Session B36: Novel Emergent Magnetism in Moiré Superlattices

Moiré skyrmions and chiral magnetic phases in twisted van der Waals magnets

Twisted van der Waals (vdW) magnets such as CrI3 have emerged as a unique platform to tune the properties of moiré superlattices. In particular, stacking dependent exchange that is prevalent in these materials can lead to new and unexpected phases as a function of twist angle. I will present a comprehensive theory of the magnetic phases in twisted vdW magnets through a combination of first-principles calculations and atomistic simulations. I will show that a wide range of noncollinear magnetic phases can be stabilized as a function of the twist angle and Dzyaloshinskii–Moriya interaction as a result of the competing interlayer antiferromagnetic coupling and the energy cost for forming domain walls. In particular, I will demonstrate that for small twist angles various skyrmion crystal phases can be stabilized in both CrI3 and CrBr3. These results provide an interpretation for the recent observation of noncollinear magnetic phases in twisted bilayer CrI3 and demonstrate the possibility of engineering further nontrivial magnetic ground states in twisted vdW magnets[1,2].

    

Interaction induced magnetism in 2D semiconductor moire superlattices

Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would open the possibility to access and control complex electronic phase diagrams on demand. Recently, moiré superlattices formed by two-dimensional materials have emerged as a promising platform for quantum engineering such phenomena. In this talk, I will present a systematic study of the emergent magnetic interactions (both antiferromagnetic and ferromagnetic) in strongly correlated transition metal dichalcogenides moire superlattices. I will show that the combination of doping, electric field, and optical excitation provide dynamic controls of the rich many-body Hamiltonian of moiré quantum matter.

    

Designer Spin Textures in Van der Waals Magnets

The recent emergence of two-dimensional magnets has provided a new playground for low-dimensional magnetism. Among various directions, the ability to general and control spin textures has been long sought after. In this talk, I will discuss two new routes towards this goal. The first route is via period strain patterning, which can give rise to topological spin textures due to the modulation of magnetic anisotropy locked onto the strain field. We show that the stability of the topological spin texture can be controlled by the strain field profile. In the second route, we show that in properly designed 2D magnet/semiconductor heterostructures, intervalley coupling can be induced by the magnetic substrate, leading to the appearance of a spin texture. These results will be discuss using both phenomenological modeling and ab initio calculations, and their experimental realization will be discussed as well.   

Exploring moiré magnetism in twisted two-dimensional magnets

Moiré superlattice develops upon the interference between two mismatched periodic lattices and has been realized in two-dimensional (2D) hetero- and homo-structures made of two vertically stacked atomic crystals. The introduction of moiré superlattice has shown tremendous impact on modifying the electronic and excitonic band structures and hence realizing novel phases of matter. In parallel with controlling charge degree of freedom (DoF), it has been recently predicted that moiré superlattice also has the power of modifying spin DoF and designing new magnetic phases. In this presentation, I will show our recent experimental effort in studying moiré magnetism in an archetypical system, twisted double bilayer CrI₃ whose building block, bilayer CrI₃, is a collinear A-type antiferromagnet with zero magnetization and strong easy-axis anisotropy. I will show how the competition between interlayer ferromagnetic (FM) and antiferromagnetic (AFM) exchange coupling within moiré supercells leads to a new magnetic state with nonzero magnetization and noncollinear spins. Given such a new magnetic ground state, I will then discuss how the spin wave excitation spectra are modified and whether any new magnetic excitations are captured. Finally, I will comment on other 2D magnets that we are working on to study moiré magnetism.   

Emerging magnetism in stacked vdW heterostructures between layered antiferromagnets

Van der Waals (vdW) magnet heterostructures have emerged as new platforms to explore exotic magnetic orders and quantum phenomena. We have observed emerging magnetism with perpendicular magnetization in stacked heterostructures between vdW inter-layer antiferromagnets (such as CrI3 or/and CrCl3) that individually no do possess such magnetization.  In one of our studies [1], we fabricated twisted double antiferromagnetic bilayer CrI3, and by magneto-optical Kerr effect microscopy demonstrate the coexistence of antiferromagnetic and ferromagnetic orders with nonzero net magnetization, which is the hallmark of moiré magnetism. Such magnetic state exhibits nonmonotonic temperature dependence and extends over a wide range of twist angles with transitions at ~0° and above 20°. We further demonstrate voltage-assisted magnetic switching and the linear magnetoelectric effect. The observed nontrivial magnetic states and unprecedented control by twist angle, temperature and electrical gating are supported by the simulated phase diagram of the moiré magnetism. The results illustrate the rich behaviors of twisted antiferromagnets and the control over them, and add significantly to the emerging study of twisted/Moire magnets exhibiting noncollinear states and novel domain structures.  In another study [2], we made heterostructures of layered antiferromagnets, CrI3 and CrCl3, with perpendicular and in-plane magnetic anisotropy, respectively. Using magneto-optical Kerr effect microscopy, we demonstrate out-of-plane magnetic order in the CrCl3 layer proximal to CrI3, with ferromagnetic interfacial coupling between the two. Such an interlayer exchange field leads to higher critical temperature than that of either CrI3 or CrCl3 alone. We further demonstrate significant electric-field control of the coercivity, attributed to the naturally broken structural inversion symmetry of the heterostructure allowing unprecedented direct coupling between electric field and interfacial magnetism. These findings illustrate another new opportunity to explore exotic magnetic phases and engineer spintronic devices in vdW heterostructures.