Magnetic Skyrmion Lattices in a Novel 2D‐Twisted Bilayer Magnet

Abstract

Magnetic skyrmions are topologically protected spin swirling vertices, which are promising in device applications due to their particle-like nature and excellent controlability. Magnetic skyrmions are extensively studied in a variety of materials and proposed to exist in the extreme 2D limit, i.e., in twisted bilayer CrI 3 (TBCI). Unfortunately, the magnetic states of TBCIs with small twist angles are disorderly distributed ferromagnetic and antiferromagnetic (AFM) domains in recent experiments, and thus the method to get rid of disorders in TBCIs is highly desirable. Here, intralayer exchange interactions up to the third nearest neighbors without empirical parameters and very accurate interlayer exchange interactions are used to study the magnetic states of TBCIs. The functions of interlayer exchange interactions obtained using first-principles calculations and stored in symmetry-adapted artificial neural networks are proposed. Based on these, the subsequent Landau-Lifshitz-Gillbert equation calculations explain the disorderly distributed FM-AFM domains in TBCIs with small twist angles and predict the orderly distributed skyrmions in TBCIs with large twist angles. This novel twisted 2D bilayer magnet can be used to design memory devices, monochromatic spin wave generators and many kinds of skyrmion lattices.

Emergence of Stable Meron Quartets in Twisted Magnets

“…Due to so-called stacking-dependent interlayer magnetism, ,− J ij ⊥ switch from FM to AFM coupling depending on the local stacking pattern between the two magnetic layers (see Supporting Information (SI)). Consequently, the interlayer exchange coupling exhibits the coexistence of AFM and FM interactions in the moiré superlattice accommodating various local stacking patterns (Figure a). − , We illustrate this behavior in Figure b through the map of the local interlayer exchange energy J i ⊥ = ∑ j J ij ⊥ computed in an FM configuration S i l = Sẑ . ,,, Specifically, in the monoclinic stacking region (red patches), J i ⊥ exhibits AFM character ( J i ⊥ > 0), indicating a tendency for the spins in the top and bottom layers to align antiparallel to each other. Conversely, in the other stacking regions, J i ⊥ exhibits FM character ( J i ⊥ < 0), signifying a preference for parallel alignment.…”

“…Consequently, the interlayer exchange coupling exhibits the coexistence of AFM and FM interactions in the moirésuperlattice accommodating various local stacking patterns (Figure 2a). [20][21][22][23][24][25]34 We illustrate this behavior in Figure 2b through the map of the local interlayer exchange energy J i ⊥ = ∑ j J ij ⊥ computed in an FM configuration S i l = Sz. 20,22,24,34 Specifically, in the monoclinic stacking region (red patches), J i ⊥ exhibits AFM character (J i ⊥ > 0), indicating a tendency for the spins in the top and bottom layers to align antiparallel to each other.…”

“…In this study, we investigate twist engineering in vdW magnets as a promising avenue to realize stable merons. By conducting atomistic spin simulations, we demonstrate that antiferromagnetic domain arrays in the twisted magnet ,,− can be utilized to localize the cores of merons along the boundaries of their respective domains (Figure a–b). This localization mechanism effectively preserves the stable spin configuration of the Meron pair by separating their cores (Figure c).…”

“…The field of twist engineering has opened up a fascinating realm of possibilities in generating topological spin textures in 2D vdW magnets. By harnessing moiré patterns, researchers have demonstrated the creation of skyrmion spin textures in lattice-mismatched heterostructures − and twisted homobilayer systems, − , with a primary focus on Ising-type easy-axis magnetic systems. However, extending this approach to XY -type easy-plane magnets holds tremendous intrigue, as it enables exploration of captivating phenomena such as the Berezinskii–Kosterlitz–Thouless transition, , the emergence of merons, , and the potential discovery of hidden magnetic phases driven by strong spin fluctuations .…”

Theoretical Investigation of Topological Magnetic Textures in Sliding Ferroelectric CrX3 (X = Cl, Br, I) Moiré Superlattices: A Multiferroic Material with Unique Magnetoelectric Coupling for Information Storage Applications

“…Among various magnetic configurations, magnetic textures with topology protection, such as skyrmions, have garnered significant attention. − Skyrmions, characterized by their strong topologically protected stability and small size, are considered the most promising candidates for next-generation information memory devices. − The moiré superlattice provides an excellent material system for investigating magnetic textures, as the twisting introduces periodic magnetization domains with complex spin texture. This results in a diverse range of physical images with noncollinear spin configurations, attributed to the modulation of interlayer exchange interaction. ,, Compared to magnetic skyrmions in bulk alloy materials, the magnetic skyrmions in vdW bilayer are significantly thinner, reaching the 2D thickness limit and paving the way for advancements in spin twistronics. − …”