Kinetically Inhibited Order in a Diamond-Lattice Antiferromagnet

Frustrated magnetic systems exhibit highly degenerate ground states and strong fluctuations, often leading to new physics. An intriguing example of current interest is the antiferromagnet on a diamond lattice, realized physically in A-site spinel materials. This is a prototypical system in three dimensions where frustration arises from competing interactions rather than purely geometric constraints, and theory suggests the possibility of unusual magnetic order at low temperature. Here we present a comprehensive single-crystal neutron scattering study of CoAl2O4, a highly frustrated A-site spinel. We observe strong diffuse scattering that peaks at wavevectors associated with Neel ordering. Below the temperature T*=6.5 K, there is a dramatic change in the elastic scattering lineshape accompanied by the emergence of well-defined spin-wave excitations. T* had previously been associated with the onset of glassy behavior. Our new results suggest instead that T* signifies a first-order phase transition, but with true long-range order inhibited by the kinetic freezing of domain walls. This scenario might be expected to occur widely in frustrated systems containing first-order phase transitions and is a natural explanation for existing reports of anomalous glassy behavior in other materials.Comment: 40 pages, 9 figures, Introduction and discussion altered and expanded. Additional section and figure added to Supplementary Informatio

Observation of different spin behavior with temperature variation and Cr substitution in a multiferroic compound YMn2_2O5_5

In this article, the collective response of the spins is explored through low field bulk magnetic measurement for the series YMn$_{2-x}$Cr$_x$O$_5$ (x= 0.0, 0.05). Low field ac susceptibility and dc magnetization of YMn$_2$O$_5$ shows multiple transition in analogy to those observed in electrical measurement of the compound. Using various time dependent magnetization protocols it has been observed that the behavior of spins in commensurate and incommensurate phase are drastically different. YMn$_{1.95}$Cr$_{0.05}$O$_5$ undergoes a ferrimagnetic ordering with an enhanced magnetic ordering temperature as compared to the parent, which undergoes an antiferromagnetic ordering. Appearance of spontaneous magnetization without any major change in the atomic structure is rather significant since the parent compound is an important multiferroic material. In addition, magnetic memory effect is observed in the Cr substituted compound whereas it is absent in the parent compound

Anisotropic physical properties of single crystal U2Rh2Sn in high magnetic fields

We report on the crystal and magnetic structures,magnetic, transport, and thermal properties of U2Rh2Sn single crystals studied in part in high magnetic fields up to 58 T. The material adopts a U3Si2 related tetragonal crystal structure and orders antiferromagnetically below TN 25 K. The antiferromagnetic structure is characterized by a propagation vector k 0,0,1 2 . The magnetism in U2Rh2Sn is found to be associated mainly with 5f states. However, both unpolarized and polarized neutron experiments reveal at low temperatures in zero field non negligible magnetic moments also on Rh sites. U moments of 0.50 2 amp; 956;B are directed along the tetragonal axis while Rh moments of 0.06 4 amp; 956;B form a noncollinear arrangement confined to the basal plane. The response to applied magnetic field is highly anisotropic. Above amp; 8764;15 K the easy magnetization direction is along the tetragonal axis. At lower temperatures, however, a stronger response is found perpendicular to the c axis. While for the a axis no magnetic phase transition is observed up to 58 T, for the field applied at 1.8 K along the tetragonal axis we observe above 22.5 T a field polarized state. A magnetic phase diagram for the field applied along the c axis is presented

Etude des propriétés magnétiques du couplage spin / réseau dans les composés multiferroïques RMnO3 hexagonaux par diffusion de neutrons

