Helical Stripes Research Articles

Emergent chiral spin textures in centrosymmetric iron garnet with spin alignment constraints

Chiral spin textures have emerged as a captivating class of topological matter. These intriguing structures harbor localized and topologically protected magnetic textures, rendering them promising candidates for novel spintronic applications. Here, the emergent chiral spin textures in an iron garnet with nanodisk configuration have been micromagnatically simulated based on the Landau-Lifshitz-Gilbert equation (LLG) of COMSOL Multiphysics. The modification of magnetic spin texture in the considered iron garnet has been controlled by introducing the interfacial Dzyaloshinskii-Moriya interaction (I-DMI) and spin alignment constraints as locally pinned spins and curvilinear defects at edges. These boundary conditions induced interesting chiral textures not commonly observed in such iron garnet due to its centrosymmetric crystal structure. The simulation results showed the presence of two distinct regions of spin texture: inside and outside the pinning boundary. By systematically varying the DMI strength (D) with the size of the pinning boundary (Rpin) and the number of defects (Ndef), the emerging chiral spin textures have been explored and reported, including the existence of ramified helical stripes, skyrmions, and skyrmions-like textures such as elongated skyrmions, chiral horseshoe domains, biskyrmions and target skyrmions (TSk) (2π-TSk and 3π-TSk). Also, it has been reported that various collapses occur in the final spin texture states, leading to the transition from one state to another when varying the values of Rpin and Ndef with D.

Experimental observation of current-driven antiskyrmion sliding in stripe domains.

Magnetic skyrmions are promising as next-generation information units. Their antiparticle-the antiskyrmion-has also been discovered in chiral magnets. Here we experimentally demonstrate antiskyrmion sliding in response to a pulsed electric current at room temperature without the requirement of an external magnetic field. This is realized by embedding antiskyrmions in helical stripe domains, which naturally provide one-dimensional straight tracks along which antiskyrmion sliding can be easily launched with low current density and without transverse deflection from the antiskyrmion Hall effect. The higher mobility of the antiskyrmions in the background of helical stripes in contrast to the typical ferromagnetic state is a result of intrinsic material parameters and elastic energy of the stripe domain, thereby smearing out the random pinning potential, as supported by micromagnetic simulations. The demonstration and comprehensive understanding of antiskyrmion movement along naturally straight tracks offers a new perspective for (anti)skyrmion application in spintronics.

Pipe to annular flow conversion: Numerical study

Large Eddy Simulation (LES) of turbulent flow at the entrance to an annular pipe section has been performed. The flow over a single circular rod, coaxially immersed in a circular pipe is considered. The Reynolds number, based on mean velocity and hydraulic diameter of the annular section, is ReDh=40530. The study aims to gain insights into the turbulence characteristics of the primary and secondary separation bubbles, developing on a side surface of the rod. Key findings include tiny toroidal vortices in the primary bubble's front, forming a vortex train with intermittently shed vortex structures. Large-scale helical stripes have been reproduced at the entrance to the annular section. Tilting of the axially-oriented streaks was observed close to the rod surface. It resulted in a slight anisotropy of the normal Reynolds stresses. The LES data could refine classic Reynolds-averaged Navier-Stokes-based turbulence models.

Effects of lattice structure on the electronic properties of helically symmetric nanotubes

In the last decades, nanotubes with varied compositions, underlying lattice structures, and symmetries have been proposed. In this work, we theoretically investigate nanotubes with similar compositions and symmetries but with different underlying lattice structures. Our aim is to understand how the underlying lattice structure of the nanotubes may affect their electronic properties. In order to do so, helically symmetric BCN nanotubes with graphene and graphenylene lattice structures were studied. In the considered structures, helical stripes of C atoms were incorporated into BN nanotubes with different helical angles. The geometric optimizations of the helical structures were carried out by using the Parametric Method 3 (PM3), whereas the electronic properties were calculated within the Green function formalism by using the Rubio–Sancho technique. Our results indicate that the stability of the two types of nanotube follows a similar pattern, with the energy per atom increasing as the fraction of C–N and C–B bonds increases. On the other hand, the type of lattice plays a critical role in the electronic properties. Although all considered helically symmetric nanotubes are semiconductors, different underlying lattice structures lead to distinct features. Graphene lattice nanotubes present a tunable band gap, whereas graphenylene lattice nanotubes present a nearly constant band gap for all helical angles. By focusing on the role of the underlying lattice structure, our work highlights how strain or stress, associated with each underlying lattice structure, may be tools to customize the properties of the nanotubes.

