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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.

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

High-quality monodisperse metallic cobalt nanorods are obtained by the reduction of carboxylate salts of CoII in 1,2-butanediol using a rapid, simple, and solid-template-free procedure. In this polyol process, particle shape can be controlled via the growth rate, which depends on three parameters: i) the nature of the cobalt carboxylate, ii) the temperature ramp, and iii) the basicity of the medium. Cobalt in the hexagonally close-packed phase favored the growth of anisotropic particles. Magnetic measurements of the cobalt nanorods indicate they are ferromagnetic at room temperature. They have a very high coercivity of 9.0 kOe at 140 K, much higher than that observed for wires prepared with solid templates. This can be attributed to their small mean diameter and high crystallinity.

Cobalt nanorods and wires were prepared by reduction of a cobalt salt in a liquid polyol. These particles crystallize with the hcp structure and the growth axis is parallel to the crystallographic c-axis. The kinetic control of the growth allows to vary the mean diameter of the rods and their aspect ratio. Dumbbell like shape particles consisting of a central rod with two conical tips were also obtained. Magnetization curves of oriented wires present very high coercivity (up to 9 kOe) resulting from both a high shape anisotropy and the high magnetocrystalline anisotropy of the hcp cobalt. Micromagnetic simulations showed that the magnetization reversal is shape dependent. The conical tips of the dumbbell particles strongly contribute to the coercivity decrease and must be precluded for permanent magnet applications.

We demonstrate in this paper the possibilities offered by Grazing Incidence Small Angle Neutron Scattering (GISANS) for the study of solid/liquid interfaces. We present experimental results obtained by Specular Neutron Reflectivity (SNR) and GISANS on a model system made of silica nanospheres adsorbed on a silicon wafer by electrostatic interactions both at solid/air interface and solid/liquid interfaces. At the solid/liquid interface, we demonstrate that grazing incidence scattering enables to discriminate the surface and the bulk scattering. The surface structure factor derived from GISANS shows that the nanospheres are organized as a repulsive liquid system, with a surface fraction occupation consistent with values obtained by SNR. This original setup highlights a direct correlation between the structure of the silica nanospheres in solution and their organization on the surface: due to the strong electrostatic repulsions between spheres, their organization at the surface is close to the projection in 2D of the 3D organization of the nanospheres in solution.

We report on the magnetic properties of the enneanuclear mixed-valent single-molecule magnet [Mn9O7(OAc)11(thme)(py)3(H2O)2] (1). Using inelastic neutron scattering (INS) spectroscopy, we have confirmed the S = 17/2 cluster ground state of 1. Furthermore, we have determined accurately the anisotropy parameters of the zero field split ground state. The cluster 1 exhibits a strong negative axial anisotropy D [ S ˆ z 2 - S ( S + 1 ) / 3 ] where D = −0.249(5) cm−1. It is necessary to include an axial higher order term ( B 4 0 ) in the effective spin Hamiltonian to account for the experimental data.

A new family of tetranuclear Ni complexes [Ni(4)(ROH)(4)L(4)] (H(2)L = salicylidene-2-ethanolamine; R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni(4)O(4)] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni(II) centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D approximately -0.93 cm(-)(1) for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions. These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.

The ground-state properties of a Co moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2]⋅40 D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin–orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as ℋ=−2Σ(JŜ1αŜ2α+JŜ2αŜ3α), where Jα are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: J=J, J=J, J=J. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: J=J=1.37, J=J=0.218, and J=J=1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3.

