天美影视

The research results of Mr. Reiji Obata (Master's degree in the Course in Functional Materials, Department of Science and Engineering, Graduate School of Science and Engineering in 2023, affiliated with Prof. Junji Haruyama's laboratory at that time) were published in the German journal "Advanced Materials" on Sunday, January 5, 2025, with an impact factor (IF)*? of about 30.

Research overview

We discovered that when a novel atomic layer magnetic material, "Fe?GeTe?"*?, which is only a few atomic layers thick, is formed by mechanically exfoliating a bulk crystal using Scotch tape, and two fragments are directly stacked while rotating, a pseudo-magnetic tunnel junction*? resistance characteristic appears, where electron spins*? interact between the fragments depending on the rotation angle. It is thought that even without intentionally forming a tunnel junction, the van der Waals gap induced by lattice mismatch between the fragments acts as a tunnel barrier layer and appears. This opens up a great path for the creation of next-generation magnetic memory elements and other devices in the future solely through the direct rotational stacking of atomic layer magnetic materials.

This research was conducted as part of a project at the Center for Advanced Technology Research and Development (CAT) of our university, in collaboration with the following organizations: the Eiji Saito Laboratory, Department of Applied Physics, Faculty of Engineering; the Shigeo Maruyama Laboratory, Department of Mechanical Engineering, Faculty of Engineering, University of Tokyo; the Kazuhiko Hirakawa Laboratory, Institute of Industrial Science, Faculty of Engineering, University of Tokyo; the Tomokazu Suenaga Laboratory, Institute of Scientific and Industrial Research, Osaka University; the National Institute for Materials Science Center for Advanced Technology（CAT）; and the Evgeny Chimbal Laboratory, Department of Physics, University of Nebraska (USA).

Comment from Reiji Obata

I am truly delighted that the research I worked on last year during my master's program has been published in a high-impact factor academic journal. It was a very challenging and difficult research project, but my perseverance has paid off.

This research is based on a technique that was previously developed for creating graphene, a material as thin as a single carbon atom, and has become popular worldwide. This technique involves creating fragments (atomic layer fragments) as thin as one or a few atoms by peeling them off the original crystal using cellophane tape. Currently, all kinds of atomic layer fragments, from metals to semiconductors, magnets, and superconductors, are being created using this method, and there is fierce research competition in both fundamental properties and device applications. In recent years, however, a method has been developed to stack multiple atomic layer fragments like stamping them together, using only van der Waals forces (the natural attractive forces between atoms), which has created a major wave in this field. In particular, the discovery of a "magic angle," in which the properties of two atomic layer fragments change dramatically when they are stacked with only a single rotation, has taken the world by storm in the last four or five years.

Therefore, I created a novel atomic layer magnetic material, Fe?GeTe? (FGT), and directly rotated and stacked two of them at significantly different angles (Figure 1). I discovered, for the first time in the world, that a structure called a magnetic tunnel junction is spontaneously created depending on the rotation angle (Figure 2). Generally, this structure and property do not occur unless a tunnel film is inserted between the two magnetic materials, but in my case, I stacked them directly without any insertion, so I was surprised by the extremely unusual result. In collaboration with Osaka University and others, we investigated the structure in detail and clarified that the gap of only a few atoms (van der Waals gap) between the fragments plays the role of this tunnel film, and that the mismatch in the crystal lattice that occurs between the fragments depending on the rotation angle changes this gap, causing this phenomenon to occur.

Because this FGT has a fairly complex crystal structure, thinning it to just a few atoms thick was extremely difficult, and stacking two of them while controlling the rotation angle was practically a craftsman's art. Moreover, these fragments are easily oxidized, so after doing this in a short time, it was necessary to cover them with an insulator called atomic layer boron nitride to prevent oxidation. I remember being so happy that I almost cried when I finally produced a sample with good properties after immersing myself in this work from morning till night in front of a special stacking microscope.

My hard work paid off when my research was published in a world-renowned academic journal. I also received the Komoda Prize from the university's graduate school upon graduation. I am deeply grateful to everyone who supported me.

Comment from the supervising professor (Professor Junji Haruyama)

I am extremely pleased that Ms. Obata, who graduated from our laboratory's Master's program last year, has had her research published in the world-renowned academic journal "Advanced Materials," with an impact factor (IF) of around 30. It is exceptional for research from a Master's student at our university to be published in a journal that typically features the work of postdoctoral researchers, and I think it is truly wonderful and something to be proud of.

In fact, Ms. Obata published three papers in the three years from her undergraduate studies to her master's degree, and two of them were published in "Advanced Materials," a remarkable achievement. This particular paper, in particular, is the result of an extremely skilled technique: thin fragments of a novel magnetic material with a complex crystalline structure, consisting of only a few atoms, are peeled off from a thick layered crystal using cellophane tape, and then the two fragments are stacked at different angles while being controlled and rotated in the plane. This is a wonderful result that could not have been achieved without tremendous effort.

Furthermore, Ms. Obata had already begun experiments during the spring break of her third year, even before officially joining our laboratory, and was commuting to the University of Tokyo, our collaborating research partner. Throughout her undergraduate and master's programs, she tirelessly dedicated herself to her craft, working from morning till night every day, achieving remarkable results. I am truly humbled by her dedication. She was a wonderful student who quietly and diligently continued the kind of behind-the-scenes effort that is rarely seen these days. She is now working as a professional, and she was very pleased to hear about the publication of this paper. I sincerely hope that this will encourage her to achieve even greater success in the future.

Remarks

*?Impact Factor (IF): One of the indicators that represents the influence of academic journals in the fields of natural science and social science, etc., provided annually by Journal Citation Reports (JCR). It shows how often papers published in a journal are cited in other journals. The IF of a typical academic journal is around 3 or less.

*? Novel atomic layer magnet "Fe?GeTe?": A magnetic material that has been actively researched in recent years, with new phenomena related to electron spin being discovered one after another. For example, "the temperature at which magnetization is exhibited is close to room temperature and is high temperature," "an extremely high tunnel magnetoresistance ratio is expected," and "spin vortex aggregate skyrmions exist at high temperatures."

*?Spin: In atoms that make up matter, electrons orbit the nucleus while rotating on their own axis like the Earth; this rotation is called spin. For example, ferromagnetic materials become magnets because all the effective electron spins are oriented in the same direction, causing magnetization. One way to control this electron spin is by applying an external magnetic field.

*?Magnetic tunnel junction: A structure in which a thin tunnel film, thin enough for electrons to pass through (tunnel) between two stacked magnetic materials. Electron spins that rotate in the same direction can easily pass through these two fragments, while spins that rotate in opposite directions repel each other and cannot easily pass through. Therefore, when the applied magnetic field is swept between positive and negative and reversed, the direction of electron spins in the two fragments becomes opposite near the zero magnetic field point, which is the reversal point, resulting in high magnetoresistance in the flow of electron spins between the fragments. The ratio of the maximum to minimum of this magnetoresistance is extremely important for applications, and the larger the ratio, the more advantageous it is for applications such as magnetic memory elements.