NUM division - Publication Highlights

Our publication highlights of the current year.

16 November 2018

Multiple-q noncollinear magnetism in an itinerant hexagonal magnet

Multiple-q spin order, i.e., a spin texture characterized by a multiple number of coexisting magnetic modulation vec- tors q, has recently attracted attention as a source of nontrivial magnetic topology and associated emergent phenome- na. One typical example is the triple-q skyrmion lattice state stabilized by Dzyaloshinskii-Moriya interactions in noncentrosymmetric magnets, while the emergence of various multiple-q states of different origins is expected according to the latest theories. Here, we investigated the magnetic structure of the itinerant polar hexagonal magnet Y3Co8Sn4, in which several distinctive mechanisms favoring multiple-q states are allowed to become active. Small- angle neutron-scattering experiments suggest the formation of incommensurate triple-q magnetic order with an in- plane vortex-like spin texture, which can be most consistently explained in terms of the novel four-spin interaction mechanism inherent to itinerant magnets. The present results suggest a new route to realizing exotic multiple-q orders and that itinerant hexagonal magnets, including the R3M8Sn4 family with wide chemical tunability, can be a unique material platform to explore their rich phase diagrams.
Facility: SINQ

Reference: R. Takagi et al, Science Advances 4, eaau3402 (2018)

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15 November 2018

Adsorption and Interfacial Layer Structure of Unmodified Nanocrystalline Cellulose at Air/Water Interfaces

Nanocrystalline cellulose (NCC) is a promising biological nanoparticle for the stabilization of fluid interfaces, which is however poorly understood due to the inability to form controlled NCC interfacial layers. Herein we present parameters that allow the adsorption of unmodified NCC at the air-water (A/W) interface. Initial NCC adsorption is limited by diffusion, followed by monolayer saturation and decrease in surface tension at the time scale of hours. These results confirm current hypotheses of a Pickering stabilization. NCC interfacial performance can be modulated by salt-induced charge screening, enhancing adsorption kinetics, surface load, and interfacial viscoelasticity. Adsorbed NCC layers were visualized by atomic force microscopy at planar Langmuir films and curved air bubbles, whereat NCC coverage was higher at curved interfaces. Structure analysis by neutron reflectometry revealed that NCC forms a discontinuous monolayer with crystallites oriented in the interfacial plane at a contact angle < 90°, favoring NCC desorption upon area compression. This provides the fundamental framework on the formation and structure of NCC layers at the A/W interface, paving the way to exploit NCC interfacial stabilization and optimize cellulose based colloidal materials.
Facility: SINQ

Reference: P. Bertsch et al, Langmuir, advanced online publication, November 2018

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6 November 2018

Linking Structure to Dynamics in Protic Ionic Liquids: A Neutron Scattering Study of Correlated and Single-Particle Motions

Coupling between dynamical heterogeneity of ionic liquids and their structural periodicity on different length-scales can be directly probed by quasielastic neutron scattering with polarization analysis. The technique provides the tools to investigate single-particle and cooperative ion motions separately and, thus, dynamics of ion associations affecting the net charge transport can be experimentally explored. The focus of this study is the structure-dynamic relationship in the protic ionic liquid, triethylammonium triflate, characterized by strong hydrogen bonds between cations and anions. The site-selective deuterium/hydrogen-isotope substitution was applied to modulate the relative contributions of different atom groups to the total coherent and incoherent scattering signal. This approach in combination with molecular dynamics simulations allowed us to obtain a sophisticated description of cation self-diffusion and confined ion pair dynamics from the incoherent spectral component by using the acidic proton as a tagged particle. The coherent contribution of the neutron spectra demonstrated substantial ion association leading to collective ion migration that preserves charge alteration on picosecond time scale, as well as correlation of the localized dynamics occurring between adjacent ions.
Reference: T. Burankova et al, Scientific Reports 8, 16400 (2018)

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26 October 2018

Design of magnetic spirals in layered perovskites: Extending the stability range far beyond room temperature

In insulating materials with ordered magnetic spiral phases, ferroelectricity can emerge owing to the breaking of in- version symmetry. This property is of both fundamental and practical interest, particularly with a view to exploiting it in low-power electronic devices. Advances toward technological applications have been hindered, however, by the rel- atively low ordering temperatures Tspiral of most magnetic spiral phases, which rarely exceed 100 K. We have recently established that the ordering temperature of a magnetic spiral can be increased up to 310 K by the introduction of chemical disorder. Here, we explore the design space opened up by this novel mechanism by combining it with a targeted lattice control of some magnetic interactions. In Cu-Fe layered perovskites, we obtain Tspiral values close to 400 K, comfortably far from room temperature and almost 100 K higher than using chemical disorder alone. Moreover, we reveal a linear relationship between the spiral’s wave vector and the onset temperature of the spiral phase. This linear law ends at a paramagnetic-collinear-spiral triple point, which defines the highest spiral ordering temperature that can be achieved in this class of materials. On the basis of these findings, we propose a general set of rules for designing magnetic spirals in layered perovskites using external pressure, chemical substitutions, and/or epitaxial strain, which should guide future efforts to engineer magnetic spiral phases with ordering temperatures suitable for technological applications.
Facility: SINQ

Reference: T. Shang et al, Science Advances 4, eaau6386 (2018)

