We report the discovery of topological magnetism in the candidate magnetic Weyl semimetal CeAlGe. Using neutron scattering we find this system to host several incommensurate, square-coordinated ...multi-kover → magnetic phases below T_{N}. The topological properties of a phase stable at intermediate magnetic fields parallel to the c axis are suggested by observation of a topological Hall effect. Our findings highlight CeAlGe as an exceptional system for exploiting the interplay between the nontrivial topologies of the magnetization in real space and Weyl nodes in momentum space.
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Specular neutron reflectometry is a powerful technique to resolve interfacial compositions and structures in soft matter. Surprisingly however, even after several decades, a universal ...modeling approach for the treatment of data of surfactant and phospholipid monolayers at the air/water interface has not yet been established. To address this shortcoming, first a systematic evaluation of the suitability of different models is presented. The result is a comprehensive validation of an optimum model, which is evidently much needed in the field, and which we recommend as a starting point for future data treatment. While its limitations are openly discussed, consequences of failing to take into account various key aspects are critically examined and the systematic errors quantified. On the basis of this physical framework, we go on to show for the first time that neutron reflectometry can be used to quantify directly in situ at the air/water interface the extent of acyl chain compaction of phospholipid monolayers with respect to their phase. The achieved precision of this novel quantification is ∼10%. These advances together enhance significantly the potential for exploitation in future studies data from a broad range of systems including those involving synthetic polymers, proteins, DNA, nanoparticles and drugs.
The vertical sample‐plane reflectometer D17 at the Institut Laue–Langevin in Grenoble, France, has undergone several major upgrades since its commissioning, which are summarized in this article. The ...three major improvements are (i) a new focusing guide, increasing the usable flux on the sample by a factor of 2.5; (ii) a new beam polarizer and new spin flippers, allowing for the use of polarized neutrons in time‐of‐flight mode; and (iii) a new detector with a particularly uniform response under homogeneous exposure, improved stability and state‐of‐the‐art detector electronics. The combination of these factors has paved the road to new possibilities in fast kinetic measurements, magnetism and off‐specular scattering. Examples and scientific references for the new capabilities are presented.
The article describes recent upgrades of the polarized neutron reflectometer D17 at the Institut Laue–Langevin, Grenoble, France, and presents recent examples of scientific studies enabled with the new options.
We present an operando neutron reflectometry study on the electrochemical incorporation of lithium into crystalline silicon for battery applications. Neutron reflectivity is measured from the ⟨100⟩ ...surface of a silicon single crystal which is used as a negative electrode in an electrochemical cell. The strong scattering contrast between Si and Li due to the negative scattering length of Li leads to a precise depth profile of Li within the Si anode as a function of time. The operando cell can be used to study the uptake and the release of Li over several cycles. Lithiation starts with the formation of a lithium enrichment zone during the first charge step. The uptake of Li can be divided into a highly lithiated zone at the surface (skin region) (x ∼ 2.5 in Li x Si) and a much less lithiated zone deep into the crystal (growth region) (x ∼ 0.1 in Li x Si). The total depth of penetration was less than 100 nm in all experiments. The thickness of the highly lithiated zone is the same for the first and second cycle, whereas the thickness of the less lithiated zone is larger for the second lithiation. A surface layer of lithium (x ∼ 1.1) remains in the silicon electrode after delithiation. Moreover, a solid electrolyte interface is formed and dissolved during the entire cycling. The operando analysis presented here demonstrates that neutron reflectivity allows the tracking of the kinetics of lithiation and delithiation of silicon with high spatial and temporal resolution.
Reflectometry is a powerful technique for determining many physical quantities of stratified media, including length scales, densities and magnetism. However, experimentally neutron reflectometry in ...particular suffers from the relatively feeble brilliance of the sources compared with those of X‐rays, for example. In this paper, a simple modification of existing data‐reduction methods is demonstrated, allowing quantitative improvements in the quality of the data. Using the same algorithm, reflections from non‐flat surfaces can be treated, leading to a full recovery of the resolution. The method involves re‐binning of the data in the linear coordinates of the raw data, which leads to substantial gains in statistical quality, equivalent to a significant flux increase, and also improved resolution.
A biskyrmion consists of two bound, topologically stable, skyrmion spin textures. These coffee‐bean‐shaped objects are observed in real space in thin plates using Lorentz transmission electron ...microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface‐confined objects, or, analogous to skyrmions in noncentrosymmetric helimagnets, 3D tube‐like structures in a bulk sample. Here, the biskyrmion form factor is investigated in single‐ and polycrystalline‐MnNiGa samples using small‐angle neutron scattering. It is found that biskyrmions are not long‐range ordered, not even in single crystals. Surprisingly all of the disordered biskyrmions have their in‐plane symmetry axis aligned along certain directions, governed by the magnetocrystalline anisotropy. This anisotropic nature of biskyrmions may be further exploited to encode information.
Biskyrmion scattering is measured in a single‐crystalline centrosymmetric MnNiGa bulk sample using small‐angle neutron scattering. The biskyrmions are not long‐range ordered, and all of the disordered biskyrmions have their in‐plane symmetry axis aligned along certain directions, governed by the magnetocrystalline anisotropy. Such an anisotropic nature of biskyrmions, representing an extra degree of freedom, may be exploited as an information carrier.
