Manipulating the Topology of Nanoscale Skyrmion Bubbles by Spatially Geometric Confinement

Real-Space Imaging of Intrinsic Symmetry-Breaking Spin Textures in a Kagome Lattice

The electronic orders in kagome materials have emerged as a fertile platform for studying exotic quantum states, and their intertwining with the unique kagome lattice geometry remains elusive. While various unconventional charge orders with broken symmetry is observed, the influence of kagome symmetry on magnetic order has so far not been directly observed. Here, using a high-resolution magnetic force microscopy, it is, for the first time, observed a new lattice form of noncollinear spin textures in the kagome ferromagnet in zero magnetic field. Under the influence of the sixfold rotational symmetry of the kagome lattice, the spin textures are hexagonal in shape and can further form a honeycomb lattice structure. Subsequent thermal cycling measurements reveal that these spin textures transform into a non-uniform in-plane ferromagnetic ground state at low temperatures and can fully rebuild at elevated temperatures, showing a strong second-order phase transition feature. Moreover, some out-of-plane magnetic moments persist at low temperatures, supporting the Kane-Mele scenario in explaining the emergence of the Dirac gap. The observations establish that the electronic properties, including both charge and spin orders, are strongly coupled with the kagome lattices.

Current-Induced Reversible Split of Elliptically Distorted Skyrmions in Geometrically Confined Fe3 Sn2 Nanotrack

Skyrmions are swirling spin textures with topological characters promising for future spintronic applications. Skyrmionic devices typically rely on the electrical manipulation of skyrmions with a circular shape. However, manipulating elliptically distorted skyrmions can lead to numerous exotic magneto-electrical functions distinct from those of conventional circular skyrmions, significantly broadening the capability to design innovative spintronic devices. Despite the promising potential, its experimental realization so far remains elusive. In this study, the current-driven dynamics of the elliptically distorted skyrmions in geometrically confined magnet Fe3 Sn2 is experimentally explored. This study finds that the elliptical skyrmions can reversibly split into smaller-sized circular skyrmions at a current density of 3.8 × 1010 A m-2 with the current injected along their minor axis. Combined experiments with micromagnetic simulations reveal that this dynamic behavior originates from a delicate interplay of the spin-transfer torque, geometrical confinement, and pinning effect, and strongly depends on the ratio of the major axis to the minor axis of the elliptical skyrmions. The results indicate that the morphology is a new degree of freedom for manipulating the current-driven dynamics of skyrmions, providing a compelling route for the future development of spintronic devices.

Topological Hall Effect Anisotropy in Kagome Bilayer Metal Fe_{3}Sn_{2}

Kagome lattice materials have attracted growing interest for their topological properties and flatbands in electronic structure. We present a comprehensive study on the anisotropy and out-of-plane electric transport in Fe_{3}Sn_{2}, a metal with bilayer of Fe kagome planes and with massive Dirac fermions that features high-temperature noncollinear magnetic structure and magnetic skyrmions. For the electrical current path along the c axis, in micron-size crystals, we found a large topological Hall effect over a wide temperature range down to spin-glass state. Twofold and fourfold angular magnetoresistance are observed for different magnetic phases, reflecting the competition of magnetic interactions and magnetic anisotropy in kagome lattice that preserve robust topological Hall effect for inter-kagome bilayer currents. This provides new insight into the anisotropy in Fe_{3}Sn_{2}, of interest in skyrmionic-bubble application-related micron-size devices.

Controllable Topological Magnetic Transformations in the Thickness-Tunable van der Waals Ferromagnet Fe5GeTe2

Recent observations of topological meron textures in two-dimensional (2D) van der Waals (vdW) magnetic materials have attracted considerable research interest for both fundamental physics and spintronic applications. However, manipulating the meron textures and realizing the topological transformations, which allow for exploring emergent electromagnetic behaviors, remain largely unexplored in 2D magnets. In this work, utilizing real-space imaging and micromagnetic simulations, we reveal temperature- and thickness-dependent topological magnetic transformations among domain walls, meron textures, and stripe domain in Fe₅GeTe₂ (FGT) lamellae. The key mechanism of the magnetic transformations can be attributed to the temperature-induced change of exchange stiffness constant within layers and uniaxial magnetic anisotropy, while the magnetic dipole interaction as governed by sample thickness is crucial to affect the critical transformation temperature and stripe period. Our findings provide reliable insights into the origin and manipulation of topological spin textures in 2D vdW ferromagnets.