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25 October 2018

Rolling dopant and strain in Y-doped BiFeO3 epitaxial thin films for photoelectrochemical water splitting

We report significant photoelectrochemical activity of Y-doped BiFeO3 (Y-BFO) epitaxial thin films deposited on Nb:SrTiO3 substrates. The Y-BFO photoanodes exhibit a strong dependence of the photocurrent values on the thickness of the films, and implicitly on the induced epitaxial strain. The peculiar crystalline structure of the Y-BFO thin films and the structural changes after the PEC experiments have been revealed by high resolution X-ray diffraction and transmission electron microscopy investigations. The crystalline coherence breaking due to the small ionic radius Y-addition was analyzed using Willliamson-Hall approach on the 2Θ-ω scans of the symmetric (00l) reflections and confirmed by high resolution TEM (HR-TEM) analysis. In the thinnest sample the lateral coherence length (L||) is preserved on larger nanoregions/nanodomains. For higher thickness values L|| is decreasing while domains tilt angles (αtilt) is increasing. The photocurrent value obtained for the thinnest sample was as high as Jph = 0.72 mA/cm2, at 1.4 V(vs. RHE). The potentiostatic scans of the Y-BFO photoanodes show the stability of photoresponse, irrespective of the film’s thickness. There is no clear cathodic photocurrent observation for the Y-BFO thin films confirming the n-type semiconductor behavior of the Y-BFO photoelectrodes.
Reference: F. Haydous et al, Scientific Reports 8, 15826 (2018)

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25 October 2018

Dynamic volume magnetic domain wall imaging in grain oriented electrical steel at power frequencies with accumulative high-frame rate neutron dark-field imaging

The mobility of magnetic domains forms the link between the basic physical properties of a magnetic material and its global characteristics such as permeability and saturation field. Most commonly, surface domain structure are studied using magneto-optical Kerr microscopy. The limited information depth of approx. 20 nanometers, however, allows only for an indirect interpretation of the internal volume domain structures. Here we show how accumulative high-frame rate dynamic neutron dark-field imaging is able for the first time to visualize the dynamic of the volume magnetic domain structures in grain oriented electrical steel laminations at power frequencies. In particular we studied the volume domain structures with a spatial resolution of ∼100 μm and successfully quantified domain sizes, wall velocities, domain annihilation and its duration and domain wall multiplication in real time recordings at power frequencies of 10, 25 and 50 Hz with ±262.5 A/m and ±525 A/m (peak to peak) applied field.
Reference: R. Harti et al, Scientific Reports 8, 15754 (2018)

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24 October 2018

Dynamics of the Coordination Complexes in a Solid-State Mg Electrolyte

Coordination complexes of magnesium borohydride show promising properties as solid electrolytes for magnesium ion batteries and warrant a thorough microscopic description of factors governing their mobility properties. Here, the dynamics of Mg(BH4)2-diglyme0.5 on the atomic level are investigated by means of quasielastic neutron scattering supported by density functional theory calculations and IR and NMR spectroscopy. Employing deuterium labeling, we can unambiguously separate all the hydrogen-containing electrolyte components, which facilitate Mg2+ transport, and provide a detailed analytical description of their motions on the picosecond time scale. The planar diglyme chain coordinating the central Mg atom appears to be flexible, while two dynamically different groups of [BH4] anions undergo reorientations. The latter has important implications for the thermal stability and conductivity of Mg(BH4)2-diglyme0.5 and demonstrates that the presence of excess Mg(BH4)2 units in partially chelated Mg complexes may improve the overall performance of related solid-state electrolytes.
Facility: SINQ

Reference: T. Burankova et al, Journal of Physical Chemistry Letters 9, 6450 (2018)

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22 October 2018

Multiple Coulomb phase in the fluoride pyrochlore CsNiCrF6

The Coulomb phase is an idealized state of matter whose properties are determined by factors beyond conventional consid- erations of symmetry, including global topology, conservation laws and emergent order. Theoretically, Coulomb phases occur in ice-type systems such as water ice and spin ice; in dimer models; and in certain spin liquids. However, apart from ice-type systems, more general experimental examples are very scarce. Here we study the partly disordered material CsNiCrF6 and show that this material is a multiple Coulomb phase with signature correlations in three degrees of freedom: charge configurations, atom displacements and spin configurations. We use neutron and X-ray scattering to separate these correlations and to deter- mine the magnetic excitation spectrum. Our results show how the structural and magnetic properties of apparently disordered materials may inherit, and be dictated by, a hidden symmetry—the local gauge symmetry of an underlying Coulomb phase.
Facility: SINQ and SLS

Reference: T. Fennell et al, Nature Physics, adv. online publication (2018)

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25 September 2018

Evidence of a Coulomb-Interaction-Induced Lifshitz Transition and Robust Hybrid Weyl Semimetal in Td-MoTe2

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of Td-MoTe2. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations. Unveiling the importance of Coulomb interaction opens up a new route to comprehend the unique properties of MoTe2, and is significant for understanding the interplay between correlation effects, strong spin-orbit coupling and superconductivity in this van der Waals material.
Facility: SLS

Reference: N. Xu et al, Physical Review Letters 121, 136401 (2018)

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17 September 2018

Thermal Critical Points and Quantum Critical End Point in the Frustrated Bilayer Heisenberg Antiferromagnet

