Tunable moiré nagnetism in twisted double bilayers of CrI3

Twistronics is not your typical dance move, exercise equipment, or passing music trend. It’s much more fascinating than that. It represents a groundbreaking development in quantum physics and material science, involving the stacking of van der Waals materials in layered structures, akin to sheets of paper in a ream that can twist and rotate while remaining flat. Quantum physicists have harnessed these stacked materials to uncover intriguing quantum phenomena.

By incorporating the concept of quantum spin into twisted double bilayers (tDB) of an antiferromagnet, it becomes possible to achieve tunable moiré magnetism. This discovery suggests a new material platform for the next stage of twistronics: spintronics. This cutting-edge science holds the potential for promising memory and spin-logic devices, opening up a whole new realm of physics with spintronic applications.

A team of researchers specializing in quantum physics and materials has recently introduced a twist to control the spin degree of freedom using CrI3, a van der Waals material with interlayer antiferromagnetic coupling. The results of their study were published in Nature Electronics on June 19, 2023.

Leading the team was Professor Yong P. Chen, a principal investigator at Tohoku University’s Advanced Institute for Materials Research (WPI-AIMR). Chen also holds professorships at Purdue University in the United States and Aarhus University in Denmark.

Dr. Guanghui Cheng, who conducted the experiments for the study and now serves as an assistant professor in Chen’s laboratory at WPI-AIMR, explains, “In this study, we fabricated twisted double bilayer CrI3, which refers to a bilayer stacked on top of another bilayer with a twist angle between them. We report moiré magnetism with diverse magnetic phases that can be significantly tuned using electrical methods.”

Chen elaborates, “We stacked and twisted an antiferromagnet onto itself, resulting in the emergence of a ferromagnet. This serves as a remarkable example of ‘twisted’ or moiré magnetism in twisted 2D materials, where the twisting angle between the layers acts as a powerful tuning knob that dramatically alters the material’s properties.”

Cheng further clarifies the fabrication process, stating, “To create twisted double bilayer CrI3, we tear one section of a bilayer CrI3, rotate it, and stack it onto the other part using a technique known as tear-and-stack.”

Through the use of magneto-optical Kerr effect (MOKE) measurements, a highly sensitive tool for probing magnetic behavior down to atomic layers, the researchers observed the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) orders, a characteristic feature of moiré magnetism. Additionally, they demonstrated voltage-assisted magnetic switching. Moiré magnetism represents a novel form of magnetism characterized by spatially varying ferromagnetic and antiferromagnetic phases that alternate periodically according to the moiré superlattice.

Until now, twistronics primarily focused on modulating electronic properties, as seen in twisted bilayer graphene. However, the research team aimed to introduce the twist to the spin degree of freedom, opting to use CrI3, a van der Waals material with interlayer antiferromagnetic coupling. The ability to twist and stack antiferromagnets onto themselves became possible by fabricating samples with different twisting angles. Once fabricated, the twist angle of each device remained fixed, and subsequent MOKE measurements were conducted.

The theoretical calculations for this experiment were performed by a group led by Pramey Upadhyaya from Purdue University, who is the corresponding author of the paper alongside Yong P. Chen. These calculations provided robust support for the observations made by Chen’s team.

Source: Tohoku University

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