The hydration and thermal response kinetics of the cross-linked thermoresponsive copolymer poly((diethylene glycol monomethyl ether methacrylate)-co-poly(ethylene glycol) methyl ether ...methacrylate), abbreviated as P(MEO2MA-co-OEGMA300), thin film on a hydrophobic polyacrylonitrile (PAN) substrate coating, which resembles a synthetic fabric, is probed by in situ neutron reflectivity (NR). The PAN and monomer (MEO2MA and OEGMA300) solutions are sequentially spin-coated onto a silicon (Si) substrate. Afterward, plasma treatment is applied to realize the cross-linking of PAN and monomers. The as-prepared cross-linked P(MEO2MA-co-OEGMA300) film on the hydrophobic PAN substrate coating presents a two-layer structure: a substrate-near layer, which is a mixture of PAN and P(MEO2MA-co-OEGMA300), and a main layer, which is composed of pure hydrophilic P(MEO2MA-co-OEGMA300). During hydration in D2O vapor atmosphere, the hydrophobic PAN component prevents the formation of D2O enrichment in the substrate-near layer. However, an additional vapor-near layer is observed on top of the main layer, which is enriched with D2O. The hydration process is constrained by the cross-linking points in the film, inducing the relaxation time to be longer than that in a spin-coated P(MEO2MA-co-OEGMA300) film. Because the as-prepared cross-linked film presents a transition temperature (TT) at 38 °C, the hydrated film switches to the collapsed state when the temperature is increased from 23 to 50 °C. The response to a thermal stimulus is also slower due to the existence of the internal cross-linking points as compared to the spin-coated film. Interestingly, no reswelling is observed at the end of the thermal stimulus, which can be also attributed to the presence of internal cross-linking points.
Magnetic skyrmions are vortex-like topological spin textures often observed to form a triangular-lattice skyrmion crystal in structurally chiral magnets with the Dzyaloshinskii-Moriya interaction. ...Recently, β-Mn structure-type Co-Zn-Mn alloys were identified as a new class of chiral magnet to host such skyrmion crystal phases, while β-Mn itself is known as hosting an elemental geometrically frustrated spin liquid. We report the intermediate composition system Co
Zn
Mn
to be a unique host of two disconnected, thermal-equilibrium topological skyrmion phases; one is a conventional skyrmion crystal phase stabilized by thermal fluctuations and restricted to exist just below the magnetic transition temperature
, and the other is a novel three-dimensionally disordered skyrmion phase that is stable well below
. The stability of this new disordered skyrmion phase is due to a cooperative interplay between the chiral magnetism with the Dzyaloshinskii-Moriya interaction and the frustrated magnetism inherent to β-Mn.
Magnetic skyrmions, topological vortex-like spin textures, garner significant interest due to their unique properties and potential applications in nanotechnology. While they typically form a ...hexagonal crystal with distinct internal magnetisation textures known as Bloch- or Néel-type, recent theories suggest the possibility for direct transitions between skyrmion crystals of different lattice structures and internal textures. To date however, experimental evidence for these potentially useful phenomena have remained scarce. Here, we discover the polar tetragonal magnet EuNiGe
to host two hybrid skyrmion phases, each with distinct internal textures characterised by anisotropic combinations of Bloch- and Néel-type windings. Variation of the magnetic field drives a direct transition between the two phases, with the modification of the hybrid texture concomitant with a hexagonal-to-square skyrmion crystal transformation. We explain these observations with a theory that includes the key ingredients of momentum-resolved Ruderman-Kittel-Kasuya-Yosida and Dzyaloshinskii-Moriya interactions that compete at the observed low symmetry magnetic skyrmion crystal wavevectors. Our findings underscore the potential of polar magnets with rich interaction schemes as promising for discovering new topological magnetic phases.
We report investigations of the magnetic textures in periodic multilayers Pt(1 nm)/(CoFeB(0.8 nm)/Ru(1.4 nm)
using polarised neutron reflectometry (PNR) and small-angle neutron scattering (SANS). The ...multilayers are known to host skyrmions stabilized by Dzyaloshinskii-Moriya interactions induced by broken inversion symmetry and spin-orbit coupling at the asymmetric interfaces. From depth-dependent PNR measurements, we observed well-defined structural features and obtained the layer-resolved magnetization profiles. The in-plane magnetization of the CoFeB layers calculated from fitting of the PNR profiles is found to be in excellent agreement with magnetometry data. Using SANS as a bulk probe of the entire multilayer, we observe long-period magnetic stripe domains and skyrmion ensembles with full orientational disorder at room temperature. No sign of skyrmions is found below 250 K, which we suggest is due to an increase of an effective magnetic anisotropy in the CoFeB layer on cooling that suppresses skyrmion stability. Using polarised SANS at room temperature, we prove the existence of pure Néel-type windings in both stripe domain and skyrmion regimes. No Bloch-type winding admixture, i.e. an indication for hybrid windings, is detected within the measurement sensitivity, in good agreement with expectations according to our micromagnetic modelling of the multilayers. Our findings using neutron techniques provide valuable microscopic insights into the rich magnetic behavior of skyrmion-hosting multilayers, which are essential for the advancement of future skyrmion-based spintronic devices.
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