Edge-Mediated Skyrmion Chain and Its Collective Dynamics in a Confined Geometry

The emergence of a topologically nontrivial vortex-like magnetic structure, the magnetic skyrmion, has launched new concepts for memory devices. There, extensive studies have theoretically demonstrated the ability to encode information bits by using a chain of skyrmions in one-dimensional nanostripes. Here, we report the first experimental observation of the skyrmion chain in FeGe nanostripes by using high resolution Lorentz transmission electron microscopy. Under an applied field normal to the nanostripes plane, we observe that the helical ground states with distorted edge spins would evolves into individual skyrmions, which assemble in the form of chain at low field and move collectively into the center of nanostripes at elevated field. Such skyrmion chain survives even as the width of nanostripe is much larger than the single skyrmion size. These discovery demonstrates new way of skyrmion formation through the edge effect, and might, in the long term, shed light on the applications.Comment: 7 pages, 3 figure

Electrical Probing of Field-Driven Cascading Quantized Transitions of Skyrmion Cluster States in MnSi Nanowires

Magnetic skyrmions are topologically stable whirlpool-like spin textures that offer great promise as information carriers for future ultra-dense memory and logic devices1-4. To enable such applications, particular attention has been focused on the skyrmions properties in highly confined geometry such as one dimensional nanowires5-8. Hitherto it is still experimentally unclear what happens when the width of the nanowire is comparable to that of a single skyrmion. Here we report the experimental demonstration of such scheme, where magnetic field-driven skyrmion cluster (SC) states with small numbers of skyrmions were demonstrated to exist on the cross-sections of ultra-narrow single-crystal MnSi nanowires (NWs) with diameters, comparable to the skyrmion lattice constant (18 nm). In contrast to the skyrmion lattice in bulk MnSi samples, the skyrmion clusters lead to anomalous magnetoresistance (MR) behavior measured under magnetic field parallel to the NW long axis, where quantized jumps in MR are observed and directly associated with the change of the skyrmion number in the cluster, which is supported by Monte Carlo simulations. These jumps show the key difference between the clustering and crystalline states of skyrmions, and lay a solid foundation to realize skyrmion-based memory devices that the number of skyrmions can be counted via conventional electrical measurements

Direct imaging of a zero-field target skyrmion and its polarity switch in a chiral magnetic nanodisk

A target skyrmion is a flux-closed spin texture that has two-fold degeneracy and is promising as a binary state in next generation universal memories. Although its formation in nanopatterned chiral magnets has been predicted, its observation has remained challenging. Here, we use off-axis electron holography to record images of target skyrmions in a 160-nm-diameter nanodisk of the chiral magnet FeGe. We compare experimental measurements with numerical simulations, demonstrate switching between two stable degenerate target skyrmion ground states that have opposite polarities and rotation senses and discuss the observed switching mechanism.Comment: 18 pages, 4 figure

Porous Pyrene Organic Cage with Unusual Absorption Bathochromic-Shift Enables Visible Light Photocatalysis

We have constructed a novel porous pyrene-based organic cage, PyTC1, through the condensation reaction of cyclohexanediamine with 5,5&prime;-(pyrene-1,6-diyl)diisophthalaldehyde. Single-crystal X-ray diffraction analysis reveals the effective intercage C&ndash;H...&pi; interaction between cyclohexanediimine and pyrene segments. Such a soft intercage C&ndash;H...&pi; interaction, rather than a classic J-aggregate with slipped &pi;&ndash;&pi;-stacking configuration, induced an unusual bathochromic shift of pyrene-based chromophore absorption from an ultraviolet region of PyTC1 in solution to the visible light region of PyTC1 in solid-state. This enabled heterogeneous visible light photocatalysis of aerobic hydroxylation of benzeneboronic acid derivatives. To the best of our knowledge, this is the first report that represents an absorption bathochromic shift caused by C&ndash;H...&pi; interaction. Such phenomenon could endow visible-light-driven photocatalysis that originally could only be achieved using UV light with the same chromophore. &nbsp;</p

