Professor Luqiao Liu in the lab at MIT
Researchers at Tohoku University and Massachusetts Institute of Technology (MIT) have unveiled anomalous dynamics of non-collinear antiferromagnet, a new class of magnetic materials for future functional devices, when driven by electric current. Their findings were published in the journal Nature Materials on August 3rd, 2023.
Magnetic materials form a foundation of today’s society. In recent years, non-collinear antiferromagnets have attracted great attention due to its intriguing properties distinct from conventional magnetic materials, where the octupole moment is introduced to describe these properties. The researchers found that the octupole moment shows unconventional response to electric current, that is, it rotates in the opposite direction to that observed in general magnets. Such anomaly was found to stem from an interaction between electron spins and unique chiral-spin structure of the non-collinear antiferromagnet.
“Non-collinear antiferromagnet is an attractive subject of research owing to its exotic physical properties and potential for industrial applications. Our findings provide fundamental basis for spintronic devices such as memories and oscillators,” said Ju-Young Yoon, the leading author of the study.
Spintronics is an interdisciplinary field, where electric and magnetic (spin) degrees of freedom of electrons are utilized simultaneously, allowing for an electrical manipulation of magnetism. Around 2000, current-induced switching of magnetization in collinear ferromagnets, broadly termed as magnets, was demonstrated. This finding has led to a recent commercialization of a high-performance memory, so-called Spin-Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM), which is expected to play a key role for future low carbon emission societies.
Non-collinear antiferromagnet is one of the recent focuses in spintronics community. This material system has a vanishingly small magnetization unlike general ferromagnets but exhibits ferromagnet-like properties such as the anomalous Hall effect originating from its chiral-spin structure (Fig. 1). Such phenomena are known to be described by introducing a concept of octupole moment, with which one can make an analogy with magnetization in ferromagnets. Although the dynamics of magnetization driven by current has been well established in the last two decades, it is not the case for the octupole dynamics, waiting for a systematic investigation.
The researchers investigated the response of octupole moment in a non-collinear antiferromagnet Mn3Sn upon magnetic-field and electric-current applications and compared it with that of magnetization in a general ferromagnet CoFeB. Figure 2 summarizes the obtained results with schematics to illustrate the findings. While the switching directions of the ferromagnet are the same between the field and current-driven cases, they are opposite for the non-collinear antiferromagnet. Through in-depth studies, they revealed that individual magnetic moments rotate in the same direction for the two systems, but the assembled effect drives the octupole moment in the opposite direction due to the unique chiral-spin structure of non-collinear antiferromagnet.
“Electrical control of magnetic materials is of paramount importance in spintronics. Our findings afford essential insights for controlling non-collinear antiferromagnets, distinguished from the well-established understanding on the electrical control of collinear ferromagnets,” said Professor Luqiao Liu in MIT. Professor Shunsuke Fukami in Tohoku University echoed and added that “commercialization of STT-MRAM was achieved by rigorous understanding of the interaction between magnetization and current. In this regard, this work should form a solid basis for development of functional devices with non-collinear antiferromagnets.”