2020 Fiscal Year Annual Research Report
Theoretical study on dynamics of topological magnets
| Project/Area Number |
19F19815
|
|---|
| Research Institution | Institute of Physical and Chemical Research |
|---|
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, グループディレクター (60164406)
|
|---|
| 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
|
|---|
| Project Period (FY) |
2019-11-08 – 2022-03-31
|
| Keywords | skyrmions / micromagnetics / simulations / topology / helimagnetism / spintronics / Nernst effect / scalar spin chirality |
|---|
| Outline of Annual Research Achievements |
In my original proposal for the fiscal year 2020, I planned to use my self-written GPU-accelerated simulation program to study the three-dimensional texture of so-called magnetic skyrmions, i.e., topologically non-trivial whirls in the magnetization and, moreover, how skyrmions move if driven by an electric current, in particular if the surrounding magnetic phase is non-uniform.
Deviating from the original plans, I used my fully self-written simulation program only for a single project which studied the dynamics of the one-dimensionally modulated so-called “helical phase” of chiral magnets. With this very fast code, we were able to explain recent experimental findings which report long-range magnetic order induced by electrical currents. In a subsequent collaboration with a group from Germany, I predict how to generate skyrmionic defects in this helical phase and how to use them as a memory device.
For the studies of three-dimensional textures, I switched to using the open source program MuMax3 but implemented updates based which incorporate features of my self-written code, which yields an even more powerful simulation tool. This combination enabled large scale simulations of three-dimensional magnetic texture which I could use to explain new surface effects observed by the group of Dr. Y. Taguchi at RIKEN.
Moreover, I also started working in a different direction, relating the chirality of the magnetic texture on the non-chiral atomic lattice to novel electric transport signatures.
|
| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
The project in which we studied the ordering dynamics of the helical phase of magnets progressed very quickly thanks to my highly custom and fast self-written code. Thus, we could identify the relevant processes and finish the project. In fact, the results turned out to be so divers that we decided to split the original manuscript into 3 manuscripts with (i) experimental confirmation of the ordering phenomena, (ii) mathematical descriptions of various instabilities that we predict, and (iii) proposals for spintronic information processing devices. At present, only part (i) is published but a collaboration with a group in Germany emerged from this work, which is soon to be finished.
For large-scale three-dimensional textures, I decided to stop updating my own code and used an existing open source code instead which I could modify to suit my needs. This saved me both time in programming existing features of this code which, in addition, are implemented at a much higher level of optimizing as I could implement it by myself. As a result, I was able to very quickly respond to new experimental observations, here a new type of sawtooth-shaped domain wall pattern on the surfaces of S4-symmetric magnets.
In addition, I could finalize a short side project which supports experiments by Dr. M. Hirschberger on the topological Nernst effect of skyrmions. This collaboration was the starting point for other electron transport-related projects which are explained by my modelling on the atomic length scale, i.e., the opposite limit of my other projects.
|
| 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 transport effects induced by magnetic textures. As a side project, I plan to finish the manuscripts on the current-driven dynamics of helical phases in chiral magnets that are left from the already published part of the study.
Given the new material with unique properties that we now have available at RIKEN, new studies on the three-dimensional texture of the magnetization in skyrmions are in the focus of my research. I plan to study the evolution of the texture as one increases the thickness of the material which I expect to lead to topological transitions between skyrmions and their anti-particles, i.e., antiskyrmions.
The third line of research that caught my interest over the course of this financial year is the effect of magnetic fluctuations on the atomic level on the transport properties of electrons but also spin waves. The latter can be detected as they carry energy in the form of heat, leading to non-trivial heat transport phenomena such as the thermal Hall effect. Related measurements have been performed by Dr. Hirschberger at RIKEN. I plan to use my new atomic scale simulations code, used for our previous collaboration, to explain such thermal transport phenomena via the scalar spin chirality, i.e., an atomic scale counterpart of the large-scale topological skyrmion winding number.
|
Research Products
(7 results)
