Investigation of magnetization dynamics of topologically non-trivial spin textures

• Applicant: Dr. Lisa-Marie Kern
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546268067

Magnetic π-skyrmions are the prototypical example of topologically non-trivial spin textures, characterized by a single uniform rotation of spins from upward to downward orientation or vice versa, and described by the winding number Q=1. Due to their nanometer scale dimension, their stability at room temperature, and their single-particle character, skyrmions have been envisioned in a variety of application concepts, from racetrack memory to unconventional computing schemes. However, beyond π-skyrmions, a zoo of topological spin textures with higher complexity remains largely unexplored in experiments. So far, both the skyrmionium (=2π skyrmion) and the target skyrmion (=3π skyrmion) have been observed mainly in geometrical confinements, while skyrmion bags with variable topological charge have only been predicted theoretically. Simulations attribute intriguing properties to these textures, such as unhampered straight motion for the skyrmionium in absence of the skyrmion Hall effect or high storage densities for close-packed skyrmion bags, whose motion will no longer be rotational-invariant and may provide new physical insights. In the proposed project, the generation, motion and dynamics of the skyrmionium, the target skyrmion and skyrmion bags will be investigated in ferromagnetic thin films. Anisotropy defects artificially created by focused helium ion irradiation will be used as preferred nucleation sites for the complex spin textures. This project has three objectives: (1) Development of a reliable generation of skyrmionium, target skyrmion and skyrmion bags integrated into the ferromagnetic thin film device at anisotropy-engineered positions. (2) Investigation of stability and motion of these higher-order skyrmions upon external stimuli. (3) Study of the involved transient magnetization dynamics under controlled conditions, interacting with the tailored defect. Additional simulations will predict the dynamics and complement the experimental results. The use of the same host material will allow a direct comparison between these complex kπ-skyrmions and the well-studied π-skyrmion in terms of nucleation parameters, their equation of motion, achievable velocity, skyrmion Hall angle and their magnetization dynamics. Electrical measurements will allow systematic studies of generation and motion parameters to efficiently detect the three types of textures. Subsequently, static and time-resolved high-resolution imaging will yield reliable information on the size and shape of the created textures, their displacement and their intrinsic dynamics. The proposed study will leverage a deeper understanding of topologically non-trivial spin textures in general and identify new routes for combined application concepts.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Geoffrey S. D. Beach