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The local magnetic structure in the [FeIII (Tp)(CN)3 ]- building block was investigated by combining paramagnetic Nuclear Magnetic Resonance (pNMR) spectroscopy and polarized neutron diffraction (PND) with first-principle calculations. The use of the pNMR and PND experimental techniques revealed the extension of spin-density from the metal to the ligands, as well as the different spin mechanisms that take place in the cyanido ligands: Spin-polarization on the carbon atoms and spin-delocalization on the nitrogen atoms. The results of our combined density functional theory (DFT) and multireference calculations were found in good agreement with the PND results and the experimental NMR chemical shifts. Moreover, the ab-initio calculations allowed us to connect the experimental spin-density map characterized by PND and the suggested distribution of the spin-density on the ligands observed by NMR spectroscopy. Interestingly, significant differences were observed between the pseudo-contact contributions of the chemical shifts obtained by theoretical calculations and the values derived from NMR spectroscopy using a simple point-dipole model. These discrepancies underline the limitation of the point-dipole model and the need for more elaborate approaches to break down the experimental pNMR chemical shifts into contact and pseudo-contact contributions.

A new crystallographic method is proposed in order to refine a spin-resolved atomic orbital model against X-ray and polarized neutron diffraction data. This atomic orbital model is applied to the YTiO3 perovskite crystal, where orbital ordering has previously been observed by several techniques: X-ray diffraction, polarized neutron diffraction and nuclear magnetic resonance. This method gives the radial extension, orientation and population of outer atomic orbitals for each atom. The interaction term between Ti3+, Y3+ cations and O2− ligands has been estimated. The refinement statistics obtained by means of the orbital method are compared with those obtained by the multipole model previously published.

A new crystallographic method is proposed in order to refine a spin-resolved atomic orbital model against X-ray and polarized neutron diffraction data. This atomic orbital model is applied to the YTiO

International audience; We report the synthesis, spectroscopic and magnetic characterisation and ab initio investigation of a new mononuclear ytterbium based complex, [YbTp2NO3], and discuss the physical mechanisms underlying the slow relaxation of its magnetisation in a moderate static field and at low temperature.

The local magnetic structure in the [Fe

The anisotropy of the magnetic properties of molecular magnets is a key descriptor in the search for improved magnets. Herein, we prove how a novel analysis approach using single crystal polarized neutron diffraction (PND) provides direct access to atomic magnetic susceptibility tensors. The technique has been applied for the first time to two Dy-based single molecule magnets, showing clear axial atomic susceptibility for both Dy(III)-ions. For the triclinic system, bulk magnetization methods are not symmetry-restricted, and the experimental magnetic easy axes from both PND, angular-resolved magnetometry (ARM) and theoretical approaches all match reasonably well. ARM curves simulated from the molecular susceptibility tensor determined with PND show strong resemblance with the experimental ones. For the monoclinic compound, a comparison can only be made with the theoretically calculated magnetic anisotropy, and in this case PND yields an easy-axis direction that matches that predicted via electrostatic methods. Importantly, this technique allows the determination of all elements of the magnetic susceptibility tensor and not just the easy-axis direction as is available from electrostatic predictions. Furthermore, it has the capacity to provide each of the anisotropic magnetic susceptibility tensors for all independent magnetic ions in a molecule, thus allowing for studies of poly-nuclear complexes and compounds of higher crystalline symmetry than triclinic.

