Surface acoustic waves-driven magnon spin Hall effect in atomically thin van der Waals antiferromagnets
e-mail: kato@issp.u-tokyo.ac.jp講演言語 : 英語
Intrinsic magnetism in two-dimensional (2D) materials had long been believed to hardly survive due to the enhanced thermal fluctuations. However, the recent discovery of exfoliated van der Waals (vdW) magnets has opened up a new avenue for 2D magnetism at finite temperatures [1,2]. Especially, transition metal phosphorus trichalcogenides are a family of easily exfoliatable vdW antiferromagnets [3]. These materials share the same honeycomb structure, but the bulk antiferromagnetic (AFM) phase varies depending on the magnetic elements. Furthermore, antiferromagnets exhibit ultrafast dynamics, null stray field, and robustness against external fields. Therefore, the investigation of these materials paves the way toward not only the understanding of 2D magnetism, but also future AFM spintronic devices.
Standard methods such as magnetization measurements and neutron diffraction, which could only access macroscopic magnetic properties, are not suitable for the study of atomically thin magnets. Especially, antiferromagnets do not have net magnetization, magneto-optical Kerr effect is not available either. Although recent studies have focused on Raman spectroscopy [4] and second-harmonic generation [5] to detect crystal symmetry lowering associated with the AFM transition, these signals do not provide clear identification in the monolayer limit. Therefore, an inclusive method which suits for exploring 2D antiferromagnets is highly desired.
Here, we propose a magnon spin Hall current driven by the surface-acoustic waves (SAWs) as a novel probe for such 2D vdW antiferromagnets [6]. Owing to extremely large mechanical flexibility of 2D materials, SAWs are ideally suited for fundamental research of them. A modulation of exchange energies due to strain mimics the role of gauge fields for magnons. The strain gauge fields work at two valley points in the opposite direction, leading to the activation of the valley degrees of freedom (DOF). Therefore, the valley DOF with the use of SAWs is a promising concept for detection of the magnetic order in 2D vdW antiferromagnets.
[1] C. Gong et al., Nature 546, 265 (2017)[2] B. Huang et al., Nature 546, 270 (2017)
[3] K. Du et al., ACS Nano 10, 1738 (2016)
[4] J. Lee et al., Nano Lett. 16, 7433 (2016)
[5] H. Chu et al., PRL 124, 027601 (2020)
[6] R. Sano et al., PRL 132. 236302(2024)