Download Report

ARTICLE IN PRESS
Physica B 356 (2005) 141–145
www.elsevier.com/locate/physb
ILL polarised hot-neutron beam facility D3
E. Lelie`vre-Bernaa,, E. Bourgeat-Lamia, Y. Giberta, N. Kernavanoisa,
J. Locatellia, T. Marya, G. Pastrellob, A. Petukhova, S. Pujola, R. Rouquesa,
F. Thomasa, M. Thomasa, F. Tasseta
a
Institut Laue-Langevin, 6, rue J. Horowitz, BP 156, Cedex 9, 38042 Grenoble, France
b
AZ-Syste`me, 38170 Seyssinet-Pariset, France
Abstract
D3 is a very comprehensive polarised beam facility at the renewed hot neutron source of the Institut Laue-Langevin
(ILL). In magnetic ﬁeld up to 10 T, it exploits the spin dependency of the neutron scattering cross-section for
determining unpaired electron magnetisation in crystals. The technique applies very successfully to molecular
compounds, heavy fermions, high-T c superconductors, transition metals and actinide alloys.
Within the frame of the ILL Millennium Programme, we have recently added polarisation analysis by taking
advantage of 3He spin ﬁlters and built a dedicated third-generation Cryopad for carrying out spherical neutron
polarimetry experiments. In the case of magnetic structures, this leads to the direct determination of the magnetic
interaction vector. Hence, D3 has become one of the most powerful tool for solving complex AF structures that had
proven to be intractable when employing other techniques. Moreover, when the magnetic and nuclear scattering occur
at the same position in the reciprocal space, it allows a precise determination of the AF magnetisation distributions.
D3 can also be used for many purposes other than diffraction experiments, e.g. the search for the T-odd asymmetry
of light particle emission in 239 Pu ternary ﬁssion.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.25.+z
Keywords: Polarised neutron diffraction; 3He neutron spin ﬁlter; Magnetism; Instrumentation
1. Introduction
Since its construction in 1974 at the Institut
Laue-Langevin (ILL), the instrument D3 applies
Corresponding author. Tel.: +33 476 20 77 48.
E-mail address: [email protected] (E. Lelie`vre-Berna).
URL: http://www.ill.fr/YellowBook/D3/.
the polarised neutron diffraction (PND) technique
[1,2] to single crystals which are magnetically
ordered in a ferro- or ferri-magnetic phase under
an applied magnetic ﬁeld for determining magnetisation distributions and form factors. Assuming
a good knowledge of the nuclear structure factors
N (i.e. the Fourier components of the density of
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2004.10.065
ARTICLE IN PRESS
142
E. Lelie`vre-Berna et al. / Physica B 356 (2005) 141–145
atomic nuclei in the unit cell), the dependence of
the elastic scattering cross-section on the initial
neutron polarisation gives access to magnetic
structure factors M (i.e. the Fourier components
of the magnetisation density). In practice, one
measures the ‘‘ﬂipping ratio’’ R between the
intensities observed for ðþÞ and ðÞ initial
polarisation states at the peak of each Bragg
reﬂection. The experimental ﬂipping ratios are
easily corrected for some instrumental imperfections, and one has to take into account extinction
which may occur in the scattering process [3]. As
shown by the study of Ce3 Al11 ; it is even possible
to determine the magnetisation density of a twined
crystal [4].
A few years ago, within the frame of the ILL
Millennium Programme and the European Neutron Polarisation Initiative, we have taken advantage of the novel 3He neutron spin ﬁlter already
available at ILL for adding polarisation analysis of
the scattered beam. In elastic mode, this is simpler
and better than using a polarising crystal. Indeed,
without the need for analysing the energy of the
scattered beam, it leads to a large increase of the
ﬂux into the detector: 6 for P3 He ¼ 70% and l ¼
( and much more at shorter wavelengths.
0:843 A;
With this spin analysis option, we can also take
advantage of the vector properties of the neutron
polarisation for recovering the signiﬁcant
directional and phase information lost when only
intensities are measured. Indeed, the changes in
direction of the neutron spin that take place
on scattering by a magnetic interaction vector
are highly dependent on their relative orientations.
Using this technique, called spherical neutron
polarimetry (SNP), it has been possible to
solve a number of magnetic structures that had
proven to be intractable [5] when employing other
techniques.
2. Instrument description
2.1. Primary spectrometer
D3 is a very modular polarised neutron instrument connected to the renewed hot neutron source
of the high ﬂux reactor of the ILL. It uses readily
exchangeable Co92 Fe8 and Heusler (Cu2 MnAl)
polarising monochromators within removable
shielded cassettes in symmetric Laue geometry.
