Achieving Low Resistance and High Performance in MTJs Using High-Entropy Oxides

An MTJ—composed of a thin insulating layer (tunnel barrier) sandwiched between two ferromagnetic layers—operates by allowing electrons to tunnel through the barrier via quantum tunneling. Magnesium oxide (MgO) is currently the most widely used tunnel barrier material due to its ability to produce a high TMR ratio, enabling significant changes in electrical resistance depending on the relative magnetization directions of the two ferromagnetic layers. However, because of its high barrier height, MgO suppresses electron tunneling, which in turn increases the overall electrical resistance of the MTJ. To address this issue, there is strong interest in developing new tunnel barrier materials that maintain high TMR ratios while reducing barrier heights, thus enhancing the tunneling current.

The NIMS research team successfully developed high-quality LiTiMgAlGaO—a high-entropy oxide composed of five metallic elements uniformly distributed at the atomic level—as a tunnel barrier material (Figure (a)). An MTJ incorporating a tunnel barrier made from this material demonstrated large perpendicular magnetic anisotropy (Figure (b)), a TMR ratio exceeding 80% and a barrier height less than half that of MgO. These properties increased the tunneling current and reduced the electrical resistance of the MTJ. These achievements are expected to contribute to the development of next-generation MRAM and higher-speed, higher-capacity HDDs.

In future studies, the research team aims to develop tunnel barrier materials with even lower resistances and higher TMR ratios by optimizing both the combinations and compositional ratios of their constituent elements. In addition, the team plans to promote more efficient and practical materials design by employing machine learning and other data-driven techniques, thereby contributing to the development of higher-capacity, higher-performance MRAM and HDDs.

• This project was carried out by a research team led by Rombang Rizky Sihombing (Postdoctoral Researcher, Spintronics Group (SG), Research Center for Magnetic and Spintronic Materials (CMSM), NIMS) and Hiroaki Sukegawa (Group Leader, SG, CMSM, NIMS). Other team members include Thomas Scheike (Visiting Researcher, SG, CMSM, NIMS), Zhenchao Wen (Senior Researcher, SG, CMSM, NIMS), Seiji Mitani (Distinguished Researcher, SG, CMSM, NIMS), Jun Uzuhashi (Principal Engineer, Electron Microscopy Unit, Materials Fabrication and Analysis Platform (MFAP), Research Network and Facility Services Division (RNFS), NIMS), Hideyuki Yasufuku (Principal Engineer, Surface and Bulk Analysis Unit, MFAP, RNFS, NIMS) and Tadakatsu Ohkubo (Deputy Director, CMSM, NIMS).
• This work was supported by the MEXT Program: Data Creation and Utilization-Type Material Research and Development Project, Digital Transformation Initiative Center for Magnetic Materials (DXMag) (grant number: JPMXP1122715503), the JSPS Grants-in-Aid for Scientific Research (grant numbers: 22H04966 and 24H00408) and the MEXT Initiative to Establish Next-Generation Novel Integrated Circuits Centers (X-NICS) (grant number: JPJ011438).
• This research was published in Materials Today, an online international journal, on July 6, 2025.