Twistronics is not your typical dance move, exercise gadget, or passing music trend. Its coolness factor surpasses all of those. Instead, it represents a captivating advancement 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. These layers can effortlessly twist and rotate while maintaining a flat configuration, leading quantum physicists to uncover fascinating quantum phenomena within these stacks.
By incorporating the concept of quantum spin into twisted double bilayers of an antiferromagnet, a remarkable possibility arises: the ability to manipulate moiré magnetism. This breakthrough introduces a fresh material platform, paving the way for the next phase in twistronics known as spintronics. This emerging field of science holds great potential for the development of cutting-edge memory and spin-logic devices. Consequently, it ushers in a new realm of physics, brimming with exciting opportunities and applications in the realm of spintronics.
Purdue University's team of quantum physics and materials researchers has made a significant breakthrough by incorporating the twist as a means to manipulate the spin degree of freedom. Their study focused on utilizing CrI3, a van der Waals (vdW) material with interlayer antiferromagnetic coupling, as their medium of investigation. The team's findings, titled "Electrically tunable moiré magnetism in twisted double bilayers of chromium triiodide," have been published in the esteemed scientific journal Nature Electronics. This research marks a crucial step forward in the exploration of twistronics and its potential for achieving electrically controllable moiré magnetism.
Dr. Guanghui Cheng, one of the co-lead authors of the publication, explains the focus of their study, stating, "In this study, we created a structure called twisted double bilayer CrI3, which consists of two bilayers stacked on top of each other with a specific twist angle between them." Dr. Cheng highlights their key findings, stating, "We observed the emergence of moiré magnetism, characterized by a wide range of magnetic phases, and demonstrated its significant tunability through electrical means." This highlights the team's successful fabrication of the twisted double bilayer CrI3 and their groundbreaking discovery of moiré magnetism, showcasing its magnetic phase diversity and the remarkable potential for electrical manipulation.
The research team behind this groundbreaking study comprises two co-lead authors, Dr. Guanghui Cheng and Mohammad Mushfiqur Rahman, who have made equal contributions to the work. Dr. Cheng was a postdoctoral researcher in Dr. Yong P. Chen's group at Purdue University and currently serves as an Assistant Professor at the Advanced Institute for Material Research (AIMR), affiliated with Tohoku University. Dr. Chen, a corresponding author of the publication, holds the prestigious position of the Karl Lark-Horovitz Professor of Physics and Astronomy at Purdue University. Additionally, he is a Professor of Electrical and Computer Engineering and serves as the Director of the Purdue Quantum Science and Engineering Institute. Dr. Upadhyaya, another corresponding author and professor at Purdue University, leads the group where Mohammad Mushfiqur Rahman is pursuing his PhD. Dr. Upadhyaya holds the position of Assistant Professor of Electrical and Computer Engineering.
Other members of the Purdue-affiliated team include Andres Llacsahuanga Allcca, a PhD student, Dr. Lina Liu and Dr. Lei Fu, both postdoctoral researchers from Dr. Chen's group, Dr. Avinash Rustagi, a postdoctoral researcher from Dr. Upadhyaya's group, and Dr. Xingtao Liu, a former research assistant at the Birck Nanotechnology Center. Their collective efforts and expertise have contributed to the successful execution of this research endeavor.
Dr. Yong P. Chen emphasizes the remarkable outcome of their experiment, stating, "We achieved the transformation of an antiferromagnet into a ferromagnet by stacking and twisting the layers onto themselves." He further highlights the significance of this result, stating, "This serves as a striking illustration of the emerging field of 'twisted' or moiré magnetism in twisted 2D materials. The twisting angle between the layers acts as a powerful tuning knob, leading to dramatic changes in the material's properties." The ability to induce such transformative changes through twistronics and exploit the moiré magnetism phenomenon showcases the immense potential of this field in manipulating and controlling material properties.
Dr. Guanghui Cheng provides insights into the fabrication process of the twisted double bilayer CrI3, stating, "To create the twisted double bilayer CrI3, we tore a portion of the bilayer CrI3, rotated it, and stacked it onto the other part using a technique known as tear-and-stack." He goes on to describe their experimental approach, saying, "By employing the magneto-optical Kerr effect (MOKE) measurement, a highly sensitive technique capable of probing magnetic behavior at the atomic layer level, we observed the coexistence of ferromagnetic and antiferromagnetic orders. This coexistence serves as a characteristic feature of moiré magnetism. Furthermore, we demonstrated voltage-assisted magnetic switching, showcasing the ability to control the magnetic state with applied voltage. The moiré magnetism we observed represents a unique form of magnetism, exhibiting spatially varying ferromagnetic and antiferromagnetic phases that alternate periodically in accordance with the moiré superlattice." This highlights the experimental techniques employed to observe and manipulate the moiré magnetism phenomenon within the twisted double bilayer CrI3 structure.
Up until now, Twistronics has primarily concentrated on manipulating electronic characteristics, particularly in materials like twisted bilayer graphene. However, the team at Purdue University decided to explore the influence of twist on the spin degree of freedom. To achieve this, they selected CrI3, a van der Waals material with interlayer antiferromagnetic coupling. By fabricating samples with varying twisting angles, they were able to achieve a unique phenomenon where stacked antiferromagnetic layers twist upon themselves. It is important to note that after fabrication, each device's twist angle remains fixed, and subsequent measurements using MOKE (Magneto-Optical Kerr Effect) are conducted.
Theoretical calculations for this experiment were performed by Upadhyaya and his team. This provided strong support for the observations arrived at by Chen's team.
"Our theoretical calculations have revealed a rich phase diagram with non-collinear phases of TA-1DW, TA-2DW, TS-2DW, TS-4DW, etc.," says Upadhyaya.
This study builds upon the continuous research efforts led by Chen's team. It aligns with their previous investigations exploring the unique physics and properties of "2D magnets." Notably, they have recently published works like "Emergence of electric-field-tunable interfacial ferromagnetism in 2D antiferromagnet heterostructures" in Nature Communications, which delve into the fascinating realm of novel phenomena exhibited by these materials. The ongoing research avenue pursued by Chen's team holds promising potential in the domains of twistronics and spintronics, presenting exciting prospects for future developments and advancements.
Chen emphasizes that the discovery of the moiré magnet introduces a new category of materials that hold immense potential for spintronics and magnetoelectronics. The observed phenomena of voltage-assisted magnetic switching and magnetoelectric effect have significant implications for the development of memory and spin-logic devices with promising capabilities. By considering the twist as a novel degree of freedom, this concept can be extended to a wide range of homo/heterobilayers consisting of van der Waals magnets. Consequently, this opens up exciting opportunities to explore new physics and pursue spintronic applications in these systems.
This research receives partial support from various funding sources. The US Department of Energy (DOE) Office of Science provides support through the Quantum Science Center (QSC), which is a National Quantum Information Science Research Center. Additionally, the Department of Defense (DOD) Multidisciplinary University Research Initiatives (MURI) program (FA9550-20-1-0322) also contributes to the funding. Cheng and Chen have received partial support from WPI-AIMR and JSPS KAKENHI grants, specifically Basic Science A (18H03858), New Science (18H04473 and 20H04623), and the Tohoku University FRiD program during the early stages of the research. Upadhyaya acknowledges support from the National Science Foundation (NSF) (ECCS-1810494).