(
Wavelength change ð0:25pl ½Ap0:84Þ
is an
automatic online operation, including the insertion
of the appropriate resonant harmonic ﬁlter. This is
particularly useful when extinction or multiple
scattering are present. The incident beam polarisation depends slightly on the in-pile collimator, the
monochromator and the wavelength used (the
transport of the polarisation is not perfectly
adiabatic at the shortest wavelengths). The incident ﬂux is about 107 n cm2 s1 with Heusler
and about 5 times less with Co92 Fe8 : However, the
peak count rates
are only 2 times better at
( 1 with Heusler because of its low
sin y=lX0:4 A
take-off angle resulting in a low resolution
(Dl=l 10%). Such a reduction is frustrating
as it comes where the magnetic signal weakens.
We will soon replace the polarising monochromators with a focusing Cu (2 0 0) monochromator
combined with a 3He neutron spin ﬁlter. With
P3 He X70%; the standard deviation of the
measured magnetic structure factors will be
reduced by 1.5–2 depending on the incident
wavelength used [6,7].
2.2. Secondary spectrometer
For measuring magnetic distributions and form
factors in ferromagnetically aligned systems, D3
provides an electromagnet combined with an
Orange cryostat ð1:4pT ½Kp300) for low-ﬁeld
measurements (Hp0:6 T) and a dedicated cryomagnet (Fig. 1) which will soon be able to host a
dilution insert (0:04pT ½Kp650) for higher ﬁelds
(1pH ½Tp10). For both conﬁgurations, a large
volume of the reciprocal space is available thanks
to the 25pn ½
p þ 5 vertical access provided to
the lifting detector.
SNP measurements are performed with a thirdgeneration zero-ﬁeld polarimeter Cryopad (Fig. 2)
for investigating complex magnetic structures and
antiferromagnetic form factors [8,9]. Polarisation
analysis is made with a 3He neutron spin ﬁlter
hosted by the magneto-static cavity Decpol, a
cylindrical solenoid with extra compensation
windings mounted inside a m-metal cylinder closed
ARTICLE IN PRESS
E. Lelie`vre-Berna et al. / Physica B 356 (2005) 141–145
Fig. 1. High-ﬁeld conﬁguration of D3. In order to prevent any
accident, a ‘‘green-house’’ avoids the presence of ferromagnetic
pieces in the large in-homogeneous stray ﬁeld produced by the
cryomagnet.
with end-covers [10]. The adiabatic rotation of the
polarisation vector is ensured at the entrance of
( (i.e. 1 eV) with symmetriDecpol down to 0:28 A
cally positioned equivalent coils of variable sections inserted into the holes of the m-metal endcovers. The detector background is reduced as
much as possible with polyethylene and boronnitride plates stacked inside the m-metal cylinder.
Because of the relaxation of the 3He polarisation
[10], the 3He cell is replaced with a fresh one every
1 or 2 days. Exchanging this cell is quite easy and
takes a few minutes: one opens the back door
containing the detector and replaces the old cell
with the fresh one in the earth ﬁeld. Indeed, the
time required to exchange the cell is so short that
no special holding ﬁeld is required.
All the equipment (electromagnet, cryomagnet,
Cryopad, lifting arm, Decpol, etc.) can be installed
in a few hours without the need for intensive
calibrations. The mechanics have been designed in
order that the sample and detector shafts do not
require re-alignment. The incident beam polarisation is known in advance and the control software
takes care of any instrumental modiﬁcation and
resets the electronics automatically (plug and play
instrument). Moreover, the instrument is so
ﬂexible that any special conﬁguration can be
installed rapidly. For example, the nature and
detailed mechanism of the T-odd correlation
observed in the 233U ternary ﬁssion, which remains
a mystery compared with P-odd and P-even effects
[11], was investigated recently on D3. During the
143
Fig. 2. Zero-ﬁeld conﬁguration of D3 (beam coming from the
right). Cryopad is ﬁxed and centred on the vertical sample axis
(Z) and the 3He spin ﬁlter cell is installed inside the detector
assembly Decpol (on the left). A dilution fridge or an Orange
cryostat can be installed inside Cryopad (not visible here).
replacement of the hot-source beam tubes, it has
even been possible to move D3 to a cold guide for
testing the Laue diffraction applied to the search
for the neutron EDM [12].
2.3. Collection and analysis of data
The control software offers a powerful graphical
user interface with automatic plotting capabilities.
It is based on IGOR Pro, an extraordinarily
powerful and extensible graphing, data analysis,
and programming (interpreted commands, compiled routines, C plug’ins, etc.) software from
WaveMetrics [13]. It runs on an Apple computer
[14] which is connected to the VME electronics
with a BIT3 card from SBS Technologies [15] and
a GPIB serial line from National Instruments [16].
All the equipment (beam shutters, diaphragms,
shafts, detectors, sample environment, etc.) is
controlled from IGOR Pro and can be remotely
controlled/updated from outside ILL.
Since the count rates for the two spin states may
be quite different and both require to be corrected
for background, the optimum strategy for the
measurement of each Bragg reﬂection is automatically adjusted in live mode which permits ﬂipping
ratios or asymmetries to be measured with the best
accuracy available in a determined measurement
ARTICLE IN PRESS
144
E. Lelie`vre-Berna et al. / Physica B 356 (2005) 141–145
time without any pre-knowledge of the ðþÞ=ðÞ or
peak/background count rates [17].
Data can be exported in order to take advantage
of the Cambridge Crystallography Subroutine
Library (CCSL) developed by Brown [18] which
provides quick reduction, sorting and averaging of
the various collected data sets. Resulting magnetic
structure factors can be Fourier-transformed for
direct visualisation of the atomic magnetisation
density maps (maximum entropy using MemSys
code [19]), and then used to reﬁne physical models
for the magnetic electrons.
3. Selected examples
Fig. 3. Stereograms showing the directions of incident and
scattered polarisations for the [10 52] and [11 32] reﬂections. The
symbols and represent, respectively, the incident and
scattered polarisation directions. The numbers are used to
identify the corresponding pairs. For each stereogram, x is
parallel to the scattering vector and z is vertical, i.e. parallel to
the sample axis.
3.1. AF ground state of KFe3 ðOHÞ6 ðSO4 Þ2
Potassium iron Jarosite is a model Kagome´
antiferromagnet for studying the behaviour of
frustrated systems. The magnetic (iron) atoms
occupy only one crystallographic site and are
distributed in three Kagome planes which are
perpendicular to the c-axis. Two different magnetic arrangements of iron moments with propagation vector 12 cn have been proposed [20,21] from
the reﬁnement of powder neutron diffraction
patterns and the rigourous determination of the
magnetic structure could only be performed with
SNP.
Two families of magnetic interaction vectors
exist in this compound, requiring two orientations
for unambiguous structural determination. The
single crystal of KFe3 ðOHÞ6 ðSO4 Þ2 was mounted
with its ½0 1 0 and later ½1 1 0 axes vertical inside
an ILL Orange cryostat and placed within the
annular zero-ﬁeld chamber of Cryopad.
In the ﬁrst orientation (Fig. 3), there is no
depolarisation of the beam and the magnetic
interaction vector is in the plane, indicating that
there is a single magnetic domain with no
component along c: In the second orientation,
the magnetic interaction vector is parallel to z i.e.
an : If we assume that all moments have the same
amplitude, these observations corroborate the
arrangement proposed by Inami et al. [21], i.e.
the Fe moments lie entirely in the (a; b) plane and
adopt the q ¼ 0 array. A planar conﬁguration is
stabilised through an ‘order to disorder’ process
with a high density of soft excitations [22].
3.2. AF magnetisation distribution of Cr2 O3
It has been shown that SNP can also be used to
determine precise values of the magnetic interaction vectors in the class of antiferromagnetic
materials in which nuclear and magnetic scattering
appear in the same reﬂections. This method has
enabled the magnetisation distribution in Cr2 O3 at
25 K to be determined with good precision [9]. The
magnetic structure factors of [h 0 ‘] reﬂections have
( 1 on D3.
been measured out to 9:4 A
The distribution can be ﬁtted to a ﬁrst approximation by a model in which the unpaired
electrons are all in those trigonal, a1 and e, 3d
orbitals of the Cr3þ ion derived from the cubic
orbitals of t2g symmetry. However the total
moment associated with each Cr3þ ion is only
2:48mB rather than the 2:97mB which would be
predicted from the measured g-factor of 1.98. The
loss of moment can be attributed to covalent
mixing of the Cr 3d electrons with O 2p orbitals in
p-type antibonding orbitals. Positive and negative
spin transferred to O from oppositely polarised
Cr3þ ions is superposed and so does not contribute
to the magnetisation. The magnitude of the
moment deﬁcit corresponds to a covalent mixing
factor of 0.18. There are some signiﬁcant
features in the magnetisation distribution which
ARTICLE IN PRESS
E. Lelie`vre-Berna et al. / Physica B 356 (2005) 141–145
145
Acknowledgements
We are grateful to P.J. Brown and J.B. Forsyth
for sharing their inestimable expertise with us. We
also thank the European Commission for ﬁnancing polarised neutron development within the
frame of the European Polarised Neutron Initiative (ENPI, HPRI-CT-1999-50016).