In this paper, we propose a simple cluster model with limited basis sets to reproduce the unpaired electron distributions in a YTiO

We report neutron-diffraction investigations of the quasi-2D Mn-II(dca)(2)(pym)(H2O) (pym = N2C4H4) compound, where high-spin Mn-II ions are bridged by dicyanamide anions, [N(CN)(2)](-) (herein abbreviated dca). Inside the layers, Mn2+ ions are connected by single or double dca bridges. The magnetic phase diagram was established by neutron diffraction on a single crystal. In the low-field phase, the Mn-II ions are antiferromagnetically ordered in the layers, with moments nearly parallel to the c axis, and the layers are antiferromagnetically coupled. The spin-flop phase corresponds to ferromagnetic coupling between the antiferromagnetic layers, in which the Mn-II moments are nearly perpendicular to the c axis. The induced spin-density distribution in the paramagnetic phase, determined by polarized neutron diffraction, visualizes the superexchange pathways through the dca ligands within the layers and through H bonding between neighboring layers. The theoretical spin density obtained by bidimensional periodic DFT calculations is compared with the experimental results. Furthermore, quantum Monte Carlo simulations have been performed to compare the DFT results with experimental susceptibility measurements.

La distribution des electrons dans un cristal peut etre reconstruite avec une grande precision au moyen de la diffraction des rayons X a haute resolution, alors que la diffraction des neutrons polarises en spin permet de retrouver la densite d’aimantation. Bien que les grandeurs ainsi mesurees soient toutes deux une traduction du comportement des electrons, ces deux types d’experience de diffusion sont de nos jours interpretees par des modeles differents. Nous retracons ici les etapes ayant conduit a la premiere determination experimentale de la densite d’electrons resolue en spin par un traitement combine des donnees de diffraction de rayons X et de neutrons polarises.

Orbital ordering below 30 K was previously observed in the ferromagnetic ${\text{YTiO}}_{3}$ compound both by polarized neutron diffraction (PND) and x-ray magnetic diffraction (XMD). In this paper we report a procedure for the joint refinement of a unique spin-density model based on both PND and XMD data. The distribution of the unpaired $3d$ electron of titanium is clearly seen on the magnetization density reconstructed by the maximum entropy method from the PND data collection at 5 K. The ${\text{Ti}}^{3+}$ $3d$ orbital populations obtained by joint model refinement are discussed in terms of the orbital ordering scheme. Small but significant magnetic moments on apical oxygen ${\text{O}}_{1}$ and yttrium atoms are found. The agreement between experimental and theoretical spin densities obtained using density functional theory is discussed.

International audience; Joint refinement of X-ray and polarized neutron diffraction data has been carried out in order to determine charge and spin density distributions simultaneously in the nitro­nyl nitroxide (NN) free radical Nit(SMe)Ph. For comparison purposes, density functional theory (DFT) and complete active-space self-consistent field (CASSCF) theoretical calculations were also performed. Experimentally derived charge and spin densities show significant differences between the two NO groups of the NN function that are not observed from DFT theoretical calculations. On the contrary, CASSCF calculations exhibit the same fine details as observed in spin-resolved joint refinement and a clear asymmetry between the two NO groups.

Prussian blue analogues (PBA) ferromagnetic nanoparticles Cs I x Ni II [Cr III (CN)6 ] z ·3(H2O) embedded in CTA+ (cetyltrimethylammonium) matrix have been investigated by magnetometry and magnetic small-angle neutron scattering (SANS). Choosing particle sizes (diameter D = 4.8 and 8.6 nm) well below the single-domain radius and comparable volume fraction of particle, we show that the expected superparamagnetic regime for weakly anisotropic isolated magnetic particles is drastically affected due to the interplay of surface/volume anisotropies and dipolar interactions. For the smallest particles (D = 4.8 nm), magnetocrystalline anisotropy is enhanced by surface spins and drives the system into a regime of ferromagnetically correlated clusters characterized by a temperature-dependent magnetic correlation length L mag which is experimentally accessible using magnetic SANS. For D = 8.6 nm particles, a superparamagnetic regime is recovered in a wide temperature range. We propose a model of interacting single-domain particles with axial anisotropy that accounts quantitatively for the observed behaviors in both magnetic regimes.

