Noncollinear phases in twisted double bilayer magnets*

Moiré superlattices formed by twisting atomically thin materials have traditionally provided a playground to study correlated electrons, and have shown remarkable properties such as superconductivity, Mott insulating states, and moiré excitons. Very recently, by combining moiré potentials with the spin degrees of freedom in twisted bilayer antiferromagnetic (AFM) systems (monolayer plus monolayer), noncollinear magnetic states have been reported [Proc. Natl. Acad. Sci. 117, 10721 (2020), Science 374, 1140 (2021), Nature Nanotechnol. 17, 143 (2022)]. In this work, we harness a new class of magnetic system, namely, the twisted double bilayer AFM (bilayer plus bilayer) to theoretically demonstrate a rich collection of magnetic ground states beyond those observed in the previous platforms. The results predicted from our theory show good qualitative agreement with our experiments [arXiv:2204.03837] and pave the way toward understanding the role of moiré potentials on spin degrees of freedom.