Press Release

May 19, 2022

Keio University

Successfully Observing the World’s Largest Tunnel Magnetocapacitanceof 426%: Prospects for Applicationsin High-performance Magnetic Sensors and Memory Devices
Researchers have successfully observed the world's largest tunnel magnetocapacitance (TMC)
ratio and explained its mechanism. The research team was comprised of Kenta Sato, a secondyear master’s student at Keio University’s Graduate School of Science and Technology, Hideo
Kaiju, Associate Professor at Keio University’s Faculty of Science and Technology, andcolleagues including Hiroaki Sukegawa, Principal Researcher at the National Institute forMaterials Science and Gang Xiao, Professor of Physics at Brown University, USA.
TMC is a phenomenon in which capacitance (electrical capacitance, or the amount of electricity
that a system can store) changes based on a magnetic field. This phenomenon is observed in
textured magnetic tunnel junctions (MTJs) with a thin insulating layer between two magneticlayers. Until now, the largest observed TMC ratio, a figure-of-merit on magnetic sensitivity, hasbeen 332%. In this study, researchers achieved the world's largest TMC ratio of 426% by using
an insulation tunneling layer and applying voltage biasing. Furthermore, they explained themechanism behind this phenomenon using dielectric theory, which incorporates quantum
mechanics and statistical theory. These results pave the way for creating new capacitancedetecting, high-performance magnetic sensors and magnetic memory devices.
The research results were published online in Scientific Reports (via Springer Nature Group)
on May 16 (BST).
1. Main Points of Research・Successfully observed the world's largest TMC ratio by devising a new type of insulating layer andapplying voltage.
・ Explained the mechanism behind this success using dielectric theory, an approach which
incorporates quantum mechanics and statistical theory.
・Opened up new avenues for the development of high-performance magnetic sensors and memory
devices.
2. Background of Research
The field of spintronics has attracted much attention in recent years. It utilizes two properties ofelectrons, "charge" (electrical charge) and "spin" (the rotation of electrons). Experts speculate thatspintronics will be involved in next-generation electronics. Within the field of spintronics,
researchers from around the world have focused their attention on studying magnetic tunnel
junctions (MTJs), which are created by sandwiching a thin layer of insulation between two magneticlayers. This interest in MTJs is due to their ability to produce large tunnel magnetocapacitance(TMC) ratios at room temperature. TMC is a phenomenon in which capacitance (electricalcapacitance, or the amount of electricity that a system can store) changes in a magnetic field. This1

rate of change in capacitance is referred to as the TMC ratio. The maximum value observed to datehas been 332% (see relevant article in references below).
3. Content of Research and Results
In this study, aiming to exceed this record 332% TMC ratio, the research group focused theirattention on the insulation layer and the device’s voltage. In magnetic tunnel junctions (MTJs),
resistance is another factor that changes with a magnetic field. Experts have been well aware of thisphenomenon, referred to as “tunnel magnetoresistance (TMR),” for some time. It is even used inmodern technology, implemented in the magnetic heads of hard disk drives (HDDs) in computers.
As the TMR ratio increases, areal densities of a magnetic recording medium of HDDs are
significantly increased. The TMR ratio, however, decreases when “voltage” is applied to MTJs.
Conversely, the tunnel magnetocapacitance (TMC) ratio increases when voltage is applied. This isdue to capacitance (the electrical storage ability of a system) generated from the spin of electrons at
the interface between the magnetic and insulating layers. By utilizing this electrical effect, this same
research group achieved a TMC ratio of 332% last year. In this study, the group further focused on
the interface structure between the magnetic and insulating layers. In previous studies involving
MTJs where an exceptionally large TMC was observed, iron-cobalt (FeCo) and magnesium oxide(MgO) were used for the magnetic and insulating layers, respectively. However, iron-cobalt andmagnesium oxide display slight differences in their crystal sizes. To remedy this, the researchers inthis study turned to spinel (MgAl2O4), an oxide of magnesium and aluminum. This is because there
is a less than 1% difference in crystal lattice size between spinel and iron. Due to this small difference,
the atoms were able to align almost perfectly at the interface between the magnetic and insulating
layers, leading researchers to anticipate improvements in the TMC ratio.
Working under this hypothesis, the group fabricated an ultra-high quality MTJ with a thin spinel
insulator film sandwiched between two iron magnetic films by using high vacuum magnetron
sputtering1 (Figure 1). The junction was then placed in a magnetic field to investigate thecapacitance dynamics. Researchers observed the resulting TMC ratio, which reached up to 426% inthis MTJ when voltage was applied (Figure 2). These results are well above the previously observed
record of 332%. Furthermore, in addition to the experimental results, subsequent theoreticalinvestigation revealed that a new calculation method incorporating quantum mechanics andstatistical theory into dielectric theory2 can account for the results researchers observed (Figure 3).
Theoretical calculations also showed that the TMC ratio could exceed 1000% if magnetic materials
with larger spin polarization rates are used.
4. Future Developments
The results of this research are expected to create opportunities for the development of highperformance magnetic sensors and memory devices. They are also expected to be applied in nextgeneration Internet of Things (IoT) technology—a major driver of the Digital Age—and to makesignificant contributions to the acquisition, accumulation, and analysis of big data. In the future,these findings are expected to be put to practical use not only in the Information
Technology/Information and Communication Technology (IT/ICT) field, but also in a wide range ofother fields including environmental energy, healthcare, health sciences, transportation, agriculture,and manufacturing.

