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Disorder is important in the study of quantum spin liquids, but its role on the spin dynamics remains elusive. Here, we explore the disorder effect by investigating the magnetic-field dependence of the low-energy magnetic excitations in a triangular-lattice frustrated magnet YbZnGaO$_4$ with inelastic neutron scattering. With an intermediate field of 2.5 T applied along the $c$-axis, the broad continuum at zero field becomes more smeared both in energy and momentum. With a field up to 10 T, which fully polarizes the magnetic moments, we observe clear spin-wave excitations with a gap of $\sim$1.4 meV comparable to the bandwidth. However, the spectra are significantly broadened. The excitation spectra both at zero and high fields can be reproduced by performing classical Monte Carlo simulations which take into account the disorder effect arising from the random site mixing of nonmagnetic Zn$^{2+}$ and Ga$^{3+}$ ions. These results elucidate the critical role of disorder in broadening the magnetic excitation spectra and mimicking the spin-liquid features in frustrated quantum magnets., Comment: Published in PRB, 10 pages, 3 figures

Inelastic neutron scattering spectra in the antiferromagnetic ferroborates ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}{\mathrm{Fe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ ($x$ = 0, 0.1, 0.2, and 1) reveal various magnetic excitations of the interacting iron and rare-earth subsystems. We observe an evolution of the magnetic system from ``easy-plane'', in the Tb-free ($x$ = 0) case, to ``easy-axis'' anisotropy for samples substituted with Tb. The spectra show hybridized Fe and Nd branches, which are determined by the Fe-Nd exchange splitting of the ground-state ${\mathrm{Nd}}^{3+}$ doublet. In the easy-plane configuration, near the Brillouin zone center, there are two different pairs of anticrossing quasiacoustic Fe and Nd modes in contrast to the easy-axis state, where the two corresponding pairs of the branches are degenerated. The high-energy (exchange) branches are similar in both spin configurations. The Ising-type anisotropy of the Tb ion prevents the magnetic moment from precession. The increasing of the Tb content changes the effective magnetic anisotropy and stabilizes the easy-axis state. The spin-wave dispersion in the substituted and pure $\mathrm{Tb}{\mathrm{Fe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ compounds, which have the same, easy-axis magnetic structure, but different crystal symmetry, strongly differ. The observed spectra were analysed in the frame of linear spin-wave theory and the exchange parameters were determined.

Combining inelastic neutron and x-ray scattering with atomic scale simulation, we report on a comprehensive study of the lattice dynamics and its relationship with the low thermal conductivity of the o-${\mathrm{Al}}_{13}{\mathrm{Co}}_{4}$ phase, a periodic approximant (about 100 atoms per cell) to a decagonal quasicrystal. The obtained experimental data, phonon lifetimes, and temperature-independent lattice thermal conductivity can only be fully described by molecular dynamics simulations, using oscillating pair potential, when the disorder of 12 Al sites is properly taken into account. Our results pave the way for a detailed understanding of materials where structural complexity and local disorder are at play.

A review of recent state-of-the-art pulsed field experiments performed on URu 2 Si 2 under a magnetic field applied along its easy magnetic axis c is given. Resistivity, magnetization, magnetic susceptibility, Shubnikov-de Haas, and neutron diffraction experiments are presented, permitting to emphasize the relationship between Fermi surface reconstructions, the destruction of the hidden-order and the appearance of a spin-density wave state in a high magnetic field.

Magnetic excitations and exchange interactions in multiferroic NdFe3(BO3)4 were studied by inelastic neutron scattering in the phase with commensurate antiferromagnetic structure. The observed spectra were analyzed in the frame of the linear spin-wave theory. It was shown that only the model, which includes the exchange interactions within eight coordination spheres, describes satisfactorily all observed dispersion curves. The calculation showed that the spin-wave dynamics is governed by the strongest antiferromagnetic intra-chain interaction and three almost the same inter-chain interactions. Other interactions, including ferromagnetic exchange, appeared to be insignificant. The overall energy balance of the antiferromagnetic inter-chain exchange interactions, which couple the moments from the adjacent ferromagnetic layers as well as within a layer, stabilizes ferromagnetic arrangement in the latter. It demonstrates that the pathway geometry plays a crucial role in forming of the magnetic structure.