While it is mostly well-known for its excellent thermoelectric properties at high temperatures, Yb${}_{14}$MnSb${}_{11}$ has also come to be characterized as a ferromagnetic semiconductor. Such systems support ferromagnetic correlations similar to metallic compounds, although with semiconducting electronic configurations. As such, they are promising materials for use in spintronic devices. The authors here use neutron scattering techniques to examine the distribution of the magnetization density and the energy scale of the magnetic exchange interactions in the Yb${}_{14}$MnSb${}_{11}$ compound. They find that there is a large ferromagnetic moment localized to the Mn site in the crystal structure with a `Kondo cloud' of negative moments distributed over the other atomic sites. The results provide long sought answers to questions regarding the valence, magnetization distribution, and energy scales of the magnetic interactions in this compound. The ferromagnetic spin-wave spectrum supports this premise and is well characterized by inclusion of nearest and next-nearest neighbor exchange interactions.

A method to map spin-resolved electron distribution from combined polarized neutron and X-ray diffraction is described and applied for the first time to a molecular magnet and it is shown that spin up density is 5% more contracted than spin down density., Since the 1980s it has been possible to probe crystallized matter, thanks to X-ray or neutron scattering techniques, to obtain an accurate charge density or spin distribution at the atomic scale. Despite the description of the same physical quantity (electron density) and tremendous development of sources, detectors, data treatment software etc., these different techniques evolved separately with one model per experiment. However, a breakthrough was recently made by the development of a common model in order to combine information coming from all these different experiments. Here we report the first experimental determination of spin-resolved electron density obtained by a combined treatment of X-ray, neutron and polarized neutron diffraction data. These experimental spin up and spin down densities compare very well with density functional theory (DFT) calculations and also confirm a theoretical prediction made in 1985 which claims that majority spin electrons should have a more contracted distribution around the nucleus than minority spin electrons. Topological analysis of the resulting experimental spin-resolved electron density is also briefly discussed.

International audience; A new charge and spin density model and the corresponding refinement software were recently developed to combine X- ray and polarized neutron diffraction experiments [1]. This joint refinement procedure allows getting access to both charge and spin densitiy distributions by refining both spin up and down parameters for magnetic atoms. The paper is focused on the refinement procedure and its application to the case of an end-to-end azido double bridged copper(II) complex. The results of this joint refinement of X-ray and polarized neutron diffraction data are presented.

A new polarized neutron diffractometer has been commissioned at the ORPHEE reactor in Saclay. Hot neutron beam of 0.8 A is obtained by Heussler monochromator. 64 position sensitive detectors cover angular range of 25°×80° in vertical and horizontal directions, which allows performing measurements of flipping ratios for dozens of reflections simultaneously. Data acquisition and data treatment programs have been developed. Possibilities of multi-crystal experiments and studies of powder samples have been tested. Ensemble of these measures should boost the data acquisition rate of magnetization densities by a factor of 5-20 depending on the crystal cell parameters.

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International audience; Polarized neutron diffraction (PND) experiments were carried out at low temperature to characterize with high precision the local magnetic anisotropy in two paramagnetic high‐spin cobalt(II) complexes, namely [CoII(dmf)6](BPh4)2 (1 ) and [CoII2(sym‐hmp)2](BPh4)2 (2 ), in which dmf=N ,N ‐dimethylformamide; sym‐hmp=2,6‐bis[(2‐hydroxyethyl)methylaminomethyl]‐4‐methylphenolate, and BPh4−=tetraphenylborate. This allowed a unique and direct determination of the local magnetic susceptibility tensor on each individual CoII site. In compound 1 , this approach reveals the correlation between the single‐ion easy magnetization direction and a trigonal elongation axis of the CoII coordination octahedron. In exchange‐coupled dimer 2 , the determination of the individual CoII magnetic susceptibility tensors provides a clear outlook of how the local magnetic properties on both CoII sites deviate from the single‐ion behavior because of antiferromagnetic exchange coupling.

