Faits marquants scientifiques

M. Warburton, J.M. Ablett, J.-P. Rueff, P. Baroni, L. Paolasini, L. Noirez

We examine the influence at room temperature of the deposit of a water layer on the phonon dynamics of a solid. It is shown that the water wetting at the surface of an Alumina monocrystal has deep effects on acoustic phonons, propagating over several hundred µm distance and taking place on a relatively long time scale. The effect of the wetting at the boundary is two-fold: a hardening of both transverse and longitudinal acoustic phonons is observed as well as a relaxation of internal stresses. These acoustic phonon energy changes were observed by inelastic X-ray scattering up to 40 meV energy loss, allowing us to probe the solid at different depths from the surface.


A. Sazonov, H. Thoma, R. Dutta, M. Meven, A. Gukasov, R. Fittipaldi, V. Granata, T. Masuda, B. Nafradi, and V. Hutanu

A detailed investigation of Ba2MnGe2O7 was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. In the paramagnetic state at 10 K, polarized neutron diffraction was applied to reveal the components of the susceptibility tensor. The crystal and magnetic structures below the antiferromagnetic transition temperature of TN≈4K were determined using unpolarized neutron diffraction. These data imply no structural phase transition from 10 K down to 2.5 K and are well described within the tetragonal space group P¯421m. We found that in zero magnetic field the magnetic space group is either Ccmc21 or Pc212121 with antiferromagnetic order along the [110] or [100] direction, respectively, while neighboring spins along the [001] axis are ordered antiferromagnetically. A noncollinear spin arrangement due to small canting within the ab plane is allowed by symmetry and observed experimentally. The ordered moment is found to be 3.24(3) μB/Mn2+ at 2.5 K and the temperature-field dependent magnetic phase diagram is mapped out by macroscopic magnetization. Distinct differences between the magnetic structure of Ba2MnGe2O7 as compared to those of Ba2CoGe2O7 and Ca2CoSi2O7 are discussed.


I. V. Golosovsky, I. A. Kibalin, A. Gukasov, A. G. Roca, A. López-Ortega, M. Estrader, M. Vasilakaki, K. N. Trohidou, T. C. Hansen, I. Puente-Orench, E. Lelièvre-Berna, J. Nogués

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3O4/Mn3O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3O4 and Mn3O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials.



Özge Azeri, Dennis Schönfeld, Bin Dai, Uwe Keiderling, Laurence Noirez and Michael Gradzielski

Block copolymers synthesized via Atom Transfer Radical Polymerization from alkyl acrylate and t-butyl acrylate and the subsequent hydrolysis of the t-butyl acrylate to acrylic acid were systematically varied with respect to their hydrophobic part by the variation in the alkyl chain length and the degree of polymerisation in this block. Depending on the architecture of the hydrophobic part, they had a more or less pronounced tendency to form copolymer micelles in an aqueous solution. They were employed for the preparation of IPECs by mixing the copolymer aggregates with the polycations polydiallyldimethylammonium chloride (PDADMAC) or q-chit. The IPEC structure as a function of the composition was investigated by Static Light and Small Angle Neutron Scattering. For weakly-associated block copolymers (short alkyl chain), complexation with polycation led to the formation of globular complexes, while already existing micelles (long alkyl chain) grew further in mass. In general, aggregates became larger upon the addition of further polycation, but this growth was much more pronounced for PDADMAC compared to q-chit, thereby leading to the formation of clusters of aggregates. Accordingly, the structure of such IPECs with a hydrophobic block depended largely on the type of complexing polyelectrolyte, which allowed for controlling the structural organisation via the molecular architecture of the two oppositely charged polyelectrolytes.


Déterminer le mécanisme à l'origine de la supraconductivité à haute température critique (Tc), découverte en 1986 dans des oxydes de cuivre , reste un défi majeur en physique au XXIe siècle. Dans ces matériaux, les corrélations entre les électrons sont telles qu’elles engendrent des propriétés électroniques et magnétiques totalement inédites, au nombre desquelles la mystérieuse phase de pseudo-gap (présentant une ouverture partielle d'un gap dans la structure électronique), de laquelle semble émerger la supraconductivité.

Une partie du voile entourant ce nouvel état de la matière vient d’être levé par la diffraction de neutrons polarisés, qui révèle au sein de la phase pseudo-gap l’existence d’une structure électronique originale, présentant des boucles de courant avec un ordre spatial complexe.

I. Mirebeau et N. Martin, J. Appl. Cryst. (2022). 55, 1173-1183

The application is discussed of neutron methods to the study of reentrant spin glasses (RSGs), close to the transition towards a `canonical' spin glass (SG). The focus is on two emblematic systems, namely Au1−xFex and amorphous a-Fe1−xMnx. A set of experimental results is presented to highlight their peculiar static and dynamic properties. The role of small-angle neutron scattering (SANS) is stressed as an important tool to unravel the structure of these complex systems over mesoscopic length scales. Finally, recent SANS results performed under an applied magnetic field in the region of the RSG → SG transition are presented. They show that vortex-like defects are present in the RSG region up to the critical line and vanish in the SG region. These defects, which develop only in a ferromagnetic medium, could be a key feature to probe the emergence of long-range magnetic order.