Spin-Orbit Torque in Bilayers of Kagome Ferromagnet Fe3Sn2 and Pt

Spin-orbit torque phenomena enable efficient manipulation of the magnetization in ferromagnet/heavy metal bilayer systems for prospective magnetic memory and logic applications. Kagome magnets are of particular interest for spin-orbit torque due to the interplay of magnetic order and the nontrivial band topology (e.g., flat bands and Dirac and Weyl points). Here we demonstrate spin-orbit torque and quantify its efficiency in a bilayer system of topological kagome ferromagnet Fe₃Sn₂ and platinum. We use two different techniques, one based on the quasistatic magneto-optic Kerr effect (MOKE) and another based on time-resolved MOKE, to quantify spin-orbit torque. Both techniques give a consistent value of the effective spin Hall angle of the Fe₃Sn₂/Pt system. Our work may lead to further advances in spintronics based on topological kagome magnets.

Stimulated Nucleation of Skyrmions in a Centrosymmetric Magnet

Understanding the dynamics of skyrmion nucleation and manipulation is important for applications in spintronic devices. In this contribution, the spin textures at magnetic domain-boundaries stimulated by application of in-plane magnetic fields in a centrosymmetric kagome ferromagnet, Fe₃Sn₂, with thickness gradient are investigated using Lorentz transmission electron microscopy. Switching of the in-plane magnetic field is shown to induce a reversible transformation from magnetic stripes to skyrmions, or vice versa, at the interface between differently oriented domains. Micromagnetic simulations combined with experiments reveal that the rotatable anisotropy and thickness dependence of the response to the external in-plane field are the critical factors for the skyrmion formation. In addition, it is shown that the helicity of skyrmions can also be controlled using this dynamic process. The results suggest that magnetic materials with rotatable anisotropy are potential skyrmionic systems and provides a different approach for nucleation and manipulation of skyrmions in spintronic devices.

Emergence of Room Temperature Magnetotransport Anomaly in Epitaxial Pt/γ'-Fe4N/MgO Heterostructures toward Noncollinear Spintronics

Noncollinear spin textures have attracted much attention due to their novel physical behaviors in heavy/ferromagnetic metal (HM/FM) systems. The transport anomaly, appearing as contrast humps in Hall resistivity curves, is the mark of noncollinear spin textures. Here, the epitaxial Pt/γ'-Fe₄N bilayers with noncollinear spin textures were obtained by facing target sputtering. Large micromagnetic Dzyaloshinskii-Moriya interaction coefficient D of 2.90 mJ/m² appears in Pt/γ'-Fe₄N/MgO systems, which is larger than 2.05 mJ/m² of Pt/Co/MgO systems with skyrmionic states. Moreover, at 300 K, magnetic bubble-like domains appear in Pt/γ'-Fe₄N bilayers that just possess a 3 nm thick ferromagnetic layer instead of [HM/FM]_n or [HM₁/FM/HM₂]_n multilayers. Additionally, a room-temperature transport anomaly appears in Pt/γ'-Fe₄N/MgO systems. The contrast humps of Pt(3 nm)/γ'-Fe₄N(t_Fe4N ≤ 4 nm)/MgO heterostructures are not sharp due to the nonuniform distributions of the magnetic bubble-like domains with various sizes and irregular shapes, as observed by the magnetic force microscopy. The discovery of epitaxial Pt/γ'-Fe₄N bilayers with noncollinear spin states is more crucial than that of polycrystalline or amorphous HM/FM systems for reducing ohmic heating, which provides a candidate for noncollinear spintronic applications.

