Uncovering the fundamental properties of discretized 3D partial skyrmion tubes in magnetic multilayers and their role in the dynamics of topological phase transitions

• Applicants: Professor Dr. Felix Büttner; Dr. Bastian Pfau
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 505818345

One of the most fascinating phenomena in physics is the possibility that qualitatively new behavior (new states of matter) can emerge from large ensembles of simple entities, especially in the presence of competing interactions. Magnetism offers a particularly rich playground in this respect because a multitude of competing interactions (e.g., interactions with short and long range, axial, directional, collinear, and chiral character) meets a vector degree of freedom by which complex, topologically non-trivial collective states can be constructed. Magnetic skyrmions are a prime example of such a topological spin texture, which exhibits collective qausi-particle behavior that is very different from the physics of the individual constituent spins (e.g., gyrotropic defection, inertia, topological damping, etc.). However, while the zoo of 2D emergent topological spin states (vortices, skyrmions, target skyrmions, etc.) is already well explored, the richness expected in 3D is just starting to become accessible. Here, we propose to investigate the physics of discretized 3D partial skyrmion tubes—the simplest siblings of skyrmions in magnetic multilayer materials. These states are characterized by a skyrmion structure in some of the magnetic layers and a topological trivial configuration in others, and as such represent a conceptual novelty compared to any texture in 2D top topological charge becomes a quasi-continuous number. We refer to these states as “discretized” because the alternatinv stacking of magnetic and non-magnetic layers in magnetic multilayer materials, which is important because it permits the existence of such states without energetically costly Bloch points. Our study is motivated by our preliminary observation of such 3D states in aperiodically stacked multilayers, and by our hypothesis that the same states are responsible for the phenomenon of ultrafast laser-induced topological phase transitions by serving as catalysts to overcome the otherwise prohibitively strong topological energy barriers in 2D systems. Our goal is to validate this hypothesis and to uncover the basic physical properties of discretized 3D partial skyrmions on the way. Our approach is characterized by interdisciplinarity between the fields of materials science (exploring the stability phase space), highly advanced coherent x-ray imaging (resolving the 2D and 3D spin textures), spintronics (confirming the quasi-particle properties of 3D partial skyrmions through their spin-orbit torque driven motion) and ultrafast science (tracking the transient gradient of magnetic properties and the vertical propagation of topological switching after ultrafast laser exposure), and the feasibility of this project arises from the unique expertise of this German-French collaboration. By thoroughly exploring the simplest of 3D topological magnetic quasi-particles, our project will become a milestone in our understanding of the field of 3D magnetism.

• DFG Programme: Research Grants
• International Connection: France
• Partner Organisation: Agence Nationale de la Recherche / The French National Research Agency
• Cooperation Partners: Vincent Cros, Ph.D.; Dr. Nicolas Jaouen