25. Acknowledgments
This research was supported by the Grant-in-Aid for Scientific Research (B), the Research Project
funded by the Center for Spintronics Research Network (CSRN) (adopted by the Ministry ofEducation, Culture, Sports, Science and Technology [MEXT] under “Roadmap 2020; Fundamental
Concepts for Promoting Large-Scale Scientific Research Projects”), Brown University, the US
National Science Foundation, and other organizations.
References
Article detailing the previous largest observed TMC ratio of 332%:
K. Ogata, Y. Nakayama, G. Xiao, and H. Kaiju:“Observation and theoretical calculations of voltage‑induced large magnetocapacitance beyond
330% in MgO‑based magnetic tunnel junctions”, Scientific Reports 11, 13807 (2021)
Details of Journal Article
Title of article: Large magnetocapacitance beyond 420% in epitaxial magnetic tunnel junctions withan MgAl2O4 barrierAuthors: Kenta Sato1, Hiroaki Sukegawa2, Kentaro Ogata1, Gang Xiao3, and Hideo Kaiju1, 4 (1KeioUniversity, Faculty of Science and Technology, Department of Applied Physics and PhysicoInformatics; 2National Institute for Materials Science; 3Brown University, Department of Physics;
4Keio University Spintronics Research Center)
Journal: published in Scientific Reports (an open access journal published by Sprinder NatureGroup)
Publication date: published online on Monday, May 16 2022 (BST)
DOI: 10.1038/s41586-021-03832-5
Glossary and References
1Magnetron sputtering
A technique that forms a thin film by bombarding inert gas (e.g., argon gas) with a material in an
ultrahigh vacuum to eject (or “sputter”) the material on a substrate. It is called magnetron sputtering
because a magnet is used to increase the frequency with which the inert gas atoms collide with thematerial.
2Dielectric theory

This theory is used to calculate the dynamic dielectric constant, and the model is called the DebyeFröhlich model. In this study, researchers developed a new theory that combines the Debye-Fröhlichmodel with the Zhang-Sigmoid theory, the parabolic barrier approximation, and the spin-dependent
drift-diffusion model. The conducted experiments from this study lined up well with the predicted
results of this new theory.

3Figures

Figure 1. Magnetic tunnel junction (left: measurement setup, right: photograph of prototype)Figure 2. 426% capacitance ratio achieved once voltage was applied4

Figure 3. Comparison of experimental and calculated results(The blue circles are the results of the TMC experiment. The blue line are the calculated results.
The red circles are the results of the TMR experiment. The red line are the calculated results.)*Please direct any requests or inquiries on press coverage to the contact information provided below
in advance.
Inquiries about research
Associate Professor Hideo Kaiju,Department of Applied Physics and Physico-Informatics,
Faculty of Science and Technology, Keio University
TEL: +81 (0)45-566-1428
Email: kaiju@appi.keio.ac.jp
Inquiries about press release
Keio University Office of Communications and Public Relations (Contact: Sawano)
TEL: +81 (0)3-5427-1541
FAX: +81 (0)3-5441-7640
Email: m-pr@adst.keio.ac.jp
https://www.keio.ac.jp/en/

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