Wen-Gen Zheng, Victor Balédent, Claire V. Colin, Françoise Damay, Jean-Pascal Rueff, Anne Forget, Dorothée Colson and Pascale Foury-Leylekian

It has been recently observed that a superconducting phase emerges under pressure in the Fe-based spin-ladders BaFe2X3 (X = S, Se). The low dimensionality of the Fe spin-ladders, which simplifies the elaboration of theoretical models, should help to understand the mechanism of superconductivity. We investigate here the frontier between magnetic and superconducting (SC) phases in BaFe2Se3 by performing challenging powder neutron diffraction (PND) and Fe Kβ x-ray emission spectroscopy (XES) under high pressure. We show that the ambient pressure ground state with a block-like magnetic order is destabilized under pressure. A pressure-induced antiferromagnetic stripe-like spin order, similar to the magnetic order of the parent superconductor BaFe2S3, is observed above 3-4 GPa. Our discovery shows that the stripe magnetic order is a key phase close to the SC dome and its particular magnetic fluctuations could be involved in the stabilization of superconductivity in Fe-based spin ladders.



Une expérience proposée par des chercheurs de l'Université de l’Académie des sciences chinoise de Pékin en collaboration avec le CEA/Irig/D-Phy/MEM et le LLB révèle que les fluctuations du spin dans un supraconducteur à base de fer ont une direction privilégiée, ce qui suggère un mécanisme potentiel pour la supraconductivité dans ces matériaux.


Emil A. Klahn, Andreas M. Thiel, Rasmus B. Degn, Iurii Kibalin, Arsen Gukassov, Claire Wilson, Angelos B. Canaj, Mark Murrie and Jacob Overgaard

We present the magnetic anisotropy of two isostructural pentagonal-bipyramidal complexes, [Ln(H2O)5(HMPA)2]I3·2HMPA (HMPA = hexamethylphosphoramide, Ln = Dy, Ho). Using ac magnetic susceptibility measurements, we find magnetic relaxation barriers of 600 K and 270 K for the Dy- and Ho-compounds, respectively. This difference is supported by polarized neutron diffraction (PND) measured at 5 K and 1 T which provides the first experimental evidence that the transverse elements in the magnetic anisotropy of the Ho-analogue are significant, whereas the Dy-analogue has a near-axial magnetic anisotropy with vanishing transverse contributions. The coordination geometries of the two complexes are highly similar, and we attribute the loss of strong magnetic axiality as expressed in the atomic susceptibility tensors from PND, as well as the smaller relaxation barrier in the Ho-complex compared to the Dy-complex, to the less favorable interaction of the pentagonal bipyramidal crystal field with the characteristics of the Ho(III) 4f-charge distribution.


Nécessaires à la réalisation de dispositifs quantiques aux propriétés originales, ou pour mettre en évidence des comportements originaux de la matière, les physiciens du solide explorent de nouveaux composés, à la recherche de matériaux aux propriétés nouvelles et spécifiquement quantiques. Un des objectifs de la physique de la matière condensée moderne est ainsi de mettre à jour et d’étudier de nouvelles phases quantiques de la matière, dont la description dépasse les modèles classiques. Une collaboration entre l'équipe NFMQ du laboratoire Léon Brillouin, l’Institut Néel et l’Université de Warwick, a ainsi mis en évidence un nouvel état magnétique, à très basse température, dans un oxyde de zirconium à structure pyrochlore : Nd2Zr2O7.


Christopher D. O’Neill, Gino Abdul-Jabbar, Didier Wermeille, Philippe Bourges, Frank Krüger, and Andrew D. Huxley

Quantum order by disorder revealed 
Ferromagnets are ubiquitous in everyday life, present in household items ranging from fridge-magnets on a fridge door to the sensors and motors inside, while dozens can be found in an average car. As temperature is raised ferromagnetic order disappears, but more rarely the formation of more fascinating states can occur in which the uniform magnetic order is replaced by a static magnetic wave. One mechanism for this is known as quantum order by disorder; the wave (the order) forms because it has more low energy excited states (the disorder) available to it than the uniform state and this lowers its energy, offsetting the energy cost of producing the wave.  In a recent paper appearing in Physical Review Letters such a static oscillation was achieved for the first time by applying a magnetic field at right angles to the easy axis moment in a ferromagnet, allowing the resulting increase in magnetic excitations to be clearly seen with neutron scattering.  In the long term the creation of such magnetic oscillations with variable pitch could be useful, for example, to make magnetic diffraction gratings for spintronics.


