Researchers from Purdue University, Pennsylvania State University and Japan's National Institute for Materials Science (NIMS) have reported electrically tunable moiré magnetism in twisted double bilayers (a bilayer on top of a bilayer with a twist angle between them) of layered antiferromagnet chromium triiodide.
Using magneto-optical Kerr effect microscopy, the team observed the coexistence of antiferromagnetic and ferromagnetic order with non-zero net magnetization—a hallmark of moiré magnetism. Such a magnetic state extends over a wide range of twist angles (with transitions at around 0° and above 20°) and exhibits a non-monotonic temperature dependence. The researchers also demonstrated voltage-assisted magnetic switching. The observed non-trivial magnetic states, as well as control via twist angle, temperature and electrical gating, are supported by a simulated phase diagram of moiré magnetism.
Twistronics, a new development in quantum physics and material science where van der Waals materials are stacked on top of each other in layers that can twist and rotate while remaining flat, is enabling scientists to discover intriguing quantum phenomena. Adding the concept of quantum spin with twisted double bilayers of an antiferromagnet, it is possible to have tunable moiré magnetism. This suggests a new class of material platform for the next step in twistronics: spintronics. This new science could lead to promising memory and spin-logic devices, opening the world of physics up to a whole new avenue with spintronic applications.
The team of scientists in this recent study has introduced the twist to control the spin degree of freedom, using CrI3, an interlayer-antiferromagnetic-coupled van der Waals (vdW) material, as their medium. “In this study, we fabricated twisted double bilayer CrI3, that is, bilayer plus bilayer with a twist angle between them,” says Dr. Guanghui Cheng, co-lead author of the publication. “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,” says Purdue's Dr. Yong P. Chen. “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.”
Twistronics up to this point has mainly focused on modulating electronic properties, such as twisted bilayer graphene. The team wanted to introduce the twist to spin degree of freedom and chose to use CrI3, an interlayer-antiferromagnetic-coupled vdW material. The result of stacked antiferromagnets twisting onto itself was made possible by having fabricated 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 for this experiment were performed, which 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 Dr. Pramey Upadhyaya.
This research folds into an ongoing research avenue by Chen’s team. It follows several related recent publications by the team related to novel physics and properties of “2D magnets” and could have exciting possibilities in the field 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.”