During my time in Zurich, I was heavily involved in studying samples provided by a research group at Spintec. Our focus was to detect skyrmions in these samples. It was like solving a puzzle every day, trying to unlock the secrets hidden within. We used NV magnetometry as our main tool, which allowed us to measure magnetic fields at the nanoscale with incredible precision. Zhewen, the ESR of our consortium at the startup, played a crucial role in helping me navigate the intricacies of the NV tool, providing invaluable insight and support along the way.
Outside of work, I made sure to explore Zurich’s academic offerings. One outstanding experience was a visit to ETH Zurich, an excellent university. I visited Pietro Gambardella’s lab where I had a good time with Marco and Niklas, the two ESRs of our consortium in that lab, and I learnt a lot about atomic force microscopy (AFM), which was quite exciting.
During one of the weekends, I decided to visit the Kunsthaus, eager to immerse myself in the world of art. Walking through the halls, I came across some important paintings from the Impressionist movement or the more recent Salvador Dalí. It was a surreal experience to see these famous pieces up close, but what really caught my attention was the work of De Chirico, an artist I have always admired. His surrealistic landscapes and enigmatic compositions have always fascinated me, and seeing them in person was a highlight of my time in Zurich.
When I was not in the lab or exploring the art scene, I took the opportunity to immerse myself in Zurich’s cultural offerings. Lazy walks along the Limmat River allowed me to take in the city’s unique blend of old and new, while exploring local markets and trying different cuisines added layers to my experience.
However, what really made Zurich special for me were the people, everyone I met was friendly and welcoming. These personal connections made my time there truly memorable.
As my time in Zurich ended, I couldn’t help but feel grateful for the experiences I’d had. From the scientific discoveries to the cultural immersion, it was a journey I won’t soon forget.
Here are two pictures that represent two memories of my experience, the first skyrmion I found and a painting of Salvador Dalí.
At the end of April, I paused my main PhD topic since I had to move in Imec (Leuven, Belgium) for my first secondment. This laboratory is full of Italians, so it is not difficult to feel immediately welcome. My main research focus is about the study of SOT-MRAM features and analyzing the role of the in-plane field in order to switch the magnetization state of the free layer. But it is not all about science! During my stay I had the luck of participating in the summer festival organized by IMEC with free food, alcohol and DJ set. I also participated in the team building event of this year consisting of paintball games.
During this period my daily supervisor was Van Dai Nguyen (see picture below). He helped me a lot by teaching me how to use the set-ups and to discuss the experimental results of my work. Fun fact, Dai was a previous PhD student in my group many years ago, so it has been a pleasure to talk with him also about my group and my current work in Spintec. I feel happy with this experience. In three months, I had the opportunity to learn (in depth) many aspects of the MTJs and SOT-MRAMs that I wouldn’t have learned by just reading papers during lazy days in Spintec.
I am glad to have had this opportunity and I would like to thank Dai for his constant supervision and patience.
Ferroelectrics are a category of material that in absence of an external applied voltage they still show a remanent polarization. The reason can be found in their atomic structure. For example the family of perovskites are ferroelectric material due to their specific crystal arrangement. Perovskite all share this similar ABX3 structure, where usually the X is oxygen and the A and B represent two different metals.
Fig.1 Phases of KNbO3 (potassium niobate) at different temperatures. It shows some structures where is possible to have a remanent polarization due to non-centro symmetry of the Niobium atom (in green) inside the cubic structure. When the Niobium atom is exactly at the center (in this case above 708 K) the material is not anymore ferroelectric [1].
One of most famous and studied is lead zirconate (PbZrxTi1-xO3) commonly called PZT. One of the biggest issues with this material is the toxicity due to the presence of the lead. For this reason, researchers focused on finding a material with similar characteristics. So many of them came out like barium titanate (BaTiO3), know as BTO or strontium titanate (SrTiO3) known as STO. In the picture here above another example of a lead-free perovskite material: potassium niobate (KNO3).
The remanent polarization properties is given by the presence of an atom inside this cubic-like structure that is not exactly at the center but slightly shifted. This non centro-symmetric structure give rise to a non-compensated positive charge of the body atom (Ti or Nb for example). The ferroelectric properties then are just given by the presence of the non-compensated charge when all the external voltages are removed. The temperature has an important role since for every material, for a given energy the structure tend to become a symmetric body centered cubic structure, hence there is a critical temperature after which the ferroelectric materials become paraelectrics. In this state they still react non-linearly to an external applied field but they do not show a remanent effect in absence of it.
Fig.2 Behaviour of a dielectric, a paraelectric and a ferroelectric material under an applied external electric field [2].
The polarization bistability for a zero external applied field is the key feature that makes ferroelectrics good candidates for memory applications. In recent year, many studies focused on integrating the ferroelectric materials in order to create new memories, more competitive from an energy computation point of view or overall faster writing and reading speed as FE-Fet [3], FE-RAM [4] or MESO [5] and FESO [6]. If for the first two the ferroelectric properties are aimed to improve the properties of already existing devices, like transistors or non-volatile random access memories; for the MESO and FESO case the aim is more ambitious. The idea is to develop a new logic based on spin controlled by ferrolectric non-volatility.
