Investigation of the propagation of isolated magnetic skyrmions

• Applicant: Professor Dr. Felix Büttner
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 288444881

• Final Report Year: 2018

Final Report Abstract

The goal of this project was to understand the physics at the interfaces of ultrathin magnetic layers and non-magnetic materials with large spin-orbit interactions. In these systems, chiral interactions can emerge and lead to the formation of topological magnetic quasi-particles, so-called skyrmions. Furthermore, other interface effects can be used to drive these skyrmions in the material. The small size, large stability, and the high mobility of these skyrmions makes them very attractive for future data storage technologies. It was the proposed goal of this project to find ways to deterministically nucleate (write), annihilate (delete), and move (process) skyrmion bits and to demonstrate these operations in a prototype device, a so-called racetrack memory. The two most important ingredients for a skyrmion racetrack memory, (i) creation and (ii) motion, were demonstrated exactly as predicted in the original proposal. Skyrmion nucleation can be done in a simple integrated device, where the racetrack has a small notch. The process is ultrafast (sub-nanoseconds) and already meets energy requirements of commercial devices. Backand-forth skyrmion motion could be reproduced exactly several tens of billions of times in the same material, which again is sufficient for a prototype device. The key remaining issue, however, is not deleting the skyrmions. It is making them smaller (sub-10 nm) and faster (>1000 m/s) at room temperature. By analytically solving all energy integrals of magnetic skyrmions, including the decade-old challenge of stray field interactions, I could show that all ferromagnetic materials are fundamentally incapable of reaching these challenging goals. At the same time, my model allowed me to predict a phase pocket for such demanding technological goals in materials with antiferromagnetic exchange interactions, in particular in ferrimagnets. Concluding this project, I have obtained preliminary proof of such ultrasmall skyrmions in ferrimagnetic GdCo by x-ray holographic imaging.

Publications

• Accurate model of the stripe domain phase of perpendicularly magnetized multilayers. Physical Review B 95, 174423 (2017)
  Lemesh, I., Büttner, F. & Beach, G. S. D.
  
• Field-free deterministic ultrafast creation of magnetic skyrmions by spin–orbit torques. Nature Nanotechnolgy 12, 1040–1044 (2017)
  Büttner, F. et al.
  
• Skyrmion Hall effect revealed by direct time-resolved X-ray microscopy. Nature Physics 13, 170–175 (2017)
  Litzius, K. et al.
  
• Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications. Scientific Reports 8, 4464 (2018)
  Büttner, F., Lemesh, I. & Beach, G. S. D.