Tb2Ti2O7 presents an ongoing conundrum in the study of rare-earth pyrochlores. Despite the expectation that it should be the prototypical unfrustrated noncollinear Ising antiferromagnet on the pyrochlore lattice, it presents a puzzling correlated state that persists to the lowest temperatures. Effects which can reintroduce frustration or fluctuations are therefore sought, and quadrupolar operators have been implicated. One consequence of strong quadrupolar effects is the possible coupling of magnetic and lattice degrees of freedom, and it has previously been shown that a hybrid magnetoelastic mode with both magnetic and phononic character is formed in Tb2Ti2O7 by the interaction of a crystal field excitation with a transverse-acoustic phonon. Here, using polarized and unpolarized inelastic neutron scattering, we present a detailed characterization of the magnetic and phononic branches of this magnetoelastic mode, particularly with respect to their composition, the anisotropy of any magnetic fluctuations, and also the temperature dependence of the different types of fluctuation that are involved. We also examine the dispersion relations of the exciton branches that develop from the crystal field excitation in the same temperature regime that the coupled mode appears, and find three quasidispersionless branches where four are expected, each with a distinctive structure factor indicating that they are nonetheless cooperative excitations. We interpret the overall structure of the spectrum as containing four branches, one hybridized with the phonons and gaining a strong dispersion, and three remaining dispersionless.

Spin wave dispersion in the frustrated fcc type-III antiferromagnet MnS

Spin wave dispersion in the frustrated fcc type-III antiferromagnet MnS$_2$ has been determined by inelastic neutron scattering using a triple-axis spectrometer. Existence of multiple spin wave branches, with significant separation between high-energy and low-energy modes highlighting the intrinsic magnetic frustration effect on the fcc lattice, is explained in terms of a spin wave analysis carried out for the antiferromagnetic Heisenberg model for this $S=5/2$ system with nearest and next-nearest-neighbor exchange interactions. Comparison of the calculated dispersion with spin wave measurement also reveals small suppression of magnetic frustration resulting from reduced exchange interaction between frustrated spins, possibly arising from anisotropic deformation of the cubic structure.

International audience

Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations are at the heart of our understanding of a whole novel class of phase transitions. In most of the cases, those transitions are controlled by a single type of topological objects. There are however some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realization of such cases is thus of considerable interest. We show here that this situation occurs in BaCo$_2$V$_2$O$_8$, a spin-1/2 Ising-like quasi-one dimensional antiferromagnet when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological objects, which are captured by different components of the dynamical structure factor., 9 pages, 4 figures; 10 additional pages of supplemental information containing 6 figures

The magnetic properties of the cobaltite BaCo2(AsO4)2, a good realization of the quasi two-dimensional frustrated honeycomb-lattice system with strong planar anisotropy, have been reinvestigated by means of spherical neutron polarimetry with CRYOPAD. From accurate measurements of polarization matrices both on elastic and inelastic contributions as a function of the scattering vector Q, we have been able to determine the low-temperature magnetic structure of BaCo2(AsO4)2 and reveal its puzzling in-plane spin dynamics. Surprisingly, the ground-state structure (described by an incommensurate propagation vector k1=(kx,0,kz), with kx=0.270±0.005 and kz≈−1.31) appears to be a quasi-collinear structure, and not a simple helix, as previously determined. In addition, our results have revealed the existence of a non-negligible out-of-plane moment component ≈0.25μB/Co2+, representing about 10% of the in-plane component, as demonstrated by the presence of finite off-diagonal elements Pyz and Pzy of the polarization matrix, both on elastic and inelastic magnetic contributions. Despite a clear evidence of the existence of a slightly inelastic contribution of structural origin superimposed to the magnetic excitations at the scattering vectors Q=(0.27,0,3.1) and Q=(0.73,0,0.8) (energy transfer ΔE≈2.3 meV), no strong inelastic nuclear-magnetic interference terms could be detected so far, meaning that the nuclear and magnetic degrees of freedom have very weak cross-correlations. The strong inelastic Pyz and Pzy matrix elements can be understood by assuming that the magnetic excitations in BaCo2(AsO4)2 are spin waves associated with trivial anisotropic precessions of the magnetic moments involved in the canted incommensurate structure.

