# 高等專題討論二 期末報告
###### tags: `學習` `完成` `高等專題討論二`

**Speaker**: `Prof. Pin-Jui Hsu 徐斌睿`
**Author**: `D06222003` `Kuan-Chia Chiu 邱冠嘉`

## Title

### Imaging and Manipulating Individual Magnetic Skyrmions by Spin-Polarized Scanning Tunneling Microscopy

## The content and My Understanding
### Introduction and Motivation
Magnetic skyrmions are a collective behavior of spins in a material formed by a domain wall. [^1] 
In general, domain wall is the boundaries of different magnetization directions. As the figure above, we can find there are two different direction (up and down). Then, the transition boundary is so-called "**domain wall**". The importance of domain wall is because it is easy to observe in many magnetic-related measurement, such as "**Magneto-optic Kerr effect**" (MOKE)
, "**X-ray magnetic circular dichroism**" (XMCD). Also, it is used to record in early industrial magnetic recording device.

In case of skyrmion, the domain wall is not formed as "edge type" as [^1] shown, but "**circular shape**" [^2].
The intrigue part of skyrmion is its movement in a electric field. It is possible to reduce the critical electric field applying in material which is easy to write information in some magnetic recording device using skyrmions.

### Experimental Results
In Prof. Hsu's work [^3], they demonstrate local electric fields can be used to switch reversibly between magnetic skyrmions. In others research, it is mostly used a in-plan electric field to drive skyrmion. However, Prof. Hsu used unconventional method: using a tip of spin-polarized scanning tunnelling microscopy (SP-STM) to erase or write a skyrmion on Fe triple layer. Besides, it can also move a distance when it is both applying electric (in-plan) and magnetic field (out-of-plan). This work may be a good try to manipulate the domain wall easily.

In the other part of this talk, Prof. Hsu demonstrated how the magnetic parameters and the corresponding magnetic period length in ultrathin Fe films may be tuned by hydrogen exposure. By SP-STM measurements performed on and Fe-DL on Ir(111) revealed two hydrogenated phases with different atomic structures. While the H2-Fe structure is ferromagnetic, H1-Fe structure displays a spin spiral ground state at zero field and skyrmions with a unique rotational sense in the presence of and applied field.[^4]

## My experience after this talk
After this talk, I had seen a new way to manipulate domain wall. It may also related to my interest. I planed to find some related article which pumps pure spin current to some material with skyrmions. Because most researches focus on electric field to change the domain wall of skyrmion or move the skyrmion. So, I may wonder if pure spin current can influence the skyrmion. In my experience, the magnetization in devices can be change by both electric field and spin-transfer toque. I thought it is similar to case of skyrmions. It may be a interesting topic.

_P.S. This report is edited by "**HackMD**", and for better readability you can check [here](https://hackmd.io/s/BJZmkP1zm#)_

## Reference

[^1]: Schematic Diagram of "**domain wall**" from [energetic forum](http://www.energeticforum.com/282153-post70.html)
[^2]: Schematic Diagram of "**skyrmion**" form [wikipeida](https://en.wikipedia.org/wiki/Magnetic_skyrmion)
[^3]: P.-J. Hsu, A. Kubetzka, A. Finco, N. Romming, K. von Bergmann, and R. Wiesendanger, "Electric field driven switching of individual magnetic skyrmions", Nat. Nano., **12**, 123-126, (2017)

[^4]: P.-J. Hsu, L. Rózsa, A. Finco, L. Schmidt, K. Palotás, E. Vedmedenko, L. Udvardi, L. Szunyogh, A. Kubetzka, K. von Bergmann, and R. Wiesendanger, "Inducing skyrmions in ultrathin Fe films by hydrogen exposure", Nat. Commun., **9**, 1571, (2018)