Anomalous dynamics of non-collinear antiferromagnets

Researchers from Tohoku University and MIT have revealed the intriguing behavior of non-collinear antiferromagnets when subjected to electric currents. These magnetic materials differ from conventional ones as their magnetic moments form finite angles between each other instead of aligning in a collinear manner. Scientists identified a single order parameter, the octupole moment, crucial for determining the unique properties of these materials.

The researchers found that the octupole moment exhibits unconventional responses to electric currents, rotating in the opposite direction to general magnets’ order parameters. This anomaly arises from the interaction between electron spins and the distinct chiral-spin structure of non-collinear antiferromagnets.

Ju-Young Yoon, the lead author and a Ph.D. student at Tohoku University, emphasized that the exotic properties of non-collinear antiferromagnets offer vast potential for applications in information technology hardware. Their findings lay a fundamental groundwork for spintronic devices such as memories and oscillators.

Spintronics, an interdisciplinary field that utilizes electrons’ spin to manipulate magnetism, holds promise for faster, smaller, and more efficient electronic devices. Current-induced switching of magnetization in collinear ferromagnets, demonstrated around 2000, has led to the commercialization of high-performance memory like the spin-transfer torque magnetoresistive random access memory (STT-MRAM), expected to play a crucial role in future low-carbon-emission technologies. The recent research opens new possibilities for advancing technology in the spintronic domain.

Experimental results and schematics for anomalous dynamics of non-collinear antiferromagnet Mn3Sn compared with that of collinear ferromagnet CoFeB. Credit: Ju-Young Yoon, Shunsuke Fukami, and Luqiao Liu

Non-collinear antiferromagnets have garnered significant attention in the spintronics community due to their remarkable ferromagnet-like properties despite having extremely low magnetization. The unique chiral-spin structure of these materials induces phenomena like a large anomalous Hall effect, which can be attributed to the octupole moment, analogous to magnetization in ferromagnets.

While current-driven magnetization dynamics have been extensively studied in the past two decades, the understanding of octupole dynamics remains limited, requiring further investigation. To address this, researchers focused on the non-collinear antiferromagnet Mn3Sn and compared its response to magnetic field and electric current with the ferromagnet CoFeB. Interestingly, while the magnetization switching direction was the same for both cases, the octupole moment exhibited an opposite response in the non-collinear antiferromagnet.

Upon closer examination, they found that individual magnetic moments in both systems rotated in the same direction. However, the collective effect resulted in the octupole moment responding oppositely due to the unique chiral-spin structure of the non-collinear antiferromagnet.

Professor Luqiao Liu from MIT highlighted the importance of electrically controlling magnetic materials in spintronics, stating that their research provides crucial insights into non-collinear antiferromagnets, distinct from the well-established control of collinear ferromagnets.

Professor Shunsuke Fukami from Tohoku University emphasized the significance of understanding the interaction between magnetization and currents, as it played a crucial role in achieving the commercialization of STT-MRAM. This new work forms a solid foundation for the development of functional devices utilizing non-collinear antiferromagnets.

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