New twist on 2D magnets

Twistronics is an exciting new development in quantum physics and material science where two-dimensional van der Waals materials are stacked on top of each other and then twisted. Quantum physicists have already used these twisted stacks to discover intriguing quantum phenomena.

Now, by adding the concept of quantum spin to the twisted double bilayers of an antiferromagnet, researchers have shown that it is possible to have tunable moiré magnetism. This suggests a new material platform for the next step in twistronics – spintronics – which could lead to promising memory and spin-logic devices.

A team of quantum physics and materials researchers, mostly from Purdue University, has utilized the twist to control the spin degree of freedom in chromium triiodide (CrI₃), an interlayer-antiferromagnetic-coupled van der Waals (vdW) material. They report their findings in a paper in Nature Electronics.

“In this study, we fabricated twisted double bilayer CrI₃, that is bilayer plus bilayer with a twist angle between them,” says Guanghui Cheng, an assistant professor in the Advanced Institute for Material Research (AIMR) at Purdue University and co-lead author of the paper. “We report moiré magnetism with rich magnetic phases and significant tunability by the electrical method.”

“We stacked and twisted an antiferromagnet onto itself and voila got a ferromagnet,” adds Yong Chen, a professor of physics and astronomy and of electrical and computer engineering, director of Purdue Quantum Science and Engineering Institute and a corresponding author of the paper. “This is also a striking example of the recently emerged area of ‘twisted’ or moiré magnetism in twisted 2D materials, where the twisting angle between the two layers gives a powerful tuning knob and changes the material property dramatically.”

“To fabricate twisted double bilayer CrI₃, we tear up one part of bilayer CrI₃, rotate and stack onto the other part, using the so-called tear-and-stack technique,” explains Cheng. “Through magneto-optical Kerr effect (MOKE) measurement, which is a sensitive tool to probe magnetic behavior down to a few atomic layers, we observed the coexistence of ferromagnetic and antiferromagnetic orders, which is the hallmark of moiré magnetism, and further demonstrated voltage-assisted magnetic switching. Such a moiré magnetism is a novel form of magnetism featuring spatially varying ferromagnetic and antiferromagnetic phases, alternating periodically according to the moiré superlattice.”

Up to this point, twistronics research has mainly focused on modulating electronic properties, such as with twisted bilayer graphene. The Purdue team wanted to apply the twist to the spin degree of freedom and chose to use CrI₃. This was made possible by fabricating samples with different twisting angles. In other words, once fabricated, the twist angle of each device becomes fixed, and then MOKE measurements are performed. Theoretical calculations also provided strong support for the observations made by Chen’s team.

This work follows recent studies by the team into the novel physics and properties of 2D magnets, as reported in a paper in Nature Communications. These studies also have exciting possibilities in the fields of twistronics and spintronics.

“The identified moiré magnet suggests a new class of material platform for spintronics and magnetoelectronics,” says Chen. “The observed voltage-assisted magnetic switching and magnetoelectric effect may lead to promising memory and spin-logic devices. As a novel degree of freedom, the twist can be applicable to the vast range of homo/heterobilayers of vdW magnets, opening the opportunity to pursue new physics as well as spintronic applications.”

This story is adapted from material from Purdue University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.