Jing Wang

Here, we predict the tetradymite-type compound MnBi2Te4 and its related materials host topologically nontrivial magnetic states. The magnetic ground state of MnBi2Te4 is an antiferromagetic topological insulator state with a large topologically non-trivial energy gap (0.2 eV). It presents the axion state, which has gapped bulk and surface states, and the quantized topological magnetoelectric effect. It has several advantages over the previous proposals on realizing the topological magnetoelectric effect. The intrinsic magnetic and band inversion further lead to quantum anomalous Hall effect in odd layer MnBi2Te4 thin film with combined inversion and time-reversal symmetry breaking, which has been recently observed in experiments. The high quality intrinsic MnBi2Te4 together with other magnetic/superconducting 2D materials provides fertile ground for exploring exotic topological quantum phenomena.

We further show the Moire superlattice of twisted bilayer MnBi2Te4 exhibits highly tunable Chern bands with Chern number up to 3. We show that a twist angle of 1 degree turns the highest valence band into a flat band with Chern number ±1, that is isolated from all other bands in both ferromagnetic and antiferromagnetic phases. This result provides a promising platform for realizing time-reversal breaking correlated topological phases, such as fractional Chern insulator and p+ip topological superconductor.

Phil King

Control over materials thickness down to the single-atom scale has emerged as a powerful tuning parameter for manipulating not only the single-particle band structures of solids, but increasingly also their interacting electronic states and phases. Recently, magnetism has emerged as the new frontier in the area of 2d materials. Here, I will show how direct measurement of the electronic structure using angle-resolved photoemission (ARPES) can lead to valuable insight not only on the question of whether a 2d material does in fact exhibit long-range magnetic order, but also on the microscopic mechanisms of the magnetic ordering when it occurs. First, I will consider the example of epitaxial monolayers of VSe₂.¹ Our ARPES measurements, combined with x-ray magnetic circular dichroism (XMCD), demonstrate that a putative magnetic order is prevented from occurring by the formation of a robust charge density wave,² which gaps the complete Fermi surface thus removing a Stonor-like channel for ferromagnetism. The instability can be expected to be nearby in phase space, however, and we show how ferromagnetism can be induced via proximity coupling with a ferromagnetic  Fe layer.³ Second, I will show ARPES results from bulk Cr₂Ge₂Te₆, which is an established van der Waals ferromagnet,⁴ where long-range order has been shown to persist to the bilayer thickness.⁵ From ARPES, we identify atomic- and orbital-specific band shifts upon cooling through T_C. From these, together with XMCD, we identify the states created by a covalent bond between the Te 5p and the Cr e_g orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t_2g states that carry the majority of the spin moment. This reflects a rather direct observation of how 90° superexchange leads to ferromagnetism, and demonstrates how an experimental band-structure perspective can give important insight even in a “local moment" magnetic system.

 

This work was performed in close collaboration with M.D. Watson, A. Rajan, K. Underwood, J. Feng, D. Biswas, D. Burn, T. Hesjedal, G. van der Laan, M. Ciomaga Hatnean, G. Balakrishnan, G. Vinai, G. Panaccione and colleagues from the Universities of St Andrews, Oxford, Warwick, Diamond, and Elettra.

 

¹ Rajan et al., Phys. Rev. Materials 4 (2020) 014003

² Feng et al., Nano Lett. 18 (2018) 4493

³ Vinai et al., Phys. Rev. B 101 (2020) 035404

⁴ Carteaux et al., J. Phys. Condens. Mat. 7 (1995) 69

⁵ Gong et al., Nature 546 (2017) 265

⁶ Watson et al., Phys. Rev. B 101 (2020) 205125

Maciej Koperski

The magnetism of chromium has been investigated for almost a century now, providing substantial knowledge about its electronic configuration. Extensive research has been conducted regarding the physics of valence electrons from d-shell, which is fundamentally important for understanding the mechanisms of magnetic ordering. Interestingly, chromium atom, exhibiting a stable electronic configuration exempting from Hund’s rules, has half-filled 3d shell, which leads to manifestation of robust magnetic effects in a variety of structures. Recently, attention has been refocused on chromium trihalides (CrCl3, CrBr3 and CrI3), which constitute a group of electrically insulating layered materials displaying magnetic ordering at low temperatures, as established by inspection of bulk crystals carried out few decades ago. The progress of mechanical exfoliation techniques, performed in a controlled argon atmosphere, enables now isolation of thin layers (down to monolayers) and their incorporation in van der Waals heterostructures.
Initial reports demonstrated layer-dependent ferromagnetic and anti-ferromagnetic order below Curie temperature using Kerr rotation measurements as magnetization probe. These appealing findings motivate further study to uncover the underlying microscopic mechanisms. One possible path to learn about the electronic structure and characteristics of electronic states via optical methods involves investigations of emission and absorption processes. Here, we present detailed optical studies of exfoliated films of CrBr3 and CrI3 to demonstrate that the emergent interband luminescence has molecular-like character (most likely due to formation of Frenkel-type excitons) and the details of the structure of emission resonances can be explained by Franck-Condon principle involving multiple phonon modes. The photoluminescence studies unveil unambiguous signatures of coupling between the magnetic moments of Cr3+ ions with band carriers, offering insight into fundamental properties of these novel magnetic structures and opening up new routes for potential applications of 2D systems.

