Inorganica Chimica Acta 361 (2008) 3365–3370

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Inorganica Chimica Acta

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Review

Neutron powder diffraction in molecule-based magnetic materials:Long- and short-range magnetic order

Peter Day *

Davy Faraday Research Laboratory, Kathleen Lonsdale Building, University College, Gower Street, London WC1E 6BT, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 January 2008Received in revised form 28 February 2008Accepted 3 March 2008Available online 18 March 2008

Dedicated to Dante Gatteschi, a pioneer ofmolecule-based magnets.

Keywords:Neutron diffractionMolecule-based magnetsMagnetic neutron scatteringNeutron polarisation analysis

Peter Day was born in 1Departmental Demonstrat1991 he was Director of tInstitution in London andin addition to numerous aNewcastle and Kent, and HTrustee of the Academia Euorganic solids.

0020-1693/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.ica.2008.03.004

* Tel.: +442076792979.E-mail address: pday@ri.ac.uk

Neutron scattering has long been the ultimate technique of choice for defining the structures and exci-tations in magnetically ordered solids. Hence, it is surprising that neutron diffraction has not played alarger part in the rapidly growing field of molecule-based magnets. There are several reasons for this:large-volume unit-cells, frequency of low symmetry, containing a relatively low concentration of mag-netic centers meaning that the magnetic contribution is a small fraction of the total scattering whilethe need to deuterate the organic ligands limits the available molecules. Nevertheless, it is possible toget information about short- and long-range ordering and this brief review summarizes the recent workon contrasting molecular magnetic materials: the weakly ferromagnetic Mn(II) organo-phosphonates;the bimetallic oxalato- and dithio-oxalato-honeycomb layer ferro- and ferrimagnets and a prototypefor p-d ferrimagnetism in ion-radical salts.

� 2008 Elsevier B.V. All rights reserved.

938. He gained his first degree and DPhil in inorganic chemistry from Oxford University, where he was subsequentlyor, University Lecturer and Ad Hominem Professor of Solid State Chemistry, and a Fellow of St. John’s College. From 1988–he Institut Laue-Langevin, Grenoble, the European neutron scattering centre and from 1991–1998, Director of the Royalits Davy-Faraday Research Laboratory, where he remains Fullerian Professor of Chemistry. He was elected FRS in 1986 and,wards from the UK Royal Society of Chemistry and the Royal Society, he holds honorary doctorates from the Universities ofonorary Fellowships of St. John’s and Wadham Colleges, Oxford, and University College, London. He is the Treasurer and aropaea. His research interests have long centred on the synthesis and electronic properties of molecular organic and metal–

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33662. Neutron powder diffractometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33663. Three-dimensional long-range magnetic order: magnetic Bragg diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33674. Separating nuclear and magnetic scattering: polarisation analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33685. Low-dimensional short-range magnetic order: diffuse magnetic scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33696. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3369

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3370References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3370

ll rights reserved.

3366 P. Day / Inorganica Chimica Acta 361 (2008) 3365–3370

1. Introduction

Quite often the essence of a highly significant discovery in sci-ence can be encapsulated and symbolised by a single graphic im-age. It is undoubtedly the case with magnetic neutron diffraction,where the two powder diffraction profiles of the simple inorganiccompound MnO, recorded at ambient and liquid nitrogen temper-ature (Fig. 1), instantly reveal the extra peaks due to the superlat-tice arising from the long-range-ordered antiferromagneticarrangement of localised atomic moments below the Neel temper-ature [1]. The potential for such a simple experiment to define themagnetic structure is obvious at once. That experiment, performednearly 60 years ago, led to a share in a Physics Nobel Prize for Clif-ford Shull and ushered in an era when the structures of literallythousands of magnetically-ordered inorganic solids were refinedby the same method. Indeed it can be stated quite firmly that neu-tron diffraction has become the ultimate method of choice for thesolid-state physicist or chemist to define such structures.