Magnetic frustration and low dimensionality can prevent long range magnetic order and lead to exotic correlated ground states. SrDy$_2$O$_4$ consists of magnetic Dy$^{3+}$ ions forming magnetically frustrated zig-zag chains along the c-axis and shows no long range order to temperatures as low as $T=60$ mK. We carried out neutron scattering and AC magnetic susceptibility measurements using powder and single crystals of SrDy$_2$O$_4$. Diffuse neutron scattering indicates strong one-dimensional (1D) magnetic correlations along the chain direction that can be qualitatively accounted for by the axial next-nearest neighbour Ising (ANNNI) model with nearest-neighbor and next-nearest-neighbor exchange $J_1=0.3$ meV and $J_2=0.2$ meV, respectively. Three-dimensional (3D) correlations become important below $T^*\approx0.7$ K. At $T=60$ mK, the short range correlations are characterized by a putative propagation vector $\textbf{k}_{1/2}=(0,\frac{1}{2},\frac{1}{2})$. We argue that the absence of long range order arises from the presence of slowly decaying 1D domain walls that are trapped due to 3D correlations. This stabilizes a low-temperature phase without long range magnetic order, but with well-ordered chain segments separated by slowly-moving domain walls.

In the spinel compound ${\mathrm{GeCo}}_{2}{\mathrm{O}}_{4}$, the ${\mathrm{Co}}^{2+}$ pyrochlore sublattice presents remarkable magnetic-field-induced behaviors that we unveil through neutron and x-ray single-crystal diffraction. The N\'eel-ordered magnetic phase is entered through a structural lowering of the cubic symmetry. In this phase, when a magnetic field is applied along a 2-fold cubic direction, a spin-flop transition of one-fourth of the magnetic moments releases the magnetic frustration and triggers magnetostructural effects. At high field, these ultimately lead to an unusual spin reorientation associated with structural changes.

International audience; The chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with a strong potential for multiferroicity. We report neutron scattering measurements for the multichiral Ba3MFe3Si2O14 (M = Nb, Ta) langasites revealing new important features of the magnetic order of these systems: the bunching of the helical modulation along the c-axis and the in-plane distortion of the 120° Fe-spin arrangement. We discuss these subtle features in terms of the microscopic spin Hamiltonian, and provide the link to the magnetically-induced electric polarization observed in these systems. Thus, our findings put the multiferroicity of this attractive family of materials on solid ground.

URu2Si2 is one of the most enigmatic strongly correlated electron systems and offers a fertile testing ground for new concepts in condensed matter science. In spite of >30 years of intense research, no consensus on the order parameter of its low-temperature hidden-order phase exists. A strong magnetic field transforms the hidden order into magnetically ordered phases, whose order parameter has also been defying experimental observation. Here, thanks to neutron diffraction under pulsed magnetic fields up to 40 T, we identify the field-induced phases of URu2Si2 as a spin-density-wave state. The transition to the spin-density wave represents a unique touchstone for understanding the hidden-order phase. An intimate relationship between this magnetic structure, the magnetic fluctuations and the Fermi surface is emphasized, calling for dedicated band-structure calculations., The strongly-correlated electron system URu2Si2 possesses a hidden-order phase whose order parameter remains unidentified. Here, the authors demonstrate the development of spin-density-wave phases in URu2Si2 under high magnetic fields, providing a potential in-road to understanding this system.