Helitronics as a potential building block for classical and unconventional computing

Magnetic textures are promising candidates for unconventional computing due to their non-linear dynamics. We propose to investigate the rich variety of seemingly trivial lamellar magnetic phases, e.g. helical, spiral, stripy phase, or other one-dimensional soliton lattices. These are the natural stray field-free ground states of almost every magnet. The order parameters of these phases may be of potential interest for both classical and unconventional computing, which we refer to as helitronics. For the particular case of a chiral magnet and its helical phase, we use micromagnetic simulations to demonstrate the working principles of all-electrical (i) classical binary memory cells and (ii) memristors and artificial synapses, based on the orientation of the helical stripes.

A Double Open-Shelled Au43 Nanocluster with Increased Catalytic Activity and Stability

Atomically precise metal nanoclusters (NCs) are an intriguing class of crystalline solids with unique physicochemical properties derived from tunable structures and compositions. Most atomically precise NCs require closed-shells and coordinatively saturated surface metals in order to be stable. Herein, we report Au43(C≡CtBu)20 and Au42Ag1(C≡CtBu)20, which feature open electronic and geometric shells, leading to both paramagnetism (23 valence e-) and enhanced catalytic activity from a single coordinatively unsaturated surface metal. The Au-alkynyl surface motifs of these NCs form five helical stripes around the inner Au12 kernel, imparting chirality and high thermal stability. Density functional theory (DFT) calculations suggest that there are minimal energy differences between the open-shelled NCs and hypothetical closed-shell systems and that the open-shelled electronic configuration gives rise to the largest band gap, which is known to promote cluster stability. Furthermore, we highlight how coordinatively unsaturated surface metals create active sites for the catalytic oxidation of benzyl alcohol to benzaldehyde, leading to high selectivity and increased conversion. This work represents the first example of an atomically precise Au NC with a double open-shelled structure and provides a promising platform for investigating the magnetic and catalytic properties of noble metal nanoparticles.

Collapse of externally pressurized elliptical steel cylinders stiffened with helical composite stripes

This study investigates the collapse of externally pressurized elliptical steel cylinders stiffened with four helically wound composite stripes. The cylinders have a length of 320 mm, a major axis of 80.5 mm, and a major-to-minor axis ratio of 1.002. The width and warping angle of the composite stripes are 58 mm and 55°, respectively. Elliptical steel cylinders without stiffening stripes are also considered. Three nominally identical elliptical steel cylinders and three nominally identical elliptical steel–composite cylinders are designed, fabricated, geometrically measured, and hydrostatically experimented. Linear eigenvalue and nonlinear RIKS analyses are performed to evaluate the collapse properties of the fabricated samples. The experimental and numerical data agree favorably. The results indicate that the reinforcement provided by the composite stripes approximately doubles the loading capacity of the steel cylinders. Furthermore, elliptical steel–composite cylinders are found to be less sensitive to imperfections than are steel-only and circular steel–composite cylinders.

Formation and annihilation of skyrmions in a bucket-shaped nanotube

In this paper, formation and annihilation of skyrmions in bucket-shaped nanotubes are studied by micromagnetic simulation. Nucleation of skyrmions in the bucket-shaped nanotube is realized by breaking the helical stripes on the left and right sides of the nanotube. With increasing of the external magnetic field, the skyrmions annihilate in pairs step by step. Skyrmions in the thinner side of the bucket first annihilate and the skyrmions in the thicker side annihilate finally. The number of skyrmion can be controlled by the external magnetic field.

