Tunable spin transport in low-dimensional systems

Abstract

As the scaling down of conventional semiconductor technologies approaches fundamental physical limits due to device miniaturization and power consumption constraints, spintronics has emerged as a promising alternative that offers new avenues for functional integration. While early spintronic research primarily focused on phenomena in relatively bulk materials, spin-dependent effects in low-dimensional systems remain significantly less explored and demand further in-depth investigation. Among these emerging platforms, organic/metallic heterointerfaces have garnered considerable interest owing to their unique ability to mediate spin transport while offering advantages such as chemical tunability, low spin–orbit scattering, and long spin coherence lengths. At the interfaces, hybridization between the π-orbitals of organic molecules and the d-orbitals of ferromagnetic metal (FM) layers induces novel spin-dependent interactions, including interfacial spin polarization and enhanced magnetic ordering. Recent studies have shown that spin selectivity and relaxation can be exquisitely tuned through molecular symmetry, anchoring chemistry, and interface dipoles. Thus, the organic/metallic interfaces provide highly adaptable platforms for developing multifunctional spintronic devices with rich interfacial spin–orbit phenomena. In parallel, van der Waals (vdW) materials have emerged as another versatile platform for spin transport, due to their atomically thin profiles, low defect concentrations, and ultraclean interfaces. The vdW materials such as graphene and transition metal dichalcogenides (TMDCs) exhibit a wide range of spin-dependent phenomena, which can be actively modulated through electrostatic gating, mechanical strain, and interfacial engineering. The absence of dangling bonds at the surfaces enables the formation of high-quality heterostructures that preserve spin information across interfaces. Graphene is particularly notable for its exceptionally long spin diffusion lengths, while TMDCs offer strong spin–orbit coupling and spin–valley locking, enabling gate-tunable phenomena such as the spin Hall effect and the Rashba–Edelstein effect. Furthermore, recent discoveries of intrinsic ferromagnetism and ferroelectricity support proximity-induced magnetism and electrically reconfigurable spin textures. The integration of spin–orbit functionalities with vdW material systems opens new directions toward realizing ultrathin, flexible, and reprogrammable spintronic devices. This dissertation shows the manipulation of spin transport in low-dimensional systems, such as organic/metallic heterointerfaces and vdW materials. It is essential to understand the tunability of the magnetic characteristics and spin-orbit coupling (SOC) in the systems. In details, organic semiconductors (OSCs) upon FM films can adjust the magnetic properties and Rashba SOC in vdW materials can be tuned by surface engineering and ferroelectricity. First, we demonstrated manipulation of magnetic characteristics of an ultrathin Co film by ‘on-surface’ absorption of OSCs, such as C60 and copper phthalocyanine (CuPc). Results indicated that surface molecular absorption tunes the magnetic characteristics of the ultrathin Co film via the transfer of Co valence electrons at the interfaces, as evidenced by in-situ ultraviolet photoelectron spectroscopy. Studies of anomalous Hall effects demonstrated that the on-surface absorption of OSCs enforced perpendicular magnetic anisotropy (PMA) and magnetic hardening of the underlying FM film. Additionally, the interfacial charge transfer significantly modifies the strength of exchange splitting and magnetic transition temperature of an ultrathin FM film. The established modulation of magnetic properties of OSC/FM interfaces will inspire fundamental research as well as practical applications of organic spinterfaces for emerging molecular spintronics. Beyond the heterointerfaces, we show that defect engineering in 2D PtSe2 films, achieved through plasma treatments, broke spatial inversion symmetry and led to the Rashba effect. The Rashba effect caused by broken spatial inversion symmetry was confirmed via the observations of directional current flow behaviors by nonreciprocal charge transport and first-principles density functional theory calculations. This approach offers a promising strategy for functional defect engineering that can couple spin and momentum of itinerant electrons, thereby facilitating novel pathways for emerging electronic device applications. Finally, we show that tuning nonreciprocal charge transport in ferroelectric few-layer WTe2 can be modulated by switching the polarization. In magneto-transport measurement, the large asymmetric magnetoresistance of WTe2 was observed under magnetic fields transverse to charge current and could be modulated by ferroelectric polarization. We confirmed that the Rashba-type spin texture of the WTe2 can be tuned by the spontaneous polarization through the nonreciprocal charge transport. Our studies inspire comprehensive understanding of magnetoelectric effect in ferroelectric system.