Two features of Fig. 1 are worth remarking: first, the principlebehind the experiment itself is very simple – a classical two-circlediffractometer of the type pioneered for X-rays by the Bragg’s al-most a century ago; second, it can be performed on a micro-crys-talline (i.e. powder) sample. Not surprisingly, fifty years ofdevelopment have brought huge improvements both in techniqueand data analysis: dedicated and optimised research reactors andspallation sources have brought vast increases in neutron flux onto the sample; neutron focussing and collimation devices meanthat the incident neutron beam can be made smaller and smaller;detectors have become more efficient. Nevertheless, in spite ofthese advances, an unbiased outside observer such as an inorganiccoordination chemist might be forgiven for thinking that in its fieldof applications, the technique has not advanced greatly beyond itsoriginal heartland of simple binary and ternary continuous-latticeinorganic solids such as oxides, chalcogenides and inter-metallicphases.

That is especially true when we come to a field of inorganicchemistry that has blossomed remarkably over the last 20 years,and where neutron diffraction might have been expected to playa much larger role, namely molecule-based magnetic materials.Several practical obstacles have inhibited the use of neutron scat-tering to study these otherwise very attractive targets. Because

Fig. 1. Neutron powder diffraction profiles of MnO at 80 K and 293 K. After Ref. [1],note the broad diffuse scattering between 5� and 20�.

they often result from quite lengthy synthetic pathways, they arerarely available in large enough amounts to get good enoughcounting statistics in reasonable lengths of beam-time. Becausethey are usually coordination complexes, in which the localisedmagnetic moment of the metal-ion is surrounded by large amountsof organic ‘coating’, they tend to have large unit-cells, normally oflow-symmetry, so many reflections at low scattering vectors over-lap, just in the region where magnetic reflections mainly occur.Furthermore, because the ordered moments are diluted by organicmaterial, the magnetic scattering is often only a small fraction ofthe nuclear scattering, from which it must be separated. Finally,when (as is usually the case) the organic ligands contain manyH-atoms, the large incoherent scattering from the latter can onlybe eliminated by replacing the H with D. Synthesising deuteratedligands is tricky and expensive, so limiting the range of complexesopens to study.

Looking on the bright side, however, two technical advances inparticular are beginning to impact on that negative picture. First,the advent of multi-detectors means that it is possible to map largeareas of Q-space simultaneously, thus building up the diffractionprofile in much shorter time than scanning a single detector ele-ment. One such instrument (GEM at the ISIS pulsed source in theRutherford Laboratory, UK [2,3]) has enabled refinable data –admittedly on standard materials like YAG – to be recorded onsamples as small as 3 mg. The D20 instrument at ILL Grenoble iscomparable and has a special capability for magnetic diffractionat longer wavelengths. Second, increased availability of polarisedneutron beams gives access to sophisticated methods for defini-tively separating magnetic scattering, even in the presence ofmuch more intense nuclear scattering.

As always in condensed matter science, though, the key toobtaining good results is to choose the right system. In this briefoverview, I want to give a few examples of molecule-based mag-netic materials where neutron powder diffraction has yieldedinformation on the magnetic order that would not have been pos-sible through any other techniques. Three quite different systemswill be described: the ferro- and ferrimagnetic bimetallic tris-oxa-lato- [4–6] and di-dithio-oxalato-metallate(II,III) salts [7,8] withtwo-dimensional honeycomb structures, the organo-phosphona-to-Mn(II) salts [9,10] (also two-dimensional) and p-d ferrimagneticion-radical salts [11–18]. These examples are chosen to exemplify,first, classical magnetic diffraction from a three-dimensional long-range-ordered magnetic lattice; second, diffuse scattering arisingfrom lower-dimensional magnetic order which is quite often foundin molecule-based materials; and third, the use of neutron polari-sation analysis to separate magnetic scattering from nuclearscattering.

2. Neutron powder diffractometers

Until the new-generation neutron spallation sources under con-struction at Oak Ridge (USA) and Tokai (Japan) enter into use, themost intense and versatile sources are in Europe: the reactorsource at the Institut Laue-Langevin, Grenoble, France (ILL) andthe spallation source ISIS at the Rutherford Laboratory, UK. Atthe former, the powder diffraction instruments used in the worksummarised below are D2B and D20 and at the latter, HRPD,GEM and OSIRIS. For polarisation analysis, D7 at the ILL is currentlythe only option. A few words follow about each of these instru-ments; more details can be found at Refs. [19,20].

At ILL, the diffractometer D2B is situated on a thermal neutronbeam. A complete diffraction pattern is obtained after about 100steps of 0.025� in 2h, since the 64 detectors are spaced at 2.5� inter-vals. Such scans take typically 30 min, repeated as many times asnecessary to obtain the desired counting statistics. D2B was also

Fig. 3. Crystal structure of (CD3PO3)Mn�D2O refined from neutron and synchrotronX-ray powder diffraction. Note that the positions of the D-atoms have been refined(Ref. [29]).