Recently, others materials showed to have ferroelectricity properties like Germanium telluride (GeTe), Indium arsenide (InAs) and many others. These materials show a simpler combination of only two atoms and that are not insulating like perovskites (sually they are metalic or semiconducting).
The bigger advantage is the possibility to pattern them in order to produce nanodevices, given by an higher durability when subjected to nanofabrication steps like etching. This one tend to destroy the crystal structure and hence the properties of the insulating perovskites . As a consequence, these new materials bring the ferroelectric-based devices a step closer to mass production and adoption.
If we take into account the case of germanium telluride, the ferroelectricity comes from the unusual bonds between germanium and tellurium layers. They tend to form a stronger bond with a neighbour layer with respect to the other forming a bilayer structure that is not symmetric (see pictures below). Similarly under an applied electric field the structure reorganize, causing the polarization to change sign (if the field is strong enough). It can be also seen as the germanium in the center of the cell moving along the larger diagonal of the deformed cubic cell (also called rhombohedral cell, left picture).
Fig. 3 Left: Cell structure of Germanium telluride (yellow Germanium, blue tellurium). Right: Switching mechanism: a) stable configuration of layer of Germanium (yellow dots) bonded to Tellurium ones (in red). b) when an electric field is applied, an unstable state appear where Germanium is bond to both top and bottom Tellurium atoms. c) Final state in which the Germanium atoms will be bond to the Tellurium atoms in the upper level with respect to the initial state [7]
So in a similar way to perovskites germanium telluride is know to show a remanent polarization at room temperature that can be controlled by an external applied electric field.
I hope you enjoyed this small talk on ferroelectrics, I will write in future a part 2 to explain the relation between ferroelectricity and spin-logic based devices. For further information you can email me at: salvatore.teresi@cea.fr
References
[1] P. Hirel et al., Phys. Rev. B 92 (2016) 214101.
[3] Stefan Ferdinand Müller (2016). Development of HfO2-Based Ferroelectric Memories for Future CMOS Technology Nodes. ISBN 9783739248943.
[4] Dudley A. Buck, “Ferroelectrics for Digital Information Storage and Switching.” Report R-212, MIT, June 1952.
[5] Manipatruni, S., Nikonov, D.E., Lin, CC. et al. Scalable energy-efficient magnetoelectric spin–orbit logic. Nature565, 35–42 (2019). https://doi.org/10.1038/s41586-018-0770-2
[6] Noël, P., Trier, F., Vicente Arche, L.M. et al. Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system. Nature 580, 483–486 (2020). [7] A. V. Kolobov, D. J. Kim, A. Giussani, P. Fons, J. Tominaga, R. Calarco, and A. Gruverman, Ferroelectric switching in epitaxial GeTe films, APL Materials 2, 066101 (2014).
[7] A. V. Kolobov, D. J. Kim, A. Giussani, P. Fons, J. Tominaga, R. Calarco, and A. Gruverman, Ferroelectric switching in epitaxial GeTe films, APL Materials 2, 066101 (2014).
This summer I was invited to a talk in San Diego, California (USA) to show a recent work of my group on Bilinear magnetoresistance effect in mercury telluride (HgTe) a topological insulator. It was an important event since I had to make a presentation in front of « experienced researchers » in the field of Spintronics.
It was a giant conference where only a part of it was dedicated to Spintronics. I had the pleasure to meet some researchers that are currently revolutionizing the spintronic field like Henry Jaffres or Dongwook Go (a young scientist working on a new phenomenon called Orbital Hall Effect). It is always a good opportunity to catch up on all the news in our field.
I took the opportunity to visit not only San Diego but also cities like Los Angeles and Santa Monica. I am not a big fan of the States but I would like to say that San Diego and the state of California is worth visiting. Furthermore in California there is the Silicon Valley, it was a big opportunity to look for a job after the PhD. For this purpose international conferences are a good opportunity to start to be known as a (hopefully) good researcher (and scientist) in the international community.
Here is below a picture of the ferris wheel in Santa Monica Pier
My PhD started incredibly well! I had the luck of finding such a wonderful group of people always available for help and suggestions. I enjoyed a lot of time with them also outside the laboratory. It is common to organise something all together and Grenoble is famous for having beautiful mountains all around and a lot of people enjoying hiking. For this reason, only few days after I joined the group, we went all together to Le Col Vert.
Here we had lunch surrounded by nature and with a beatiful view of Grenoble’s surroundings.
About the everyday time on the lab: at the moment I am checking through the ST FMR technique to evaluate the charge to spin conversion of a topological insulator at different temperatures. The idea is to extract the efficiency of the conversion from the measurements, but this step is not straightforward. Everything is working well only thanks to the experienced members of my team, that they do not hesitate in helping and discussing the results. I spent also a lot of time in cleanroom to prepare the samples through depositions and etching. I am really happy in spending my time at work, since I am learning a lot of things and the experience is exciting everyday.