Fig. 4. Maximum-entropy reconstruction of the density corresponding to the difference between the observed magnetisation
distribution and that calculated from the multipole model. The
section shown is perpendicular to ½0 1 0 and passes through the
origin. The contours are logarithmically spaced with a factor of
two between
successive levels. The highest contour is at
( 3 ; negative contours are dashed. The ﬁlled triangles
1:0mB A
mark the Cr3þ ion positions; the one farthest to the right in the
diagram has positive spin.
are not accounted for by the ionic model; they
occur in regions where the Cr and O radial wave
functions overlap strongly and are hence probably
due to covalent overlap (Fig. 4).
The magnetisation distribution has a gradient at
the Cr3þ sites which is consistent with a parallel
magneto-electric coefﬁcient with the same sign as
that implied by the combination of magnetic and
electric ﬁelds needed to stabilise the domain in
question.
4. Conclusion
Combining novel devices such as the Cryopad
and the 3He neutron spin ﬁlter, the ILL has a
uniquely versatile beam facility taking full advantage of the good neutron ﬂux available at short
wavelength. Featuring a 10 T cryomagnet for spindependent cross-sections measurements and a
dedicated zero-ﬁeld neutron polarimeter for
three-dimensional polarisation analysis experiments, D3 remains at the forefront for determining
the form factors and magnetisation distributions
of molecular magnets, superconducting systems,
antiferromagnets, etc., as well as the non-trivial
magnetic arrangements expected from the geometric frustration in antiferromagnetic interactions systems.
References
[1] R. Nathans, C.G. Shull, G. Shirane, et al., J. Phys. Chem.
Solids 10 (1959) 138.
[2] G.L. Squires, Introduction to the Theory of Neutron
Scattering, University Press, Cambridge, 1978.
[3] J. Schweizer, F. Tasset, J. Phys. F 10 (1980) 2799.
[4] A. Mun˜oz, F. Batallan, J.-X. Boucherle, et al., J. Phys.:
Condens Matter 7 (46) (1995) 8821.
[5] P.J. Brown, Physica B 297 (2001) 198.
[6] E. Lelie`vre-Berna, F. Tasset, Physica B 267–268 (1999) 21.
[7] K.H. Andersen, R. Chung, V. Guillard, H. Humblot, D.
Jullien, E. Lelie`vre-Berna, A. Petukhov, F. Tasset, Physica
B, this issue. doi:10.1016/j.physb.2004.10.057.
[8] E. Lelie`vre-Berna, E. Bourgeat-Lami, P. Fouilloux, B.
Geffray, Y. Gibert, K. Kakurai, N. Kernavanois, B.
Longuet, F. Mantegezza, M. Nakamura, S. Pujol, L.-P.
Regnault, F. Tasset, M. Takeda, M. Thomas, X. Tonon,
Physica B, this issue. doi:10.1016/j.physb.2004.10.063.
[9] P.J. Brown, J.B. Forsyth, E. Lelie`vre-Berna, F. Tasset, J.
Phys.: Condens. Matter 14 (2002) 1957.
[10] E. Lelie`vre-Berna, in: I.S. Anderson, B. Gue´rard (Ed.),
Advances in Neutron Scattering Instrumentation; Proc.
SPIE 4785 (2002) 112.
[11] P. Jesinger, A. Koetzle, A. Gagarski, et al., in: G. Fioni, et
al. (Eds.), AIP Conf. Proc. 447, (1998) 384.
[12] V.V. Fedorov, E.G. Lapin, E. Lelie`vre-Berna, V.V.
Nesvizhevsky, A.K. Petoukhov, S. Yu. Semenikhin, T.
Soldner, F. Tasset, V.V. Voronin, Nucl. Instr. and Meth.
in Phys. Res. B (2004), in press.
[13] WaveMetrics—http://www.wavemetrics.com/.
[14] Apple Computer—http://www.apple.com/.
[15] SBS Technologies—http://www.sbs.com/.
[16] National Instruments—http://www.ni.com/.
[17] M. Portes de Albuquerque, Mesure optimise de densits
d’aimantation, Ph.D. Thesis, Institut Laue-Langevin,
Grenoble, 1994.
[18] P.J. Brown, Institut Laue-Langevin, [email protected], http://
www.ill.fr/dif/ccsl/html/ccsldoc.html.
[19] R.J. Papoular, B. Gillon, Europhys. Lett. 13 (1990) 429;
R.J. Papoular, Acta Crystallogr. A 48 (1992) 244.
[20] M.G. Townsend, G. Longworth, E. Roudaut, Phys. Rev.
B 33 (7) (1986) 4919.
[21] T. Inami, S. Maegawa, M. Takano, J. Magn. Magn.
Mater. 177–181 (1997) 752.
[22] A. Harisson, E. Lelie`vre-Berna, et al., not yet published.