P. Day / Inorganica Chimica Acta 361 (2008) 3365–3370 3367

designed for work on magnetism and high resolution of very larged-spacings using wavelengths of between 2.4 Å and 6 Å. D20, alsoon the same thermal beam, is a very high intensity 2-axis diffrac-tometer equipped with a large microstrip detector. Due to the ex-tremely high neutron flux, it opens up new possibilities for real-time experiments on very small samples. The complete diffractionpattern is captured in a matter of seconds, and may be followed asa function of temperature, pressure or other parameters.

At ISIS, as its name implies, HRPD (High Resolution PowderDiffractometer) has an exceptionally high d-spacing resolution(4 � 10�4), because of its very long neutron flight-path (100 M),making it specially suitable for crystal structure determination ofmaterials with large unit-cells. It is situated on a beam of the liquidCH4 moderator. In contrast, the special feature of GEM (GeneralMaterials diffractometer) is the extremely wide range of solid an-gles round the sample subtended by the banks of ZnS detectors.That gives it an exceptionally high rate of data acquisition, suitablefor small samples or repeated scans as a function of temperature,pressure, etc. Access to diffraction at large d-spacings, such as mag-netic peaks, is provided by OSIRIS, operating in diffractometermode, because it views the liquid H2 moderator and hence has apeak flux on the sample at 6 A.

As far as neutron polarisation analysis is concerned, the gen-eral-purpose spectrometer D7 at ILL was designed primarily tostudy the diffuse scattering from materials with only short-rangenuclear or magnetic order. It features a unique combination of 3-directional (XYZ) polarisation analysis with a multi-detector array.The incident neutron beam is monochromated to the wavelengthsof 3.1 Å, 4.8 Å or 5.8 Å and polarised using a focusing super-mirrorpolarizer. The neutron spins are manipulated using a flipper, fol-lowed by a set of XYZ field coils situated around the sampleposition.

3. Three-dimensional long-range magnetic order: magneticBragg diffraction

In cases where the magnetic order is long-range in three dimen-sions, the magnetic neutron diffraction is of classical Bragg typeand the magnetic reflections have the same half-width as the nu-clear ones. Except close to TN, as described in the next section,the alkyl- and aryl-phosphonate salts represent such a case andof these, the methyl- [21] and phenyl-derivatives [22] are the eas-iest to synthesise in deuterated form, via the Wittig reaction. How-ever, in the space group Pna21, in which the methyl saltcrystallises, some of the magnetic reflections coincide with nuclearBragg peaks, as can be seen in Fig. 2, recorded on D20 between 1.7

Fig. 2. Neutron powder diffraction profiles of (CD3PO3)Mn�D2O recorded between1.6 and 18 K (Ref. [21]).

and 17 K, well above TN = 14.3 K. The contrast between the qualityand quantity of the data in Figs. 1 and 2 serves to illustrate the ad-vance in neutron diffraction techniques over the last 50 years. Sub-tracting the highest and lowest temperature profiles in Fig. 2reveals a number of magnetic Bragg peaks, whose widths are lim-ited by the resolution of the diffractometer. They define the pre-dominantly antiferromagnetic structure unambiguously with themoments localised on the Mn and oriented perpendicular to thelayers. In fact, bulk magnetisation measurements show that themoments are actually slightly canted, though the effect is too small(a few degrees) to be observed using neutrons.

Should the long-range order be ferromagnetic, then the mag-netic propagation vector is q = 0 and all the magnetic Bragg reflec-tions coincide with the nuclear ones. That renders it even moredifficult to observe the magnetic intensity since, as already pointedout, the nuclear scattering is much more intense. Subtracting theprofiles measured above and below Tc therefore yields a differenceprofile with much poorer statistics since it represents the differ-ence between two large numbers, each with counting errors pro-portional to the square root of the number of counts. Such acase is represented by the honeycomb-layer salt [P(C6D5)4][FeIICrIII-(C2S2O2)3], where we recorded profiles at 2 and 20 K and then tookthe difference, but the signal-to-noise was not very satisfactory[7,8].

