News and posts

Bart van Wees

In recent years it was demonstrated that magnons can be efficient transporters of spins, making new devices and functionalities possible with (insulating) magnonic systems. I will give an introduction into magnon spin transport in ferro/ferri/and anti-ferrro magnetic systems. I will discuss how charge current information can be transformed into (electronic) spin information by the spin Hall effect, which can then generate a magnon spin current in the ferrimagnetic insulators yttrium iron garnet (YIG) [1]. Magnon spins can then be detected via the inverse spin Hall effect, and converted back into a charge signal. These experiments have led to a better understanding of electrically and  thermally induced magnon currents (spin Seebeck effect) and emphasize the role of the nonequilibrium magnon chemical potential as the driving force for magnon currents [2] Based on these concepts a magnon transistor geometry was fabricated in which the magnon density was controlled by a magnon injecting gate electrode [3]. It was also shown that magnons in antiferromagnets can effectively transport spins, and experiments demonstrated this in multilayer 2D Van der Waals antiferromagnets[4]  I will discuss our recent results on magnon spin caloritronics, including magnon spin Seebeck effect and anomalous Nernst effects, in CrBr3 based ferromagnetic van der Waals systems.

• J. Cornelissen et al., Nat. Phys. 11, 1022 (2015)
• J. Cornelissen et al., Phys. Rev. B94, 014412 (2016)
• J. Cornelissen et al., Phys. Rev. Lett. 120, 097702 (2018)
• Xing et al., Phys. Rev. X9, 011026 (2019)
• Liu et al., Phys. Rev. B 101, 205407 (2020)

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.

Jianting Ye

Many recent discoveries on novel electronic states were made on 2D materials. Especially, by making artificial bilayer systems, new electronic states such as superconductivity and ferromagnetism have been reported. This talk will discuss quantum phase transitions and Ising superconductivity induced in 2D transition metal dichalcogenides. Using ionic gating, quantum phases such as superconductivity can be induced by field-effect on many 2D materials. In transition metal dichalcogenides, Ising-like paring states can form at K and K’ point of the hexagonal Brillouin zone. Also, we will discuss how to couple two Ising superconducting states through Josephson coupling by inducing superconductivity symmetrically in a suspended bilayer. This method can access electronic states with broken local inversion symmetry while maintaining the global inversion symmetry [3]. Controlling the Josephson coupling and spin-orbit coupling is an essential step for realizing many exotic electronics states predicted for the coupled bilayer superconducting system with strong spin-orbit interactions.

[1] Lu, J. M. Zheliuk O, et al., Science 350 1353 (2015).
[2] Lu, J. M. Zheliuk O, et al., Proceedings of the National Academy of Sciences 115 3551 (2018).
[3] Zheliuk O, Lu, J. M., et al., Nature Nanotechnology 14 1123 (2019).

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.