We consider the finite-temperature phase diagram of the S=1/2 frustrated Heisenberg bilayer. Although this two-dimensional system may show magnetic order only at zero temperature, we demonstrate the presence of a line of finite-temperature critical points related to the line of first-order transitions between the dimer-singlet and -triplet regimes. We show by high-precision quantum Monte Carlo simulations, which are sign-free in the fully frustrated limit, that this critical point is in the Ising universality class. At zero temperature, the continuous transition between the ordered bilayer and the dimer-singlet state terminates on the first-order line, giving a quantum critical end point, and we use tensor-network calculations to follow the first-order discontinuities in its vicinity.
Reference: J. Stapmanns et al, Physical Review Letters 121, 127201 (2018)

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14 September 2018

Disordered skyrmion phase stabilized by magnetic frustration in a chiral magnet

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 Co7Zn7Mn6 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 Tc, and the other is a novel three-dimensionally disordered skyrmion phase that is stable well below Tc. 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.
Reference: K. Karube et al, Science Advances 4, eaar7043 (2018)

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12 September 2018

Solid deuterium surface degradation at ultracold neutron sources

Solid deuterium (sD2) is used as an efficient converter to produce ultracold neutrons (UCN). Itis known that the sD2 must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD2 material must be high because crystal inhomogeneities limit the mean freepath for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment (“conditioning”) of the sD2. We show that, in addition to the quality of the bulk sD2, the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D2 frost-layers underpulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD2 frost formation on initially transparent sD2 in offline studies with pulsed heat input at the North Carolina State University UCN source, results in a consistent description of the UCN yield decrease.
Facility: UCN

Reference: A. Anghel et al, European Physical Journal A 54, 148 (2018), featured as EPJ A highlight

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11 September 2018

Giant Pressure Dependence and Dimensionality Switching in a Metal-Organic Quantum Antiferromagnet

We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [CuF2(H2O)2]2 pyrazine. At zero pressure, this material realizes a quasi-two-dimensional spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the quasi-two-dimensional lattice into weakly coupled chains. We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction studies with quantitative modeling using spin-polarized density functional theory.
Facility: SLS

Reference: B. Wehinger et al, Physical Review Letters 121, 117202 (2018)

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28 August 2018

Anomalous Hall effect in Weyl semimetal half-Heusler compounds RPtBi (R = Gd and Nd)

GdPtBi and NdPtBi belong to the Heusler family of compounds and are conventional antiferromagnets below 9 and 2.1 K, respectively. We present evidence for magnetic-field–induced Weyl physics in these compounds, namely, a chiral anomaly (negative magnetoresistance) and an anomalous Hall effect (AHE) with a large anomalous Hall angle over a wide range of temperature. The AHE and chiral anomaly have a similar temperature dependence, indicating their common origin. These studies plus band structure calculations reveal that GdPtBi and NdPtBi develop Weyl points in the presence of an external magnetic field that arises from an exchange field. Our observations open the path to the realization of the quantum AHE in antiferromagnetic Heusler thin films.
Facility: SμS

Reference: C. Shekhar et al, Proceedings of the National Academy of Sciences, 201810842 (2018)

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22 August 2018

Magnetoelectric inversion of domain patterns

The inversion of inhomogeneous physical states has great technological importance; for example, active noise reduction relies on the emission of an inverted sound wave that interferes destructively with the noise of the emitter1, and inverting the evolution of a spin system by using a magnetic-field pulse enables magnetic resonance tomography2. In contrast to these examples, inversion of a distribution of ferromagnetic or ferroelectric domains within a material is surprisingly difficult: field poling creates a single-domain state, and piece-by-piece inversion using a scanning tip is impractical. Here we report inversion of entire ferromagnetic and ferroelectric domain patterns in the magnetoelectric material Co3TeO6 and the multiferroic material Mn2GeO4, respectively. In these materials, an applied magnetic field reverses the magnetization or polarization, respectively, of each domain, but leaves the domain pattern intact. Landau theory indicates that this type of magnetoelectric inversion is universal across materials that exhibit complex ordering, with one order parameter holding the memory of the domain structure and another setting its overall sign. Domain-pattern inversion is only one example of a previously unnoticed effect in systems such as multiferroics, in which several order parameters are available for combination. Exploring these effects could therefore advance multiferroics towards new levels of functionality.
Reference: N. Leo et al, Nature 560, 466 (2018)

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21 August 2018

Modifying the contact angle of anisotropic cellulose nanocrystals: Effect on interfacial rheology and structure

Cellulose nanocrystals (CNC) are an emerging natural material with the ability to stabilize fluid/fluid interfaces. Native CNC is hydrophilic and does not change the inter- facial tension of the stabilized emulsion or foam system. In this study, rod-like cellulose particles were isolated from hemp and chemically modified to alter their hydrophobicity, i.e. their surface-activity, which was demonstrated by surface tension measurements of the particles at the air/water interface. The build-up and mechanical strength of the interfacial structure was investigated using interfacial shear and dilatational rheometry. In contrast to most particle or protein-based interfacial adsorption layers, we observe in shear flow a Maxwellian behavior instead of a glass-like frequency response. The slow and reversible build-up of the layer and its unique frequency dependence indicate a weakly aggregated system, which depends on the hydrophobicity and, thus, on the contact angle of the CNC particles at the air/water interface. Exposed to dilatational flow the weakly aggregated particles cluster and form compact structures. The inter- facial structure generated by the different flow fields is characterized by the contact angle, immersion depth, and layer roughness obtained by neutron reflectometry with contrast variation while the size and local structural arrangement of the CNC particles was investigated by AFM imaging.
Facility: SINQ