A systematic neutron-scattering study of large near-optimally doped single crystals of the cuprate superconductor, Bi2Sr2CaCu2O8+δ, indicates that its magnetic properties are governed by localized magnetic moments, and not by itinerant quasiparticles, as widely expected. Much of what we know about the electronic states of high-temperature superconductors is due to photoemission1,2,3 and scanning tunnelling spectroscopy4,5 studies of the compound Bi2Sr2CaCu2O8+δ. The demonstration of well-defined quasiparticles in the superconducting state has encouraged many theorists to apply the conventional theory of metals, Fermi-liquid theory, to the cuprates6,7,8,9. In particular, the spin excitations observed by neutron scattering at energies below twice the superconducting gap energy are commonly believed to correspond to an excitonic state involving itinerant electrons10,11,12,13,14. Here, we present the first measurements of the magnetic spectral weight of optimally doped Bi2Sr2CaCu2O8+δ in absolute units. The lack of temperature dependence of the local spin susceptibility across the superconducting transition temperature, Tc, is incompatible with the itinerant calculations. Alternatively, the magnetic excitations could be due to local moments, as the magnetic spectrum is similar to that in La1.875Ba0.125CuO4 (ref. 15), where quasiparticles16 and local moments17 coexist.

Quasicrystals are long-range-ordered materials that lack translational invariance, so the study of their physical properties remains a challenging problem. Here, we have carried out inelastic-X-ray- and neutron-scattering experiments on single-grain samples of the Zn-Mg-Sc icosahedral quasicrystal and of the Zn-Sc periodic cubic 1/1 approximant, with the aim of studying the respective influence of the local order and of the long-range order (periodic or quasiperiodic) on lattice dynamics. Besides the overall similarities and the existence of a pseudo-gap in the transverse dispersion relation, marked differences are observed, the pseudo-gap being larger and better defined in the approximant than in the quasicrystal. This can be qualitatively explained using the concept of a pseudo-Brillouin-zone in the quasicrystal. These results are compared with simulations on atomic models and using oscillating pair potentials, and the simulations reproduce in detail the experimental results. This paves the way for a detailed understanding of the physics of quasicrystals.

Inelastic neutron scattering is a powerful technique that can measure magnetic correlations in a large momentum and energy range. In strongly correlated electronic systems, where spin, orbital, lattice and charge degrees of freedom are entangled, it is currently used to study the magnetic properties and shed light on their role in the appearance of the exotic electronic properties, such as unconventional superconductivity. In this article, we focus on the observation by inelastic neutron scattering technique of unconventional spin triplet collective modes in the superconducting state of high temperature superconducting cuprates and its interplay with anomalies in the charge excitation spectrum. The triplet spin mode is interpreted as a spin exciton , within a spin band model. Alternative scenarii based on localized or dual (itinerant localized) models are also mentioned. To cite this article: Y. Sidis et al., C. R. Physique 8 (2007).

The rotation of the neutron polarisation vector that takes place on scattering is determined with polarimeters which can measure the three components of the scattered polarisation for any chosen incident polarisation direction. Cryopad zero-field polarimeters are used on the D3/ILL diffractometer and the IN14/ILL, IN20/ILL, IN22/CEA and TAS-1/JAEA three-axis spectrometers. MuPAD zero-field polarimeters have also recently been built for use at the cold neutron beam MIRA/FRM-II and the three-axis spectrometer TASP/SINQ. We present the experimental strategy of this technique called spherical neutron polarimetry (SNP), develop the equations that relate the polarisation components to the CRYOPAD rotation angles, and explain the uncertainties that must be taken into account when carrying out SNP experiments.

Quantum communication relies on the ability to entangle quantum states. Experiments now show that this is possible in a bulk material, with unpaired spins at the ends of antiferromagnetic spin chains entangled over long distances.

We explore the spin dynamics emerging from the N\'eel phase of the chain compound antiferromagnet ${\mathrm{BaCo}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$. Our inelastic neutron scattering study reveals unconventional discrete spin excitations, so-called Zeeman ladders, understood in terms of spinon confinement, due to the interchain attractive linear potential. These excitations consist of two interlaced series of modes, respectively, with transverse and longitudinal polarization. The latter, which correspond to a longitudinal fluctuation of the ordered moment, have no classical counterpart and are related to the zero-point fluctuations that weaken the ordered moment in weakly coupled quantum chains. Our analysis reveals that ${\mathrm{BaCo}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, with moderate Ising anisotropy and sizable interchain interactions, remarkably fulfills the conditions necessary for the observation of discrete long-lived longitudinal excitations.

