Spintronics: Soft X-ray microscopy unravels the nature of domain walls

Experiments at BESSY II allow to observe the formation of skyrmions in ferrimagnetic thin films of dysprosium and cobalt.

Magnetic skyrmions are tiny vortices-like of magnetic spin textures that - in principle - can be used for spintronic devices, for example very fast and energy-efficient data storage devices. But at the moment it is still difficult to control and manipulate skyrmions at room temperature. A new study at BESSY II analyses the formation of skyrmions in ferrimagnetic thin films of dysprosium and cobalt in real time and with high spatial resolution. This is an important step towards characterising suitable materials with skyrmions more precisely in the future.

Isolated magnetic skyrmions are topologically protected spin textures that are in the focus of research interest today, also because of their potential applications in information technology. Skyrmions of particular interest occur in ferrimagnetic rare earth-transition metal (RE-TM) materials. They exhibit tunable ferromagnetic properties with antiferromagnetically coupled sublattices. By choosing elements from the rare earth and transition metal group, they provide a playground for controlling magnetisation and perpendicular magnetic anisotropy, which are key parameters for stabilising topological ferrimagnetic textures.

One class of ferrimagnetic alloys has a stronger perpendicular magnetic anisotropy, including a compound of dysprosium (Dy) and cobalt (Co). These materials could store information in a much more stable way, but their magnetic properties and structures have hardly been studied so far. A team led by HZB physicist Dr. Florin Radu has now analysed DyCo3 samples using X-ray microscopy methods at BESSY II and determined the spin structures.

They used scanning transmission X-ray microscopy with both X-ray magnetic circular dichroism and X-ray magnetic linear dichroism as element specific contrast mechanisms. The key feature exploited here is that the linear dichroism of RE materials is much stronger than that of the TM materials. “This allowed us to directly observe isolated ferrimagnetic skyrmions in high density and to accurately determine their domain wall type," Radu reports. The results show that the ferrimagnetic skyrmions are of the Néel type and can be clearly distinguished from the other domain walls, the Bloch walls. Thus, for the first time, the type of domain walls can now be reliably determined by X-ray investigations. This is an important step towards the application of this interesting class of materials for real spintronic devices.

Published in Communications Physics (DOI: 10.1038/s42005-023-01341-7 )

https://www.nature.com/articles/s42005-023-01341-7

Image Caption: The two upper rows show the expected magnetic image contrast for Bloch type and Néel type skyrmions when using circular, linear horizontal (LH) and linear vertical (LV) polarized x-rays. The results of experimental scanning transmission x-ray microscopy (STXM) are shown in the lower row, they ressemble the simulation of Néel type skyrmions. (Credit: Florin Radu/HZB)