Raman spectroscopy and MCD of magnetic moiré superlattices formed by twisted double bilayer CrI₃

We fabricate twisted double bilayers (tDB) of a 2D magnet, chromium triiodide (CrI₃), and demonstrate the successful twist engineering of 2D magnetism in them. We identify Raman signatures of a new magnetic ground state that is distinct from those in natural two-layer (2L) and four-layer (4L) CrI₃. We show that for a very small twist angle, this emergent magnetism can be well approximated by a weighted linear superposition of those of 2L and 4L CrI₃, whereas for a large twist angle, it mostly resembles that of isolated 2L CrI₃. However, at an intermediate twist angle, there is a finite net magnetization that emerges because spin frustrations are introduced by competition between ferromagnetic and antiferromagnetic exchange coupling within individual moiré supercells. Magnetic circular dichroism (MCD) spectroscopy measurements reveal the evidence of noncollinear spin texture in tDB CrI₃. Our results establish the emergence of noncollinear spins from magnetic moiré superlattices and provide a versatile platform to explore nontrivial magnetism with noncollinear spins.