International audience; Inelastic neutron scattering is used to study the finite-temperature scaling behavior of the local dynamic structure factor in the quasi-one-dimensional quantum antiferromagnet Ni[N, N'-bis(3-aminopropyl) propane-1,3-diamine](mu-NO2)ClO4 (NTENP), at its field-induced Ising quantum critical point. The validity and the limitations of the theoretically predicted scaling relations are tested.

The metal monoxides NiO and CoO both order antiferromagnetically. In NiO, the moments are perpendicular to the propagation vector. In the case of CoO, some discrepancies exist as far as the direction of the moments is concerned. We have taken advantage of the high sensitivity of the spherical neutron polarimetry as implemented in CRYOgenic Polarization Analysis Device (CRYOPAD) to re-investigate their magnetic structures. The results of these experiments are presented and discussed.

Within the frame of the ILL millennium programme, the European ENPI network and the ILL-ASRC/JAERI Memorandum of Understanding, a third-generation Cryopad has been developed and built in three copies. The aim of this collaboration is to open new fields of investigation on the D3/ILL diffractometer, the IN22/CEA and TAS-1/JAERI three-axis spectrometers: complex antiferromagnetic structures, precision determination of antiferromagnetic distributions, magnetic-lattice excitations spectra, search for the neutron electric dipole moment, etc. We present the progress performed with the new-generation devices and show how easy and reliable it is today to carry out spherical neutron polarimetry measurements with Cryopad.

Polarization analysis permitted to verify very promptly that the plane of the helix in the incommensurate phase of CuO was not ( a * , c * ) as first proposed but another one containing without equivoque the b * -axis. Inelastic polarization analysis under applied magnetic field permitted to study the triplet magnon-like mode of spin-Peierls CuGeO3. This mode splits in three, as expected. Intensities of inelastic neutron scattering measurements with polarization analysis have been collected in both spin-flip and nonspin-flip channels. This Zeeman splitting revealed that two out of the three processes are purely spin-flip excitations, while the third undisplaced one is a nonspin-flip process in which the neutron conserves its spin orientation.

CRYOgenic Polarization Analysis Device (CRYOPAD) has been introduced to the triple-axis spectrometer TAS-1 at JRR-3 in JAERI. ILL, CEA-Grenoble and JAERI assembled three third-generation CRYOPADs in the framework of EC/ENPI (ILL and CEA-Grenoble), and in the framework of the MoU between ILL and JAERI. One is installed on the ILL instrument D3 (Millennium Project) and the other on the CEA-Grenoble instrument IN22. The JAERI CRYOPAD is now in commissioning. The current status of the CRYOPAD on TAS-1 is reported.

It is now well admitted that the orbital degrees of freedom play an important role in the occurrence of non-conventional properties in many low-dimensional highly-correlated electron systems. Thus, in quasi-2D high-T, cuprates the existence of staggered orbital currents has been invoked to explain the peculiarities of the pseudo-gap phase. In these materials, the quantitative understanding requires to take into account the multi-spin exchange interactions. as well. Among other effects, electron circulations or exchanges should give rise to anomalous magnetic correlation functions, which can be accurately investigated by means of polarized neutron scattering. This will be illustrated by recent studies carried out on the spin-ladder Sr 14 Cu 24 O 41 and on the quasi-2D helimagnet BaCo 2 As 2 O 8 .

In this paper we would like to review the zero field inelastic neutron scattering results obtained by using both the thermal and cold neutron triple axis instrument. The first single triplet state at around 3 meV is split due to the Dzyaloshinski-Moriya (DM) inter-dimer interaction with both the out-of-plane and in-plane components. The splitting between the S z = ±1 states is of the order of 0.3 meV. The two triplets bound states also form a very flat bands at 4.9. 5.5 and 6.3 meV with finite line width of the order and/or twice of the DM splitting. Finally the higher energy range up to 13 meV can be also characterized by three rather broad, but flat bands at around 8, 10 and 12 meV with the line width roughly three times the DM splitting. These excitations can be probably assigned to three and more triplets bound state excitations.

