Author Archives: Dongwook

Young Researcher’s Workshop

In this week, I participated in SPICE Young Research Leaders Group Workshop, which was organized by Alex and Helena. It was held in Ingelheim, a charming village located not so far from Mainz and famous for many wineries in the neighborhood. As the workshop title says, only junior-level researchers could join the workshop, and the participants were mostly postdocs and junior/assistant professors. The workshop provided a unique opportunity not only to discuss recent progress in magnetism in depth, especially from the point of view of young researchers, but also to get to know each other and build a strong connection that goes beyond “collaborators”. During the workshop, we chatted on many different topics, from funny and pleasant things to more serious stuffs such as career/job and frustrating and challenging aspects in academia.

Sure, scientific contents of the workshop was undoubtly excellent, and many questions were raised in every talk. Also, listening to talks from other junior colleagues gave me nice stimulus and inspiration.

On my side, I presented the usual “orbitronics”, with emphasis on main general ideas, opportunities, impact on other fields, and future prospects. The slide that I presented can be downloaded from this link. I think my talk went pretty well although I slightly went overtime. Most questions raised were serious enough and important, for which I do not have clear answer either, but it was very good to openly discuss (instead of pretending to know…).

I hope I can participate in the nexst workshop too, if possible, but I think it’s fair for other young researchers to have a chance as well. In any case, I hope the workshop continues to occur every year.

DFT lectures

JGU Mainz campus

In the last week, just before the Easter holidays, I visited Mainz for giving lectures on the density functional theory. It was an intensive course style, rather than a broad introduction over a semester, and I dealt with lots of practical aspects. I skipped all formal stuffs such as the Hohenberg-Kohm theorem and the Kohn-Sham formulation. I think these are basics that PhD students should know already (especially if one is planning to do theory). So, I started from the discussion on the choice of basis sets (e.g. plane wave, localized, augmented plane wave, etc.) and spent most of the time on the properties of the LAPW basis, which is the implementation that I use within the FLEUR code.

My motivation was to teach that an “optimal” choice of computation parameters is not really something that can only be learned only “by practice” (still required though) but something that naturally follows by understanding properties of the basis set. If one first encounters a DFT code that implements the FLAPW method, it is quite often that one is overwhelmed by so large number of parameters that has to be set. Well, the accuracy of the method comes at a cost of sophisticated parametrization. But if one understands the propertiees of the LAPW basis and its implementation, it becomes quite obvious that only a few paramters are really essential and the rest of them naturally follow.

I think my lecture sent quite well, and people seemed to be satisfied. Well, this has been only the first part, and the second part will continue in 2 weeks. My goal is for everyone to be able to calculate the anomalous Hall effect and spin-orbit torques at the end of the lecture series 🙂

Trip to Marseille

In the previous week, I visited a colleague of mine in Marseille. Although I have been to several workshops within Germany in recent years, I just realized that this was my first busienss trip to a foreign country since 2020, which excited me a bit. As I am trying to be more responsible for my consequences on the environment, I took trains from Aachen to Marseille, instead of taking airplanes. It took roughly 10 hours. But it wasn’t that bad as I had an internet and I could work during the trip.

The place where I visited was CINaM. It is located a bit far from the city center, but it’s surrounded by the beautiful nature. The institute covers wide area of “nanoscience”, which include chemists, material scientists, and physicists, etc. So, it was a bit challenge for me to give a seminar in front of people with various interests and backgrounds (but it went well, I think).

I spent effectively 4 days in Marseille, and I had great time there. Sure, scientific discussion we had were fantastic. It was so great to discuss in depth, which have totally missed during covid times. I got quite impressed that we discussed for 3 hours with 15 slides. This somehow reminded me of my PhD time. It was also a kind of small group, and we discussed quite intensively (still relaxed though) without worrying much about the time in group meetings. In fact, the atmosphere of the group was really similar to my former group!

By the way, the members of the group took me out for dinner and drinks in the evenings. I must admit that everyone was so nice to me, and I am quite happy that I had a chance to know nice people. We discussed not just science but also our lives and many other stuffs. Thanks to them, I could try some local restaurants with great food, away from tourists. Sure, I was very glad that I could try nice fresh seafood, which is hard to find in Germany.

