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.

Above-Room-Temperature Ferromagnetism in Thin van der Waals Flakes of Cobalt-Substituted Fe5GeTe2

“…Hence, our results pave the way for creating energy-efficient spintronic devices at room temperature with thin CFGT flakes, such as SOT-magnetic random-access memory and spin-torque oscillator. Lastly, our work also opens up the direction toward engineering novel magnetic phases in thin 2D magnets at or near room temperature, e.g., noncollinear texture or skyrmion via the proximity effect or twist engineering. − , …”

“…Also known as two-dimensional (2D) magnets, they embody a high degree of tunability (e.g., via gating or strain), the ability to form custom-designed, multifunctional heterostructures, and the availability of a rich array of interfacial phenomena. These unique properties position 2D magnets as excellent platforms for the fundamental study of low-dimensional spin physics, engineering novel quantum phases via proximity effects or Moire structures, − and studying complex magnon behaviors. , The potential to realize these novel effects, together with the ability for highly efficient interfacial control, present 2D magnets as promising platforms to implement a wide range of important applications from spintronics to magnon-based hybrid quantum systems. , …”

Coexistence of ferroelectricity and antiferroelectricity in 2D van der Waals multiferroic

“…2d shows the RMCD loops of the trilayer NiI2 sweeping between +3 T and -3 T at 10 K. The RMCD loops show a highly nonlinear behavior with magnetic field and plateau behaviors for the out-of-plane magnetization. The RMCD intensity near 0 T is suppressed and approaches zero, suggesting the vanishing remnant magnetization, which indicates a compensation of the out-of-plane magnetization and non-collinear AFM orders in the trilayer NiI2 30 . And the gradual increases of the RMCD signal are observed with increasing magnetic field between ±1.2 and ±2.6 T, suggesting a spinflop process.…”

Observation of unconventional van der Waals multiferroics near room temperature

“…Figure 2b shows the temperature-dependent RMCD loops in a non-bimeron-like area of the trilayer NiI2 sweeping between +3 T and -3 T. The RMCD loops show a highly nonlinear behavior with magnetic field 30 and plateau behaviors for the out-of-plane magnetization. At 10 K, the RMCD intensity near 0 T is suppressed and approaches zero, suggesting the vanishing remnant magnetization, which indicates a compensation of the out-of-plane magnetization and noncollinear AFM coupling in the trilayer NiI2.…”