Altermagnetic RuO2 Boosts Tunnel Magnetoresistance

Abstract

Altermagnets exhibit characteristics akin to antiferromagnets, with spin-split anisotropic bands in momentum space. RuO2 has been considered as a prototype altermagnet; however, recent reports have questioned altermagnetic ground state in this material. In this Letter, we demonstrate spin-dependent tunneling magnetoresistance (TMR) in RuO2-based magnetic tunnel junctions, which suggests the spin-splitted anisotropic band structure of our RuO2 films. The observed TMR is contingent on the direction of the Néel vector of RuO2 and reverse its sign by the inversion of the Néel vector. These results reflect the altermagnetic nature of RuO2 and highlight its potential for spintronic applications, leveraging the combined strengths of ferromagnetic and antiferromagnetic systems.

A research team, affiliated with UNIST announced the successful development of a novel semiconductor device that utilizes a new class of materials, known as altermagnetism. This breakthrough is expected to significantly advance the development of ultra-fast, energy-efficient AI semiconductor chips.

Jointly led by Professor Jung-Woo Yoo from the Department of Materials Science and Engineering and Professor Changhee Sohn from the Department of Physics at UNIST, the team succeeded in fabricating magnetic tunnel junctions (MTJs) using altermagnetic ruthenium oxide (RuO₂). They also measured a practical level of tunneling magnetoresistance (TMR) in these devices, demonstrating their potential for spintronic applications.

MTJs are critical components in magnetic random access memory (MRAM) devices. While MRAM offers advantages like non-volatility and low power consumption, its widespread use has been limited by reliance on ferromagnetic materials, which require significant energy for spin reversal, have limited switching speeds, and are sensitive to external magnetic interference.

The researchers developed an altermagnetic material-based device capable of overcoming these limitations. Unlike ferromagnets, altermagnetic materials can store information via electron spin while being less affected by external magnetic fields, enabling ultra-fast switching.

In this study, the team used RuO₂, one of the most studied altermagnetic candidates, though its properties have been debated. They synthesized RuO₂ thin films with atomic precision under high-vacuum conditions and fabricated MTJs by depositing insulating and ferromagnetic layers sequentially. When the magnetic orientation of the ferromagnetic layer was changed, a variation in TMR was observed, providing experimental evidence of the device's potential as a magnetic memory element.

This research marks the first experimental confirmation that TMR varies depending on the spin direction in altermagnetic MTJs, representing a major step toward realizing altermagnetic AI memory semiconductors. The team is now working to enhance the magnitude of TMR effects in future device designs.

Conducted in less than a year and supported by the Challenging Limits R&D Project-launched in September 2024 and modeled after the US DARPA program-this project aims to rapidly achieve high-impact breakthroughs in fundamental science in Korea.

DongHo Kim of the National Research Foundation of Korea (NRF), overseeing the project, stated, "This achievement reflects the dedication of researchers exploring the largely uncharted field of altermagnetism, supported by the Challenging Limits R&D Project." He added, "We will continue to support this technology, which could be a significant leap forward for the semiconductor industry."

The research was led by Seunghyun Noh from the Department of Materials Science and Engineering and Kyuhyun Kim from the Department of Physics at UNIST. The findings were published in Physical Review Letters on June 20, 2025.

Journal Reference

Seunghyeon Noh, Gye-Hyeon Kim, Jiyeon Lee, et al., "Tunneling Magnetoresistance in Altermagnetic RuO2-Based Magnetic Tunnel Junctions," Phys. Rev. Lett., (2025).