Recent Progress on Topological Structures in Ferroic Thin Films and Heterostructures

Topological spin/polarization structures in ferroic materials continue to draw great attention as a result of their fascinating physical behaviors and promising applications in the field of high-density nonvolatile memories as well as future energy-efficient nanoelectronic and spintronic devices. Such developments have been made, in part, based on recent advances in theoretical calculations, the synthesis of high-quality thin films, and the characterization of their emergent phenomena and exotic phases. Herein, progress over the last decade in the study of topological structures in ferroic thin films and heterostructures is explored, including the observation of topological structures and control of their structures and emergent physical phenomena through epitaxial strain, layer thickness, electric, magnetic fields, etc. First, the evolution of topological spin structures (e.g., magnetic skyrmions) and associated functionalities (e.g., topological Hall effect) in magnetic thin films and heterostructures is discussed. Then, the exotic polar topologies (e.g., domain walls, closure domains, polar vortices, bubble domains, and polar skyrmions) and their emergent physical properties in ferroelectric oxide films and heterostructures are explored. Finally, a brief overview and prospectus of how the field may evolve in the coming years is provided.

Tilted X-Ray Holography of Magnetic Bubbles in MnNiGa Lamellae

Nanoscopic lamellae of centrosymmetric ferromagnetic alloys have recently been reported to host the biskyrmion spin texture; however, this has been disputed as the misidentication of topologically trivial type-II magnetic bubbles. Here we demonstrate resonant soft X-ray holographic imaging of topological magnetic states in lamellae of the centrosymmetric alloy (Mn_1-xNi_x)_0.65Ga_0.35 (x = 0.5), showing the presence of magnetic stripes evolving into single core magnetic bubbles. We observe rotation of the stripe phase via the nucleation and destruction of disclination defects. This indicates the system behaves as a conventional uniaxial ferromagnet. By utilizing the holography with extended reference by autocorrelation linear differential operator (HERALDO) method, we show tilted holographic images at 30° incidence confirming the presence of type-II magnetic bubbles in this system. This study demonstrates the utility of X-ray imaging techniques in identifying the topology of localized structures in nanoscale magnetism.

Isotropic Nature of the Metallic Kagome Ferromagnet Fe3Sn2 at High Temperatures

Anisotropy and competing exchange interactions have emerged as two central ingredients needed for centrosymmetric materials to exhibit topological spin textures. Fe3Sn2 is thought to have these ingredients as well, as it has recently been discovered to host room temperature skyrmionic bubbles with an accompanying topological Hall effect. We present small-angle inelastic neutron scattering measurements that unambiguously show that Fe3Sn2 is an isotropic ferromagnet below T C ≈ 660     K to at least 480 K - the lower temperature threshold of our experimental configuration. Fe3Sn2 is known to have competing magnetic exchange interactions, correlated electron behavior, weak magnetocrystalline anisotropy, and lattice anisotropy; all of these features are thought to play a role in stabilizing skyrmions in centrosymmetric systems. Our results reveal that at elevated temperatures, there is an absence of magnetocrystalline anisotropy and that the system behaves as a typical exchange ferromagnet with a spin stiffness D T = 0   K = 271 ± 9   meV     Å 2 .

Target Bubbles in Fe3Sn2 Nanodisks at Zero Magnetic Field

We report a vortex-like magnetic configuration in uniaxial ferromagnet Fe₃Sn₂ nanodisks using differential phase contrast scanning transmission electron microscopy. This magnetic configuration is transferred from a conventional magnetic vortex using a zero-magnetic-field warming process and is characterized by a series of concentric cylinder domains. We termed them as "target bubbles" that are identified as three-dimensional depth-modulated magnetic objects in combination with numerical simulations. Target bubbles have room-temperature stability even at zero magnetic field and multiple stable magnetic configurations. These advantages render the target bubble an ideal bit to be an information carrier and can advance magnetic target bubbles toward functionalities in the long term by incorporating emergent degrees of freedom and purely electrically controllable magnetism.

