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

Measurement of the neutron electric dipole moment by crystal diffraction

An experiment using a prototype setup to search for the neutron electric dipole moment by measuring spin-rotation in a non-centrosymmetric crystal (quartz) was carried out to investigate statistical sensitivity and systematic effects of the method. It has been demonstrated that the concept of the method works. The preliminary result of the experiment is $d_{\rm n}=(2.5\pm 6.5)\cdot 10^{-24}$ e$\cdot$cm. The experiment showed that an accuracy of $\sim 2.5\cdot 10^{-26}$ e$\cdot$cm can be obtained in 100 days data taking, using available quartz crystals and neutron beams.Comment: 13 pages, 4 figure

Optimization of multi-channel neutron focusing guides for extreme sample environments

In this work, we present and discuss simulation results for the design of multi-channel neutron focusing guides for extreme sample environments. A single focusing guide consists of any number of supermirror-coated curved outer channels surrounding a central channel. Furthermore, a guide is separated into two sections in order to allow for extension into a sample environment. The performance of a guide is evaluated through a Monte-Carlo ray tracing simulation which is further coupled to an optimization algorithm in order to find the best possible guide for a given situation. A number of population-based algorithms have been investigated for this purpose. These include particle-swarm optimization, artificial bee colony, and differential evolution. The performance of each algorithm and preliminary results of the design of a multi-channel neutron focusing guide using these methods are described. We found that a three-channel focusing guide offered the best performance, with a gain factor of 2.4 compared to no focusing guide, for the design scenario investigated in this work

Spherical neutron polarimetry applied to spin echo and time of flight spectroscopy

The changes in direction of the neutron spin that take place on scattering by a magnetic interaction vector are highly dependent on their relative directions. When the magnetic interaction vector is complex and or when there are nuclear magnetic interferences, the rotation of the incident polarisation can be of any angle around or toward a speci amp; 64257;c direction. In such circumstances, without zero amp; 64257;eld polarimeter, it is impossible to distinguish between a simple depolarisation and a rotation of the polarisation vector. Motivated by the investigation of chiral magnetic amp; 64258;uctuations, we have implemented the third generation zero amp; 64257;eld polarimeter Cryopad on the Neutron Spin Echo spectrometer SPAN at the HCB. We present the method and the limitations of this novel technique that is now available on IN15 at the ILL. The huge progress accomplished with 3 He neutron spin amp; 64257;lters amp; 64258;ippers are going to facilitate the exploitation of polarised beams at spallation sources. Zero amp; 64257;eld polarimeters like Cryopad are used routinely at several steady state sources but their design would be ine amp; 64259;cient at a pulse source. We have investigated the possibility to implement a zero amp; 64257;eld polarimeter on a time of amp; 64258;ight spectrometer. We propose a design that would lead to a better e amp; 64259;ciency and present the amp; 64257;nite element calculations

Magnetization densities in UCoAl studied by polarized neutron diffraction

UCoAl crystallizes in the ZrNiAl-type hexagonal structure, exhibits a paramagnetic ground state, but a metamagnetic transition to a ferromagnetic state with uranium magnetic moments parallel to the c axis occurs at low temperatures when a field of 1 T is applied in this direction. We present the results of a polarized neutron experiment on a UCoAl single crystal. Experimental data have been analyzed by a maximum entropy method and within an atomic model. The main magnetic contribution originates from the uranium atoms. The ratio μL/μS between the orbital and spin moment is slightly reduced in comparison to the uranium free ion value and remains nearly unchanged between 1 and 8 T. Induced magnetic moments of nearly same magnitude and similar field dependence are observed on the two cobalt sites. An additional magnetization density is observed around the aluminum positions. The results are discussed in context of former studies on isostructural UTX compounds