Port area of Marseille in the night

I got covid

Finally, covid has come to me. I started to cough and have headache. For a few days in the beginnin, all the self-tests that I performed were negative, but I was quite certain that this is covid because it has been around since a few weeks ago, at the workplace and accoreding to the contact-tracing app (Corona-Warn). I got my first positive result, 3 days after I got symptoms. So, I could get a PCR test (which also showed positive).

The first thing that I did was to tell my friends because I hanged out with them a day before I had symptoms. Fortunately, nobody got it after me. Then, I decided to live and sleep in the livingroom in my flat so that my partner does not get covid because of me. Fortunately, she didn’t get covid yet, so I guess this strategy has been successful so far.

I somehow felt that I would get covid quite soon. The omiron variant is quite invasive to the immune system despite multiple vaccinations. The infection rate has been quite high in NRW Germany in the last months. Also, I have travelled Ireland recently, and hanged out in bars without wearing masks with my friends. Well, I wasn’t cautious enough as in the beginning of the lockdown. Probably I am getting tired of this constraint social life, or simply because I got bolder as I got a booster shot.

Fortunately, my symptoms have been very mild except for the first 1-2 days. I feel completely normal although I get tired easily. Today I felt much better in the morning and it’s been already 4 days, so I did test. But the result was so obvious positive yet. On this Wednesday, it becomes officially a week, when I can be free from the isolation if I get tested negative.

I just spend my days working at home. In the evening, I suddenly have a lot of time alone, so I also try to read many papers. I am particularly into old, milestone papers nowadays.

Vacation in Ireland

In the last weekend, I visited Ireland for a short vacation as covid restrictions were mostly lifted. I had great time there. To be honest, I didn’t know much about the country. Thanks to the trip, I had a chance to learn the people, their land, and their culture. At the end, I fell in love with Irish culture. Pubs in every cornerof the street, which also serve very nice Irish specialties. Inside pubs, it’s quite noisy and full of friendly people and I could also hear traditional Irish music and songs. But once I went outside of the busy urban area, I discovered wonderful nature that they have, from wide green field, where you can easily spot several cows and sheep, and breathtaking views of cliffs and coastlines. Although this trip was quite short, I will definitely visit this country again and spend more time.

Dublin city center

Trinity college and long room library

Amazing view of Moher cliff and west coast line

Galway, a cultural capital of western part of Ireland

Sure, one cannot miss their favourite sports, rugby, and great Irish beers.

Why I like giving seminar talks in small groups

In 2022, so far I have given three group seminars, roughly one per month. I think all of them went quite nice, not only for delivering the knowledge but also for exchanging ideas. What I like in particular was “friendly atmosphere” in which everyone asks questions and shares opinions freely (Probably it’s because they are in their home ground?). Many of them readily shared their preliminary data and asked me for my comments even though I am not sure if I am really helpful or not. Anyway, for this reason, a seminar talk often works as a trigger for the initiation of the collaboration.

I enjoy giving a talk because arranging various concepts and topics in a coherent manner gives me a fresh look on the subject that I thought I am quite familiar with. Also, each research group has different interest and strength. This means that I tend to look at the topic from their point of view, and I often discover new perspectives.

But preparation of a seminar takes at least a half to one day of working hour, and a new collaboration project starts. Well, I think I am quite swamped by too many projects, but at the same time, I am also always excited to learn new things by working with other groups. At least, I am trying to secure my private time for my partner and hobby…

I still enjoy dirty works in science

Nowadays I spend more time in supervising someone than in my own project, and I am also involved in many other collaboration projects. A good thing is that as I discuss on various topics in spin-orbitronics, this really helps me to have a broader and comprehensive perspective on the field. However, I cannot spend too much time in every projects, and I rather try to understand the gist of ideas and motivations. As human brains are not developed for multi-tasking, situation of having multiple projects simultaneously make me exhausted. Importantly, it seems to take away a “fun” part of doing science. In my experience, even for a seemingly boring project, at the end I find a lot of interesting aspects as I start to understand more things that are not so obvious from the surface. Often we get a result that goes against our intuitiion and we are surprised by that. After spending some time, there is a moment when everything perfectly makes sense.

