A cross-sectional transmission electron microscope image of the atomic arrangement of the non-collinear antiferromagnet Mn3Sn (bright points represent atoms) and the chiral-spin structure composed of Mn atoms. Image: Ju-Young Yoon, Shunsuke Fukami, and Luqiao Liu.Researchers at Tohoku University in Japan and Massachusetts Institute of Technology (MIT) have investigated the anomalous dynamics at play when an electric current is applied to a new class of magnetic materials called non-collinear antiferromagnets. They report their findings in a paper in Nature Materials.
Magnetic materials are fundamental to today's society. In recent years, non-collinear antiferromagnets have attracted great interest due to their intriguing properties, which are distinct from those of conventional magnetic materials. In such materials, known as collinear ferromagnets, the magnetic moments align in a collinear fashion. In non-collinear antiferromagnets, by contrast, the moments form finite angles between each other. Scientists describe these non-collinear arrangements in the form of a single-order parameter – the octupole moment – which has proved critical for determining the exotic properties of these materials.
In this study, the researchers found that this octupole moment shows unconventional responses to electric currents, rotating in the opposite direction to the rotation that is induced when it is exposed to magnetic fields. They also found that this anomaly stemmed from an interaction between electron spins and the unique chiral-spin structure of the non-collinear antiferromagnet.
"Non-collinear antiferromagnet's exotic physical properties give it wide-ranging potential for applications in information technology hardware," said Ju-Young Yoon, a PhD student at Tohoku University and lead author of the paper. "Our findings provide a fundamental basis for spintronic devices such as memories and oscillators."
Spintronics is an interdisciplinary field that utilizes the spin of electrons to electrically manipulate magnetism, potentially leading to electronic devices that are faster, smaller and more efficient. The current-induced switching of magnetization in conventional collinear ferromagnets was demonstrated in 2000. This finding led to the recent commercialization of so-called Spin-Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM), which is expected to play a key role in future low-carbon-emission societies.
Non-collinear antiferromagnets have also become a major focus of the spintronics community. Despite the vanishingly small magnetization of non-collinear antiferromagnets, their chiral-spin structure induces significant ferromagnet-like properties, such as a large anomalous Hall effect. Such phenomena are known to be described by the octupole moment, with is analogous to the magnetization in ferromagnets. Although current-driven magnetization dynamics have been well established in the last two decades, much is still unknown about octupole dynamics.
To remedy this, the researchers examined the response of the octupole moment in a non-collinear antiferromagnet made of manganese and tin (Mn3Sn). By applying a magnetic field and an electric current, they compared it with the magnetization in a ferromagnet made of cobalt, iron and boron (CoFeB). In the ferromagnet, they found that, whether they applied a magnetic field or an electric current, its magnetization would always switch in the same direction. But in the non-collinear antiferromagnet, the magnetic field and electric current would cause the octupole moment to switch in opposite directions.
Through deeper analysis, the researchers 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 the non-collinear antiferromagnet.
"Electrical control of magnetic materials is of paramount importance in spintronics," said Luqiao Liu from MIT. “We have provided essential insights for controlling the non-collinear antiferromagnet, which is distinguished from its well-established counterpart, the electrical control of collinear ferromagnets.”
"Commercialization of STT-MRAM was achieved by a rigorous understanding of the interaction between magnetization and currents,” added Shunsuke Fukami from Tohoku University. “In this regard, this work should form a solid basis for the development of functional devices with non-collinear antiferromagnets."
This story is adapted from material from Tohoku University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.