Reference: M. van den Berg et al, Langmuir 34, 10932 (2018)

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17 August 2018

Three-Dimensional Fermi Surface of Overdoped La-Based Cuprates

We present a soft x-ray angle-resolved photoemission spectroscopy study of overdoped high- temperature superconductors. In-plane and out-of-plane components of the Fermi surface are mapped by varying the photoemission angle and the incident photon energy. No kz dispersion is observed along the nodal direction, whereas a significant antinodal kz dispersion is identified for La-based cuprates. Based on a tight- binding parametrization, we discuss the implications for the density of states near the van Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La2−xSrxCuO4 cannot be assigned to the van Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.
Facility: SLS

Reference: M. Horio et al, Physical Review Letters 121, 077004 (2018)

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7 August 2018

Pauling Entropy, Metastability, and Equilibrium in Dy2Ti2O7 Spin Ice

Determining the fate of the Pauling entropy in the classical spin ice material Dy2Ti2O7 with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice—the dipolar spin ice model—predicts an ordering transition at T ≈ 0.15K, but recent experiments by Pomaranski et al. suggest an entropy recovery over long timescales at temperatures as high as 0.5K, much too high to be compatible with the theory. Using neutron scattering and specific heat measurements at low temperatures and with long timescales (0.35K / 106s and 0.5K / 105s, respectively) on several isotopically enriched samples, we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Furthermore, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez et al. are, after all, essentially correct: The short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.
Facility: SINQ

Reference: S.R. Giblin et al, Physical Review Letters 121, 067202 (2018)

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20 July 2018

Collective magnetism in an artificial 2D XY spin system

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.
Facility: SμS, SLS

Reference: N. Leo et al, Nature Communications 9, 2850 (2018)

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11 July 2018

Direct electric field control of the skyrmion phase in a magnetoelectric insulator

Magnetic skyrmions are topologically protected spin-whirls currently considered as promising for use in ultra-dense memory devices. Towards achieving this goal, exploration of the skyrmion phase response and under external stimuli is urgently required. Here we show experimentally, and explain theoretically, that in the magnetoelectric insulator Cu2OSeO3 the skyrmion phase can expand and shrink significantly depending on the polarity of a moderate applied electric field (few V/μm). The theory we develop incorporates fluctuations around the mean-field that clarifies precisely how the electric field provides direct control over the free energy difference between the skyrmion and the surrounding conical phase. The quantitative agreement between theory and experiment provides a solid foundation for the development of skyrmionic applications based on magnetoelectric coupling.
Facility: SINQ

Reference: A.J. Kruchkov et al, Scientific Reports 8, 10466 (2018)

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29 June 2018

Odd and Even Modes of Neutron Spin Resonance in the Bilayer Iron-Based Superconductor CaKFe4As4

We report an inelastic neutron scattering study on the spin resonance in the bilayer iron-based superconductor CaKFe4As4. In contrast to its quasi-two-dimensional electron structure, three strongly L-dependent modes of spin resonance are found below Tc = 35 K. The mode energies are below and linearly scale with the total superconducting gaps summed on the nesting hole and electron pockets, essentially in agreement with the results in cuprate and heavy fermion superconductors. This observation supports the sign-reversed Cooper pairing mechanism under multiple pairing channels and resolves the long-standing puzzles concerning the broadening and dispersive spin resonance peak in iron pnictides. More importantly, the triple resonant modes can be classified into odd and even symmetries with respect to the distance of Fe-Fe planes within the Fe-As bilayer unit. Thus, our results closely resemble those in the bilayer cuprates with nondegenerate spin excitations, suggesting that these two high-Tc superconducting families share a common nature.
Reference: T. Xie et al, Physical Review Letters 120, 267003 (2018)

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26 June 2018

Isotope effect on the spin dynamics of single-molecule magnets probed by muon spin spectroscopy

Muon spin relaxation (μSR) experiments on a single molecule magnet enriched in different Dy isotopes detect unambiguously a slowing down of the zero field spin dynamics for the non-magnetic isotope. This occurs in the low temperature regime dominated by quantum tunnelling, in agreement with previous ac susceptibility investigations. In contrast to the latter, however, μSR is sensitive to all fluctuation modes affecting the lifetime of the spin levels.
Facility: SμS

Reference: L. Tesi et al, Chemical Communications 54, 7826 (2018)

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21 June 2018

Magnetic Anisotropy Switch: Easy Axis to Easy Plane Conversion and Vice Versa

The rational design of the magnetic anisotropy of molecular materials constitutes a goal of primary importance in molecular magnetism. Indeed, the applications of molecular nanomagnets, such as single-molecule magnets and molecular magnetic refrigerants, depend on the full control over this property. Axially anisotropic magnetic systems are frequently classified as easy axis or easy plane, depending on whether the lowest energy is obtained by application of a magnetic field parallelly or perpendicularly to the unique axis. Here, the magnetic aniso-tropy of three lanthanide complexes is studied as a function of magnetic field and temperature. It is found that for two of these the type of magnetic aniso-tropy switches as a function of these parameters. Thus, this paper experimen-tally demonstrates that the magnetic anisotropy is not uniquely defined by the intrinsic electronic structure of the systems in question but can also be reversibly switched using external stimuli: temperature and magnetic field.
Facility: SINQ

