Skyrmion topology quantified in 3D

Magnetic skyrmions are particle-like topological solitons, identifying a perpendicularly-magnetized region of opposite orientation with respect to its surroundings, separated by a chiral domain wall. Skyrmions are distinguished from their topologically trivial counterparts (magnetic bubbles) through the calculation of the skyrmion (or winding) number: 0 for a topologically trivial bubble, and 1 for a skyrmion.

The description above assumes that skyrmions are intrinsically 2D objects. However, real systems, especially those of interest for the development of 3D spintronics, exhibit thicknesses of the magnetic material that cannot be neglected. There, it cannot be assumed that the 2D magnetic configurations simply extend rigidly to the third dimension, and the access to the third spatial dimension opens therefore opportunities to explore and tailor 3D topological devices with functionalities not accessible in 2D. 

A prerequisite for the investigation of 3D magnetic and topological configurations is advanced characterization techniques able to investigate the 3D configuration at the nanoscale. In this research work, published within the journal Science Advances, an international collaboration of researchers from the United States of America and Switzerland has imaged the three-dimensional magnetic configuration of a magnetic skyrmion in 3D by means of soft X-ray magnetic laminography. The measurements were performed at the PolLux endstation of the Swiss Light Source, and a voxel size of 20 nm was achieved in the reconstructed 3D images. The data allowed for the calculation of the 3D distribution of the topological charge density of the magnetic skyrmion, allowing the researcher team to observe variations in the topological charge density, and to directly correlate them to variations in parameters such as the magnitude of the magnetic anisotropy and of the Dzyaloshinskii-Moriya interaction in the magnetic material. The experimental images were then directly compared with micromagnetic simulations using a similar voxel size.

The work yielded two main results: the first is the demonstration that the soft X-ray magnetic laminography imaging technique has reached the level of maturity necessary to probe magnetic systems at 3D resolutions comparable to those used in micromagnetic simulations, allowing for a direct comparison between the two. The second is providing the foundations for nanoscale magnetic metrology, aimed at future tailored spintronic devices where the topological charge density of the device can be tailored at the nanoscale.