Exchange mechanisms in the high-spin molecular cluster [Ni II {Ni II (CH 3 OH) 3 } 8 (μ-CN) 30 {W V (CN) 3 } 6 ] · 15CH 3 OH have been investigated by a combination of polarised neutron diffraction studies on a single crystal and theoretical calculations. The experimental spin density has been deduced from a model refinement on the basis of magnetic structure factors collected at 1.6 K under a field of 5 T. Positive spin populations were observed on both, the nickel and the tungsten sites, which provides an evidence of ferromagnetic Ni II –W V exchange interactions through the cyano bridge. A positive spin density was also observed on nitrogen atoms of the bridging cyano groups indicating a spin delocalisation mechanism. The mean experimental spin populations are in good agreement with the calculated ones, obtained by using the NBO method and the B3LYP functional. The theoretical evidence for ferromagnetic Ni II –W V interactions was also given by calculations of the exchange coupling constants on the whole complex.

We have followed the photo-transformation, the light-induced thermal hysteresis, and the light-induced phase separation (LIPS) into like-spin phases on a [Fe(ptz)6](BF4)2 single crystal, in the quenched rhombohedral phase, through the analysis of neutron Laue diffraction patterns. In agreement with the analysis of previous magnetic measurements, phase separation was observed in the close vicinity of the static light-induced instability, but not otherwise. The positions of the Laue spots were analysed in terms of variation in the a/c lattice-parameter ratio which is well correlated with the variation of the high-spin fraction predicted by a simple homogeneous mean-field model. Using a mapped version of the same model, we analyzed the line shapes of the Laue spots during photo-excitation and relaxation experiments so as to determine the width of the distribution of irradiation intensities, a crucial parameter in phase separation. The predictions of the mapped model failed to reproduce the rapid initial kinetics of the phase separation process, but were in agreement with the further evolution. These features are attributed to a nucleation and growth process, for which the crossover from microscopic to macroscopic regimes has been observed.

A new end-to-end azido double-bridged copper(II) complex [Cu(2)L(2)(N(3))2] (1) was synthesized and characterized (L=1,1,1-trifluoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato). Despite the rather long Cu-Cu distance (5.105(1) A), the magnetic interaction is ferromagnetic with J= +16 cm(-1) (H=-JS(1)S(2)), a value that has been confirmed by DFT and high-level correlated ab initio calculations. The spin distribution was studied by using the results from polarized neutron diffraction. This is the first such study on an end-to-end system. The experimental spin density was found to be localized mainly on the copper(II) ions, with a small degree of delocalization on the ligand (L) and terminal azido nitrogens. There was zero delocalization on the central nitrogen, in agreement with DFT calculations. Such a picture corresponds to an important contribution of the d(x2-y2) orbital and a small population of the d(z2) orbital, in agreement with our calculations. Based on a correlated wavefunction analysis, the ferromagnetic behavior results from a dominant double spin polarization contribution and vanishingly small ionic forms.

International audience; We have determined by polarized neutron diffraction (PND) the low-temperature molecular magnetic susceptibility tensor of the anisotropic low-spin complex PPh4 [Fe(III) (Tp)(CN)3 ]⋅H2 O. We found the existence of a pronounced molecular easy magnetization axis, almost parallel to the C3 pseudo-axis of the molecule, which also corresponds to a trigonal elongation direction of the octahedral coordination sphere of the Fe(III) ion. The PND results are coherent with electron paramagnetic resonance (EPR) spectroscopy, magnetometry, and ab initio investigations. Through this particular example, we demonstrate the capabilities of PND to provide a unique, direct, and straightforward picture of the magnetic anisotropy and susceptibility tensors, offering a clear-cut way to establish magneto-structural correlations in paramagnetic molecular complexes

We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor FM SC UCoGe crystallizing in the orthorhombic TiNiSi type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC 6.5 and 7.5 K, respectively, which is considerably higher than the TC 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment amp; 956;spont 0.11 amp; 956;B f.u. and amp; 956;spont 0.21 amp; 956;B f.u., respectively in comparison to the tiny spontaneous moment of UCoGe about 0.07 amp; 956;B f.u. . No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction PND and for UCo0.88Ru0.12Ge also by soft X ray magnetic circular dichroism XMCD . The analysis of the PND results indicates that the observed enhancement of ferromagnetism is mainly due to the growth of the orbital part of the uranium 5f moment amp; 956;UL, reflecting a gradual localization of the 5f electrons with Ru substitution. In addition, the parallel orientation of the U and Co moments has been established in both substituted compounds. The results are discussed and compared with related isostructural ferromagnetic UTX compounds T transition metals, X Si, Ge in the context of a varying degree of the 5f ligand hybridization

