An EMPIR project provides new insight on topologically-protected spin structures

Potential to form improved memory storage and novel microwave devices: Skyrmions, nanosized magnetic vortexe.

Topology is the study of the properties of geometric configurations which are unaltered by certain elastic transformations such as a stretching, bending or twisting. In recent years research in this area has led to the discovery of magnetic skyrmions – which are nanosized (≤ 100 nm), vortex-like areas of magnetism that often stand out perpendicular to a material surface. Magnetic skyrmions are relatively stable due to protection from the local topology and hence are also known as “topologically-protected spin structures” or TSS.

If skyrmions can be controllably created or erased, it raises the possibility of using these in new, highly compact forms of electronic memory where their presence or absence are equivalent to ‘1’ or ’0’ in conventional computers.

An important factor in skyrmion formation is what is known as the Dzyaloshinskii-Moriya interaction. This can be detected by optical means taking advantage of certain light-matter interaction effects, such as the Magneto-Optical Kerr Effect (MOKE) or Brillouin light scattering (BLS).

However, despite intensive research in this field several high-level metrology requirements are still lacking before these structures can be used in novel devices. This includes validated measurement methods enabling the identification and manipulation of multiple and individual TSS.

Completed EMPIR project Metrology for topological spin structures (17FUN08, TOPS) addressed many outstanding measurements on TSS, helping to identify key parameters that determine the formation, size, and stability of TSS.

World firsts

Little work has been performed to compare measurement techniques for TSS on the same material. During the project the first international round robin comparison on the Dzyaloshinskii-Moriya interaction was performed amongst five partners on MOKE and BLS. The comparison revealed that these techniques consistently returned different results. A member of the consortium will be presenting the key points from this project comparison at the international Intermag Conference in May 2024.

Real-world use, in things such as improved memory systems, requires the manipulation of individual skyrmions. To this end the project undertook a large variety of thermoelectric and transport measurements and could demonstrate individual skyrmion manipulation by local magnetic field gradients. Additionally, the signature of individual skyrmions could be demonstrated in thermoelectric measurements.
Reproduceable writing and deleting of skyrmions will advance their use in information storage applications. The project studied, for the first time, the dynamics of the so-called ‘metastable skyrmion phase’, where skyrmions exist in an extended parameter range at room temperature, in a bulk material. In these measurements both optically-induced creation and deletion of skyrmion patches has been demonstrated.
Ferromagnetic resonance measurements have been used to obtain new insight into the dynamical properties of skyrmion lattices. Such information is a prerequisite for using skyrmions in microwave applications.

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Other project results.

Skyrmion properties have been investigated using very different experimental tools such as quantitative magnetic force microscopy, scanning transmission x-ray microscopy, etc.
The project developed software for analysis of Dzyaloshinskii-Moriya interaction using Magneto-Optical Kerr Effect which is now publicly available.
The project published over 20 articles in peer reviewed journals which are available on the project's webpage many of which were in prestigious journals.

The coordinator of the TOPS project Mark Bieler (PTB) said about the work: “This project nicely highlighted the importance of metrology in basic science. On example is the Round Robin on the measurement of the Dzyaloshinskii-Moriya-Interaction (DMI) constant, that created awareness about the reliability and unreliability of such measurements in the scientific community.”

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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