Cheng Gong

Magnetism, one of the most fundamental physical properties, has revolutionized significant technologies such as data storage and biomedical imaging, and continues to bring forth new phenomena in emerging materials and reduced dimensions. The recently discovered magnetic 2D van der Waals materials (hereafter abbreviated as “2D magnets”) provide ideal platforms to enable the atomic-thin, flexible, lightweight magneto-optic and magnetoelectric devices. The seamless integration of 2D magnets with dissimilar electronic and photonic materials further opens up exciting possibilities for unprecedented properties and functionalities. In this tutorial, I will start with the fundamentals on 2D magnetism, and continue to speak on our experimental observation of 2D ferromagnet, analyze the current progress and the existing challenges in this emerging field, and show how we push the boundary by exploring the potential of 2D antiferromagnets for spintronics.

Kang L Wang

Layered materials have recently been investigated for exploring magnetic properties.  This talk will discuss the magnetism of layered materials including those of magnetic doped materials as well the interface proximity-induced ferromagnetism by proximity with antiferromagnetic materials.  We will begin by describing layered magnetic doped topological insulators (TI), SbBiTe, for achieving quantum anomalous Hall.  Then we will discuss the proximity-induced magnetism in doped and undoped TIs when interfaced with different kinds of antiferromagnets, such as CrSb and MnTe, with a perpendicular and an in-plane Nel orders, respectively.  Antiferromagnets interfaced with a magnet is shown being to yield skyrmions, whose topological charge can be controlled by cooling under applied magnetic fields.   Atomically thin 2-D van der Waals magnetic materials (FeGeTe and alike) also have drawn significant interests. We observed interface Nel-type skyrmions in FeGeTe/WeTe₂ heterostructures from the topological Hall effect below the temperature of 150 K, with the varying sizes for different temperatures, and the skyrmions were also confirmed by Lorentz transmission electron microscopy. A Dzyalosinskii-Moriya interaction with an energy of 1.0 mJ/m², obtained from the aligned and stripe-like domain structure, is shown to be sufficiently large to support and stabilize the skyrmions.

Wei Han

The two-dimensional (2D) van der Waals magnets have provided new platforms for exploring quantum magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices.

In this talk, I will discuss the spin transport in magnetic 2D materials and their heterostructures. Firstly, I will discuss magnon-mediated spin transport in an insulating 2D van der Waals antiferromagnetic MnPS₃. Long distance magnon transport over several micrometers is observed in quasi-2D MnPS₃. The transport of magnons could be described using magnon-dependent chemical potential, and long magnon relaxation length of several micrometers are obtained. Then, I will discuss the spin transport in a metallic 2D van der Waals ferromagnetic Fe_0.29TaS₂ and its heterostructures. Via systematically measuring Fe_0.29TaS₂ devices with different thickness, it is found that the dominant AHE mechanism is skew scattering in bulk single crystal, and the contribution from intrinsic mechanism emerges and become more relevant as the Fe_0.29TaS₂ thickness decrease. The spin-dependent scattering at the Fe_0.29TaS₂/superconductor interface will be also discussed, which reveals a large magnetoresistance that can be explained by the anisotropic Andreev reflection.

José H. Garciá A. 

The manifestations of spin-orbit coupling in two-dimensional materials with reduced symmetries, such as MoTe2 or WTe2 in their 1T' or 1Td phases, can lead to hitherto unexplored ways to control the electronic spins. In this talk, I will present numerical simulations that demonstrate that due to a combination of a persistent canted spin texture and hotspot of the berry curvature, transition metal dichalcogenides show a tunable canted spin Hall effect. The canting angle depends on the microscopic spin-orbit coupling parameters and can be tuned through the electronic environment. Moreover, the persistent spin texture spam over a broad energy range allowing for long spin relaxations even in the metallic regime.  These findings vividly emphasize how crystal symmetry governs the intrinsic spin phenomenology and how it can be exploited to broaden the range and efficiency of spintronic functionalities. We also propose specific experimental guidelines for the confirmation of the effect.