Critical sample aspect ratio and magnetic field dependence for antiskyrmion formation in Mn1.4PtSn single crystals

Mn1.4PtSn is the first material in which antiskyrmions have been observed in ultra-thin single crystalline specimens. While bulk crystals exhibit fractal patterns of purely ferromagnetic domain ordering at room temperature, ultra-thin Mn1.4PtSn lamellae clearly show antiskyrmion lattices with lattice spacings up to several $\mu$m. In the work presented here, we systematically investigate the thickness region from 400 nm to 10 $\mu$m using 100 $\times$ 100 $\mu$m$^2$ -wide Mn1.4PtSn plates, and identify the critical thickness-to-width aspect ratio $\alpha_0 = 0.044$ for the ferromagnetic fractal domain to the non-collinear texture phase transition. Additionally, we also explore these non-collinear magnetic textures below the critical aspect ratio $\alpha_0$ above and below the spin-reorientation transition temperature $T_{SR}$ while applying variable external magnetic fields. What we find is a strong hysteresis for the occurrence of an antiskyrmion lattice, since the antiskyrmions preferentially nucleate by pinching them off from helical stripes in the transition to the field polarized state.

Skyrmion driven by rotary magnetic field on the surface of magnetic nanotube: a Monte Carlo study

We report a Monte-Carlo simulation of the formation of skyrmions under a rotary magnetic field on a nanotube. The zero-field magnetic state is characterized as helical stripe domains swirling on the nanotube, with one to three periods depending on the ratio of Dzyaloshinskii–Moriya to ferromagnetic interaction and tubular size. Under a rotary magnetic field, the formation of skyrmions is in pair and the skyrmion number can be tuned. The movement of skyrmions is neither synchronous along with the rotary field, nor along a helical trajectory perpendicular to the rotary field. It is ascribed to that within a skyrmion pair, on one hand, the coupling between skyrmions is nonnegligible; on the other hand, different skyrmion pairs side by side are decoupled. This work predicts a way of nanotube-based skyrmion manipulation, and might develop the rotary information storage on energy- and space-saving modes or an edgeless racetrack memory.

Direct visualization of nucleation intermediate state of magnetic skyrmion from helical stripes assisted by artificial surface pits

Magnetic skyrmion is a particle-like spin texture promising for future novel spintronic devices. For such applications, microscopic mechanism of nucleation of skyrmion is essential to control the unique spin texture. Although real-space visualization techniques with high spatial resolution could be powerful to investigate the mechanism at the nanometer length scale, a direct visualization of skyrmion nucleation process from the helical stripe state is still technically challenging because not only a high spatial resolution but also a high temporal resolution is required to resolve the transient state. Furthermore, the location of the intermediate state is likely to move along the stripe, which hampers a successful visualization. Here, taking advantage of the high spatial resolution of aberration-corrected differential phase contrast scanning transmission electron microscopy, we observe the nucleation of skyrmion from helical stripes in a thin plate of FeGe1-xSix (x ~ 0.05). By slowing down the kinetics with a careful control of both magnetic field and temperature, while pinning the helical stripe by artificial surface pits fabricated on the surface of the thin plate specimen, we directly visualize the nucleation intermediate state of skyrmion. We find the theoretically predicted antiskyrmion-like spin texture is actually created as the intermediate state during nucleation.

Stability phase diagrams and tuning of magnetic skyrmionium and other states

Topological magnetic states such as skyrmions have widely been investigated as a suitable candidate for memory bits in next generation data storage device applications including random access memory (RAM) or race-track memory as well as spin-torque nano-oscillators etc. Energy efficient multistate switching of these states based on electric control of magnetization involves the electric field tuning of perpendicular magnetic anisotropy (PMA). By using micromagnetic simulation study, we demonstrate the strain-mediated electric manipulation of various topological magnetic states in nanodisc with [Pt/Co/Ta]n trilayer stacks residing on a piezoelectric substrate. A stability phase diagram is derived for the representation of regions of the lowest energy states against the applied strain as a function of disc dimensions. Utilizing this derived phase diagram, various electric field-induced strain-mediated switching scenarios including non-volatile reversible switching of skyrmion into skyrmionium, 3π state and helical stripe domain state, the creation of skyrmionium from uniform single-domain magnetic state and its annihilation and volatile reversible switching between skyrmionium and vortex are achieved. Moreover, an extended phase diagram based on the estimated critical values of the applied strain for different domain structures within the nanodisc is also built and further implemented in designing a multistate sequential switching between various magnetic states triggered by a continuously varying strain pulse. These findings hold a potential for contributing in the development of low-power high-density multistate magneto-electric or magneto-elastic memory and logic device applications involving simple architectural concept applicable to future microelectronic integrated device strategies.