In this week, I had a chance to work on a tight-binding model and symmetry reduction of the Hamiltonian. In the beginning, this has been done by my master student, but he found that the final result does not make any sense: We have seen a net orbital magnetic moment even though the system posesses the time-reversal symmetry. Quite often, this signals that there must be a rather dumb mistake. Since it’s quite difficult to find a mistake from one’s own notes, I decided to work on the problem independently from the master student. To be honest, I was a bit impatient in the beginning as I had many other works which have priority. But once I started to work on it, I forgot about it and started to enjoy doing science. Especially, now that I knew most of the technical details, we could have nice discussion on the same page (so far, quite often I only checked the final result and see whether it makes sense in terms of symmetry or microscopics, etc.). At the end, we end up having hours of stimulating discussion, although I left a dirty part of the calculation to the student.

This little moment in this week started to make me realize that doing science is not only about making general statements. Every scientific arguments should be based on solid data, which requires understanding of technical side of the work as well. Details and technical aspects really matter in many cases, if you want to really understand and “feel” a physical mechanism that we study. Sometimes it’s frustrating, but I do enjoy dirty works in science, still!

Recording a conference presentation

In the last week, I recorded my presentation for the upcoming Joint MMM-Intermag conference. I was invited to give a talk in the symposium Frontiers of Orbital Physics: Statics, Dynamics, and Transport of Orbital Angular Momentum, and my presentation is supposed to cover theoretical aspects of the orbital current, which is my main research expertise.

In the beginning of virtual conferences due to travel restriction with the surge of COVID, I clearly remember that I struggled for more than 5 hours to record a 30 min presentation. There were many reasons.

First, most importantly, it felt very strange to talk in front of my camera with no audience. This is something I am not completely used yet. Even in live online seminars, I do not see people’s faces anymore as the slide show starts unless I have a second monitor. I think I just got used to it by time although I’m not sure if my presentation improved or not. As usual, in every presentation, I try to apply the two simple rules: (1) Connect with audience first before I strat my story, (2) Be myself such that I truely enjoy developing scientific thoguths. Well, at the end, we do what we can.

The second difficulty was more about technical side. I wasn’t used to using any software for the screen recording. In the beginning of the pandemic, I also somehow noticed that conference organizers were also a bit lost and were not sure which option is the best. After 2 years, most conferences now provide a very clear guideline and tips for recording. For me, I do not use any fancy software. I simply use OBS, which is a free and open-source software. Importantly, it supports all different kinds of platforms, including linux. Then, I use QuicktimePlayer to trim the video.

Even after I successfully record my presentation, what I am not sure is how people would react to my presentation. Some conferences provide a comment section to make it more interactive, but it didn’t seem to work quite well. Most people, including myself, seem to be less active. In offline conferences, one can easily talk to the speaker after the presentation even if one misses a chance to ask a question at the end of the talk. In many previous conerences, I found it super nice to have a long conversation with colleagues in the coridoor after the session.

It’s not easy. Recording a presentation takes more time than preparing for a live presentation, and it is much less effective when it comes to exchanging knowledges and having networks. But we do what we can for now until we overcome the difficult time.

Two more experimental papers on the orbital torque

This month, two experimental papers, in which I also contributed, have been published:

  • Soogil Lee, Min-Gu Kang, Dongwook Go, Dohyoung Kim, Jun-Ho Kang, Taekhyeon Lee, Geun-Hee Lee, Nyun Jong Lee, Sanghoon Kim, Kab-Jin Kim, Kyung-Jin Lee, Byong-Guk Park
    Efficient conversion of orbital Hall current to spin current for spin-orbit torque switching
    Communications Physics 4,234 (2021)
  • Dongjoon Lee, Dongwook Go, Hyeon-Jong Park, Wonmin Jeong, Hye-Won Ko, Deokhyun Yun, Daegeun Jo, Soogil Lee, Gyungchoon Go, Jung Hyun Oh, Kab-Jin Kim, Byong-Guk Park, Byoung-Chul Min, Hyun Cheol Koo, Hyun-Woo Lee, OukJae Lee, Kyung-Jin Lee
    Orbital torque in magnetic bilayers
    Nature Communications 12, 6710 (2021)

These papers propose ways to experimentally measure the orbital current contribution to the torque on the magnetization in nonmagnet/ferromagnet heterostructures: Cr/(Pt)/FM in the first study and (Ta,Pt)/FM in the second study, respectively. The main idea is if there is an orbital Hall effect in the nonmagnetic layer, the measured torque will substantially deviate from the theoretical prediction based solely on the spin Hall effect contribution. This can lead to gigantic enhancement (first paper) or a sign change (second paper) of the torque efficiency compared to a case when there is only a spin-injection contribution to the torque.

