A graphene sandwich — deposited or transferred?

Spintronics devices will be key to realizing faster and more energy-efficient computers. To give us a better understanding of how to make them, a Kobe University team now showed how different manufacturing techniques influence the material properties of a key component.

Electronic devices could be made more efficient and faster if electrons could carry more information at once. This is the basic idea behind spintronics, where researchers try to use the electrons’ spin in addition to charge in data storage, processing and sensor devices to significantly improve our computers. One component for such devices is the “magnetic tunnel junction,” which may be used, for example, for neuron-like behavior in information processing or in a new type of fast and non-volatile memory. They consist of two ferromagnets, usually a nickel-iron alloy, sandwiching a thin insulating layer such as graphene. Kobe University electronic engineer ONO Tomoya says, “The issue is that we know very little about how the interface between the materials behaves, so there are many unknowns in how to produce these devices.”

Ono and his team realized that the way these materials are produced probably changes the electronic structure of the interface. Being experts in first-principle calculations, that is, calculating the properties of materials based on the behavior of its electrons, they investigated, first, how metal and insulator would align on an atomic scale depending on the production technique; and second, how this would influence the magnetic properties at the interface that are relevant for spintronics applications. For a part of their calculations, they used the Kobe-based RIKEN supercomputer Fugaku, which was the world’s fastest supercomputer until 2022.

In the Journal of Applied Physics, the Kobe University team now published their results. They showed that the surface of the ferromagnet is different when the insulator is transferred to it compared to when the ferromagnetic crystal is grown on a flake of the insulator. More specifically, a bulk nickel-iron magnet, onto which graphene can be transferred, usually has more nickel on its surface, whereas a magnet grown on a flake of graphene will feature a layer of iron. This makes a difference for a magnetic tunnel junction’s behavior whether they are for sensors or for storage devices.

The reason these two situations differ lies in how the electrons of the insulator’s carbon atoms interact with the metal atoms’ electrons. “At the interface between nickel-iron and graphene we studied, the electrons of the iron atoms and the carbon atoms mix, or ‘hybridize,’ as we say. Electrons of carbon and nickel don’t do that. This influences how the junction as a whole behaves,” explains Ono.

The results don’t only pave the way for better control over the manufacture of this specific component. Since the researchers found a basic mechanism governing the interface structure between ferromagnetic metals and two-dimensional materials, they extend to other material systems, too. Ono says: “Our goal is to develop high-performance magnetic tunneling junctions made from other materials, too. We believe we have achieved fundamental and valuable results that will advance studies throughout the field.”

This research was funded by the Japan Society for the Promotion of Science (grants JP22H05463, JP24K06922, JP24H01196, JP24K01346, JPJSCCA20230005) and the Japan Science and Technology Agency (grant JPMJCR22B4).

Kobe University is a national university with roots dating back to the Kobe Higher Commercial School founded in 1902. It is now one of Japan’s leading comprehensive research universities with nearly 16,000 students and nearly 1,700 faculty in 11 faculties and schools and 15 graduate schools. Combining the social and natural sciences to cultivate leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society’s challenges.