Nucleation and stability of skyrmions in three-dimensional chiral nanostructures

We studied the magnetization evolution in three-dimensional chiral nanostructures, including nanotubes and circularly curved thin films, by micromagnetic simulations. We found that in a nanotube skyrmions can be formed by broken of the helical stripes on the left and right sides of the nanotube, and the formation of skyrmions doesn’t correspond to any abrupt change of topological number. Skyrmions can exist in a large range of magnetic field, and the thinner nanotube has a larger field range for skyrmion existence. The configuration of a skyrmion in nanotubes is different from the one in thin film. From the outer to the inner circular layer, the size of the skyrmion becomes larger, and the deformation becomes more obvious. In circularly curved magnetic films with fixed arc length, there are three kinds of hysteresis processes are found. For the curved films with a large radius, the magnetization evolution behavior is similar to the case in two-dimensional thin films. For the curved films with a small radius, the skyrmions are created by broken of the helical stripes on the left and right sides of the curved film. For the curved film with a medium radius, no skyrmion is formed in the hysteresis process.

Chirality and Surface Bonding Correlation in Atomically Precise Metal Nanoclusters.

Chirality is ubiquitous in nature and occurs at all length scales. The development of applications for chiral nanostructures is rising rapidly. With the recent achievements of atomically precise nanochemistry, total structures of ligand-protected Au and other metal nanoclusters (NCs) are successfully obtained, and the origins of chirality are discovered to be associated with different parts of the cluster, including the surface ligands (e.g., swirl patterns), the organic-inorganic interface (e.g., helical stripes), and the kernel. Herein, a unified picture of metal-ligand surface bonding-induced chirality for the nanoclusters is proposed. The different bonding modes of M-X (where M = metal and X = the binding atom of ligand) lead to different surface structures on nanoclusters, which in turn give rise to various characteristic features of chirality. A comparison of Au-thiolate NCs with Au-phosphine ones further reveals the important roles of surface bonding. Compared to the Au-thiolate NCs, the Ag/Cu/Cd-thiolate systems exhibit different coordination modes between the metal and the thiolate. Other than thiolate and phosphine ligands, alkynyls are also briefly discussed. Several methods of obtaining chiroptically active nanoclusters are introduced, such as enantioseparation by high-performance liquid chromatography and enantioselective synthesis. Future perspectives on chiral NCs are also proposed.

Spontaneous nanometric magnetic bubbles with various topologies in spin-reoriented La1−xSrxMnO3

Topological zero-field nanometric domains and their capability to be manipulated by external fields show potential applications in spintronics. Here, the spontaneous magnetic bubbles (≈100 nm in diameter) are observed at zero field in a ferromagnetic manganite La1−xSrxMnO3 (0.15 < x < 0.2) by using Lorentz transmission electron microscopy. The spin reorientation as a function of temperature drives the magnetic domain transition from traditional 180° in-plane domains to helical stripes and bubbles, resulting in rich magnetic configurations with various topologies. It directly demonstrates that the dynamic motion of Bloch lines in bubbles introduces the topologic transition under the application of magnetic fields.

Turbulent bifurcations in intermittent shear flows: From puffs to oblique stripes

Localised turbulent structures such as puffs or oblique stripes are building blocks of the intermittency regimes in subcritical wall-bounded shear flows. These turbulent structures are investigated in incompressible pressure-driven annular pipe flow using direct numerical simulations in long domains. For low enough radius ratio $\eta$, these coherent structures have a dynamics comparable to that of puffs in cylindrical pipe flow. For $\eta$ larger than 0.5, they take the shape of helical stripes inclined with respect to the axial direction. The transition from puffs to stripes is analysed statistically by focusing on the axisymmetry properties of the associated large-scale flows. It is shown that the transition is gradual : as the azimuthal confinement relaxes, allowing for an azimuthal large-scale component, oblique stripes emerge as predicted in the planar limit. The generality of this transition mechanism is discussed in the context of subcritical shear flows.