Both experiemnts have been performed in the beginning of 2019 and the key reuslts were obtained during the summer in 2019. Around that time, the idea of “orbital torque” was still very new idea for us and we did not have good intuition on how the orbital torque would behave and how large it would be. It is because the only theoretical study available was a case study on a tight-binding model that I have used in the original orbital torque paper.

Fortunately, that was the time when I came to Forschungszentrum JĂĽlich as a postdoc in order to develop a code and perform first-principles calculations of the orbital torque. Ideally, I wanted to investigate Cr or Ta, which were used in the experiments, but I did not have good experience in the simulation of real materials yet, and I was not sure which structure I should assume (it wasn’t clear from the experiment either). Instead, when I just arrived at JĂĽlich, I heard from my colleagues that they were already investigating Fe/W(110) because of on-going collaborations with STM and ARPES group. Thus, I decided to take an W(110) film as a “prototypical example”. It was a good choice because W has been predicted to have the opposite signs for the orbital and spin Hall effects, which is exactly what we wanted. Detailed computational study took more than I expected but we ended up publishing a very good and well-explained new method + new result theory paper (at least in my opinion). In this study, we have formulated a general theory of angular momentum transfer when a spin-orbit coupled system is driven out of equilibrium and reach a steady state by an external electric field and have shown from first-principles implementations that a competition bewteen the spin injection and orbital-to-spin conversion in the ferromagnet can compete each other in the dynamics of the magnetic order parameter. I was very glad that the speculation that I have proposed in the preivious was demonstrated in real materials.

However, in the meantime, I and the experimental team thought that it would take too long if we wait for the theoretical support and decided to submit the experimental paper although a theory backup is not complete. That is why in the second paper, a theoretical analysis presented in the Supplemental Material does not use an up-to-date method that has been developed during 2019-2020. It rather presents “the best thing that we we could do at that time”, which is not exact but at the same time not too wrong.

On the other hand, the experimentalists who led the first paper decided to spend more time in order to design more reliable experiments to draw a conclusion with less ambiguity. Since a similar study was already submitted (the second paper), we had to make one/two step further in order to claim the originality of the work. As usual, it took quite longer than we planned, but fortunately that gave me more time to perform theoretical calculation. This situation made one-to-one comparisions between the theory and experiment possible. During this process, I truely enjoyed listening to opinions from the experimentalists and designing a logic that can verify their hypotheses from my theoretical calculations (computer simulation is at the end a kind of “virtual” experiment, so designing a logic is very important), which is otherwise not so easy to check in experiments alone.

On the experimental side, we also tried to measure the magnetic torque in various different ways. Although the quantiative results can be slightly different depending on the method, we checked that the main conclusion can be generally drawn independently of the measurement method. Also, many possibilities that can lead to an artifact have been rasied, which we checked thoroughly to make sure.

As a result, although the initial data was obtained in the summer of 2019, it took us more than 2 years to have the work published. It was because these papers report the orbital Hall torque in transition metal systems for the first time, and this led to lengthy peer review processes. Most of the criticisms have been very helpful in the sense that they made us look at the data in a more objective point of view (which is indeed the most crucial role of the peer review in science). At the end, this made our paper more convincing and well-recognized in the community. I believe that it is just our first step at the entrance of the area “orbitronics”. I don’t know exactly what we will encounter in the future (though I have my own speculations). I would be happy to see the progress of the field and catch the holy grail of the orbital current in the near future.

Orbitronics review paper

Continuing from the previous post, I recently wrote a perspective article on orbitronics, which can be found in this link. So far, there hasn’t been much update and I confirmed that this article contains most of the up-to-date summary of the current understanding of the field. It’s just a 7-page mini-review, so just take 20-30 min break after lunch and have a look if you’re intersted in orbital transport 🙂