Masaki Nakano

 

Bottom-up molecular-beam epitaxy (MBE) provides a complementary approach to top-down mechanical exfoliation in 2D materials research. A great success lies in the application of MBE-grown large-area monolayer films to ARPES and STM/STS studies, unveiling emergent monolayer properties of various 2D materials. Considering the research history of semiconductors and oxides, however, one of the biggest advantages of MBE-based approach should be to create novel material systems unachievable by bulk-based approach and examine their transport phenomena, although such examples are very much limited presumably due to difficulties in making high-enough quality samples.

 

We have recently developed a fundamental route to layer-by-layer epitaxial growth of a wide variety of 2D materials and their heterostructures on insulating substrates by MBE [1-7], opening a door for exploration of emergent transport phenomena of 2D materials arising at the monolayer limit and at the interface between dissimilar materials even based on hardly-cleavable, chemically-unstable, and/or thermodynamically-metastable compounds. In this presentation, I will introduce our recent achievements in particular on the MBE-grown 2D magnets and their heterostructures, including observation of the emergent itinerant 2D ferromagnetism with intrinsic spin polarization in hardly-cleavable compound that are missing in its bulk counterpart [4], as well as control of its magnetic properties by the magnetic proximity effect across the van der Waals interface [7].

 

Reference:

[1] M. Nakano et al., Nano Lett. 17, 5595 (2017).

[2] Y. Wang et al., Appl. Phys. Lett. 113, 073101 (2018).

[3] Y. Kashiwabara et al., Adv. Funct. Mater. 29, 1900354 (2019).

[4] M. Nakano et al., Nano Lett. 19, 8806 (2019).

[5] Y. Tanaka et al., Nano Lett. 20, 1725 (2020).

[6] H. Matsuoka et al., Rhys. Rev. Research 2, 012064(R) (2020).

[7] H. Matsuoka et al., submitted.

Alexey Kaverzin

With a reference to our experimental observations I will discuss how proximity effects modify the spin transport phenomena in graphene when it is placed in the vicinity of other layered materials. For example, the combination of graphene and TMDs results in the presence of large spin-orbit interaction imprinted from TMD into graphene with the emergence of spin manipulation mechanisms including conversion between spin and charge currents. I will highlight our recent results obtained on a van der Waals heterostructure of graphene and anti-ferromagnetic material CrSBr. The presence of CrSBr introduces a large exchange interaction in graphene such that its conductivity becomes spin polarised with the polarisation of approximately 14%. This implies that spin current is generated in graphene on CrSBr with efficiency close to that of the conventional ferromagnetic materials. Overall, we experimentally demonstrate the functionality of various building blocks that can be used for assembly of spin-based devices made out of layered materials only.

Ivan Verzhbitskiy

Control of magnetism via electric fields is a long-standing exciting challenge of fundamental significance for future spintronic devices. Recent discovery of two-dimensional magnetism in van der Waals systems such as CrI₃, Fe₃GeTe₂, and Cr₂Ge₂Te₆ (CGT) highlights their unique potential as a platform to probe the interplay between charge and magnetic ordering [1]. Here, we report the first observation of carrier-induced ferromagnetic order in heavily doped thin crystals of CGT [2]. Upon degenerate electron doping, the CGT transistor exhibits clear hysteresis in the magnetoresistance (MR), which is a distinctive signature of ferromagnetism. Surprisingly, the hysteresis persists up to 200 K, which is in contrast to undoped CGT whose Curie temperature is only 61 K. We demonstrate that the Curie temperature can be modulated over 140 K by altering the electron density. Further, we find the magnetic easy-axis of doped CGT to lie within the plane of the crystal. This is in stark contrast to the out-of-plane magnetic easy-axis of the pristine CGT. We attribute these changes to emergence of the double-exchange interaction mediated by free carriers. This mechanism dominates over superexchange interaction, which is responsible for the ferromagnetic order in undoped CGT. Our calculations show that the magnetic anisotropy energy changes sign in degenerate doping limit, in agreement with our experimental observations. Our findings reveal a unique role of the electric field in tailoring the magnetic anisotropy and leading exchange interaction in semiconducting 2D ferromagnets.

[1] Gibertini, M.; Koperski, M.; Morpurgo, A. F. & Novoselov, K. S. Magnetic 2D materials and heterostructures. Nat. Nanotechnol. 14, 408–419 (2019).

[2] Verzhbitskiy, I.; Kurebayashi, H.; Cheng, H.; Zhou, J.; Khan, S.; Feng, Y. P. & Eda, G. Controlling the magnetic anisotropy in Cr₂Ge₂Te₆ by electrostatic gating. Nat. Electron. (2020), DOI:https://doi.org/10.1038/s41928-020-0427-7

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).