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate compounds Na-4[V(8)(IV)V(4)(V)A(8)(III)O(40)(H2O)].23H(2)O, Na-4[(V8V4As8O40)-V-IV-As-V-O-III(D2O)].16.5D(2)O, and (NHEt3)(4)[(V8V4As8O40)-V-IV-As-V-O-III(H2O)].H2O were investigated by an inelastic neutron scattering (INS) study using cold neutrons. In addition, the synthesis procedures and the single-crystal X-ray structures of these compounds have been investigated together with the temperature dependence of their magnetic susceptibilities. The magnetic properties below 100 K can be described by simply taking into account an antiferromagnetically exchange coupled tetramer, consisting of four vanadium(IV) ions. Up to four magnetic transitions between the cluster S = 0 ground state and excited states could be observed by INS. The transition energies and the relative INS intensities could be modeled on the basis of the following exchange Hamiltonian: (S) over cap (ex) = -2J(12)(xy)((S) over cap (1x)(S) over cap (2x) + (S) over cap (3x)(S) over cap (4x) + (S) over cap (1y)(S) over cap (2y) + (S) over bar (3y)(S) over bar (4y)) - 2J(12)(z)((S) over bar (1z)(S) over bar (2z) + (S) over bar (3z)(S) over bar (4z)) - 2J(23)(xy)((S) over bar (2x)(S) over bar (3x) + (S) over bar (1x)(S) over cap (4x)+ (S) over cap (2y)(S) over cap (3y)+ (S) over cap (1y)(S) over cap (4y)) - 2J(23)(z)((S) over cap (2z)(S) over cap (3z) + (S) over cap (1z)(S) over cap (4z)). The following sets of parameters were derived: for Na-4[V12As8O40(H2O)].23H(2)O, J(12)(xy) = J(12)(z) = -0.80 meV, J(23)(xy) = J(23)(z) = -0.72 meV; for Na-4[V12As8O40(D2O)].16.5D(2)O, J(12)(xy) = J(12)(z) = J(23)(xy) = J(23)(z) = -0.78 meV; for (NHEt3)(4)[V12As8O40(H2O)].H2O, J(12)(xy) = -0.80 meV, J(12)(z) = -0.82 meV, J(23)(xy) = -0.67 meV, J(23)(z) = -0.69 meV. This study of the same {V12As8}-type cluster in three different crystal environments allows us to draw some conclusions concerning the applicability on INS in the area of nondeuterated molecular spin clusters. In addition, the effects of using nondeuterated samples and different sample container shapes for INS were evaluated.

International audience; The implementation of nano-objects in numerous emerging applications often demands their integration in macroscopic devices. Here we present the bottom-up epitaxial solution growth of high-density arrays of vertical 5 nm diameter single-crystalline metallic cobalt nanowires on wafer-scale crystalline metal surfaces. The nanowires form regular hexagonal arrays on unpatterned metallic films. These hybrid heterostructures present an important perpendicular magnetic anisotropy and pave the way to a high density magnetic recording device, with capacities above 10 Terabits/in2. This method bypasses the need of assembling and orientating free colloidal nanocrystals on surfaces. Its generalization to other materials opens new perspectives toward many applications.

The molecular spin cluster V15 is a polyoxovanadate with 15 spins s=1/2 but total spin ground state S=1/2 due to strong antiferromagnetic (AFM) interactions. Below 1 K, the relaxation of the magnetisation exhibits a very peculiar behaviour in connection of the multi-spin character of the cluster and the ground state is composed of two non-degenerate S=1/2 Kramers doublets separated by a gap of about 0.3 K. The mechanism leading to such gap formation in the ground state has been elucidated using inelastic neutrons scattering (INS) under magnetic field. The observation and characterisation of several field-dependent INS transitions enable a comprehensive understanding of the low-energy quantum spin states. The energy gap between the two Kramers doublets is caused by slight deviations from the trigonal symmetry (symmetry lowering) of the cluster. The INS data are successfully mapped onto an S=1/2 AFM Heisenberg triangle with scalene distortion and a quantitative description of the wavefunction mixing (entanglement) within the ground state is derived. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; Prussian blue analogues magnetic nanoparticles (of radius R0 = 2.4–8.6 nm) embedded in PVP (polyvinylpyrrolidone) or CTA+ (cetyltrimethylammonium) matrices have been studied using neutron diffraction and small angle neutron scattering (SANS) at several concentrations. For the most diluted particles in neutral PVP, the SANS signal is fully accounted for by a “single-particle” spherical form factor with no structural correlations between the nanoparticles and with radii comparable to those inferred from neutron diffraction. For higher concentration in PVP, structural correlations modify the SANS signal with the appearance of a structure factor peak, which is described using an effective “mean-field” model. A new length scale R* ≈ 3R0, corresponding to an effective repulsive interaction radius, is evidenced in PVP samples. In CTA+, electrostatic interactions play a crucial role and lead to a dense layer of CTA+ around the nanoparticles, which considerably alter the SANS patterns as compared to PVP. The SANS data of nanoparticles in CTA+ are best described by a core-shell model without visible inter-particle structure factor.