La thermoélasticité décrit la variation des propriétés élastiques d'un corps solide en fonction de sa température. Pour un fluide incompressible, les coefficients thermoélastiques, dilatation isobare et compressibilité isotherme, sont en pratique nuls. Pour être non nuls, il est nécessaire que des interactions à longue portée soient présentes, mais ceci est a priori exclu de par la définition même de l’état liquide.

Une équipe du LLB vient cependant de mettre en évidence des propriétés thermoélastiques pour un liquide dans des conditions usuelles de pression. Ils observent qu'un liquide ordinaire présente une modulation de température sous l'application d'une contrainte mécanique de cisaillement à basse fréquence (~1 Hz) : le liquide se divise en bandes thermiques chaudes et froides, de plusieurs dixièmes de microns de large et variant de manière synchrone avec la déformation. Ce couplage thermomécanique ainsi mis en évidence est une preuve que l'énergie de l’onde de cisaillemente n'est pas totalement dissipée au niveau moléculaire du fait de la viscosité du fluide, mais qu'une partie est convertie adiabatiquement (i.e. sans échange avec l’extérieur) en états thermodynamiques locaux. Ceci est en accord avec de nouveaux modèles théoriques, pour lesquels les liquides ont des propriétés élastiques non-extensives qui s’étendent jusqu’à l’échelle de plusieurs dizaines de microns*.

En outre, la transformation quasi-instantanée de l'énergie de déformation, sous la forme d'une modulation locale de température, implique que les fluctuations (thermiques) de densité sont corrélées à longue distance. Le couplage thermomécanique ainsi mis en évidence a des implications directes pour l’étude des liquides physiologiques. Il pourrait également permettre de réaliser de nouveaux convertisseurs de température, en particulier en microfluidique.

*Tel que "les modèles k-gap", où les modes acoustiques présentent un gap dans l'espace réciproque [7].


K. Beauvois, V. Simonet, S. Petit, J. Robert, F. Bourdarot, M. Gospodinov, A.A. Mukhin, R. Ballou, V. Skumryev, and E. Ressouche

The research field of magnetic frustration is dominated by triangle-based lattices but exotic phenomena can also be observed in pentagonal networks. A peculiar noncollinear magnetic order is indeed known to be stabilized in Bi2Fe4O9 materializing a Cairo pentagonal lattice. We present the spin wave excitations in the magnetically ordered state, obtained by inelastic neutron scattering. They reveal an unconventional excited state related to local precession of pairs of spins. The magnetic excitations are then modeled to determine the superexchange interactions for which the frustration is indeed at the origin of the spin arrangement. This analysis unveils a hierarchy in the interactions, leading to a paramagnetic state (close to the Néel temperature) constituted of strongly coupled dimers separated by much less correlated spins. This produces two types of response to an applied magnetic field associated with the two nonequivalent Fe sites, as observed in the magnetization distributions obtained using polarized neutrons.


La recherche de nouveaux états de la matière, allant au-delà de la description classique "à la Landau" suscite un très fort engouement en physique. Dans cette perspective, les travaux théoriques orientent ces recherches vers les systèmes présentant des "ordres topologiques", tels que certains "liquides de spin quantiques" et autres états fortement corrélés, caractérisés en particulier par l’absence de symétrie brisée.

Sur le plan expérimental, c’est la recherche d’une contrepartie quantique des "glaces de spins" qui a retenu l’attention. Ces composés forment un analogue magnétique de la glace d’eau, où le comportement des spins reflète exactement celui du désordre des protons dans H2O.

Une équipe internationale formée de chercheurs du PSI (Suisse), du Stanford Institute for Materials and Energy Science (USA), de l’Institut Néel à Grenoble et du LLB à Saclay a mis en évidence par diverses techniques, dont la diffusion des neutrons, un exemple de cette contrepartie quantique des glaces de spin. Plus précisément, il s’agit d’un état "glacé" particulier, où la distribution octupolaire de la densité électronique joue le rôle des moments magnétiques dans les glaces de spins classiques. L’étude des interactions montre que l’état fondamental est constitué d’une superposition quantique d’états intriqués, confirmant ainsi les prédictions théoriques sur les liquides de spins quantiques.

M. Ruminy, S. Guitteny, J. Robert, L.-P. Regnault, M. Boehm, P. Steffens, H. Mutka, J. Ollivier, U. Stuhr, J. S. White, B. Roessli, L. Bovo, C. Decorse, M. K. Haas, R. J. Cava, I. Mirebeau, M. Kenzelmann, S. Petit, and T. Fennell

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.

https ://doi.org/10.1103/PhysRevB.99.224431

Béatrice Gillon, Albert Hammerschmied, Arsen Gukasov, Alain Cousson, Thomas Cauchy, Eliseo Ruiz, John A. Schlueter, Jamie L. Manson

We report neutron‐diffraction investigations of the quasi‐2D MnII(dca)2(pym)(H2O) (pym = N2C4H4) compound, where high‐spin MnII 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 MnII 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 MnII 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.



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