Evidence of Noncollinear Spin Texture in Magnetic Moiré Superlattices

Abstract Moiré magnetism, parallel with moiré electronics that has led to novel correlated and topological electronic states, emerges as a new venue to design and control exotic magnetic phases in twisted magnetic two-dimensional (2D) crystals. Noncollinear spin texture emerging from twisted 2D magnets with collinear spins is the most profound consequence of moiré magnetism and forms the basis for realizing novel magnetic orders and excitations. Yet, no direct experimental observation of noncollinear spins in moiré magnets have been made despite recent theoretical and experimental efforts. Here, we report the evidence of noncollinear spin texture in 2D twisted double bilayer (tDB) magnet chromium triiodide (CrI3). By studying the magnetic field dependent magnetic circular dichroism (MCD) in tDB CrI3, we distinguished the noncollinear spins with a gradual spin flop process from the collinear spins with sudden spin flip transitions, and identified a net magnetization emerging from the collinear spins. By examining the twist angle dependence, we demonstrated that both noncollinear spins and net magnetization are present at twist angles from 0.5^o to 5^o, but are most prominent in the 1.1^o tDB CrI3. By tracking the temperature dependent MCD of the 1.1^o tDB CrI3, we further resolved a new critical temperature of 22 K, for the onset of the net magnetization and the softening of the noncollinear spins, which is dramatically suppressed from the Néel temperature of 45 K for natural few layers. Our results establish the emergence of noncollinear spins from magnetic moiré superlattices and provide a versatile platform to explore nontrivial magnetism with noncollinear spins.