On the modulated structure of La2Co1.7:

Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

On the modulated structure of La Co :    a neutron Laue di!raction study C. Wilkinson 
, P. Schobinger-Papamantellos*, D. Myles, L.D. Tung, K.H.J. Buschow Department of Physics, King's College London Strand, London WC2R 2LS, UK
Institut Laue Langevin (ILL), Avenue des Martyrs, BP156, 38042 Grenoble, Cedex 9, France Laboratorium fu( r Kristallographie, ETHZ CH-8092 Zu( rich, Switzerland European Molecular Biology Laboratory (EMBL), Avenue des Martyrs, BP156, 38042 Grenoble, Cedex 9, France International Training Institute for Materials Science-ITIMS, 1 Dai Co Viet Str., Hanoi, Viet Nam Van der Waals-Zeeman Institute, University of Amsterdam Valckenierstr. 65 1018 XE Amsterdam, Netherlands Received 7 December 1999; received in revised form 20 March 2000

Abstract A preliminary analysis is presented of neutron Laue di!raction data from a single crystal of La Co at temperatures    between 295 and 15 K, recorded on neutron-sensitive image plates using a thermal neutron beam. The data can be interpreted in terms of a charge (nuclear) density wave with a propagation vector q"(q , 0, q ) with q "0.113(1)a* and V X V q "0.203(2)c*. It has also been con"rmed by using a band-pass "lter with a cold neutron beam that antiferromagnetic X Bragg re#ections which have a magnetic propagation vector q "(, , 0) exist below 144(1) K and correspond to   a cobalt atom moment of 0.80(15) l . Magnetic satellite re#ections with wave vectors corresponding to q$q have also been observed on the Laue diagrams below this temperature. The values of the propagation vector and magnetic moment have been con"rmed from neutron powder di!raction diagrams taken at 150 and 1.5 K.  2000 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ez; 74.72.Dn; 74.72.Hs; 75.25.#z Keywords: Neutron di!raction; Laue diagrams; Single crystals

1. Introduction The magnetic and structural properties of La Co have been investigated by several   

* Corresponding author. Tel.: #41-1-632-3773; fax: #41-1632-1133. E-mail address: [email protected], [email protected] (P. Schobinger-Papamantellos).

authors [1}6]. It has been reported by Gignoux et al. [1] that the material (which is isomorphous with Pr Co and Nd Co ) is a hexagonal antifer      romagnet with a"4.89 As , c"4.31 As and a paramagnetic to anti-ferromagnetic phase transition at 146 K resulting in Bragg re#ections which can be described by a propagation vector (, , 0). Al though La Co has been reported [2] to have an    incommensurate structure associated with chains of Co atoms along the hexagonal axis, a very

0304-8853/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 3 2 8 - 0

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detailed determination has so far not been made concerning the exact nature of the modulation. Traditionally, magnetic structures have been studied by monochromatic powder or single-crystal neutron di!raction experiments. Powder di!raction works well in the case where the structure is relatively simple and the di!raction lines, which are additional to those due to the nuclear structure can be easily resolved. If the structure is more complex (e.g. that of La Co ), single-crystal di!raction    techniques provide a precise tool to enable the changes in the development of individual re#ections to be followed closely and integrated accurately. However, even at a high #ux neutron beam reactor such as the Institute Laue}Langevin (ILL) in Grenoble, the collection of a full monochromatic data set up to a limit of sin h/j&0.5 from a single crystal with many thousands of possible satellite re#ections would take several weeks of beam time. This is impractical at a user-oriented Institute if it is necessary to measure the structure at several temperatures. One way to speed up data collection is to use a Laue (white beam) technique, allowing a greater range of the neutron spectrum to fall on the specimen. Using the technique and a large detector, the data collection rate is speeded up by over an order of magnitude and a full data set can be collected for a particular temperature within a matter of hours. The technique has the additional advantage that it gives a complete survey of reciprocal space, so that any additional re#ections present, which are not predicted in a conventional monochromatic data collection strategy, are evident.