Sometimes it also happens that the exigencies of the neutrondiffraction experiment lead to useful information about the crystalstructure as a by-product of the magnetic study. For example, inthe case of (CD3PO3)Mn � D2O, a bonus arising from the deuterationis that high-resolution powder neutron diffraction (HRPD) enablesthe crystal structure to be refined, including the D atoms of theCD3-groups (Fig. 3) [21]. That is of more than academic interest, gi-ven that the TN in the series (n-CnH2n+1PO3)Mn � H2O (n = 1–4)show an alternation reminiscent of that seen in the unit-cellparameters of n-alkanes, which is due to the different modes ofpacking of odd- and even-chain lengths [23]. Another example is[P(C6D5)4][FeIICrIII(C2S2O2)3], where the use of the high-intensity

Fig. 5. The crystal structure of a p-d ferrimagnet (BEDT-TTF)[Cr(NCS)4(iso-quinoline)2].

3368 P. Day / Inorganica Chimica Acta 361 (2008) 3365–3370

diffractometer GEM to follow the magnetic scattering as a functionof temperature gives detailed information on the evolution of theunit-cell constants and hence the temperature coefficients of lat-tice expansion. Perpendicular to the layers, the latter is comparableto the values found for organic molecular materials(76 � 10�6 K�1) while parallel it is four times less and in the rangefor inorganic salts (18 � 10�6 K�1) [7,8].

4. Separating nuclear and magnetic scattering: polarisationanalysis

The classical method for separating the nuclear and magneticcontributions to the measured diffraction in a powder experimentis to record profiles well above and well below the magnetic order-ing temperature and take the difference [24], as in the previoussection. This is not reliable, however, if the magnetic contributionis only a small fraction of the total. There is also the problem thatthe underlying crystal lattice may change, shifting the diffractionpeaks and creating spurious features in the difference profile. Neu-tron polarisation analysis is an elegant way to overcome that prob-lem [24]. Since the scattering from the ordered magnetic momentsis a spin-dependent process, using a polarised incident beam andanalysing the polarisation of the diffracted beam should enablethe nuclear and magnetic scattering to be definitively separated,with no problems about changing temperatures. The drawback ofthe technique is that neutrons are lost at the polarisation and anal-ysis steps, so counting times are long. Hence, it is only in the lastfew years, with improved instrument technology, that it can be ap-plied to any other than very simple prototype magnets. Here, twoexamples will be given to show the potential of the method formolecule-based magnets (see Fig. 4).

Fig. 4. Differential coherent scattering cross-section of [P(C6D5)4][MnIIFeIII(C2O4)3]measured at 1.5 K; (top) total cross-section, (bottom) magnetic cross-section (Ref.[26]).

Fig. 6. Magnetic neutron diffraction profile of d-TTF[Cr(NCS)4(phenanthroline)]measured by polarisation analysis at temperatures between 1.7 and 15 K.

The honeycomb layer salts A[MM0(C2O4)3] may be ferro- or fer-ri-magnetic, depending on M and M0 [4–6]. When M = MnII,M0 = FeIII, A = P(C6D5)+ it is the latter, but most of the magneticreflections coincide with the nuclear ones, of which there are agreat number [25]. Neutron polarisation analysis carried out onD7 (ILL) separated them completely (Fig. 5) [26]. Furthermore,integrating the coherent magnetic scattering over the Q-range cov-ered by the experiment (0.25–2.5 A�1) we found that the totalmagnetic cross-section was 3.5(4) barn/sr fmu, which should becompared with the calculated value of 3.38 for two S = 5/2 mo-ments per unit cell. Thus, the material is essentially fully orderedat the temperature of the measurement (1.5 K), with practicallyall magnetic scattering present in the Q-range covered by theexperiment.

Another instance where the magnetic Bragg intensity is only asmall part of the total occurs in the series of ion-radical saltsD+[MIII(NCX)4L]�, where D is an organic donor-molecule like tetra-thiafulvalene (TTF), M is a 3d ion, X is S or Se and L is an aromaticN-ligand like 1,10-phenanthroline (phen) or iso-quinoline [11–18].Fig. 5 shows the crystal structure of one of these. These salts areunusual ferrimagnets, in that one magnetic sub-lattice is formedby the pp-electrons of the radical cation and the other by the 3d-electrons of the anion. A prototype suitable for neutron studies is

P. Day / Inorganica Chimica Acta 361 (2008) 3365–3370 3369

TTF[Cr(NCS)4(phen)] because it can be synthesised by metathesisfrom deuterated starting materials [28]. Magnetisation and heatcapacity measurements confirm long-range magnetic order at8.3 K but difference plots from un-polarised neutron diffractiondata measured on GEM and D20 showed no convincing features.In contrast, neutron data from D7 with polarisation analysis clearlyreveal a peak at a d-spacing of 9.3 A which disappears above theordering temperature (Fig. 6). Of special note is the fact that thesedata were obtained with only 0.5 g of sample.