Ce manuscrit présente une étude des composés multiferroïques RMnO3 hexagonaux (R=Ho, Y, Yb, ln, Sc). L'accent est mis sur les corrélations spin/spin statiques et dynamiques et sur le couplage entre degrés de liberté de spin et de réseau. Cette étude s'appuie sur de nombreux résultats de diffusion de neutrons aussi bien sur poudres que sur monocristaux. La diffraction permet de déterminer précisément les positions atomiques et la structure magnétique. Nous avons pu développer un modèle rendant compte de la très grande variété de comportements observés dans ces systèmes en nous basant sur la frustration des interactions d'échange interplans. La frustration des interactions magnétiques n'est pas simplement due au réseau triangulaire de Mn mais aussi aux interactions entre Mn de plans adjacents. Cette frustration est directement controlée par la position du Mn par rapport à une valeur seuil x= 1/3 dans la maille élémentaire. Les mesures de diffusion inélastiques ont permis de valider ce modèle en vérifiant certaines prédictions à partir de mesures de dispersion d'onde de spins et d'un calcul numérique solide (signe de la constante d'échange interplan, transitions de réorientation). Elles ont également confirmé le rôle primordial des interactions RMn sur la dynamique de spin des Mn. Enfin des mesures complexes de diffusion inélastique de neutrons polarisés ont mis en évidence une hybridation entre excitations de réseau et de spin. Un modèle basé sur l'interaction de Dzyaloshiinski-Moriya est proposé pour rendre compte des observations expérimentales. Ce mode hybride est a rapprocher de l'électromagnon observé dans de nombreux systèmes, et semble être une composante inhérente à l'existence de la multiferroïcité.This manuscript is devoted to the study of magnetic properties and spin/lattice coupling in the hexagonal RMnO3 compounds which belong to the multiferroic family. Most of the results were obtained from neutron scattering experiments both on powder and single crystal samples. The neutron powder diffraction data allowed us to highlight the strong coupling between atomic position and the magnetic structure stabilized. These results are discussed in terms of magnetic frustration, and we propose a model based on interplane exchange frustration. The atomic position of Mn ions relatively to a x=1/3 threshold is crucial in this framework. Our inelastic neutron scattering data confirmed our model by giving us access to the numerical values of some parameters from spinwave dispersions and confirmed the important role of RMn interaction on the spin dynamic. Finally, complex inelastic neutron scattering of polarized neutrons experiments showed an hybridation of spin and lattice excitations. This hybridation is interpreted within the Dzyaloshiinski-Moriya framework and numerical calculations are presented to reproduce our experimental data. This hybridation is closely related to the electromagnon observed in several multiferroic systems and seems to be inherently associated to multiferroicity.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Magnetic structures of RE PdBi half-Heusler bismuthides ( RE = Gd, Tb, Dy, Ho, Er)

We present results of neutron diffraction on single crystals of several equiatomic ternary compounds of rare-earth elements with palladium and bismuth, crystallizing with cubic MgAgAs-type structure (half-Heusler phases). Band structure calculations showed that many members of that family possess electronic band inversion, which may lead to occurrence of topological insulator or topological semimetal. But even for the compounds without intrinsic band inversion another way of topologically non-trivial state realization, through a specific antiferromagnetic order, has been theoretically proposed.Our results show that the antiferromagnetic structures of all studied bismuthides are characterized by the propagation vector, allowing for antiferromagnetic topological insulator state. Therefore, the antiferromagnetic representatives of half-Heusler family are excellent candidates for extended investigations of coexistence of superconductivity, magnetic order and non-trivial topology of electronic states

CSPEC: The cold chopper spectrometer of the ESS, a detailed overview prior to commissioning

International audienceCSPEC is the cold chopper spectrometer of the European Spallation Source (ESS) and will come online with the ESS beam on the target. CSPEC will be the first cold chopper spectrometer on a long pulsed spallation source, which provides great opportunities in terms of signal to noise and novel measuring schemes. We provide a detailed overview of the instrument, scientific design considerations, and engineering requirements

Inter-Individual and Inter-Strain Variations in Zebrafish Locomotor Ontogeny

Zebrafish exhibit remarkable alterations in behaviour and morphology as they develop from early larval stages to mature adults. In this study we compare the locomotion parameters of six common zebrafish strains from two different laboratories to determine the stability and repeatability of these behaviours. Our results demonstrate large variability in locomotion and fast swim events between strains and between laboratories across time. These data highlight the necessity for careful, strain-specific controls when analysing locomotor phenotypes and open up the possibility of standardising the quantification of zebrafish behaviour at multiple life stages