Motion and Dynamics of Nanoscale Skyrmions in Ferrimagnetic Multilayers

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Magnetic skyrmions are topologically protected chiral spin textures with novel characteristics which are highly promising for future data storage and information technology applications, such as so-called racetrack memories. During the past years, these swirling magnetic patterns have been discovered and extensively studied in a wide range of materials. Very recently, experimental studies have confirmed the stabilization of skyrmions in magnetic multilayers even at room temperature and above. However, there exist fundamental limitations to the practical realization of skyrmion-based devices. In particular, skyrmions in ferromagnets are subject to an undesirable topological effect, the skyrmion Hall effect, which causes a transverse motion with respect to the current flow direction and thus can result in a strongly reduced propagation velocity or even in the annihilation of skyrmions at the device edges. In consideration of these limitations, the performance and reliability of a skyrmion-based memory device would not be competitive. However, first theoretical and experimental works have already demonstrated a significant reduction of the skyrmion Hall effect in ferrimagnetic multilayers, where the angular momentum is compensated. At this point, research is still in its infancy and there is a great need for systematic experimental work on ferrimagnetic multilayer systems of different compositions. Therefore, the main goal of this research project is to engineer skyrmions to move at velocities exceeding 1000 m/s and reducing their spatial extent to 10 nm or less. For this purpose, a detailed understanding of the skyrmion velocity dependence on the size, current density, material composition and concentration of defects will be developed. In addition, the dynamic excitations of skyrmion states in ferrimagnetic rare earth/transition metal composite materials will be investigated by means of broadband microwave absorption spectroscopy, spin-torque ferromagnetic resonance and spatially-resolved Brillouin light scattering spectroscopy. The results will provide valuable information about the underlying magnetic interactions and thereby allow for a further optimization of the multilayer design with regard to the realization of competitive devices. In this respect, novel techniques and protocols for a reproducible generation and reliable detection of individual skyrmions will be developed.