Magnetic Fluctuations and Correlations in MnSi - Evidence for a Skyrmion Spin Liquid Phase

We present a comprehensive analysis of high resolution neutron scattering data involving Neutron Spin Echo spectroscopy and Spherical Polarimetry which confirm the first order nature of the helical transition and reveal the existence of a new spin liquid skyrmion phase. Similar to the blue phases of liquid crystals this phase appears in a very narrow temperature range between the low temperature helical and the high temperature paramagnetic phases.Comment: 11 pages, 16 figure

Magnetic Fluctuations, Precursor Phenomena and Phase Transition in MnSi under Magnetic Field

The reference chiral helimagnet MnSi is the first system where skyrmion lattice correlations have been reported. At zero magnetic field the transition at $T_C$ to the helimagnetic state is of first order. Above $T_C$, in a region dominated by precursor phenomena, neutron scattering shows the build up of strong chiral fluctuating correlations over the surface of a sphere with radius $2\pi/\ell$, where $\ell$ is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above $T_C$, both of general relevance to chiral magnetism, remain an open question

Length scale dependence of dynamical heterogeneity in a colloidal fractal gel

We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final decay of the average intensity autocorrelation function is well described by $g\_2(q,\tau)-1 \sim \exp[-(\tau/\tau\_\mathrm{f})^p]$, with $\tau\_\mathrm{f} \sim q^{-1}$ and $p$ decreasing from 1.5 to 1 with increasing $q$. We show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements. We quantify the dynamical fluctuations resulting from intermittency by means of the variance $\chi(\tau,q)$ of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility $\chi\_4$ studied in glass formers. The amplitude of $\chi$ is found to grow linearly with $q$. We propose a simple --yet general-- model of intermittent dynamics that accounts for the $q$ dependence of both the average correlation functions and $\chi$.Comment: Revised and improved, to appear in Europhys. Let

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

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

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

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