Magnetic skyrmions have so far been treated as two-dimensional spin structures characterized by a topological winding number. However, in real systems with the finite thickness of the device material being larger than the magnetic exchange length, the skyrmion spin texture extends into the third dimension and cannot be assumed as homogeneous.
A 3D reconstruction of a skyrmion derived from X-ray images. Credit: Berkeley Lab
Researchers at Lawrence Berkeley National Laboratory, Swiss Light Source (Paul Scherrer Institute) and Western Digital Research Center have used soft x-ray laminography to reconstruct, with about 20-nanometer spatial (voxel) size, the full three-dimensional spin texture of a skyrmion in an 800-nanometer-diameter and 95-nanometer-thin disk patterned into a 30× [iridium/cobalt/platinum] multilayered film.
A quantitative analysis finds that the evolution of the radial profile of the topological skyrmion number is nonuniform across the thickness of the disk. Estimates of the micromagnetic energy densities suggest that the changes in topological profile are related to nonuniform competing energetic interactions.
Peter Fischer, a senior researcher at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), led this project to make 3D X-ray images of skyrmions that can characterize or measure the orientations of spins inside the whole object.
Magnetic skyrmions can be thought of as spinning circles of magnetism, explains David Raftrey, a student researcher in Fischer’s team who was the lead author of this study. At the center, the magnetic spin is pointing upward, while moving out from the center, the magnetism twists and pulls in a downward direction. What’s more, skyrmions are stable, small, fast, and not easily unfolded, a trait materials scientists call “topological.”
These spin directions are part of the appeal for skyrmions because they might be used to carry and store information in much the same way that electrons carry and store information in current devices. “However, relying on the charge of the electron, as it is done today, comes with inevitable energy losses. Using spins, the losses will be significantly lower,” Fischer said.
But theoretical knowledge of skyrmions has been based on descriptions of them as 2D objects. In the real world of electronics and silicon wafers — no matter how thin — skyrmions have to be dealt with as 3D objects. To put skyrmions to work, or perhaps to one day synthesize custom skyrmions, researchers must be able to examine and understand their spin characteristics throughout the whole 3D object.
If you are looking at a skyrmion magnetic whirlpool from the top and start slicing off layers, you might think that each successive layer would be the same. “But that’s not the case,” Raftrey said. “And we said, okay, how can we get our arms around this? How do we actually demonstrate this?”
Raftrey took a thin magnetic layer, which was synthesized by colleagues from Western Digital, and patterned a nanodisk using the Molecular Foundry’s nanofabrication facility. To obtain 3D tomographic images he traveled to Switzerland to use a novel imaging technique called magnetic X-ray laminography at a microscopy beamline at the Swiss Light Source.
With X-ray laminography, “You can basically reconfigure and reconstruct [the skyrmion] from these many, many images and data,” Raftrey said. It was a process that took months, finally yielding a better understanding of skyrmion spin structures.
A full understanding of skyrmions’ 3D spin texture “opens opportunities to explore and tailor 3D topological spintronic devices with enhanced functionalities that cannot be achieved in two dimensions,” Fischer said.
The teams' results provide a foundation for nanoscale metrology for spintronics devices using topology as a design parameter.