Magnetic Skyrmions in an FeGe Nanostripe Revealed by in situ Electron Holography

ntense research interest in magnetic skyrmions is presently driving the development of new fundamental concepts and applications1. Magnetic skyrmions are particle-like, topologically protected swirling spin textures, in which the peripheral spins are oriented vertically, the central spins are oriented in the opposite direction and the intermediate spins rotate smoothly between these two opposite orientations, as shown in the inset to Fig. 1(a). In a range of applied magnetic fields, skyrmion lattices form in certain chiral magnets, such as B20-type magnets, in which a lack of inversion symmetry and spin-orbit coupling gives rise to the Dzyaloshinskii-Moriya interaction. The typical sizes of skyrmions are between 3 and 100 nm. For technically relevant applications, a full understanding of skyrmion formation, stability, manipulation and annihilation is required. Recent experiments have demonstrated the formation of magnetic skyrmion chains in geometrically confined nanostructures2, as shown schematically in Fig. 1(b). A critical step towards real-world device applications involves the development of an approach that can be used to controllably create, manipulate and annihilate skyrmions in magnetic nanostructures, including wire-like geometries.Real-space imaging of complex skyrmion spin configurations using Lorentz microscopy (LM) in the transmission electron microscope (TEM) has enabled the direct observation of skyrmion lattice formation and transformations between different magnetic states with nanometre spatial resolution3. However, the finite size and the inherently weak magnetization of such magnetic nanostructures imposes great experimental challenges for LM. In particular, Fresnel fringe contrast at the specimen edge makes extremely difficult to use LM to obtain magnetic signals in samples that have lateral dimensions of below 10 nm. In contrast, off-axis electron holography (EH) in the TEM, which allows electron-optical phase images to be recorded directly with nanometre spatial resolution and high phase sensitivity, provides easier access to magnetic states in nanostructures. Digital acquisition and analysis of electron holograms and sophisticated image analysis software are then essential in studies of weak and slowly varying phase objects such as magnetic skyrmions4.Here, we use both LM and EH to study magnetic skyrmions in a B20-type FeGe nanostripe. The use of liquid nitrogen specimen holder (Gatan model 636) allows the specimen temperature to be varied between 95 and 370 K, and the objective lens of the microscope (FEI Titan 60-300) can be used to apply magnetic fields to the specimen of 0 to 1.5 T. The aim of our study is to resolve the fine magnetic structures of geometrically confined skyrmions and to understand their formation process. Figures 2(a-b) show Lorentz images of a typical FeGe nanostripe, in which a helix to skyrmion transition occurs in response to an applied magnetic field. Figure 2(c) shows a colour-contour composite map derived from a phase image recorded using EH. The slight asymmetry of the contours results from the wedge-shaped specimen thickness profile. Artefacts associated with local changes in specimen thickness in such images can be removed from such images by separating the mean inner potential contribution from the magnetic contribution to the phase, for examples by evaluating the difference between phase images recorded at two different specimen temperatures

Spin-dimensionality change induced by Co-doping in the chiral magnet Fe

Dimensionality is one of the most important parameters in the determination of the physical properties. Therefore, tuning of effective dimensionality is of significant importance for modulating the functionality of materials. In this work, we find that the spin-dimensionality can be changed by Co-doping in the Fe1−xCoxSi system. Investigation of the critical behavior shows that the effective critical exponents for x = 0.3 agree with the three-dimensional (3D) Heisenberg model with $\{d:n=3:3\}$ (d is the spatial-dimensionality, and n is the spin-dimensionality). With the increase of Co-content, the effective critical exponents for x = 0.5 fulfill the 3D-XY model with $\{d:n=3:2\}$ , while those for x = 0.6 approach the 3D-Ising model with $\{d:n=3:1\}$ . These results indicate the lowering of the spin-dimensionality with the increase of Co-content in Fe1−xCoxSi. We suggest that the modulation of the spin-dimensionality in Fe1−xCoxSi should result from the enhancement of the anisotropic magnetic interaction induced by the doping of Co

Magnetic Skyrmion Formation at Lattice Defects and Grain Boundaries Studied by Quantitative Off-Axis Electron Holography

We use in situ Lorentz microscopy and off-axis electron holography to investigate the formation and characteristics of skyrmion lattice defects and their relationship to the underlying crystallographic structure of a B20 FeGe thin film. We obtain experimental measurements of spin configurations at grain boundaries, which reveal inversions of crystallographic and magnetic chirality across adjacent grains, resulting in the formation of interface spin stripes at the grain boundaries. In the absence of material defects, we observe that skyrmions lattices possess dislocations and domain boundaries, in analogy to atomic crystals. Moreover, the distorted skyrmions can flexibly change their size and shape to accommodate local geometry, especially at sites of dislocations in the skyrmion lattice. Our findings provide a detailed understanding of the elasticity of topologically protected skyrmions and their correlation with underlying material defects

Quantification of Magnetic Surface and Edge States in an FeGe Nanostripe by Off-Axis Electron Holography

Whereas theoretical investigations have revealed the significant influence of magnetic surface and edge states on Skyrmonic spin texture in chiral magnets, experimental studies of such chiral states remain elusive. Here, we study chiral edge states in an FeGe nanostripe experimentally using off-axis electron holography. Our results reveal the magnetic-field-driven formation of chiral edge states and their penetration lengths at 95 and 240 K. We determine values of saturation magnetization MS by analyzing the projected in-plane magnetization distributions of helices and Skyrmions. Values of MS inferred for Skyrmions are lower by a few percent than those for helices. We attribute this difference to the presence of chiral surface states, which are predicted theoretically in a three-dimensional Skyrmion model. Our experiments provide direct quantitative measurements of magnetic chiral boundary states and highlight the applicability of state-of-the-art electron holography for the study of complex spin textures in nanostructures

Enhanced Stability of the Magnetic Skyrmion Lattice Phase under a Tilted Magnetic Field in a Two-Dimensional Chiral Magnet

The magnetic skyrmion is a topologically stable vortex-like spin texture that offers great promise as information carriers for future spintronic devices. In a two-dimensional chiral magnet, it was generally considered that a tilted magnetic field is harmful to its formation and stability. Here we investigated the angular-dependent stability of magnetic skyrmions in FeGe nanosheets by using high-resolution Lorentz transmission electron microscopy (Lorentz TEM). Besides the theoretically predicted destruction of skyrmion lattice state by an oblique magnetic field as the temperature closes to its magnetic Curie temperature <i>T</i>_c ∼ 278 K, we also observed an unexpected reentry-like phenomenon at the moderate temperatures near the border between conical and skyrmion phase, <i>T</i>_t ∼ 240 K. This behavior is completely beyond the theoretical prediction in a conventional two-dimensional (2D) system. Instead, a three-dimensional (3D) model involving the competition between conical phase and skyrmions is likely to play a crucial role