Electric-field-driven non-volatile multi-state switching of individual skyrmions in a multiferroic heterostructure

Electrical manipulation of skyrmions attracts considerable attention for its rich physics and promising applications. To date, such a manipulation is realized mainly via spin-polarized current based on spin-transfer torque or spin-orbital torque effect. However, this scheme is energy consuming and may produce massive Joule heating. To reduce energy dissipation and risk of heightened temperatures of skyrmion-based devices, an effective solution is to use electric field instead of current as stimulus. Here, we realize an electric-field manipulation of skyrmions in a nanostructured ferromagnetic/ferroelectrical heterostructure at room temperature via an inverse magneto-mechanical effect. Intriguingly, such a manipulation is non-volatile and exhibits a multistate feature. Numerical simulations indicate that the electric-field manipulation of skyrmions originates from strain-mediated modification of effective magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Our results open a direction for constructing low-energy-dissipation, non-volatile, and multistate skyrmion-based spintronic devices.

Ternary compound HfCuP: An excellent Weyl semimetal with the coexistence of type-I and type-II Weyl nodes

In most Weyl semimetal (WSMs), the Weyl nodes with opposite chiralities usually have the same type of band dispersions (either type-I or type-II), whereas realistic candidate materials hosting different types of Weyl nodes have not been identified to date. Here we report for the first time that, a ternary compound HfCuP, is an excellent WSM with the coexistence of type-I and type-II Weyl nodes. Our results show that, HfCuP totally contains six pairs of type-I and six pairs of type-II Weyl nodes in the Brillouin zone, all locating at the H-K path. These Weyl nodes situate slightly below the Fermi level, and do not coexist with other extraneous bands. The nontrivial band structure in HfCuP produces clear Fermi arc surface states in the (1 0 0) surface projection. Moreover, we find the Weyl nodes in HfCuP can be effectively tuned by strain engineering. These characteristics make HfCuP a potential candidate material to investigate the novel properties of type-I and type-II Weyl fermions, as well as the potential entanglements between them.

Observation of Magnetic Skyrmion Bubbles in a van der Waals Ferromagnet Fe3GeTe2

Two-dimensional (2D) van der Waals (vdW) magnetic materials have recently been introduced as a new horizon in materials science, and they enable potential applications for next-generation spintronic devices. Here, in this communication, the observations of stable Bloch-type magnetic skyrmions in single crystals of 2D vdW Fe₃GeTe₂ (FGT) are reported by using in situ Lorentz transmission electron microscopy (TEM). We find the ground-state magnetic stripe domains in FGT transform into skyrmion bubbles when an external magnetic field is applied perpendicularly to the (001) thin plate with temperatures below the Curie temperature T_C. Most interestingly, a hexagonal lattice of skyrmion bubbles is obtained via field-cooling manipulation with magnetic field applied along the [001] direction. Owing to their topological stability, the skyrmion bubble lattices are stable to large field-cooling tilted angles and further reproduced by utilizing the micromagnetic simulations. These observations directly demonstrate that the 2D vdW FGT possesses a rich variety of topological spin textures, being of great promise for future applications in the field of spintronics.

Current-Induced Helicity Reversal of a Single Skyrmionic Bubble Chain in a Nanostructured Frustrated Magnet

Helicity indicates the in-plane magnetic-moment swirling direction of a skyrmionic configuration. The ability to reverse the helicity of a skyrmionic bubble via purely electrical means has been predicted in frustrated magnetic systems; however, it has been challenging to observe this experimentally. The current-driven helicity reversal of the skyrmionic bubble in a nanostructured frustrated Fe₃ Sn₂ magnet is experimentally demonstrated. The critical current density required to trigger the helicity reversal is 10⁹ -10¹⁰ A m^-2 , with a corresponding pulse-width varying from 1 µs to 100 ns. Computational simulations reveal that both the pinning effect and dipole-dipole interaction play a crucial role in the helicity reversal process.

Palladium oxide: an excellent topological electronic material with 0-D and 1-D band crossings and definite nontrivial surface states

PdO is an excellent topological semimetal with coexisting 0-D and 1-D band crossings and clear surface states.