The 2020 Skyrmionics Roadmap

Abstract: The notion of non-trivial topological winding in condensed matter systems represents a major area of present-day theoretical and experimental research. Magnetic materials offer a versatile platform that is particularly amenable for the exploration of topological spin solitons in real space such as skyrmions. First identified in non-centrosymmetric bulk materials, the rapidly growing zoology of materials systems hosting skyrmions and related topological spin solitons includes bulk compounds, surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topological winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.

• The paper presents a comprehensive review of skyrmion research, integrating both theoretical frameworks and experimental realities.
• It details challenges in achieving high transition temperatures, small skyrmion sizes, and enhanced stability for practical applications.
• The roadmap outlines future directions including multilayer integrations, emergent electrodynamics, and novel spintronic technologies.

An Overview of the 2020 Skyrmionics Roadmap

The "2020 Skyrmionics Roadmap" paper provides a comprehensive examination of the state-of-the-art in the field of magnetic skyrmions, covering various perspectives from fundamental properties to potential applications. This review is structured to cover both theoretical frameworks and experimental realities, providing a landscape where the future directions and current challenges in skyrmion research are critically evaluated by a consortium of experienced researchers.

Skyrmions in Non-Centrosymmetric Bulk Materials

The paper highlights the role of magnetic skyrmions, particularly in non-centrosymmetric magnets which are stabilized by the Dzyaloshinskii-Moriya interaction (DMI). These skyrmions demonstrate robustness against external perturbations due to topological protection. The existence of Bloch-type, Néel-type, and recently discovered anti-skyrmions showcases the diversity within non-centrosymmetric bulk materials. Isotopic examples include B20 metals such as MnSi, Co-Mn alloys, and polar compounds like GaV$_4$S$_8$. These materials demonstrate skyrmions at ambient conditions, which underlines their potential for practical application in data storage devices.

Challenges and Future Directions

The roadmap identifies several key challenges in skyrmionics:

1. Combining high transition temperatures with small skyrmion sizes.
2. Enhancing skyrmion (meta)stability to ensure they're operable outside the limited phase diagrams usually constrained by specific temperature and magnetic field conditions.
3. Investigating skyrmions’ three-dimensional dynamics.
4. Developing reliable control over the skyrmion lifecycle (creation, annihilation, motion).

Skyrmions in Achiral and Frustrated Magnets

The review considers the possibilities of stabilizing skyrmions in magnets without intrinsic chirality but with competing interactions, such as Heisenberg exchange interactions. It suggests that the presence of achiral skyrmions may lead to novel collective dynamics and topological states due to additional degrees of freedom.

Skyrmionics and Spintronics Applications

Epitaxial films derived from skyrmion-hosting bulk materials and multilayered structures exploiting interfacial DMI have proven viable for RT applications. The paper discusses the promising results in reading, writing, and deleting single skyrmions. The recalibration of skyrmion characteristics is essential in multilayers to optimize for size, mobility, and stability, paving the way for applications in racetrack memory devices leveraging spin-orbit torques.

Skyrmions in Atomic-Layer and Interface Systems

Application-driven research into skyrmions in atomic layers emphasizes the importance of understanding spin-polarized currents and non-collinear magnetic textures. Model systems facilitate the exploration of spin-transfer effects and the emergence of skyrmions, offering insights into fundamental physics and improving the prediction accuracy for complex systems.

Topological Protection and Emergent Electrodynamics

Topological protection is a cornerstone of skyrmion research, with emergent electrodynamics showing practical significance. These aspects are intricately linked with phenomena such as the Topological Hall Effect and provide insights into controlling skyrmion dynamics. The coupling of emergent electrodynamics with advanced materials and techniques reinforces efforts toward applications in electronics and data storage.

Conclusion and Speculative Directions

The roadmap suggests that while considerable progress has been achieved, numerous experimental and theoretical challenges remain before skyrmions can be fully harnessed in technological applications. The synthesis of skyrmions with other material systems such as topological insulators and superconductors presents a compelling future direction. This approach speaks to a broader engagement between disparate areas of condensed matter physics to capitalize on the unique properties of skyrmions. As research progresses, the roadmap calls for interdisciplinary efforts to bridge the gap between fundamental research and applied technology, including potential new applications in magnonics and neuromorphic computing.

In summary, the "2020 Skyrmionics Roadmap" functions as both a detailed survey of current skyrmion research and a prescient guide for future experimental and theoretical investigations, emphasizing the need for improved materials, refined theoretical models, and innovative technological implementations.