Oleg A. Tretiakov

Nontrivial topological spin textures, such as magnetic skyrmions, are of great interest due to their potential use as information carriers in spintronic memory and logic. Here, we theoretically predict an asymmetric antibimeron (AAB)—a topological texture in an in-plane magnetized chiral ferromagnet with D 2 d symmetry, which has yet to be observed experimentally. We show that AABs can be stabilized by an anisotropic interfacial Dzyaloshinskii-Moriya interaction, characteristic of materials with D 2 d symmetry. Using energy considerations, we explain the origin of its asymmetric shape, composed of an antivortex and a crescent-shaped vortex with opposite core polarizations. Furthermore, we demonstrate that AABs of opposite topological charge can coexist within the same ferromagnetic film, in contrast to skyrmions in out-of-plane magnetized systems. Employing micromagnetic simulations and an analytical approach based on an extension of Thiele's equation to a system of two elliptic merons comprising the AAB, we are able to describe the current-driven dynamics of AABs. In particular, we show that AAB collisions enable controlled manipulation of the system's topological charge. Our findings shed light on fundamental understanding of asymmetric topological magnetic solitons and provide a roadmap for their experimental observation in in-plane magnetized ferromagnets.

Two-dimensional (2D) van der Waals (vdW) magnets and devices have garnered significant attention owing to the stabilization of long-range magnetic order down to atomic limit, and the prospect for quantum devices with unique functionalities. To achieve this objective, clarification of magnetotransport properties and understanding of the relevant interactions with lowering of dimensions are of extreme importance. Here, the magnetotransport properties of few atomic layer Fe3GeTe2 and (Co0.25Fe0.75)3GeTe2 nanoflake devices have been investigated. Magnetotransport investigations with applied magnetic field along the easy-axis show anomalous Hall effect, while that for applied magnetic field along the hard-axis reveal an unusual behavior. Atomistic calculations considering the presence of antiferromagnetic, ferromagnetic, and local symmetry-breaking interactions reveal the critical role of magnetic frustration effect assisted by thermal fluctuations, leading to a non-zero scalar spin chirality manifesting in an unconventional Hall effect. The present result clarifies the underlying interactions in few-layer 2D vdW ferromagnetic material system, important for the understanding of non-collinear spin configurations in vdW magnets for 2D spintronic devices.

With topological materials being billed as the key to a new generation of nanoelectronics via either functional real-space topological structures (domain walls, skyrmions etc.) or via momentum-space topology (topological insulators), tailored and controllable topological properties are of paramount significance, since they lead to topologically protected states with negligible dissipation, enabling stable and non-volatile information processing. Here, we report on the evolution of topological magnetic textures in the proximity of other topological defects, i.e., antiferromagnetic domain walls in the topological insulator MnBi2Te4. The transition from the antiferromagnetic ground state to a canted antiferromagnetic state at finite magnetic fields is accompanied by the formation of chiral bobbers - bulk-terminated topological defects adjacent to the domain walls in this system, leading to a topological proximity effect.

The chiral nature of active matter plays an important role in the dynamics of active matter interacting with chiral structures. Skyrmions are chiral objects, and their interactions with chiral nanostructures can lead to intriguing phenomena. Here, we explore the random-walk dynamics of a thermally activated chiral skyrmion interacting with a chiral flower-like obstacle in a ferromagnetic layer, which could create topology-dependent outcomes. It is a spontaneous mesoscopic order-from-disorder phenomenon driven by the thermal fluctuations and topological nature of skyrmions that exists only in ferromagnetic and ferrimagnetic systems. The interactions between the skyrmions and chiral flowers at finite temperatures can be utilized to control the skyrmion position and distribution without applying any external driving force or temperature gradient. The phenomenon that thermally activated skyrmions are dynamically coupled to chiral flowers may provide a new way to design topological sorting devices.

The world is full of fluids that flow. The fluid nature of flowing skyrmionic quasiparticles is of fundamental physical interest and plays an essential role in the transport of many skyrmions. Here, we report the laminar and transiently disordered dynamic behaviors of many magnetic skyrmions flowing in a pipe channel. The skyrmion flow driven by a uniform current may show a lattice structural transition. The skyrmion flow driven by a nonuniform current shows a dynamically varying lattice structure. A large uniform current could result in the compression of skyrmions toward the channel edge, leading to the transition of the skyrmion pipe flow into an open-channel flow with a free surface. Namely, the width of the skyrmion flow could be adjusted by the driving current. Skyrmions on the free surface may form a single shear layer adjacent to the main skyrmion flow. In addition, although we focus on the skyrmion flow dynamics in a clean pipe channel without any pinning or defect effect, we also show that variation of the magnetic anisotropy in the pipe channel could lead to more complicated skyrmion flow dynamics and path lines. Our results reveal the fluid nature of skyrmionic quasiparticles, which may motivate future research on the complex flow physics of magnetic textures.

Electric field control of topologically nontrivial magnetic textures, such as skyrmions, provides a paradigm shift for future spintronics beyond the current silicon-based technology. While significant progress has been made by X-ray and neutron scattering studies, direct observation of such nanoscale spin structures and their dynamics driven by external electric fields remains a challenge in understanding the underlying mechanisms and harness functionalities. Here, using Lorentz transmission electron microscopy combined with in situ electric and magnetic fields at liquid helium temperatures, we report the crystallographic orientation-dependent skyrmion responses to electric fields in thin slabs of magnetoelectric Cu₂OSeO₃. We show that electric fields not only stabilize the hexagonally packed skyrmion lattices in the entire sample in a hysteretic manner but also induce the rotation of their reciprocal vector discretely by 30°. The nonvolatile and energy-efficient skyrmion lattice control by electric fields demonstrated in this work provides an important foundation for designing skyrmion-based qubits and memory devices.