We feature our recent work in the field of single-molecule magnets (SMMs) using inelastic neutron scattering (INS). The term "pressure" in the title has a triple meaning. First, there is the expectation from research-funding agencies and the public to make some significant steps towards applications. Second, the synthesis of new compounds and the applications of physical techniques for their understanding are being pushed to their limits. And third, by applying hydrostatic pressure, valuable insight into the mechanisms behind the SMM phenomena can be gained. Examples from our research have been taken to illustrate these points. After a brief introduction to the technique, the strength of INS for the accurate determination of exchange and anisotropy interactions in SMMs is highlighted. We hope to demonstrate that, by pushing INS measurements to their limits, i.e., using the best available neutron sources and instrumentation, combined with high quality samples, new insights into the relevant physical processes in SMMs can be gained.

We report inelastic neutron scattering (INS) studies on a series of Mn(12) derivatives, [Mn(12)O(12)(O2CC6F5)16(H2O)4]z, in which the number of unpaired electrons in the cluster is varied. We investigated three oxidation levels: z = 0 for the neutral complex, z = -1 for the one-electron reduced species and z = -2 for the two-electron reduced complex. For z = 0, the ground state is S = 10 as in the prototypical Mn12-acetate. For z = -1, we have S = 19/2, and for z = - 2, an S = 10 ground state is retrieved. INS studies show that the axial zero-field splitting parameter D is strongly suppressed upon successive electron reduction: D = -0.45 cm(-1) (z = 0), D = -0.35 cm(-1) (z = -1), and D approximately -0.26 cm(-1) (z = -2). Each electron reduction step is directly correlated to the conversion of one anisotropic (Jahn-Teller distorted) Mn3+ (S = 2) to one nearly isotropic Mn2+ (S = 5/2).

The synthesis and structural characterisation of three small nickel(II) cages are reported, all stabilised by pivalate ligands. The magnetic properties of the cages have been studied by a combination of susceptibility measurements and inelastic neutron scattering. For a dinuclear cage, [Ni2(μ-OH2)(O2CCMe3)4(HO2CCMe3)4] 1 the ground state is S = 2, with a ferromagnetic exchange interaction between the Ni(II) centres of J = 0.32 meV and DS=2 = −0.09 meV in the ground state. For a tetranuclear heterocubane cage, [Ni4(μ3-OMe)4(O2CCMe3)4(MeOH)4] 2, two ferromagnetic exchange interactions are found and an S = 4 ground state observed. While the zero-field splitting of this state cannot be determined unambigiously the most likely value is DS=4 = −0.035 meV. For a tetranuclear nickel butterfly, [Ni4(μ3-OH)2(O2CCMe3)6(EtOH)6] 3, three exchange interactions are required, two anti-ferromagnetic and one weakly ferromagnetic; the resulting ground state is S = 0. The data enable us to estimate the zero-field splitting of single Ni(II) ions involved in the cage as Di = +1.0 meV. Both 1 and 2 are therefore expected to be new single molecule magnets.

Nine members of a new family of polynuclear ferric complexes have been synthesized and characterized. The reaction of Fe(O(2)CMe)(2) with polydentate Schiff base proligands (H(2)L) derived from salicylidene-2-ethanolamine, followed in some cases by reaction with carboxylic acids, has afforded new complexes of general formulas [Fe(2)(pic)(2)(L)(2)] (where pic(-) is the anion of 2-picolinic acid), [Fe(3)(O(2)CMe)(3)(L)(3)], [Fe(4)(OR)(2)(O(2)CMe)(2)(L)(4)], and [Fe(5)O(OH)(O(2)CR)(4)(L)(4)]. The tri-, tetra-, and pentanuclear complexes all possess unusual structures and novel core topologies. Mössbauer spectroscopy confirms the presence of high-spin ferric centers in the tri- and pentanuclear complexes. Variable-temperature magnetic measurements suggest spin ground states of S = 0, 1/2, 0, and 5/2 for the bi-, tri-, tetra-, and pentanuclear complexes, respectively. Fits of the magnetic susceptibility data have provided the magnitude of the exclusively antiferromagnetic exchange interactions. In addition, an easy-axis-type magnetic anisotropy has been observed for the pentanuclear complexes, with D values of approximately -0.4 cm(-)(1) determined from modeling the low-temperature magnetization data. A low-temperature micro-SQUID study of one of the pentanuclear complexes reveals magnetization hysteresis at nonzero field. This is attributed to an anisotropy-induced energy barrier to magnetization reversal that is of molecular origin. Finally, an inelastic neutron scattering study of one of the trinuclear complexes has revealed that the magnetic behavior arises from two distinct species.