phosphor combined with an organic binder mounted on a #exible plastic backing sheet. The photostimuable material most commonly used in BaFBr doped with Eu> ions and when irradiated with X-rays or c-rays, electrons liberated by ionising Eu> to Eu> are trapped in Br vacancy states just below the conduction band to form colour centres. In order to use the plates to detect neutrons, it is necessary to have a neutron converter and the most obvious candidate is Gd, which has a high crosssection for thermal neutrons and produces conversion electrons and prompt c-rays. The plates which we have used were fabricated by Fuji [7] and have a Gd O content of 34% by weight, a detective   quantum e$ciency of &20% and a point-spread function of &100 lm. These neutron-sensitive image plates were used to construct LADI, a neutron di!ractometer [8] shown in Fig. 1. The image plates which cover an area of 800;400 mm are mounted on the outside of an aluminium drum of thickness 4 mm, width 400 mm and circumference 1000 mm, subtending an angle at the specimen of $1443 in 2h and $523 in l. During exposure, neutrons enter the cylinder through a hole and are di!racted from the stationary specimen crystal located on a rotation axis ( ) coincident with the cylinder axis. The Bragg re#ections, which arise pass through the drum and are stopped in the gadolinium-containing phosphor. In order to cool the specimen, an Edwards Displex cryostat capable of reaching a temperature of 11.5 K can be used.

2. The image plate di4ractometer To bene"t fully from the increased number of re#ections active when using the Laue technique, it is necessary to use a large position-sensitive detector. Attractive candidates are the newly developed neutron-sensitive image plates, which are e!ectively electronic "lms and have a dynamic range &10. The neutron-sensitive plates are based upon X-ray image plates, which consist of a thin (&150 lm) layer of powdered photostimulable

Fig. 1. The neutron image plate di!ractometer LADI.

C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

57

For readout, the drum is rotated at 350 rpm and the beam from a red diode laser is passed over the plates via a read head which collects the blue light emitted from the storage phosphor. The read head containing a blue-sensitive photomultiplier tube is moved slowly (80 mm per min) in a direction parallel to the rotation axis, giving a read time of 5 min. The two motions are coupled and the integration time of the photomultiplier signal is arranged to give square pixels of 200 mm on edge.

3. Laue data collection The crystal used in the experiment was a specimen of approximately 3 mm;3 mm;3 mm cut from a single-crystalline rod of La Co grown    by means of a modi"ed tri-arc Czochralski technique. To characterise the magnetic properties of the specimen, susceptibility measurements were made on a SQUID magnetometer in the temperature range 4.2}300 K. These results which are shown in Fig. 2 gives a NeH el temperature of 145(1) K and are in good agreement with the data reported in Ref. [4]. The di!ractometer was installed at S34 station the end of the thermal neutron guide H22 at the ILL, with neutrons in the wavelength range &0.8}4.5 As incident upon the specimen. Exposures times of 30 min were su$cient to give the Laue diagrams in which the fundamental re#ections were very strong. Weaker (a few % of fundamental intensities) satellites were also visible. Exposures taken at temperatures of 295, 155, 80 and 15 K with the same crystal orientation showed satellite distributions of similar intenisty. One-half of an image taken at 15 K ( "03 rotation) is shown in Fig. 3. The cylindrical detector has been opened out so that the vertical direction on the image corresponds to the direction of the rotation axis, the horizontal direction to half of the circumference. Full data sets as 295 K and 15 K were taken using Laue exposures with 303 rotation steps over a range of 1803. In order to measure the satellite re#ections the exposure times were raised to 120 min. With a wide wavelength band (0.8}4.5 As in this case) there is energy (wavelength) overlap

Fig. 2. Temperature dependence of the reciprocal susceptibility of a La Co single crystal measured along the a-axis (full    symbols) and perpendicular to the c-axis (open symbols).

between low-order re#ections (e.g. re#ection h k l at wavelength j is superposed on 2h 2k 2l and j/2). While in general this can be shown to be a tractable problem [9], with atleast 83% of all re#ections resolved for a wide waveband, the overlap can be particularly serious for low-order re#ections [10]. In order to resolve magnetic re#ections with propagation vector (, , 0) from higher-order nu clear (fundamental) re#ections, quasi-Laue di!raction data were taken with the LADI di!ractometer on the S42 cold neutron guide at ILL, employing a NiTi multilayer "lter module normally used in protein data collection [11]. The NiTi module was oriented to divert neutrons with wavelengths between &2.5 and 3.5 As at a small angle to the main beam. Using these neutrons, data were collected with exposure times of 30 min per frame, when the magnetic and nuclear re#ections were strong and

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C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

Fig. 3. Laue diagram of La Co at 15 K. The main beam position is at the centre of the hole visible at the right-hand side of the image.    Strong fundamental re#ections and hexagonal groups of satellites are visible.

well resolved. With the smaller bandpass, it was necessary to take spectra at 73 steps in in order to cover the whole of reciprocal space. Full data sets were collected at 15 K and 60 K out to sin h/j&0.35, the limit set by the neutron wavelengths used. In addition, the temperature variation of an intense magnetic re#ection was traced between 15 K and 157 K by taking a series of 15 min exposures at close temperature intervals at a constant

angle.

