2021 Fiscal Year Annual Research Report

Magnon-phonon coupling at the van der Waals interface and its heterostructure

Research Project

Project/Area Number	21F21774
Research Institution	Institute of Physical and Chemical Research
Principal Investigator	大谷義近国立研究開発法人理化学研究所, 創発物性科学研究センター, チームリーダー (60245610)
Co-Investigator(Kenkyū-buntansha)	LYONS THOMAS 国立研究開発法人理化学研究所, 創発物性科学研究センター, 外国人特別研究員
Project Period (FY)	2021-11-18 – 2024-03-31
Keywords	magnon-phonon coupling / spintronics / magnonics
Outline of Annual Research Achievements	Work commenced in December 2021. Since then, multiple new samples have been fabricated consisting of antiferromagnetic chromium trichloride (CrCl3) exfoliated flakes on top of lithium niobate (LiNbO3) piezoelectric substrates with lithographically defined interdigital transducers, allowing generation and detection of surface acoustic waves (SAWs). Resonant coupling between SAWs and the acoustic magnon mode of CrCl3 has been demonstrated at 1.1GHz, representing a first step towards the study of magnon-phonon coupling in van der Waals magnetic materials. The absorption of SAWs by the CrCl3 indicates excitation of magnons, and is observed to disappear upon heating the sample towards the Neel temperature, likely confirming the dependence on magnetic order.
Current Status of Research Progress	Current Status of Research Progress 2: Research has progressed on the whole more than it was originally planned. Reason The initial objective of the fellowship has been achieved, that is, magnon excitation by SAW irradiation in van der Waals antiferromagnetic material. However, this result has been achieved much later than anticipated, owing to an unforeseen low temperature response of LiNbO3 acoustic devices. This has caused significant problems in terms of reproducibility, with only a small percentage of fabricated samples showing the magnon-phonon coupling behaviour in experiments. Despite causing delays, the low temperature issues are being actively investigated and ways to circumvent the problems have been devised. Alternative piezoelectric substrates, quartz and aluminium nitride (AlN) wafers have been purchased to replace LiNbO3, which will hopefully alleviate the reproducibility concerns.
Strategy for Future Research Activity	The short-term plan is two-pronged. 1. Alternative acoustic devices will be fabricated using quartz and AlN substrates, and their low temperature response to GHz-range acoustic driving will be assessed for suitability for magnon-phonon coupling experiments. 2. Further work will be carried out on currently working LiNbO3 CrCl3 devices. They will be measured in a vector magnet system capable of performing a magnetic field angle dependence study, which will provide new information on the nature of the magnon-phonon interaction. Later, magnon-phonon strong coupling will be studied with CrCl3 in acoustic cavities. Otherwise, a collaboration with the University of Exeter, UK (I. Luxmoore) will begin, aiming to achieve acoustic control of colour centres in hexagonal boron nitride flakes.