Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of Fe1−x_{1-x}Cox_xSi

We present a comprehensive Small Angle Neutron Scattering (SANS) and Neutron Spin Echo Spectroscopy (NSE) study of the structural and dynamical aspects of the helimagnetic transition in Fe$_{1-x}$Co$_x$Si with $x$ = 0.30. In contrast to the sharp transition observed in the archetype chiral magnet MnSi, the transition in Fe$_{1-x}$Co$_x$Si is gradual and long-range helimagnetic ordering coexists with short-range correlations over a wide temperature range. The dynamics are more complex than in MnSi and involve long relaxation times with a stretched exponential relaxation which persists even under magnetic field. These results in conjunction with an analysis of the hierarchy of the relevant length scales show that the helimagnetic transition in Fe$_{1-x}$Co$_x$Si differs substantially from the transition in MnSi and question the validity of a universal approach to the helimagnetic transition in chiral magnets

X-ray Near Field Speckle: Implementation and Critical Analysis

We have implemented the newly-introduced, coherence-based technique of x-ray near-field speckle (XNFS) at 8-ID-I at the Advanced Photon Source. In the near field regime of high-brilliance synchrotron x-rays scattered from a sample of interest, it turns out, that, when the scattered radiation and the main beam both impinge upon an x-ray area detector, the measured intensity shows low-contrast speckles, resulting from interference between the incident and scattered beams. We built a micrometer-resolution XNFS detector with a high numerical aperture microscope objective and demonstrate its capability for studying static structures and dynamics at longer length scales than traditional far field x-ray scattering techniques. Specifically, we characterized the structure and dynamics of dilute silica and polystyrene colloidal samples. Our study reveals certain limitations of the XNFS technique, which we discuss.Comment: 53 pages, 16 figure

A Chiral Paramagnetic Skyrmion-like Phase in MnSi

We present a comprehensive study of chiral fluctuations in the reference helimagnet MnSi by polarized neutron scattering and Neutron Spin Echo spectroscopy, which reveals the existence of a completely left-handed and dynamically disordered phase. This phase may be identified as a spontaneous skyrmion phase: it appears in a limited temperature range just above the helical transition Tc and coexists with the helical phase at Tc.Comment: PRL accepte

La multifonctionnalité de l'agriculture : un concept d'avenir ?

International audienceLa multifonctionnalité de l'agriculture : un concept d'avenir ? Etude par Luc Bodiguel chargé de recherche, UMR CNRS 3128, Droit et changement social (DCS)chargé d'enseignement à la faculté de droit de Nantes et d'Angers Accès au sommaire Comment comprendre aujourd'hui le concept de multifonctionnalité de l'agriculture ? Les réformes française et communautaire sont-elles venues lui donner un véritable contenu juridique ? Une autre portée politique ? Ou, au contraire, la multifonctionnalité de l'agriculture est-elle restée au stade théorique, au concept, voire, at -elle été dépassée par d'autres concepts plus influents, plus opérationnels ou plus à la mode ? Enfin, à quoi peut bien encore servir le concept de multifonctionnalité de l'agriculture d'un point de vue politique et juridique ? Afin de tenter de répondre à ces questions, Luc Bodiguel nous propose d'observer tout d'abord les fondements et règles de droit qui peuvent aujourd'hui être liés au concept de multifonctionnalité pour ensuite s'interroger sur l'avenir du concept au vu du droit français, communautaire et de l'organisation mondiale du commerce

Comparative analysis of IL6 and IL6 receptor gene polymorphisms in mastocytosis

Mastocytosis is a rare disease with reported high interleukin-6 (IL6) levels influencing disease severity. The present study investigated polymorphisms within the genes that encode IL6 and its receptor (IL6R) in relation to mastocytosis development in a case-control design. Analysis of the IL6R Asp358Ala polymorphism showed that carriers of the AA genotype had a 2.5-fold lower risk for mastocytosis than those with the AC or CC genotypes. No association with mastocytosis was found for the IL6-174G/C polymorphism, however, it may influence the effect of IL6R polymorphism. To the best of our knowledge this is the first study analysing IL6/IL6R polymorphisms in mastocytosis

Direct measurement of antiferromagnetic domain fluctuations

Measurements of magnetic noise emanating from ferromagnets due to domain motion were first carried out nearly 100 years ago and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise, but this must be sampled at spatial wavelengths of order several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present the first direct measurement of the fluctuations in the nanometre-scale spin- (charge-) density wave superstructure associated with antiferromagnetism in elemental Chromium. The technique used is X-ray Photon Correlation Spectroscopy, where coherent x-ray diffraction produces a speckle pattern that serves as a "fingerprint" of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micron distances. While the domain wall motion is thermally activated at temperatures above 100K, it is not so at lower temperatures, and indeed has a rate which saturates at a finite value - consistent with quantum fluctuations - on cooling below 40K. Our work is important because it provides an important new measurement tool for antiferromagnetic domain engineering as well as revealing a fundamental new fact about spin dynamics in the simplest antiferromagnet.Comment: 19 pages, 4 figure

Skyrmions and spirals in MnSi under hydrostatic pressure

The archetype cubic chiral magnet MnSi is home to some of the most fascinating states in condensed matter such as skyrmions and a non-Fermi liquid behavior in conjunction with a topological Hall effect under hydrostatic pressure. Using small angle neutron scattering, we study the evolution of the helimagnetic, conical and skyrmionic correlations with increasing hydrostatic pressure. We show that the helical propagation vector smoothly reorients from $\langle 111 \rangle$ to $\langle100\rangle$ at intermediate pressures. At higher pressures, above the critical pressure, the long-range helimagnetic order disappears at zero magnetic field. Nevertheless, skyrmion lattices and conical spirals form under magnetic fields, in a part of the phase diagram where a topological Hall effect and a non-Fermi liquid behavior have been reported. These unexpected results shed light on the puzzling behavior of MnSi at high pressures and the mechanisms that destabilize the helimagnetic long-range order at the critical pressure