4. Data treatment and analysis 4.1. Nuclear structure The Laue diagrams were displayed and indexed with the Laue programs developed at Daresbury

laboratory [12] as part of the CCP4 program suite for X-ray crystallography, with minor modi"cations to take account of the change from #at plate to cylindrical geometry. Programs have also been written to index and integrate Laue diagrams from incommensurate structures, including the prediction of satellite positions and the re"nement of propagation vectors. The diagrams are similar at all temperatures between room temperature and 15 K, showing that the modulation is not speci"cally linked to any transition observed at 146 K and it is proposed that it is due to a charge (nuclear) density wave in the material. The satellite re#ections can be indexed using a propagation vector q"(q , 0, q ) with V X least-squares "tted values q "0.113(1)a* and q " V X 0.203(2)c*. This remains unchanged up to 295 K, where the values of q "0.112(1)a* and q "0.203 V X (2)c* are found. This is consistent with data

C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

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Fig. 4. (a) Satellite prism around fundamental re#ections. Each &star' comprises 12 re#ections due to six hexagonal domains; (b) $satellites of the (1 2 2) fundamental re#ection at 15 K. The (1 2 2)\ satellites are much stronger than (122)>.

measured at 150 K on a powdered sample of La Co on the high-resolution neutron powder    di!ractometer D1A at ILL at a wavelength of 1.91 As (see Section 4.3). There are "ve other equivalent (hexagonal) domains having vectors q"(q , !q , q ), q " G V V X  (0, !q , q ), q "(!q , 0, q ), q "(!q , q , q ), V X  V X  V V X and q "(0, !q , q ).  V X Since each propagation vector gives rise to two ($q) satellite re#ections, this produces a characG teristic pattern of a six-fold prism of satellite re#ections (Fig. 4a) around a fundamental re#ection position, which can be seen in projection in the Laue diagrams. The area near the (1 2 2) nuclear re#ection is shown in Fig. 4b, with the satellite positions indicated. In this case (and in general), the !q satellites (nearer the reciprocal lattice origin) G are much stronger than the #q satellites. These G satellites have been observed for re#ections with all values of l between $6. While the groups of "rstorder satellites remain the most prominent features of the diagrams (after the fundamental re#ections), there are additional re#ections, which can be indexed as second-order satellites. These are observed to be generally much weaker than those of "rst order, and disappear completely at higher temperatures. Second (and higher)-order satellites can occur as a result of an anharmonic atomic site

occupation wave, or a harmonic or anharmonic atomic displacement wave in the material. It is common for both e!ects to be simultaneously present. The observed e!ects are therefore consistent with the presence of a charge (nuclear) density wave in the material. While the intensities of both fundamental and satellite re#ections are only properly described by the sums of Bessel functions, an approximate "t to the average structure can be obtained by a classical crystallographic least-squares re"nement. Adopting the hexagonally averaged structure proposed for La Co (Fig. 5(a)), and "xing the Co    site occupation at 1.7, a preliminary "t was made of a scale factor and temperature factors for the Co and La atoms to the intensities of the 15 K fundamental re#ections. The Cambridge crystallographic subroutine library (CCSL) [13] was used for re"nement and gave an R factor of 11.7% on structure factor. The re"nement was based on 422 observations of h k l Bragg re#ections, with h, k lying in the range $6 and l in the range $7, although many low-order re#ections were not used due to the overlap e!ects described in Section 3. The results (Table 1) show an acceptable temperature factor for the La atoms, but a large anisotropy in the temperature factor components for the Co atoms (the c* component is &50 times larger than the planar

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C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

Fig. 5. (a) The hexagonally averaged structure of La Co (Space group P6 /mmc); (b) the magnetic structure of La Co .       

Table 1 Structural parameters from a hexagonally averaged "t of the fundamental nuclear intensities observed at 15 K of La Co . Space group    P6 /mmc. Unit cell dimensions a"4.8807(1) As , c"4.2723(1) As  Atom

Position

Occ.