Reference: M. Perfetti et al, Advanced Functional Materials, 1801846 (2018)

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14 June 2018

Observation of Anomalous Meissner Screening in Cu/Nb and Cu/Nb/Co Thin Films

We have observed the spatial distribution of magnetic flux in Nb, Cu/Nb, and Cu/Nb/Co thin films using muon-spin rotation. In an isolated 50-nm-thick Nb film, we find a weak flux expulsion (Meissner effect) which becomes significantly enhanced when adding an adjacent 40 nm layer of Cu. The added Cu layer exhibits a Meissner effect (due to induced superconducting pairs) and is at least as effective as the Nb to expel flux. These results are confirmed by theoretical calculations using the quasiclassical Green’s function formalism. An unexpected further significant enhancement of the flux expulsion is observed when adding a thin (2.4 nm) ferromagnetic Co layer to the bottom side of the Nb. This observed cooperation between superconductivity and ferromagnetism, by an unknown mechanism, forms a key ingredient for developing superconducting spintronics.
Facility: SμS

Reference: M.G. Flokstra et al, Physical Review Letters 120, 247001 (2018), Editors' suggestion

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8 June 2018

Breakdown of Magnetic Order in the Pressurized Kitaev Iridate β-Li2IrO3

Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate β-Li2IrO3 is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The Néel temperature of β-Li2IrO3 increases with the slope of 0.9 K/GPa upon initial compression, but the reduction in the polarization field Hc reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition, giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate the classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange Γ under pressure.
Facility: SμS

Reference: M. Majumder et al, Physical Review Letters 120, 237202 (2018)

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4 June 2018

Observation of ttH Production

The observation of Higgs boson production in association with a top quark-antiquark pair is reported, based on a combined analysis of proton-proton collision data at center-of-mass energies of √s = 7,8, and 13 TeV, corresponding to integrated luminosities of up to 5.1, 19.7, and 35.9 fb-1, respectively. The data were collected with the CMS detector at the CERN LHC. The results of statistically independent searches for Higgs bosons produced in conjunction with a top quark-antiquark pair and decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets are combined to maximize sensitivity. An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis. The corresponding expected significance from the standard model for a Higgs boson mass of 125.09 GeV is 4.2 standard deviations. The combined best fit signal strength normalized to the standard model prediction is 1.26+0.31-0.26.
Facilities: Particle Physics Facilities

Reference: A.M. Sirunyan et al (CMS collaboration), Physical Review Letters 120, 231801 (2018), Editors' suggestion

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1 June 2018

Clogging in staked-in needle pre-filled syringes (SIN-PFS): Influence of water vapor transmission through the needle shield

Staked-in needle pre-fillable syringes (SIN-PFS) are a convenient delivery system widely established in the growing pharmaceutical market. Under specific storage conditions, the needle of PFS containing high concentration drug product (DP) solution is prone to clogging, which prevents administration of the liquid. The purpose of this study is to clarify the clogging phenomenon of SIN-PFS and to elucidate the role of water vapor transmission via the needle shield.
The presence of liquid within needles is a prerequisite condition for clogging and was investigated non-invasively by neutron imaging (NI) to confirm that liquid can migrate into the needle under certain processing conditions. The water vapor transmission rate (WVTR) of different needle shields was measured and the impact of temperature and relative humidity (rH) on the WVTR was investigated on sheets with the same composition as used in commercial needle shields. Our study clearly showed that the partial vapor pressure difference (ΔPP) across the needle shield is the dominant driving factor for water vapor transmission. A linear correlation between ΔPP and WVTR was found and a model to predict the water vapor transmission for PFS under specific storage conditions was developed. The impact of the WVTR on needle clogging was confirmed by clogging tests performed on SIN-PFS stored under different conditions. Thereby, we clearly show that high water loss induced by higher WVTR can be correlated to an increased occurrence of needle clogging. In conclusion, the WVTR of the needle shield plays a key role in needle clogging and the established WVTR model can be employed to assess the clogging risk for product development.
Facility: SINQ

Reference: M. De Bardi et al, European Journal of Pharmaceutics and Biopharmaceutics 127, 104 (2018)

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7 May 2018

Observation of two types of fractional excitation in the Kitaev honeycomb magnet

Quantum spin liquid is a disordered but highly entangled magnetic state with fractional spin excitations. The ground state of an exactly solved Kitaev honeycomb model is perhaps its clearest example. Under a magnetic field, a spin flip in this model fractionalizes into two types of anyon, a quasiparticle with more complex exchange statistics than standard fermions or bosons: a pair of gauge fluxes and a Majorana fermion. Here, we demonstrate this kind of fractionalization in the Kitaev paramagnetic state of the honeycomb magnet α-RuCl3. The spin excitation gap determined by nuclear magnetic resonance consists of the predicted Majorana fermion contribution following the cube of the applied magnetic field, and a finite zero-field contribution matching the predicted size of the gauge flux gap. The observed fractionalization into gapped anyons survives in a broad range of temperatures and magnetic fields, which establishes α-RuCl3 as a unique platform for future investigations of anyons.
Facility: SINQ