We review our recent work in the field of molecular spin clusters and single-molecule magnets, showing how inelastic neutron scattering (INS) can be used to determine magnetic exchange interactions and anisotropy splittings. A general introduction to neutron scattering precedes selected examples, building upon the first determination of exchange coupling in a transition metal complex using INS, through anisotropic exchange in cobalt(II) spin clusters to the determination of exchange interactions in a dodecanuclear nickel(II) wheel. The strength of INS for the accurate determination of anisotropy splittings in single-molecule magnets is revealed. Not only can one determine the axial zero-field splitting parameter D, which plays a key role in single-molecule magnet behavior, but also higher-order terms important in understanding the quantum tunneling behavior. Finally, we review two of our synthetic approaches towards new single-molecule magnets based on nickel, manganese, and iron.

The ground-state properties of a Co3II moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2].40D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as H = -2 sigma a = x,y,z (Ja12 S1a S2a + Ja23 S2a S3a), where Ja are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: Jx12 = Jy23, Jy12 = Jx23, Jz12 = Jz23. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: Jx12 = Jy23 = 1.37, Jy12 = Jx23 = 0.218, and Jz12 = Jz23 = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3.

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate cluster compound (NHEt3)(4)[V12As8O40(H2O)] x H2O were investigated by a detailed inelastic neutron scattering study using cold neutrons. The data show clear evidence for the presence of a magnetic anisotropy within the cluster. Exchange parameters are accurately determined.

CoNi magnetic nanowires with a mean diameter of 7 nm and a mean length in the range 100–300 nm have been prepared by reduction of a mixture of cobalt and nickel salts in a sodium hydroxide solution in a liquid polyol. The shape control is related to the particle growth rate which is strongly dependent on the basicity and on the Co–Ni composition. We show that high growth rate favours cobalt urchin-like particles while lower growth rate favours dumbbell-like bimetallic cobalt–nickel particles. Wire formation corresponds to an optimization of the growth conditions and is obtained in a narrow range of basicity and Co–Ni composition. Structural studies showed well crystallized wires with their long axis corresponding to the crystallographic c-axis of the hexagonal close-packed (hcp) phase. Multipods are also observed in which several wires have grown from a single nucleus presenting a cubic structure. Addition of surfactants (trioctyl phosphine and oleic acid) allows the modification of the wire surface and favours their dispersion in toluene. Square hysteresis curves are obtained on wires aligned under magnetic field and frozen in toluene or in PMMA with remanence to saturation ratio close to 1 and coercivity of 6.5 kOe. This value is much higher than was previously obtained on anisotropic particles prepared by the polyol process and results from an improvement of the shape control and the good crystallinity of the wires.

The cover picture shows the principles of inelastic neutron scattering (INS) in magnetic molecular clusters. An incoming monochromatic neutron beam is scattered by a target sample (here a tetranuclear nickel(II) complex) both elastically (no energy transfer) and inelastically (positive or negative energy transfer). Scattered neutrons of varying velocity are then collected in detector banks that span a wide angular range. The interactions between the magnetic moments of the sample and the neutrons that occur during INS allow the investigation of the magnetic properties of the material and the accurate determination of the relevant parameters, including those associated with exchange interactions and magnetoanisotropy. This is nicely illustrated by the inelastic peaks, observed on both sides of the elastic line, which correspond to magnetic energy transfer between the neutrons and the sample. INS is a unique technique that affords a detailed insight into the magnetic levels over a large energy range (up to 1000 cm−1) and thus can provide crucial information for a wide variety of magnetic materials. For more information, see the review by Gudel and co-workers on pages 910–926.

A cyclic complex [Ni(12)(chp)(12)(O(2)CMe)(12)(thf)(6)(H(2)O)(6)] (1) has been synthesised and studied (chp=6-chloro-2-pyridonate). Complex 1 exhibits ferromagnetic exchange between the S=1 centres, giving an S=12 spin ground state. Detailed studies demonstrate that it is a single-molecule magnet with an energy barrier of approximately 10 K for reorientation of magnetisation. Resonant quantum tunnelling is also observed. The field between resonances allows accurate measurement of D, which is 0.067 K. Inelastic neutron scattering studies have allowed exchange parameters to be derived accurately, which was impossible from susceptibility data alone. Three exchange interactions are required: two ferromagnetic nearest neighbour interactions of approximately 11 and 2 cm(-1) and an anti-ferromagnetic next nearest neighbour interaction of -0.9 cm(-1).