The field-induced magnetic phase occurring above H c 1 ≃1.5 T in Cs3Cr2Br9 has been investigated by means of elastic and inelastic neutron scattering. The magnetic structure was determined at H=6 T and the field dependence of both the Cr3+ average magnetic moment and the propagation vector were measured. The spin dynamics study reveals that the lowest excitation remains gapped above the critical field Hc1.

The recent availability at the ILL of two three-axis spectrometers, IN20B and IN22, offering high flux of polarized neutrons in the thermal-energy range has opened a new field of investigation of magnetic and lattice excitations. In this short article, we will show recent technical developments concerning the installation and the optimization of the spherical neutron polarimetry (SNP) on the three-axis spectrometer IN22.

X-ray magnetic scattering experiments have been performed in CeSb at the Ce and Sb L edges. In the non-resonant regime, we observe charge satellites reflecting the lattice modulation associated with the periodicity of paramagnetic Γ 8 planes. At the Ce L 2 edge we observe strong magnetic resonances, due to the antiferromagnetic stacking of the ferromagnetic Γ 7 . The study at the Sb L 1 edge shows a magnetic dipole resonance, which supports the model of strong p–f mixing, used to explain the origin of the long-range magnetic order of CeSb.

The existence at the ILL of three-axis spectrometers (TAS) offering high flux of polarized neutrons in the thermal range (“CRG” IN22 and IN20B) has open a new field of investigation of magnetic and lattice excitation spectra. In this paper, we will show recent technical developments concerning the installation and the optimization of the spherical neutron polarization analysis on TAS IN22.

We will review recent results obtained by longitudinal polarization analysis (LPA) and spherical neutron polarimetry (SNP) on typical "low-dimensional" strongly correlated electronic systems (spin-Peierls CuGeO3, High-T-c YBa2Cu3O6+x...), which all demonstrate the importance of polarized neutrons for studies of the magnetic and structural inelastic contributions. In such systems, the hybridization between the structural and magnetic degrees of freedom is expected to give rise to non-negligible "hybrid" correlation functions, which can be detected and investigated accurately by neutron polarimetry. The interest of the SNP in inelastic scattering will be discussed through measurements on CuGeO3.

We present inelastic neutron scattering data on optimally doped Tl2Ba2CuO6+δ (T c =92 K ), a single-layer copper oxide with undistorted copper oxide planes. The data indicate a magnetic resonant mode below Tc, as previously observed in YBa2Cu3O6+δ and Bi2Sr2CaCu2O8+δ. The mode is thus a generic feature of the copper oxide superconductors.

We explore the spin dynamics emerging from the Néel phase of the chain compound antiferromagnet BaCo(2)V(2)O(8). Our inelastic neutron scattering study reveals unconventional discrete spin excitations, so-called Zeeman ladders, understood in terms of spinon confinement, due to the interchain attractive linear potential. These excitations consist of two interlaced series of modes, respectively, with transverse and longitudinal polarization. The latter, which correspond to a longitudinal fluctuation of the ordered moment, have no classical counterpart and are related to the zero-point fluctuations that weaken the ordered moment in weakly coupled quantum chains. Our analysis reveals that BaCo(2)V(2)O(8), with moderate Ising anisotropy and sizable interchain interactions, remarkably fulfills the conditions necessary for the observation of discrete long-lived longitudinal excitations.

The low-energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on a synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon excitable only by the electric field of an electromagnetic wave. Using a comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electroactive crystal field transition.