Reference: N. Jansa et al, Nature Physics 14, 786 (2018)

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7 May 2018

Topological quantum phase transition in the Ising-like antiferromagnetic spin chain BaCo2V2O8

Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations have been at the heart of our understanding of a whole novel class of phase transitions. In most cases, those transitions are controlled by a single type of topological objects. There are, however, some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realizations of such cases is thus of considerable interest. We show here that this situation occurs in BaCo2V2O8, a spin-1/2 Ising-like quasi-one-dimensional antiferromagnet, when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological object, which are captured by different components of the dynamical structure factor.
Facility: SINQ

Reference: Q. Faure et al, Nature Physics 14, 716 (2018)

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3 May 2018

Searching for New Physics with b → sτ+τ-

In recent years, intriguing hints for the violation of lepton flavor universality (LFU) have been accumulated in semileptonic B decays, both in the charged-current transitions b → cl-ν-l (i.e., RD, RD∗, and RJ/Ψ and the neutral-current transitions b → sl+l- (i.e., RK and RK∗. Hints for LFU violation in RD(∗) and RJ/Ψ point at large deviations from the standard model (SM) in processes involving tau leptons. Moreover, LHCb has reported deviations from the SM expectations in b → sμ+μ- processes as well as in the ratios RK and RK∗, which together point at new physics (NP) affecting muons with a high significance. These hints for NP suggest the possibility of huge LFU-violating effects in b → sτ+τ- transitions. In this Letter, we predict the branching ratios of B → Kτ+τ-, B → Kτ+τ-, and Bs → Φτ+τ-, taking into account NP effects in the Wilson coefficients Cττ9(') and Cττ10('). Assuming a common NP explanation of RD, RD(∗), and RJ/Ψ, we show that a very large enhancement of b → sτ+τ- processes, of around 3 orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of Bs → τ+τ-, Bs → Φτ+τ-, and B → K(∗)τ+τ- under these assumptions are in the observable range for LHCb and Belle II.
Reference: B. Capdevila et al, Physical Review Letters 120, 181802 (2018)

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1 May 2018

Dirac and Chiral Quantum Spin Liquids on the Honeycomb Lattice in a Magnetic Field

Motivated by recent experimental observations in α-RuCl3, we study the Κ-Γ model on the honeycomb lattice in an external magnetic field. By a slave-particle representation and variational Monte Carlo calculations, we reproduce the phase transition from zigzag magnetic order to a field-induced disordered phase. The nature of this state depends crucially on the field orientation. For particular field directions in the honeycomb plane, we find a gapless Dirac spin liquid, in agreement with recent experiments on α-RuCl3. For a range of out-of-plane fields, we predict the existence of a Kalmeyer- Laughlin-type chiral spin liquid, which would show an integer-quantized thermal Hall effect.
Reference: Z-X Liu et al, Physical Review Letters 120, 187201 (2018)

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30 April 2018

Experimental signatures of emergent quantum electrodynamics in Pr2Hf2O7

In a quantum spin liquid, the magnetic moments of the constituent electron spins evade classical long-range order to form an exotic state that is quantum entangled and coherent over macroscopic length scales. Such phases offer promising perspectives for device applications in quantum information technologies, and their study can reveal new physics in quantum matter. Quantum spin ice is an appealing proposal of one such state, in which the fundamental ground state properties and excitations are described by an emergent U(1) lattice gauge theory. This quantum-coherent regime has quasiparticles that are predicted to behave like magnetic and electric monopoles, along with a gauge boson playing the role of an artificial photon. However, this emergent lattice quantum electrodynamics has proved elusive in experiments. Here we report neutron scattering measurements of the rare-earth pyrochlore magnet Pr2Hf2O7 that provide evidence for a quantum spin ice ground state. We find a quasi-elastic structure factor with pinch points - a signature of a classical spin ice - that are partially suppressed, as expected in the quantum-coherent regime of the lattice field theory at finite tem- perature. Our result allows an estimate for the speed of light associated with magnetic photon excitations. We also reveal a continuum of inelastic spin excitations, which resemble predictions for the fractionalized, topological excitations of a quantum spin ice. Taken together, these two signatures suggest that the low-energy physics of Pr2Hf2O7 can be described by emergent quantum electrodynamics. If confirmed, the observation of a quantum spin ice ground state would constitute a concrete example of a three-dimensional quantum spin liquid—a topical state of matter that has so far mostly been explored in lower dimensionalities.
Reference: R. Sibille et al, Nature Physics 14, 711 (2018)