International audience; A large up-field shift (−763 ppm) has been observed for the carboxyl carbons of the dichlorido complex TBA[Ru2(O2CCH3)4Cl2] (TBA+ = tetra(n-butyl)ammonium cation) in the 13C NMR spectrum (CD2Cl2 at 25 °C). The DFT calculations showed spin delocalization from the paramagnetic Ru25+ core to the ligands, in agreement with the large up-field shift.

A design of infinite two- and three-dimensional cyano-bridged networks with useful properties has attracted a great deal of attention in contemporary science. A proposal described some investigations in crystal engineering, performed over the past few years on supramolecular coordination polymers based on [Mo(CN) n ] 4− ( n =7,8) building blocks and on their magnetic and intercalation properties.

Some spin-crossover solids can show photo-convertibility at low temperatures, which generally makes this class of compounds interesting for studies towards application as optical switches and memories in numerical displays. The mechanism of the optical switching is the light-induced excited spin state trapping (LIESST) and the reverse-LIESST, as reported before [Inorg. Chem. 24 (1985) 2174; J. Phys. Chem. 94 (1991) 2741]. These processes can be combined with the relaxation in order to cycle the excited state, which involves temperature scanning of the title system [Polyhedron 20 (2001) 1599]. In this paper, we report the spin density map in the [Fe(ptz) 6 ](BF 4 ) 2 single crystal in its undercooled R 3 i space group in the photo-induced metastable high-spin state, which is paramagnetic, at T =2 K. The experiment was carried out on the 5C1 polarised neutron diffractometer at LLB newly equipped with an optical fiber for the light-coupled measurements. The photo-excitation reaches a steady state within an hour and a half. The lifetime of the photo-induced high-spin state at 40 K exceeds 10 h, as in accordance with previous spectroscopic, magnetic and Mossbauer measurements. The first results obtained on [Fe(ptz) 6 ](BF 4 ) 2 can serve as a base to future investigations of other photo-convertible materials.

International audience; The spin population distribution in the ferromagnetically coupled hetero-bimetallic chain compound [MnNi(NO(2))(4)(en)(2)] (en = 1,2-ethanediamine) has been investigated by means of polarized neutron diffraction experiments, and the results compared with those from theoretical estimates obtained via calculations based on density functional theory on dinuclear molecular models of the chain. The spin distributions obtained from experiment and from theory are consistent and reflect a larger spin delocalization from the Ni atom due to the more covalent character of the Ni-N bonds compared to the Mn-O ones. Also a nearly isotropic spin distribution is observed for the more ionic d(5) Mn(2+) ion and a clearly anisotropic distribution for the d(8) Ni(2+) ion. The use of dinuclear molecular models for the calculation of the exchange coupling constant between Ni and Mn provide upper and lower limits (+17.6 and -4.2 cm(-)(1)) for the experimentally determined value (+1.3 cm(-)(1)), depending on how the missing part of the chain is simulated, but yield essentially the same spin distribution. The Mn(II)-Ni(II) weak ferromagnetic coupling in the chain is interpreted in a spin delocalization mechanism as resulting from the weakness of the overlap between the magnetic orbitals centered on nickel and those centered on manganese which are only weakly delocalized on the ligands.

