2019 Fiscal Year Annual Research Report
Theoretical study on dynamics of topological magnets
| Project/Area Number |
19F19815
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| Research Institution | Institute of Physical and Chemical Research |
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Principal Investigator |
永長 直人 国立研究開発法人理化学研究所, 創発物性科学研究センター, 副センター長 (60164406)
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| Co-Investigator(Kenkyū-buntansha) |
MASELL JAN 国立研究開発法人理化学研究所, 創発物性科学研究センター, 外国人特別研究員
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| Project Period (FY) |
2019-11-08 – 2022-03-31
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| Keywords | スキルミオン / トポロジカルなスピン構造 |
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| Outline of Annual Research Achievements |
In my original proposal for the fiscal year 2019, I planned to study the lifetime of tiny whirls in the magnetization, called “skyrmions”. I wanted to start developing a new numerical technique which was supposed to combine Monte Carlo sampling of decay paths with minimal energy path calculations (MC+GNEB). The goal was to more accurately calculate the lifetime of skyrmions, which is a complex problem, and, for the first time, test the validity of an often-used, simple method. Moreover, I planned to investigate the lifetime of skyrmions in new setups that had never been considered before.
However, I did not achieve any of the defined goals. I did not manage to develop the MC+GNEB method in time. In February 2020, another group has published a paper where they did almost precisely what I had planned: They compared results from an elaborate path sampling algorithm to the simple methods and the agreement turned out to be good. Moreover, I started to upgrade my simulation package, which I use for transition path calculations, to be able to run on GPU accelerated hardware. After first tests, the software indeed showed an extreme speedup but other projects seemed more interesting and therefore the implementation of the transition path calculation is not yet completed.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
As described above, large parts of one major pillar of my work were in the meantime published by another group, making this part of the project significantly less attractive. The excellent research environment in RIKEN, however, made me soon start other interesting projects and join existing projects, all of which feature experimental support.
I could finish a collaboration with the group of Karin Everschor-Sitte from the University of Mainz in Germany. We studied theoretically the stability of current-driven skyrmions which revealed a new type of instability. The preprint was finished just after the end of the fiscal year.I was invited to write a chapter for an upcoming Springer book on the dynamics of magnetic solitons, such as skyrmions and domain walls. I finished also this work just after the end of the fiscal year.
Recent experiments by Dr. X. Yu in RIKEN revealed the projected magnetization texture of topological defects in skyrmion tubes. Only with simulation results of my newly developed code, it was possible to explain the experimental finding. Also, this project was just finished after the fiscal year.
I showed that the helical phase of chiral magnets can be manipulated by electric currents and that the dynamics in the bulk of the sample are completely different from the dynamics which are imposed by the edges. Moreover, my theoretical results seem to match the experimental observations by Dr. X. Yu.Moreover, I could contribute with my theoretical support to even more experiments of which some will also be finished in the fiscal year 2020.
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| Strategy for Future Research Activity |
In the future, I plan to focus my work more on the two very appealing new projects: The three-dimensional texture of magnetic skyrmions and the current-driven dynamics of helical phases in chiral magnets.
The three-dimensional texture of skyrmions is still an open question due to the pack of proper experimental techniques. Various attempts have been made in the past to unveil the texture but none was able to provide sufficient resolution or only two-dimensional projections are measured which do not uniquely clarify the three-dimensional magnetization. I plan to improve the existing experimental technique based on our previous project, continuing the great collaboration with Dr. X. Yu.
Moreover, our present results for the dynamics of current-driven helical phase show already a plethora of interesting features, that have not been reported so far even though the helical phase is thought to be much simpler and experimentally more easily accessible than skyrmions. I plan to continue my study on this project to clarify the nature of the dynamics and their potential use for applications.
Finally, I plan to continue my collaborations within RIKEN which, already, turn out to be very fruitful. Nevertheless, besides these new projects, the transitions from topologically non-trivial skyrmions to trivial phases are still an interesting subject that I plan to investigate in the future, however, with less priority than the two new main projects.
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