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19 April 2018

Structure and Interaction of Nanoparticle–Protein Complexes

The integration of nanoparticles with proteins is of high scientific interest due to the amazing potential displayed by their complexes, combining the nanoscale properties of nanoparticles with the specific architectures and functions of the protein molecules. The nanoparticle–protein complexes, in particular, are useful in the emerging field of nanobiotechnology (nanomedicine, drug delivery, and biosensors) as the nanoparticles having sizes comparable to that of living cells can access and operate within the cell. The understanding of nanoparticle interaction with different protein molecules is a prerequisite for such applications. The interaction of the two components has been shown to result in conformational changes in proteins and to affect the surface properties and colloidal stability of the nanoparticles. In this feature article, our recent studies exploring the driving interactions in nanoparticle–protein systems and resultant structures are presented. The anionic colloidal silica nanoparticles and two globular charged proteins [lysozyme and bovine serum albumin (BSA)] have been investigated as model systems. The adsorption behavior of the two proteins on nanoparticles is found to be completely different, but they both give rise to similar phase transformation from one phase to two phase in respective nanoparticle–protein systems. The presence of protein induces the short-range and long-range attraction between the nanoparticles with lysozyme and BSA, respectively. The observed phase behavior and its dependence on various physiochemical parameters (e.g., nanoparticle size, ionic strength, and solution pH) have been explained in terms of underlying interactions.
Facility: SINQ

Reference: S. Kumar et al, Langmuir 34, 5679 (2018)

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9 April 2018

Quantitative 3D determination of self-assembled structures on nanoparticles using small angle neutron scattering

The ligand shell (LS) determines a number of nanoparticles’ properties. Nanoparticles’ cores can be accurately characterized; yet the structure of the LS, when composed of mixture of molecules, can be described only qualitatively (e.g., patchy, Janus, and random). Here we show that quantitative description of the LS' morphology of monodisperse nanoparticles can be obtained using small-angle neutron scattering (SANS), measured at multiple contrasts, achieved by either ligand or solvent deuteration. Three-dimensional models of the nanoparticles' core and LS are generated using an ab initio reconstruction method. Characteristic length scales extracted from the models are compared with simulations. We also characterize the evolution of the LS upon thermal annealing, and investigate the LS morphology of mixed-ligand copper and silver nanoparticles as well as gold nanoparticles coated with ternary mixtures. Our results suggest that SANS combined with multiphase modeling is a versatile approach for the characterization of nanoparticles' LS.
Facility: SINQ

Reference: Z. Luo et al, Nature Communications 9, 1343 (2018)

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6 April 2018

No-Go Theorem for Nonstandard Explanations of the τ → KSπντ CP Asymmetry

The CP asymmetry in τ → KSπντ, as measured by the BABAR collaboration, differs from the standard model prediction by 2.8 σ. Most nonstandard interactions do not allow for the required strong phase needed to produce a nonvanishing CP asymmetry, leaving only new tensor interactions as a possible mechanism. We demonstrate that, contrary to previous assumptions in the literature, the crucial interference between vector and tensor phases is suppressed by at least 2 orders of magnitude due to Watson’s final-state- interaction theorem. Furthermore, we find that the strength of the relevant CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and D—D¯. These observations together imply that it is extremely difficult to explain the current τ → KSπντ measurement in terms of physics beyond the standard model originating in the ultraviolet.
Reference: V. Cirigliano et al, Physical Review Letters 120, 141803 (2018)

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5 April 2018

Magnetic Field Control of Cycloidal Domains and Electric Polarization in Multiferroic BiFeO3

The magnetic field induced rearrangement of the cycloidal spin structure in ferroelectric monodomain single crystals of the room-temperature multiferroic BiFeO3 is studied using small-angle neutron scattering. The cycloid propagation vectors are observed to rotate when magnetic fields applied perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value of ∼5T. In light of these experimental results, a phenomenological model is proposed that captures the rearrangement of the cycloidal domains, and we revisit the microscopic origin of the magnetoelectric effect. A new coupling between the magnetic anisotropy and the polarization is proposed that explains the recently discovered magnetoelectric polarization perpendicular to the rhombohedral axis.
Facility: SINQ

Reference: S. Bordacs et al, Physical Review Letters 120, 147203 (2018)

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30 March 2018

Dipolar Spin Ice States with a Fast Monopole Hopping Rate in CdEr2X4 (X = Se, S)

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr2Se4 is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three orders of magnitude faster than the canonical spin ice Dy2Ti2O7. In this Letter we use diffuse neutron scattering to show that both CdEr2Se4 and CdEr2S4 support a dipolar spin ice state—the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to those in Dy2Ti2O7, i.e., dilute and uncorrelated, and so cannot provide three orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er3+ ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr2X4 (X = Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr2X4 offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.
Facility: SINQ

Reference: S. Gao et al, Physical Review Letters 120, 137201 (2018)

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14 March 2018

Spin-liquid-like state in a spin-1/2 square-lattice antiferromagnet perovskite induced by d10 – d0 cation mixing

A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Néel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor interaction dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use d10 – d0 cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr2Cu(Te0.5W0.5)O6, where the isostructural end phases Sr2CuTeO6 and Sr2CuWO6 are Néel and columnar type antiferromagnets, respectively. We show that magnetism in Sr2Cu(Te0.5W0.5)O6 is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr2Cu(Te0.5W0.5)O6 - similar to several spin liquid candidates—a plateau in muon spin relaxation rate and a strong T-linear dependence in specific heat. Our observations for Sr2Cu(Te0.5W0.5)O6 highlight the role of disorder in addition to magnetic frustration in spin liquid physics.
Facility: SμS

Reference: O. Mustonen et al, Nature Communications 9, 1085 (2018)