Polarized-neutron-diffraction (PND) measurements were carried out using a pseudo-single-phase powder sample of ferromagnetic ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ nanoparticles. For the well-identified ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ phase, sizes of the magnetic moments at the three crystallographic Fe sites were determined in the absolute scale. The agreement between the magnetization value deduced from the present PND and that measured by a magnetometer $({M}_{\mathrm{VSM}})$ supports the hypothesis that ${M}_{\mathrm{VSM}}$ is primarily caused by the magnetization value in the target ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$; thus there is no evidence for macroscopic giant saturation magnetization, at least for ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ nanoparticles. On the basis of the large magnetic moment size at one of the Fe sites, a possible coexisting state of localized spins and itinerant electron spins is inferred. Drawing a distinction between thin films and nanoparticles is currently necessary because of their divergent magnetic evolutions.

We have investigated by inelastic neutron scattering (INS), neutron diffraction, andmagnetometry the magnetic properties of the mononuclear complex [Mn3+(pyrol)3(tren)] in both high-spin (5E, HS, S = 2) and low-spin (3T1, LS, S = 1) states. The system presents a spin transition (ST) around 47 K with a small hysteresis width (TST,↑ = 47.5 K and TST,↓ = 46 K) characteristic of an efficient collective transition process. In the HS state, the INS spectrum at 56 K and zero magnetic field is accounted for by a zero-field splitting with D = −5.73(3) cm−1 and |E| = +0.47(2) cm−1 which may be the result of a dynamic Jahn-Teller effect reported in the literature.In the LS state, a single magnetic peak at 4.87 meV is observed, still at zero field. Despite the existence of an unquenched orbital moment (L = 1) in the ground 3T1 state, we argue that it may be described by a genuine S = 1 spin Hamiltonian owing to the existence of a strong trigonal distortion of the Mn3+ coordination octahedron. The observed peak corresponds to a transition M = +1 within the S = 1 ground state split by a large single-ion anisotropy term D = +39.3 cm−1. A full spin-Hamiltonian model is proposed based on these first INS results obtained in a thermal ST molecular magnetic system.

International audience; The magnetic anisotropy is a prerequisite for a metal complex to behave as a single-molecule magnet (SMM). Unfortunately, today we do not fully understand the relationships between the local structural parameters and the magnetic anisotropy that results at the molecular level. This is an issue that has become recursive in this area. Out of the synthesis work which is still important, but generates a multiplication of SMMs with frustrating properties, there are various studies to understand these relationships among which most are theoretical studies. In this context, we believe that polarized neutron diffraction (PND) can provide an experimental and complementary point of view to these theoretical studies. PND is indeed well known to allow an accurate determination of the spin density in magnetic compounds and in the field of molecular magnetism it has provided unique information on the pathways and the nature of intra- or intermolecular magnetic coupling [1]. In the case of highly anisotropic paramagnetic materials, where local magnetic moments cannot be aligned by an external magnetic field, that is more tricky, but a method based on local magnetic susceptibility tensor, has been recently developed that allows now analysing the data in this case and obtaining the magnetization distribution [2]. This approach was first used for inorganic compounds. Our idea has been to use this approach to go beyond the reconstruction of spin density to study the magnetic anisotropy in molecular systems. In this paper, we present the results of such an approach applied for the first time to metal complexes that are simple mono and dinuclear cobalt(II) complexes.

This article details the local spin density determination in the cyano-bridged, two-dimensional, molecular based magnet K 2 Mn 3 (H 2 O) 6 [Mo(CN) 7 ] 2 ·6 H 2 O ( T c = 39 K). The crystal structure, determined at room temperature by X-ray diffraction, was redetermined at 50 K using unpolarised neutron diffraction. The importance of intermolecular hydrogen-bonding interactions is clearly demonstrated in this study, previously characterised with X-rays. The local spin density was determined from polarised neutron diffraction data at 4 K with an applied field of 3 T. Positive spin densities were observed on the manganese sites, consistent with high-spin d 5 ions in octahedral fields, whilst a negative spin density was found on the molybdenum sites, signifying delocalisation onto the cyano ligands. The alternating sign of the spin populations on the metal sites, suggests that the primary Mo III –Mn II interactions are antiferromagnetic in nature and the delocalisation onto the cyano-bridges clearly demonstrates the role of the ligand bridges in the magnetic exchange pathway.