Scattering Length (cm\)

Re"ned temperature factor (As )

(a) All (h k l) re#ections re"ned with anisotropic temperature factor for Co Co

La

(0, 0, 0) (0, 0, ) 

(,  (,  Scale factor s"120(3)

,  , 

)  ) 

0.85

0.25

1.0

0.83

B "B "0.6(6)   B "29(3),  B "!2.6(3)  B"0.45(5)

Agreement on structure factor R(F)"s("F "!s"F )/s"F ""0.116 
  
 (b) Co (isotropic temperature factor) contributes only to h k 0 re#ections, La to all re#ections Co La

(0, 0, 0) (0, 0,) 0.85 0.25  (, , ) 1.0 0.83    (, , )    Scale factor s"118(3) Agreement on structure factor R(F)"s("F "!s"F )/s"F ""0.112 
  


component). A large B component ("29(3))  means that the Co contributions to re#ections with non-zero l are almost insigni"cant, which is consistent with the composite model proposed in

B"0.6(6) B"0.44(5) B"0.45(5)

Ref. [2], where the intensities of (h k 0) re#ections contain La as well as Co contributions while those of (h k l) re#ections comprise exclusively La contributions. The equivalent "t for this composite

C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

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Fig. 7. Moduli of the vector structure factors for magnetic re#ections observed at 60 K and 15 K. The quantity k (h k l$q )¹(h k l$q ) is plotted as a function of sin h/j. ! Fig. 6. Variation in the integrated intensity of the (, !, 0)   re#ection as a function of temperature.

model is shown in Table 1b, from which it can be seen that the "t and the scale factor are almost unchanged. An average &classical' "t is clearly insu$cient to describe accurately that details of the structure, but is adequate to obtain a preliminary scale factor to apply to the observed magnetic intensities. A fuller and more appropriate "t of the fundamental and satellite re#ections is being undertaken using the JANA suite of programs [14] which employs four-dimensional space groups for the description and re"nement of incommensurable structures. 4.2. Magnetic structure Below 146 K, magentic re#ections described by a propagation vector q "(, , 0) were found in the  quasi-Laue di!raction patterns collected using a neutron band pass from 3.5 A to 4.5 A. Re#ections in positions h k l$(, , 0) were observed, strong  with l"0 and weak otherwise. The measured variation of the intensity of the (, !, 0) re#ection   is shown in Fig. 6, giving a NeH el temperature of 148(2) K. There appear to be short-range order e!ects, however, which make this value higher than

that (145 K) obtained from the magnetisation measurements. A more recent lSR measurement [15] gives a value 143.86$0.79 K which might be considered as the most precise. Weak magnetic satellite re#ections were also detected in positions h k l$q $q(i"1, 6) with G q "0.113a* and q "0.203c*, corresponding to V X the six-fold prism shown in Fig. 4(a), in this case about the magnetic re#ections h k l$(, , 0). These  were observed only for non-zero l, were strongest for l"2 and as in the case of the nuclear scattering satellites with !q were generally stronger than G the #q satellites. These weak satellite re#ections G were absent above the magnetic transition temperature. The h k l$q intensities can be "tted to the triangular magnetic structure proposed by Gignoux et al. [1,3], in which the Co moments rotate within the (0 0 1) plane by p/3 along [1 0 0] and [0 1 0] directions, and 2p/3 along the [1 1 0] direction (Fig. 5(b)). A simplifed forumulation for the modulus of the vector magnetic structure factor according to this model is given (in Bohr magnetons) by "F(h k l$q )""(1.7 k ) f (h k l$q )  ! ;¹(h k l$q )k (l even), "0(l odd),

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C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

where k is the moment, f(h k l$q ) is the mag! netic form factor and ¹(h k l$q )"exp (! dHB dH) is a temperature term associated  H GH G with individual Co atoms. Using a scaling factor derived from the hexagonally averaged nuclear re#ection "t and the amplitudes of h k l (nuclear) re#ections observed on the same quasi-Laue spectra as the magnetic re#ections enables values of the quantity k f(h k l$q )T(h k l$q ) to be plot!

ted as a function of sin h/j ("d*(h k l$q )). These  are shown for temperatures of 15 and 60 K for l"0 re#ections in Fig. 7. Weak re#ections with l"$1, $2, $3 were also observed but have not been included in the diagram as their structure factors are very small due to the large value of B . These  re#ections are predicted to have zero intensity when B is in"nite, as implied in the simple com posite model [2]. The existence of weak re#ections

Fig. 8. Observed and calculated powder patterns of La Co measured on D1A with a neutron wavelength of 1.9114 As at (a) 150 K    paramagnetic state (top part). Arrows denote the strongest satellites; (b) 1.5 K magnetically ordered state (bottom part). The few weak magnetic re#ections pertaining to q "(, , 0) are denoted by arrows. 