Jing Shi

2D van der Waals (vdW) magnetic materials offer exciting new opportunities to study interfacial phenomena arising from or enhanced by the atomically flat interfaces. I will present our recent studies on three types of bilayer systems composed of vdW magnet and Pt: Cr₂Ge₂Te₆/Pt, Fe₃GeTe₂/Pt, and Pt/CI₃.  In each bilayer, the exfoliated vdW magnet consists of 10’s atomic layer units and the sputtered 5 nm Pt layer is either below or above the vdW magnet. In Cr₂Ge₂Te₆/Pt and Pt/CI₃, both Cr₂Ge₂Te₆ and CI₃ are insulating, we use induced magneto-transport properties in Pt to probe the spin states and magnetic domains in the insulating magnets [1-3]. Unlike these two insulating magnets, Fe₃GeTe₂ is a metallic ferromagnet which has the highest Curie temperature among all 2D vdW magnets, strong perpendicular magnetic anisotropy, and more resistive than Pt; therefore, it is an excellent 2D magnet for investigating the spin-orbit torque effects. We demonstrate that Fe₃GeTe₂/Pt has a spin-orbit torque efficiency comparable with that in the best bilayers made with 3D magnets and the Fe₃GeTe₂ magnetization can be switched with a relatively low critical current density [4]. These excellent properties show great potential of 2D materials for spintronic applications.

1. B. Niu, T. Su, et al., Nano Lett. 20, 553 (2020). DOI: 10.1021/acs.nanolett.9b04282.
2. M. Lohmann, et al., Nano Lett. 19, 2397 (2019). DOI: 10.1021/acs.nanolett.8b05121.
3. T. Su, et al., 2D Materials 7, 045006 (2020). DOI:10.1088/2053-1583/ab9dd5.
4. M. Alghamdi, M. Lohmann, et al., Nano Lett. 19, 4400 (2019). DOI: 10.1021/acs.nanolett.9b01043.

Tatiana G. Rappoport

The field of spintronics blossomed in the last decade, driven by the use of spin-orbit coupling to generate and manipulate spin currents in non-magnetic materials. In these systems, the efficient conversion between charge and spin currents is mediated by spin-orbit. Great progress in the manipulation of the orbital angular momentum of light has also been achieved in the last decades, leading to a large number of relevant applications. Still, electron orbitals in solids were less exploited, even though they are known to be essential in several underlying physical processes in material science. The orbital-Hall effect (OHE), similarly to the spin-Hall effect (SHE), refers to the creation of a transverse flow of orbital angular momentum that is induced by a longitudinally applied electric field. The OHE has been explored mostly in three dimensional metallic systems, where it can be quite strong. However, several of its features remain unexplored in two-dimensional (2D) materials.

We then investigate the OHE in multi-orbital 2D insulators, such as transition metal dichalcogenides. We show that the OHE in these systems is associated with exotic momentum-space orbital textures. This intrinsic property emerges from the interplay between orbital attributes and crystalline symmetries and does not rely on the spin-orbit coupling. Our results indicate that multi-orbital 2D materials can display robust OHE that may be used to generate orbital angular momentum accumulation, and produce strong orbital torques that are of great interest for developing novel spin-orbitronic devices.

Seonghoon Woo

Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing properties and integration with other 2D family for unique heterostructures. In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy (PMA). Such a strong SOC could also lift the chiral degeneracy, leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). In this talk, we present the experimental observation of Néel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet Fe₃GeTe₂ (FGT). We demonstrate the ability to drive individual skyrmion by short current pulses along a vdW heterostructure, FGT/h-BN, as highly required for any skyrmion-based spintronic device. Using first principle calculations supported by experiments, we unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states. Our finding opens the door to topological spin textures in the 2D vdW magnet and their potential device application.