The compound 2-{3-[4-methyl-1,2,4-triazolyl]}-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide, abbreviated as Metrz-Nit, crystallizes in the non-centrosymmetric space group P2 1 2 1 2 1 . The investigation of the magnetic properties has revealed the occurrence of intermolecular ferromagnetic interactions. The crystal structure has been refined by neutron diffraction at 11 K. The spin density distribution has been determined from polarized neutron diffraction experiments carried out at 1.5 K under a magnetic field of 5 T. As expected, the main contributions of the spin distribution have been observed in the 2p π orbitals of the nitrogen and oxygen atoms of the two NO groups, and a significant negative spin population has been detected on the sp 2 carbon atom of the nitronyl nitroxide moiety. The spin distribution is slightly dissymmetrical, so that the sp 3 carbon atoms in α-position of the nitro nitrogen atoms carry spin populations of opposite signs. Concerning the triazole ring, the main spin population, of negative sign, has been found on the nitrogen atom occupying the 3-position. The carbon atom of the methyl group attached to the 4-position has been also found to carry a significant negative spin population. The spin populations on the hydrogen atoms have been determined. These experimental data have been compared to the results of LSD calculations performed on an isolated molecule. The role of intermolecular interactions in the spin distribution has been discussed.

The determination of the spin density map in molecular magnetic compounds provides crucial informations about the phenomena playing a role in magnetic interactions, like spin delocalization and spin polarization. Polarized neutron diffraction on single crystals is the unique experimental technique which allows to visualize the spin distribution over a whole molecule. A review of spin density studies in bimetallic compounds will be presented as well as very recent results obtained on the induced spin density distribution along a ferromagnetic Mn(II)Ni(II) bimetallic chain.

The heavy-fermion quadratic compound UIr2Si2 presents an antiferromagnetic ordering below T=6 K, with a very small moment of 0.10 (1)μB. Below this temperature, a metamagnetic transition is induced by an applied field. The induced moment is then 0.36 (1)μB. To try to understand this behavior, we have performed magnetization density measurements on a single crystal with polarised neutrons, above and below the metamagnetic transition. The results have been analyzed by the maximum entropy method and by form factor least-squares refinements. In both cases, the whole magnetic density is located on the uranium atoms. This density is mainly due to 5f electrons, but a small positive density of d-type is also present. The analysis of the uranium form factor indicates that, especially above the metamagnetic transition, the orbital contribution to the 5f density is smaller, compared to the spin one, than expected for the ground J multiplet.

The magnetization density has been studied by polarized neutron scattering in the ferromagnetic La0.8Sr0.2MnO3 manganite at 20 K and at 290 K, close to the Curie point. A weak preponderance of t2g Mn wave functions is observed, as well as a positive polarization on oxygen. At low temperature, the density on Mn corresponds to a well-localized moment of 3 μB and a more diffuse component. Heating the sample leads to a non-uniform renormalization of the magnetization density.

This paper is devoted to the determination of the spin density in the ferrimagnetic ground state of the bimetallic chain compound MnCu(pba)(H2O)(3) . 2H(2)O, with pba = 1,3-propylenebis(oxamato). The crystal structure, previously determined at room temperature through X-ray diffraction, was redetermined at 10 K through unpolarized neutron diffraction (orthorhombic system, space group Pnma, a = 12.727(11) Angstrom, b = 21.352(19) Angstrom, c = 5.153(3) Angstrom, Z = 4). The experimental spin density has been deduced from polarized neutron diffraction data recorded at 10 K under 50 kOe. Positive spin densities were observed on the manganese side and negative spin densities on the copper side. The delocalization of the spin density from the metal centers toward the oxamato bridging ligand was found to be more pronounced on the copper side than on the manganese side, so that the nodal surface (of zero spin density) is closer to manganese than to copper. The experimental spin distribution has been compared to the theoretical distributions deduced from density functional theory (DFT) calculations, using both DGauss and DMol programs. The experimental results for the title chain compound have also been compared to the spin distribution for the binuclear compound [Mn(Me-6-[14]ane-N-4)Cu(oxpn)](CF3SO3)(2) with Me-6-[14]ane-N-4 = (+/-)-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, and oxpn = N,N'-bis(3-aminopropyl)oxamido, recently reported. The most striking difference between pair and chain compounds concerns the positive P+ and negative P- spin populations carried by the manganese and copper sides, respectively. For the pair compound P+ was found as 4.67(8) mu(B), and P- as -0.67(8) mu(B) while for the chain compound these values are 5.05(7) mu(B) and -1.05(10) mu(B), respectively. The spin distribution for the ferrimagnetic chain compound is very close to a Neel state (P+ = 5 mu(B) and P- = -1 mu(B)).

