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A NIMS research team has developed a magnetic tunnel junction (MTJ) featuring a tunnel barrier made of a high-entropy oxide composed of multiple metallic elements. This MTJ simultaneously demonstrated stronger perpendicular magnetization, a higher tunnel magnetoresistance (TMR) ratio (i.e., the relative change in electrical resistance when the magnetization directions of the two ferromagnetic layers switch between parallel and antiparallel alignments) and lower electrical resistance.

These properties may contribute to the development of smaller, higher-capacity and higher-performance hard disk drives (HDDs) and magnetoresistive random access memory (MRAM).

This research is in Materials Today.

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.

Key findings

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. An MTJ incorporating a tunnel barrier made from this material demonstrated large perpendicular magnetic anisotropy, 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.

Future outlook

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.