An international team led by Tohoku University and MIT have shown a representative effect of the anomalous dynamics involved when an electric current is applied to a new class of magnetic materials called non-collinear antiferromagnets (NCAFMs). The findings provide a fundamental basis to electrically manipulate this new class of magnetism to make spintronic devices such as memories and oscillators operate in a faster, smaller and more efficient way.
There has been much interest in NCAFMs in recent years because of their interesting physical properties. While with traditional collinear magnets the magnetic moments align in a collinear fashion, in non-collinear ones the moments form finite angles between each other. Such arrangements are described by a single order parameter known as the octupole moment, which is key to determining the useful properties of the materials.
Implementing new magnetic materials in spintronic devices requires an understanding of the dynamics of magnetic order parameters driven by current-induced spin torques. NCAFMs have spin structures distinct from conventional magnetic materials, a difference that inspired the investigation into the unique current-driven dynamics of the NCAFM.
As reported in the journal Nature Materials [Yoon et al. Nat. Mater. (2023) DOI: 10.1038/s41563-023-01620-2], the octupole moment was found to exhibit unconventional responses to electric currents by rotating in the opposite direction to the order parameters of general magnets. This anomalous finding was due to an interaction between electron spins and the chiral-spin structure of the NCAFM.
The response of the octupole moment in the NCAFM manganese–tin (Mn3Sn) was assessed. On applying a magnetic field and electric current, it was compared to the magnetization in a ferromagnet cobalt–iron–boron (CoFeB). While the switching directions of the magnetization were the same between the field and current-driven cases, those of the octupole moment were the opposite for the NCAFM.
It was found that in response to the injected spin current, the octupole moment rotates in the opposite direction to the individual magnetic moments, leading to a spin-orbit torque (SOT) switching polarity distinct to general magnets. As researcher Jiahao Han told Materials Today, “We term this effect handedness anomaly, an unprecedented effect that is unique to NCAFMs.” Showing this representative effect of the anomalous spin dynamics in NCAFM under SOT is the foundation for understanding and implementing the electrical manipulation of NCAFM.
The team hope this study will highlight the uniqueness of NCAFMs and help further exploration of characteristic behaviors of complex spin structures beyond the conventional magnetic systems. They are now looking to further improve the switching efficiency guided by the essential understanding of the handedness anomaly, and to build NCAFM-based devices, including advanced non-volatile memory, nano-oscillators, and random number generators.
"Non-collinear antiferromagnets are a new class of magnetic materials attracting growing interest, because they possess intriguing spin structures distinct from conventional magnetic materials"Jiahao Han
A cross-sectional TEM image of the atomic arrangement of NCAFM Mn3Sn (bright points for atoms) and chiral-spin structure composed of Mn atoms