International audience; Since the first works of Stewart on modelling charge density [1], huge improvements of X-ray sources, detectors and software has significantly increased the resolution and the qualities of diffraction data allowing an accurate determination of the charge density of a growing number of molecules. However, despite the technological improvement, no dramatic change of the experimental model was reached since the multipolar model of Hansen & Coppens in 1978 [2]. At the same time polarised neutron diffraction (PND) experiments were developed [3] to get access to the spin density at the molecular scale and the multipolar Hansen & Coppens model was adapted to model this quantity. These two quantities (charge and spin densities) are described by a similar multipolar atom centred model with a common parameterization, therefore a combined treatment of X-ray diffraction (XRD) and PND data, is not only possible but also useful as stated by Becker & Coppens in 1985 [4]. Recently an extended Hansen & Coppens model and the corresponding refinement program were developed [5, 6] in order to allow the joint refinement of data sets coming from three different experiments (X-ray, unpolarised and polarised neutron diffraction). By combining different data sets, the new model gives access to electron density with spin up (ρ↑) and electron density with spin down (ρ↓) separately. These two quantities (ρ↑ and ρ↓) can be observed experimentally for the first time, and this observation allows a further comparison with theoretical models. In a first part the presentation will focus on the common model and the refinement procedure. The second part will describe its application to the case of an end-to-end azido double-bridged copper(II) complex (Cu2L2(N3)2 where L=1,1,1-triuoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato) [7]. The experimental results will be presented and compared to the theoretical densities. Acknowledgments This work has been supported by l’Agence Nationale de la Recherche (CEDA project); M.D. thanks CNRS for PhD fellowship.

This paper is devoted to the determination of the spin density in the S = 2 ground state of [Mn(Me6-[14]ane-N4)Cu(oxpn)](CF3SO3)2 with Me6-[14]ane-N4 = (±)-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraa...

Single crystals of the novel phase MnCu(pbaOH)(H2O)3·2H2O (A) have been obtained; pbaOH stands for 2-hydroxy-1,3-propylenebis(oxamato). A crystallizes in the monoclinic system, space group P21/c,with a = 21.212(4) ,b = 5.033(1), c = 14.246(2) A , β = 100.72(2)° and Z = 4. The structure consits of oxamato-bridged Mn(II)Cu(II) bimetallic chains running along the a-axis direction. The shortest metal-metal separations between adjacent chains related by, respectively, the unit cell translation along the b-axis direction and the glide translation along the c-axis direction involve metal ions of the same type, in contrast with what was found in the previously described phase MnCu(pbaOH)(H2O)3 (B). According to our interpretation of the three-dimensional (3-D) magnetic effects in bimetallic chain compounds, the topology of the spin carriers in A should favor a 3-D antiferromagnetic ordering, while B exhibits a 3-D ferromagnetic ordering. The study of the temperature dependence of the magnetic susceptibility as well as the temperature and field dependences of the magnetization have confirmed this prediction. The antiferromagnetic ordering occurs at 2.4 K. A magnetic field of 0.9 kOe is sufficient to overcome the weak interchain interactions, so that the compound may be described as a metamagnet built from ferrimagnetic chains.