Matthias Batzill

In this tutorial talk, I am introducing the concept of van der Waals epitaxy of transition metal dichalcogenides (TMDs) and the endeavor of finding potentially ferromagnetic 2D materials. Epitaxial mono- or few-layer films allow detailed measurement of electronic structure by angle resolved photoemission and thus determine layer dependent properties and the role of interlayer interactions of the properties. In addition, scanning tunneling microscopy can give information on the growth process and defect structures in the films. We discuss selected cases of TMDs. For VSe₂ we suggest a competition between charge density and ferromagnetic ordering for the ground state. While in CrTe₂ the ground state may be the semiconducting 1H-phase rather than the sought metallic and possibly ferromagnetic 1T-phase. Formation of ultrathin intercalation compounds are also discussed as a potential ultrathin ferromagnets. Finally, we discuss properties of defects in TMDs and how these may help in inducing magnetic properties. The incorporation of magnetic dopants may be one approach and recent reports suggest the possibility of diluted ferromagnetic 2D semiconductors. While there are many unanswered questions, a controlled vacuum synthesis and characterization of monolayer materials is an important aspect to find new materials.

Juan F. Sierra

In recent years, spin-based technologies, in which information is carried by spin instead of charge, have become promising for “beyond-CMOS” devices. Graphene and other two dimensional materials have rapidly established themselves as intriguing building blocks for spintronics applications [1]. Because of the graphene intrinsic low spin-orbit interaction, spins can flow snugly through its crystal lattice over long distances, resulting in an ideal spin channel. At the same time, the graphene’s low spin-orbit interaction inhibits the manipulation of spins, which is the cornerstone for successfully implementing spin-based devices. Nevertheless, this bottleneck can be overcome by combing graphene with other layered materials in artificial van der Waals heterostructures. In this talk, I will present a set of experiments where we study the spin-relaxation in graphene-transition metal dichalcogenides heterostructures [2]. In such van der Waals systems, spin-orbit coupling in graphene is enhanced by proximity effects. As a consequence, the spin dynamics becomes anisotropic [2, 3], with a spin relaxation that depends on the spin orientation. Furthermore, we demonstrate an efficient spin-charge interconversion driven by the Spin Hall effect and inverse spin galvanic effect at room temperature [4].
1. W. Han et al., Nature Nanotechnology 9, 794 (2014).
2. L. A. Benítez, J. F. Sierra et al., Nature Physics 14, 303 (2018).
3. L. A. Benítez, J. F. Sierra et al., APL Materials 7, 120701 (2019).
4. L. A. Benítez, W. Savero Torres, J. F. Sierra, et al., Nature Materials 19, 170 (2020).

Young Hee Lee

The ferromagnetic state in van der Waals two-dimensional (2D) materials has been reported recently in the monolayer limit. Intrinsic CrI₃ and CrGeTe₃ semiconductors reveal ferromagnetism but the Tc is still low below 60K. In contrast, monolayer VSe₂ is ferromagnetic metal with Tc above room temperature but incapable of controlling its carrier density. Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at room temperature. The key research target is to realize the long-range order ferromagnetism, Tc over room temperature, and semiconductor with gate tunability. Here, Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~10⁵ at 0.1% V-doping concentration. The V-substitution to W siteskeep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission-electron-microscopy. More importantly, the ferromagnetic order is clearly modulated by applying a back gate. We also observe a ferromagnetic hysteresis loop together with oscillatory behavior at room temperature in diluted V-doped WSe₂, while maintaining the semiconducting characteristic of WSe₂ with a high on/off current ratio of five orders of magnitude. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics.

Ahmet Avsar

EPFL

Defects are ubiquitous in solids and often introduce new properties that are absent in pristine materials especially at their low-dimensional limits [1]. For example, atomic-scale disorder in two-dimensional (2D) transition metal dichalcogenides is often accompanied by local magnetic moments, which can conceivably induce long-range magnetic ordering in these otherwise non-magnetic materials. In this talk, I will present magneto-transport properties of ultrathin PtSe2 crystals down to monolayer thickness and demonstrate the emergence of such extrinsic magnetism [2]. Electrical measurements supported by first-principles calculations and aberration-corrected transmission electron microscopy imaging of point defects show the existence of either ferromagnetic or anti-ferromagnetic ground state orderings depending on the number of layers in this ultra-thin material. By combining this defect-induced magnetism with unique thickness-dependent electronic properties of PtSe2 emerging from the strong coupling between layers [3], I will discuss its potential integration into several 2D spintronics device applications [4].

Tobias Roedel

Nature

Scientific publishing is an integral part of the scientific endeavor. The number of publications has increased steadily over the last years due to a metrics-driven ‘publish-or-perish’ culture and an increase in research funding in certain countries. At Nature, we strive to filter only the most significant advances and to communicate their impact to a broad audience – beyond the specialized community. Our assessment of significance is based on objective criteria, but our final decisions are not devoid of subjectivity. As our editorial decisions can be controversial, I will present the editorial process behind these decisions. Feel free to contact me at ‘tobias.roedel@nature.com’ if you have questions or criticism.

Efrat Lifshitz

Technion

Magnetism is a topic of a wide interest since the discoveries of motors/generators, through magneto-resistance and up to modern times, where low dimensional materials offer a support for new magnetic phenomena. The talk will focus on the influence of magnetic moments and magnetism on the optical magneto-properties of semiconductors in an ultimate two-dimensional limit found in van der Waals transition metal phosphorous tri-chalcogenides. A few types of magnetic properties will be discussed: the long-range magnetic order, ferromagnetism, anti-ferromagnetism or special spin textures; an interfacial developed Rashba spin-orbit effect; nuclear spin Overhauser effect; magnetic polaron, all gaining special stabilization by the size confinement and a shape anisotropy. The mentioned intrinsic fields lead to a lift of energy or momentum degeneracy at band-edge states with selective spin orientation in the ground or/and excited state, being of a special interest in emerging technologies of spin-electronics and quantum computation. The lecture will include the study of long-range magnetic order and valley effects in single layer of metal phosphor tri-chalcogenide compounds. Metal phosphor tri-chalcogenides with the general chemical formula MPX3 (M=metal, X=chalcogenide) closely resembling the metal di-chalcogenides, but the metal being paramagnetic elements, while one-third of them are replaced by phosphor pairs. The metal ions within a single layer produce a ferromagnetic or anti-ferromagnetic arrangement, endowing those materials with unique magnetic and magneto-optical properties. Most recent magneto-optical measurements will be reported, exposing the existence of valley degree of freedom in a few MPX3 (e.g., FePS3, MnPS3), that reveals a protection of the spin helicity of each valley however, the coupling to an anti-ferromagnetism lifts the valleys' energy degeneracy. The phenomenon was also examined in magnetically doped diamagnetic MPX3 layers. The results indicated the occurrence of coupling between photo-generated carriers and magnetic impurities and the formation of magnetic polaron.