5. Low-dimensional short-range magnetic order: diffusemagnetic scattering

The organo-phosphonato salts already referred, are layer com-pounds so lower-dimensional spin-correlations can be expectedto be important at temperatures higher than the onset of three-dimensional order. In these circumstances, we expect to see diffusemagnetic scattering, distinguished from Bragg scattering by itsmuch greater width in Q, which decreases with temperature asthe correlation length builds up. Actually such scattering can beclearly seen in the room temperature diffraction profile of MnO,as measured by Shull and his colleagues in 1951 (Fig. 1, near 5�scattering angle) although they make no mention of it in the origi-nal article. The high flux available at long-wavelength on D20,combined with the high data acquisition rate due to the multi-detector set-up, enable diffuse scattering to be observed in thephosphonate case and analysed as a function of temperature

Fig. 7. Magnetic neutron diffraction profiles of (CD3PO3)Mn�D2O measured at(a) 11 K, (b) 14 K (Ref. [21]). The lines are fits to a combination of Gaussian andWarren (Ref. [29]) line-shapes.

Fig. 8. Diffuse magnetic scattering in [P(C6D5)4][FeIIFeIII(C2O4)3] as a function oftemperature (Ref. [30]). The lines are fits to the Warren function (Ref. [29]).

[21]. In particular, it is clear (Fig. 7) that approaching TN from be-low, the [001] magnetic reflection develops a strongly asymmetricbroad line-shape characteristic of low-dimensional spin correla-tions, conforming to the well-known Warren function describingtwo-dimensional ordering [29]. Similar intensity is also foundabove TN.

A similar case of diffuse scattering due to two-dimensionalshort-range correlations is found in the layer honeycomb ferrimag-net [P(C6D5)4][FeIIFeIII(C2O4)3] in an even more extreme form, be-cause the competing single-ion anisotropies of the FeII and FeIII

actually prevent the establishment of true long-range order atany temperature, so there are no Bragg peaks [30]. Here, the largewidth of the magnetic scattering means that the difference methodcannot be used reliably, thus necessitating recourse to polarisationanalysis, as described in the previous section. Fig. 8 shows the tem-perature dependence of the diffuse scattering, again fitted to thetwo-dimensional Warren function line-shape. Nevertheless, justas with the corresponding MnIIFeIII salt mentioned above, the totalmagnetic cross-section over the Q-range covered (2.7(3) barn/sr fmu) is consistent with expectation for one S = 2 and one S = 5/2 ion per formula unit (2.85 barn/sr fmu), so we can be confidentthat all the magnetic scattering has been captured [26,27].

6. Conclusions

With a small number of examples, I have tried to show howneutron powder diffraction can give valuable information aboutthe magnetic ordering, both long- and short-range in molecule-based magnetic materials where the unit-cell volumes are larger

3370 P. Day / Inorganica Chimica Acta 361 (2008) 3365–3370

and the space-group symmetry is lower than the usual in moreconventional continuous-lattice magnets. There is no doubt thatimproving technology for guiding, focussing, polarising and detect-ing neutrons has brought this exciting new class of ‘chemical’magnets within the realm of neutron scattering. With the third-generation spallation sources coming on stream in the USA andJapan, and the second target station at ISIS, optimised for longwavelengths, we can confidently expect neutron scattering to playa much greater role in the study of molecule-based magnets than ithas in the past.

Acknowledgements

The experiments summarised here were carried out by manytalented co-workers whose names appear in the references. Wehave also been grateful for help from instrument scientists at ILL(Thomas Hansen, Ross Stewart, Ken Andersen) and ISIS (RichardIbberson, Paolo Raddaelli). Our group has been supported by theUK Engineering and Physical Science Research Council and theEuropean Commission Network of Excellence MAGMANet.

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