Transitional structures in annular Poiseuille flow depending on radius ratio

The transitional regime of incompressible pressure-driven flows inside an annular pipe is investigated using accurate direct numerical simulation in long computational domains. At marginally low friction Reynolds number $Re_{{\it\tau}}$, turbulence occurs in the form of intermittent localised structures. Different types of localisation are identified as the aspect ratio is varied from ${\it\eta}=0.8$ to $0.1$. These coherent structures vary from helical turbulence at ${\it\eta}=0.8$ to streamwise-localised puffs at ${\it\eta}=0.1$. They are respectively analogous to the stripe patterns and puffs formerly identified in plane channel flow and cylindrical pipe flow. Helical turbulence has been tracked down to ${\it\eta}=0.3$, accompanied by a monotonic reduction of the pitch angle. For ${\it\eta}=0.3$ and marginally low $Re_{{\it\tau}}$, these turbulence structures localise in the streamwise direction, giving rise to a new regime of helical puffs with chirality. The present results suggest that helical puffs mediate the transition from globally axisymmetric puffs to helical stripe patterns.

Solitons and nonlinear waves in the spiral magnetic structures

This paper presents a procedure to integrate the sine-Gordon model against the background of the stripe domain structure. The nonlinear dynamics of solitons and dispersive waves in the helical (stripe domain) structure of a ferromagnet with the easy plane anisotropy in the magnetic field, which is perpendicular to the spiral axis, has been investigated in detail. It has been shown that the formation and motion of solitons are accompanied by the local translations of the stripe structure and by the oscillations of its domain walls, which manifest themselves as “precursors” and “tails” of the solitons. The large time behavior of the weak-nonlinear dispersive wave field generated by an initial localized perturbation of the structure has been investigated. The ways of observing and exciting the solitons in the spiral structure of magnets and multiferroics are discussed.

Chiral magnetization textures stabilized by the Dzyaloshinskii-Moriya interaction during spin-orbit torque switching

We study the effect of the Dzyaloshinskii-Moriya interaction (DMI) on current-induced magnetic switching of a perpendicularly magnetized heavy-metal/ferromagnet/oxide trilayer both experimentally and through micromagnetic simulations. We report the generation of stable helical magnetization stripes for a sufficiently large DMI strength in the switching region, giving rise to intermediate states in the magnetization confirming the essential role of the DMI on switching processes. We compare the simulation and experimental results to a macrospin model, showing the need for a micromagnetic approach. The influence of the temperature on the switching is also discussed.

Structural Basis for the Interaction between Yeast Spt-Ada-Gcn5 Acetyltransferase (SAGA) Complex Components Sgf11 and Sus1

Sus1 is a central component of the yeast gene gating machinery, the process by which actively transcribing genes such as GAL1 become associated with nuclear pore complexes. Sus1 is a component of both the SAGA transcriptional co-activator complex and the TREX-2 complex that binds to nuclear pore complexes. TREX-2 contains two Sus1 chains that have an articulated helical hairpin fold, enabling them to wrap around an extended α-helix in Sac3, following a helical hydrophobic stripe. In SAGA, Sus1 binds to Sgf11 and has been proposed to provide a link between SAGA and TREX-2. We present here the crystal structure of the complex between Sus1 and the N-terminal region of Sgf11 that forms an extended α-helix around which Sus1 wraps in a manner that shares some similarities with the Sus1-Sac3 interface in TREX-2. However, the Sus1-binding site on Sgf11 is somewhat shorter than on Sac3 and is based on a narrower hydrophobic stripe. Engineered mutants that disrupt the Sgf11-Sus1 interaction in vitro confirm the importance of the hydrophobic helical stripe in molecular recognition. Helix α1 of the Sus1-articulated hairpin does not bind directly to Sgf11 and adopts a wide range of conformations within and between crystal forms, consistent with the presence of a flexible hinge and also with results from previous extensive mutagenesis studies (Klöckner, C., Schneider, M., Lutz, S., Jani, D., Kressler, D., Stewart, M., Hurt, E., and Köhler, A. (2009) J. Biol. Chem. 284, 12049–12056). A single Sus1 molecule cannot bind Sgf11 and Sac3 simultaneously and this, combined with the structure of the Sus1-Sgf11 complex, indicates that Sus1 forms separate subcomplexes within SAGA and TREX-2.