C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

can only be explained by a detailed incommensurate description of the Co atomic positions. To put them on the same scale, the 60 K values have been mulitplied by 1.10, the average experimental scale factor between the amplitudes for identical re#ections observed at 15 and 60 K. Extrapolating the moment value to sin h/j"0 gives an intercept of 0.80(15) l at 15 K. 4.3. High-resolution powder data For comparison, using the re"ned single-crystal wave vector, two sets of powder data of La Co ,    previously collected at 150 K and 1.5 K on the high-resolution neutron powder di!ractometer D1A at ILL at a wavelength of 1.91 As (Fig. 8) were indexed and re"ned using the composite model proposed in Ref. [2]. As expected, the main characteristic of the patterns is the presence of very strong fundamental re#ections and very weak satellites. The model used in the re"nement of the main intensity re#ections was the composite structure proposed in Ref. [2] (re#ections with l"0 have both Co and La contributions, those with non-zero l have La contributions only). The Co and La atomic positions were "xed (values given in Table 1 and Co occupation-re"ned, leading to the formula La  Co . The re"ned isotropic temperature factors   were B"0.66(1)[As ] for La and 0.06(1) [As ] for Co. The weak satellites present in the 150 K pattern were "tted by the pro"le-matching procedure available in the program Fullprof [16], using the wave vector obtained from the single-crystal Laue measurements. An extracted pro"le of the satellite re#ections (which is displayed in Fig. 8 below the observed/calculated main intensity pro"les) shows how weak they are compared to the main re#ections. The reliability factors for the two sets of nuclear re#ections, the satellites and the weighed and expected pro"le values are as follows: R "  2.2%, R "3.3%, R "1.5%, R "12.5%,    R "7.3%, s"2.9.  The re"nement of the 1.5 K data obtained in the magnetically ordered regime was e!ected using the wave vector (, , 0), and the R-factors obtained  were similar to those for the 150 K re"nement. Due to the very weak magnetic re#ections the re"nement of the magnetic intensities was carried out on

63

the di!erence diagram obtained by subtracting the 150 K data from the 1.5 K data. A moment value 0.64(1) l /Co atom was obtained.

5. Discussion In the idealised atomic structure of La Co the    two La atoms are located at (, , ) and (, , ),   while the two cobalt atoms are located at (0, 0, 0) and (0, 0, ) and form chains along the c-axis in  which the Co atoms are in contact. However, the c lattice constant (4.31 As ) is too short to accommodate two cobalt atoms of radius 2.37 As in close contact. Schweizer et al. [2] suggest that the Co atoms form continuous chains along c with an interatomic distance of 2.37 As . However, the correlation between these Co chains is weak, giving rise to di!use planes of intensity that were observed by the latter authors to be perpendicular to the c-axis. Ballou [6] has reported that these di!use layers are formed by Bragg peaks which can be indexed as (h$l)a*#2l(l!e)c*, with l"0.112(2) and e"0.082(2). While the a* component corresponds exactly with that observed in our experiment, the variation of the c* component (in particular with l index) cannot be made to "t with our observations, where there is a constant $q X component of 0.203(2)c* associated with each l layer. The observation by Ballou of satellite re#ections at a distance q "0.164(4)c* below the second X fundamental layer (the l"$1 satellite with e"0.082), or 0.426(1) As \ from the reciprocal origin, correspond to a real space distance of 2.35(1) As . The strongest nuclear satellites which we observe in our experiment have indices of the type (h k 2)!q, lying below the fundamental re#ections with l"2, at a distance of 0.421(1) As \ from the reciprocal origin. This corresponds to a real lattice distance of 2.38(1) As , in reasonable agreement with the interpretation by Ballou et al. [3] of a Co atom spacing 2.35 As . The di!erence in the two explanations lies in the correlation between the columns of Co atoms, which are three-dimensionally ordered under the in#uence of the charge density wave. A complete structural description of the system and an explanation for the existence of these satellite

