Beyond skyrmions: Review and perspectives of alternative magnetic quasiparticles (2005.01390v2)

Abstract: Magnetic skyrmions have attracted enormous research interest since their discovery a decade ago. The non-trivial real-space topology of these nano-whirls leads to fundamentally interesting and technologically relevant consequences - the skyrmion Hall effect of the texture and the topological Hall effect of the electrons. Furthermore, it grants skyrmions in a ferromagnetic surrounding great stability even at small sizes, making skyrmions aspirants to become the carriers of information in the future. Still, the utilization of skyrmions in spintronic devices has not been achieved yet, among other reasons, due to shortcomings in their current-driven motion. In this review, we present recent trends in the field of topological spin textures that go beyond skyrmions. The majority of these objects can be considered a combination of multiple subparticles, such as the bimeron, or the skyrmion analogues in different magnetic surroundings, such as antiferromagnetic skyrmions, as well as three-dimensional generalizations, such as hopfions. We classify the alternative magnetic quasiparticles - some of them observed experimentally, others theoretical predictions - and present the most relevant and auspicious advantages of this emerging field.

• The paper examines alternative magnetic quasiparticles that address skyrmion limitations such as the skyrmion Hall effect.
• It categorizes these quasiparticles into distinct classes like intermediate skyrmions, antiskyrmions, bimerons, and skyrmioniums with unique dynamic properties.
• The review highlights their potential in spintronics, offering stable motion and improved data encoding for advanced device architectures.

Review and Perspectives of Alternative Magnetic Quasiparticles

This paper by B{\"o}rge G{\"o}bel et al. presents an extensive review on magnetic quasiparticles that extend beyond the widely studied skyrmions, which are topologically non-trivial spin structures with promising applications in spintronics. Despite the potential benefits of skyrmions, such as their stability and small size, their practical application is hindered by challenges like the skyrmion Hall effect, leading researchers to explore alternative magnetic quasiparticles.

The paper categorizes these alternative quasiparticles into several classes: fundamental excitations, variations, and extensions, each presenting unique features and potential use cases in spintronic devices. Among these, intermediate skyrmions and antiskyrmions offer significant promise due to their capacity for motion parallel to applied currents, thereby mitigating the skyrmion Hall effect. Antiskyrmions are distinguished by an anisotropic DMI, which results in a unique form favorable for racetrack applications, offering the potential for more stable and dependable data encoding mechanisms.

Moreover, the authors explore composite objects like bimerons, biskyrmions, and skyrmioniums. The bimeron, which can be understood as an in-plane analog of a skyrmion, provides unique emergent electrodynamic properties that decouple the topological Hall effect from other Hall contributions, allowing for purer signal detection. Skyrmioniums, characterized by their zero net topological charge, can move without a skyrmion Hall effect, yet offer challenges like deformation under motion. Antiferromagnetic skyrmions, which have been realized in synthetic antiferromagnet systems, promise high-speed motion and negligible stray fields, making them excellent candidates for future densely packed, high-speed data storage solutions.

Three-dimensional structures such as chiral bobbers and hopfions are also discussed. Chiral bobbers present an interesting possibility due to their coexistence with skyrmion tubes, permitting complex data encoding schemes in three-dimensional racetracks. Hopfions bring the additional complexity of a second topological invariant, the Hopf number, and exhibit distinctive transport properties owing to their unique emergent field configurations.

These discussions on alternative magnetic quasiparticles emphasize their potential to solve key limitations associated with traditional skyrmions. The paper's insights into the stabilizing mechanisms, emergent dynamics, and potential for integration into devices highlight the evolving landscape of spintronics research. This review serves as a platform for further exploration and experimental verification of these theoretical predictions, paving the way for more sophisticated magnetic storage and computing technologies. Future work could focus on operational stability, precise control mechanisms for manipulation, and the integration of these quasiparticles into existing magnetic and electronic infrastructure, further refining their implementation in practical applications.