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28 February 2018

Multi-q Mesoscale Magnetism in CeAuSb2

We report the discovery of a field driven transition from a single-q to multi-q spin density wave (SDW) in the tetragonal heavy fermion compound CeAuSb2. Polarized along c, the sinusoidal SDW amplitude is 1.8(2)μB/Ce for T<<TN=6.25(10)K with a wave vector q1=(η,η,1/2) [η=0.136(2)]. For H || c, harmonics appearing at 2q1 evidence a striped magnetic texture below μ0H1=2.78(1) T. Above H1, these are replaced by coupled harmonics at q1+q2=(2η,0,0)+c* until μ0H2=5.42(5)T, where satellites vanish and magnetization nonlinearly approaches saturation at 1.64(2)μB/Ce for μ0H≈7T.
Facility: SINQ

Reference: G.G. Marcus et al, Physical Review Letters 120, 097201 (2018)

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20 February 2018

Quasistatic antiferromagnetism in the quantum wells of SmTiO3/SrTiO3 heterostructures

High carrier density quantum wells embedded within a Mott insulating matrix present a rich arena for exploring unconventional electronic phase behavior ranging from non-Fermi-liquid transport and signatures of quantum criticality to pseudogap formation. Probing the proposed connection between unconventional magnetotransport and incipient electronic order within these quantum wells has however remained an enduring challenge due to the ultra-thin layer thicknesses required. Here we address this challenge by exploring the magnetic properties of high-density SrTiO3 quantum wells embedded within the antiferromagnetic Mott insulator SmTiO3 via muon spin relaxation and polarized neutron reflectometry measurements. The one electron per planar unit cell acquired by the nominal d0 band insulator SrTiO3 when embedded within a d1 Mott SmTiO3 matrix exhibits slow magnetic fluctuations that begin to freeze into a quasistatic spin state below a critical temperature T*. The appearance of this quasistatic well magnetism coincides with the previously reported opening of a pseudogap in the tunneling spectra of high carrier density wells inside this film architecture. Our data suggest a common origin of the pseudogap phase behavior in this quantum critical oxide heterostructure with those observed in bulk Mott materials close to an antiferromagnetic instability.
Facility: SμS

Reference: R.F. Need et al, npj Quantum Materials 3, 7 (2018)

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16 February 2018

Crystal-to-Crystal Transition of Ultrasoft Colloids under Shear

Ultrasoft colloids typically do not spontaneously crystallize, but rather vitrify, at high concentrations. Combining in situ rheo–small-angle-neutron-scattering experiments and numerical simulations we show that shear facilitates crystallization of colloidal star polymers in the vicinity of their glass transition. With increasing shear rate well beyond rheological yielding, a transition is found from an initial bcc-dominated structure to an fcc-dominated one. This crystal-to-crystal transition is not accompanied by intermediate melting but occurs via a sudden reorganization of the crystal structure. Our results provide a new avenue to tailor colloidal crystallization and the crystal-to-crystal transition at the molecular level by coupling softness and shear.
Facility: SINQ

Reference: J. Ruiz-Franco et al, Physical Review Letters 120, 078003 (2018)

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2 February 2018

Three Dimensional Polarimetric Neutron Tomography of Magnetic Fields

Through the use of Time-of-Flight Three Dimensional Polarimetric Neutron Tomography (ToF 3DPNT) we have for the first time successfully demonstrated a technique capable of measuring and reconstructing three dimensional magnetic field strengths and directions unobtrusively and non-destructively with the potential to probe the interior of bulk samples which is not amenable otherwise. Using a pioneering polarimetric set-up for ToF neutron instrumentation in combination with a newly developed tailored reconstruction algorithm, the magnetic field generated by a current carrying solenoid has been measured and reconstructed, thereby providing the proof-of-principle of a technique able to reveal hitherto unobtainable information on the magnetic fields in the bulk of materials and devices, due to a high degree of penetration into many materials, including metals, and the sensitivity of neutron polarisation to magnetic fields. The technique puts the potential of the ToF time structure of pulsed neutron sources to full use in order to optimise the recorded information quality and reduce measurement time.
Reference: M. Sales et al, Scientific Reports 8, 2214 (2018)

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19 January 2018

Low-Field Bi-Skyrmion Formation in a Noncentrosymmetric Chimney Ladder Ferromagnet

The real-space spin texture and the relevant magnetic parameters were investigated for an easy-axis non-centrosymmetric ferromagnet Cr11Ge19 with Nowotny chimney ladder structure. Using Lorentz transmission electron microscopy,we report the formation of bi-Skyrmions,i.e., pairs of spin vortices with opposite magnetic helicities. The quantitative evaluation of the magnetocrystalline anisotropy and Dzyaloshinskii-Moriya interaction (DMI) proves that the magnetic dipolar interaction plays a more important role than the DMI on the observed bi-Skyrmion formation. Notably, the critical magnetic field value required for the formation of bi-Skyrmions turned out to be extremely small in this system, which is ascribed to strong easy-axis anisotropy associated with the characteristic helix crystal structure. The family of Nowotny chimney ladder compounds may offer a unique material platform where two distinctive Skyrmion formation mechanisms favoring different topological spin textures can become simultaneously active.
Facility: SINQ

Reference: R. Takagi et al, Physical Review Letters 120, 037203 (2018)

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