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C. Wilkinson et al. / Journal of Magnetism and Magnetic Materials 217 (2000) 55}64

re#ections must await the more detailed JANA analysis of the data, which is presently being undertaken. We "nd no measurable variation of the propagation vector"(0.113(1), 0, 0.203(2)) between low and ambient temperatures, but there does appear to be some variation in the relative intensities of re#ections, in both the fundamentals and the satellites within this temperature range. We are presently studying the amplitude, form and direction of the Co atom displacements for the data taken at 295, 155, 80 and 15 K in order to explain the observed fundamental and satellite re#ection intensities at these temperatures. The previously reported [1}5] transition from a paramagnetic to an antiferromagnetic structure near 146 K has been con"rmed by our observed NeH el temperatures within a 3p limit of 148(2) K from neutron data, 145(1) K from the magnetic measurements and 144(1) K by lSR and a value of 0.80(15) l obtained for the magnetic moment of the Co atoms at 15 K. The relatively large error estimate on the moment value arises from an uncertainty in the magnetic domain population (demonstrated by the scatter in Fig. 7 of nominally equivalent magnetic re#ections) and the possibililty of an anomaly [3] at low sin h/j in the form factor for Co. The value is rather larger than the moment of 0.64(1) l from our powder di!raction measurements at 1.5 K and also larger than that of 0.7(1) l found by Gignoux et al. [1] from powder measurements taken at 5 K. The 0.96(3) l reported from the single-crystal study of Ballou et al. [3] is higher, although this is partly due to the assumption of an anomaly in the magnetic form factor. In addition, there is evidence from single-crystal X-ray studies described by Ballou [6] and also Dusek [17] that there may be variations in the incommensurable ordering of the Co atoms from one specimen of La Co to another, and this could a!ect the aver   age moment value. It is interesting that the hexagonal prism of magnetic satellite re#ections ($q (i"1, 6)) is observG able below 144 K in association with magnetic re#ections positions h k l$q $q where l is not G equal to zero. These satellites are expected from di!raction theory, since the magnetic moments associated with the Co atoms will conform to the

same spin density wave as the charge density wave, which describes the positions of their nuclei. The detailed interpretation of the intensities of these (and all other) re#ections must therefore await the description of the displacements of the Co atoms under the in#uence of the charge density wave. Acknowledgements The authors are very grateful to F. Cipriani, J.C. Castagna and L. Claustre of EMBL for technical assistance. References [1] D. Gignoux, R. Lemaire, R. Mendia-Monterroso, J.M. Moreau, J. Schweizer, Physica B 130 (1985) 376. [2] J. Schweizer, K.A. Strnat, J. Tsui, Ninth Rare Earth Research Conference Blacksburg, VA, USA, 1971. [3] R. Ballou, D. Gignoux, R. Lemaire, J. Schweizer, Acta Phys. Polonica A 72 (1987) 25. [4] R. Ballou, D. Gignoux, R. Lemaire, R. Mendia-Monterroso, J. Schweizer, J. Magn. Magn. Mater. 54}57 (1986) 499. [5] D.N.H. Nam, L.D. Tung, P. Nordblad, N.P. Thuy, N.X. Phuc, J. Magn. Magn. Mater. 177}181 (1998) 1135. [6] R. Ballou, Thesis, L'Universite Scienti"que et Medicale et L'Institut National Polytechnique de Grenoble, 1987. [7] N. Niimura, Y. Karasawa, I. Tanaka, J. Miyahara, K. Takahashi, H. Saito, S. Koizimi, M. Hidaka, Nucl. Instr. and Methods A 359 (1994) 521. [8] F. Cipriani, J.-C. Castagna, C. Wilkinson, P. Oleinek, M.S. Lehmann, J. Neutron Res. 4 (1996) 79. [9] D.W.J. Cruickshank, J.R. Helliwell, K. Mo!at, Acta Crystallogr. A 43 (1987) 656. [10] S. Weisgerber, J.R. Helliwell, J. Chem. Soc. Faraday Trans. 89 (1993) 2667. [11] N. Niimura, Y. Minezaki, T. Nonaka, J.-C. Castagna, P. Hoghoj, M.S. Lehmann, C. Wilkinson, Nature Struct. Biol. 4 (1997) 909. [12] J.W. Campbell, Q. Hao, M.M. Harding, N.D. Nguti, C. Wilkinson, J. Appl. Crystallogr. 31 (1998) 496. [13] J.C. Matthewman, P. Thompson, P.J. Brown, CCSL library, J. Appl. Crystallogr. 15 (1982) 167. [14] V. Petricek, M. Dusek, JAN'98, Crystallographic system for Standard, Modulated and Composite Crystals, Institute of Physics, Praha, Czech Republic, 1998. [15] A. Schenck, private communication. [16] J. RodrmH guez-Carvajal, Physica B 192 (1993) 55. The manual of FullProf can be obtained from a Web browser at http://www-llb.cea.fr/fullweb/powder.htm. [17] M. Dusek, private communication.