 min). The unit cell of the ZrNiAl-type hexagonal structure contains three rare earth atoms at (x,0,1 /2), (0,x, l/2) and ( --x, - .\-,I /2) positions, three Al atoms at ( y,O.O), (O,v,O) and c-y, - y,O) and three Ni atoms at (l/3,2/3,0), (2/3,1/3.0) and (O,O, l/2) positions. At T = IO K, only nuclear reflections of the studied compounds were detected. Refinement of these data for NdNiAl gives the lattice parameters a = 7.00 k 0.01 A and c = 4.07 k 0.01 A and the atomic position parameters xNd = 0.578 * 0.001 and J,, = 0.241 k 0.003. These values are comparable to those of other RNiAl compounds [2,6-g]. In the case of
the PrNiAl sample, several additional peaks of Pr oxides were observed. Thus we could not separate the nuclear peaks well and evaluate the structure parameters precisely. At T= I.8 K. numerous additional magnetic reflections appear in the neutron powder patterns of both compounds. The poor statistics due to the short measurement time prevented us analyzing details of the temperature dependence of the magnetic reflections. Nevertheless. we note that the intensities of all magnetic reflections continuously decrease with increasing temperature and vanish at the proposed T,. For the PrNiAl sample. all newly observed (magnetic) retlections at I .X K belong to the studied phase. and not to the oxides. This conclusion is also corroborated by the fact that all these peaks can be indexed by a single propagation vector (see following). In agreement with results of bulk measurements [3,4]. no clear indications of any additional magnetic phase transition below T, have been obtained. The lack of any additional intensity on top of the nuclear peaks down to I .8 K confirms the absence of a ferromagnetic component in the magnetic structure of NdNiAl and PrNiAl. All the magnetic reflections (see Figs. 3 and 4) can be described by a propagation vector k = (1/2,0.~/) with q = 0.46 (NdNiAl) or 0.41 (PrNiAl). In order to sort out all possible orientations of the magnetic moments. we used group theory [ 11.12].
110 -~-~~
Fig, 3. Neutron
NdNiAl
o 0.46) I
diffraction
patterns for NdNiAl
experimental
points, (-1
( .t = 0.57X.
1’= 0.241 ): mapetic
inset only.
k = (0.5
calculated
pattern\ reflection\
at T =
I .8 K:
(
.)
of nuclear retlectiom are indexed
in the
Sl
Following
’ -
7
Fig. 1. Differential
I ,OK): the
PrNiAl k = (0.5 0 0.41)
neutron difl’raction
most intensive
rctlections
the procedure
patterns for PrNiAI
( I, hh -
are indexed.
described in Ref. [I I] for or 0.41) we obtained the same final results as for k = (I /2.0. I /2). which are described in detail in Ref. [ 131. There are six possible solutions if all the rare earth magnetic moments (negligible Ni moments are considered) propagate with the same propagation vector. They are all oriented along the c-axis or they lie in the basal plane. Moments at (x.0.1/2) positions are coupled ferromagnetically or antiferromagnetically with the nearest neighbour moments at ( -x. - .Y,I /2) positions. Furthermore, one has to also take into account the possibility that the rare earth moments at different positions propagate with different propagation vectors k = (1/2.O.q), k’ = (O.l/2,q) or k” = (- I /2,1/2.4) which are equivalent in hexagonal symmetry. Due to the relatively weak intensities of the magnetic reflections in both compounds we could not make a final decision as to the magnetic moment directions and magnitudes. However, we can make certain qualitative conclusions. The intensities of the (07 0) + k and (0 I 1) + k reflections are relatively very intense for NdNiAl, but almost absent for PrNiAl. According to the representation theory results, and also taking into account the intensities of (000) + k. (OOT)+k, (OlO)+ k and (Oli)+k reflections (which are relatively strong), it follows that magnetic moments at (.~.0.1/2) and ( -x, - s. I /2) are, if they propagate with the same propagation vector, coupled antiferromagnetically in NdNiAl and ferromagnetically in PrNiAl.
