- Email: [email protected]
ILL polarised hot-neutron beam facility D3
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
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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
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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
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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.
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.
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