The time dependence of switching multiferroic domains in ${\mathrm{MnWO}}_{4}$ has been studied by time-resolved polarized neutron diffraction. Inverting an external electric field inverts the chiral magnetic component within rise times ranging between a few and some tens of milliseconds in perfect agreement with macroscopic techniques. There is no evidence for any faster process in the inversion of the chiral magnetic structure. The time dependence is well described by a temperature-dependent rise time suggesting a well-defined process of domain reversion. As expected, the rise times decrease when heating towards the upper boundary of the ferroelectric phase. However, switching also becomes faster upon cooling towards the lower boundary, which is associated with a first-order phase transition.

The low-energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on a synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon excitable only by the electric field of an electromagnetic wave. Using a comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electroactive crystal field transition.

Dans cette partie du cours, nous presenterons les principaux aspects associes a l'utilisation de la polarimetrie en diffusion inelastique des neutrons sur des entites magnetiques. Les aspects generaux relatifs aux excitations magnetiques dans les solides et a la diffusion inelastique des neutrons seront respectivement traites dans les cours de B. Fak (CEA-Grenoble) et B. Dorner (ILL). Le plan du cours "application des neutrons polarises a l'etude des excitations magnetiques" est le suivant: Generalites et definitions (neutrons polarises et analyse de polarisation) La polarimetrie longitudinale: sections efficaces spin-flip et non spin-flip La polarimetrie spherique Exemples d'utilisation des neutrons polarises et de l'analyse de polarisation

Neutron scattering investigations of the magnetic ordering and spin waves have been carried out on the bilayer manganite La1.2Sr1.8Mn2O7 which shows colossal magnetoresistance (CMR) effect close to the ferromagnetic phase transition TC≈128 K. The dispersions of both the acoustic and the optic modes along [100] and [001] have been measured and by using a effective localised spin model the complete set of exchange parameters has been determined. The strong anisotropy in the exchange interactions suggests that La1.2Sr1.8Mn2O7 can be considered as a quasi two-dimensional ferromagnet consisting of weakly coupled ferromagnetic bilayers. Although an effective localised spin model can explain the spin wave dispersion for small momentum and energy transfers close to the zone centre, it is no longer valid for larger momentum and energy transfers close to the zone boundary. The energy widths of the spin waves are found to be much larger than the instrument resolution showing the dual localised–itinerant character of the electronic system.

Commensurate antiferromagnetic ordering has been observed in the superconducting high-$Tc$ cuprate ${\rm YBa_{2}Cu_{3}O_{6.5}}$ (${\rm T_{c}}$=55 K) by polarized and unpolarized elastic neutron scattering. The magnetic peak intensity exhibits a marked enhancement at $T_{c}$. Zero-field $\mu $SR experiments demonstrate that the staggered magnetization is not truly static but fluctuates on a nanosecond time scale. These results point towards an unusual spin density wave state coexisting with superconductivity.

A polarized neutron di!raction experiment was performed in a slightly Si-doped CuGeO compound. The data were analyzed using both the maximum-entropy method to obtain the magnetization density map and a wave function re"nement to quantify the moment carried by each type of atom. These data treatments evidenced a magnetization density essentially carried by the Cu atoms, described by 3d plus possibly 3d orbitals, some sizeable contributions on the Ge and O1 atoms, but nothing on the O2 atoms, although they belong to the CuO chains. 2001 Elsevier Science B.V. All rights reserved.

The current capabilities for carrying out research using polarised 3He gas at the ILL are summarised. The gas production facility is briefly reviewed, as are the instrument implementations that are either in use or under development. A route map of recent scientific study that has been carried at the ILL is given along with an overview of the design of a new filling station that is currently under construction.

A number of magnetic and structural phase transitions in EuMn2O5 is observed below TN ≃ 40 K by neutron diffraction on a single crystal. The magnetic structure just below TN is incommensurate with the propagation vector ( 1 2 0 0.3) . A ferroelectric transition occurs at TN with the polar displacements of Mn3+ ions along axis a . Three-dimensional polarization analysis has shown, that magnetic moments lie in a – c plane down to T1≃ 22 K. Second structural transition, mainly due to the polar shifts of the Mn4+ ions along c , is observed at T1. It is accompanied by a magnetic phase transition and by appearance of the b-component of spins.