k = (I /2.O.q) (q = 0.46
The antiferromagnetic coupling of moments at (x,0,1/2) and (-x,-x,1/2) is a common behaviour of all previously studied RNiAl (R = Tb. Ho, Er) compounds [6-8,131. From this point of view, PrNiAl is an exception. Here, we would like to point out that PrNiAl orders magnetically at higher temperature than NdNiAl, although, according to the de Gennes factor (0.8 and 1.841 for Pr3+ and Nd3+ ions, respectively), it should be the other way around. For other RNiAl, the ordering temperatures roughly follow the de Gennes factor [3]. Comparing models of magnetic structure with the rare earth magnetic moments lying in the basal plane or along the c-axis, better agreement between calculated and observed intensities can be achieved, for both compounds, with models of magnetic structure where Nd (Pr) magnetic moments lie in the basal plane. However, the obtained refinement factors of Rln = 20% are not satisfactory. This conclusion about the direction of the magnetic moments is corroborated by the observed magnetization curves: magnetization of the ‘polycrystal’ is almost the same as magnetization of the ‘free powder’. In the case of uniaxial anisotropy (magnetic moments along the c-axis), magnetization of the ‘polycrystal’, above the metamagnetic transition as long as the field is not too large, should be significantly lower and could be reduced to nearly 50% of the values obtained with the ‘free powder’. However, in order to come to a final decision about the direction and magnitude of the magnetic moments in PrNiAl and NdNiAl, single crystal experiments are necessary.
4. Conclusions NdNiAl and PrNiAl exhibit antiferromagnetic ordering below TN = 2.4 and 6.5 K, respectively. The magnetic structure of these compounds between 1.8 K and TN is incommensurate, described by the prop-
agation vector k = ( l/2,0,4) with q = 0.46 and 0.4 1 for NdNiAl and PrNiAl, respectively. The antiferromagnetic structure is destroyed at metamagnetic transitions at B = 1 and 6 T, respectively. A single crystal experiment is suggested in order to determine further details of the magnetic structures of NdNiAl and PrNiAl.
Acknowledgements One of the authors (P.J.) acknowledges the CENG and MRES for hospitality and financial support. Part of this work was supported by the Grant Agency of the Czech Republic (grant No. 202/96/0207) and the Grant Agency of Charles University (grant No. 145/95).
References [II A.E. Dwight et al.. Trans. Metall. Sot. AIME 242 (1968) 2075.
[21 H. Oeaterreicher,
J. Less-Common Metals 30 (1973) 225. [31 N.C. Tuan. V. Sechovski, M. DiviS. P. Svoboda, H. Nakotte, F.R. de Boer and N.H. Kim-Ngan, J. Appl. Phys. 73 (1993) 5677. [41 N.C. Tuan, Ph.D. thesis, Charles University, Prague, 1992. [51 P. JavorskG. N.C. Tuan, M. Divi’s, L. Havela, P. Svoboda, V. Sechovsk? and G. Hilscher, J. Magn. Magn. Mater. 140-144 (1995) 1139. [61 H. Maletta and V. Sechovskk, J. Alloys Compounds 207/208 (1994) 254. [71 P. Javorski, P. Burlet, E. Ressouche, V. Sechovskk and G. Lapertot. J. Magn. Ma&n. Mater. 159 ( 1996) 324. [81 P. Javorskk, P. Burlet, E. Ressouche, V. SechovskG. H. Michor and G. Lapertot, Physica B 225 (1996) 230. FULLPROF, version 2.4 (1993). ILL, 191 J. Rodriguez-Carvajal, unpublished. [lOI V.F. Sears, Neutron News 3 (I 992) 26. [I 11 E.F. Bertaut, J. Phys. (Paris) 32 (I 97 I) C I-462; E.F. Bertaut, Acta Crystallogr. A 24 (1968) 217. [121 E.R. Hovestreydt, M.I. Aroyo and H. Wondratschek, J. Appl. Crystallogr. 25 (1992) 544. A.V. Andreev, J. [I31 P. Javorsk9, P. Burlet. V. Sechovsk?. Brown and P. Svoboda, to be published.