Orbitronics with two-dimensional rare-earth dichalcogenides from first principles

RWTH Publications (RWTH Aachen)

Abstract

The emerging field of orbitronics, which exploits the orbital degree of freedom of electrons rather than spin for information processing, offers a promising platform for next-generation eco-friendly electronic devices. It is also regarded a viable alternative to spintronics, with orbital degree of freedom playing a central role in transport phenomena and magnetization manipulation. A major focus of this work is the exploration of orbital-related effects in two-dimensional (2D) magnetic materials, which remain underexplored in the context of orbitronics. In this study the electronic, magnetic, and transport properties of 2D rare-earth dichalcogenides (REDs) are investigated using first-principles density functional theory (DFT). By utilizing the strong spin-orbit coupling (SOC) and localized 4$f$ electrons of rare-earth elements, REDs exhibit a rich interplay of magnetism, orbital Hall effects (OHE), and spin-orbit torques (SOT), making them ideal candidates for topological spintronic and orbitronic applications. This thesis first investigates the spin and orbital Hall effects in nonmagnetic monolayer and bilayer transition-metal dichalcogenides (TMDs) and their heterostructures. It is demonstrated that the OHE is a fundamental phenomenon, independent of SOC, and often dominates the spin Hall effect (SHE) in 2D materials. This finding suggests that the OHE could serve as a more efficient mechanism for generating spin currents in spintronic devices. The study is then extended to magnetic 2D materials, focusing on a novel family of magnetic REDs based on europium (Eu) and gadolinium (Gd). It is shown that electronic structure engineering through $p$--$d$--$f$ hybridization in 2D materials can lead to a prominent topological orbital response, which is highly sensitive to the details of crystal lattice design. Furthermore, the results reveal that strain-induced modifications in the electronic structure of Gd-based REDs can enable quantized charge, spin, and orbital transport. Finally, this thesis explores the role of orbital magnetism in mediating SOT and orbital pumping in 2D magnets based on $f$-electrons. Using low-symmetry Janus-type REDs, characterized by strong magnetism, significant SOC, and pronounced orbital polarization, we demonstrate that the current-induced torque acting on 4$f$ states is purely orbital in nature and reaches magnitudes comparable to those in leading 2D magnetic materials. Furthermore, we predict that magnetization dynamics can drive in-plane orbital angular momentum currents via orbital pumping, with the response showing a non-trivial dependence on direction and polarization due to structural asymmetry. We also identify the origin of the orbital pumping as an effect of orbital-to-orbital current conversion, establishing these REDs as promising platforms for spintronic applications based on in-plane spin and orbital angular momentum injection.

Authors 1

1. Mahmoud Zeer corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen