Shu Hsuan Su

Pairing a ferromagnet with the MnBi${}_{2}$Te${}_{4}$ offers a promising route to efficient spin-to-charge conversion, but how well it performs across temperature had remained unmapped. Combining ferromagnetic resonance with inverse spin-Hall voltage measurements from 20--290 K, the authors show here that the spin-Hall angle stays nearly constant below approximately 130 K, then becomes unreliable as spin rectification and shifting damping take over. The results establish the material as a robust platform for cryogenic spin-orbit and topological spintronics.

This study explores the temperature-dependent magnetization dynamics in permalloy-topological insulator heterostructures using temperature-resolved ferromagnetic resonance. NiFe/Bi2Se3 heterostructures are compared to NiFe/Pt and NiFe/W systems to identify unique damping behaviors. This work uncovers an unprecedented damping profile in NiFe/Bi2Se3 at temperatures below 90 K, shedding light on the interplay between spin current backflow and spin reorientation transitions, mechanisms previously unexplored in this context. Additionally, significant differences in inhomogeneous linewidth broadening (ΔH0) and effective magnetization (Meff) are observed, further emphasizing the distinct spin dynamics in NiFe/Bi2Se3 compared to heavy metal-based heterostructures. The findings reveal that topological insulators like Bi2Se3 play a critical role in modulating spin-pumping processes, particularly through the Rashba effect and spin–orbit coupling. These results offer deeper insight into the mechanisms underlying magnetic damping and provide a foundation for the optimization of spintronic memory and logic devices. The findings provide a critical foundation for designing advanced spintronic devices, particularly for low-temperature applications in spin–orbit torque magnetic random-access memory and magnonic systems.

Spin-to-charge conversion at the interface between magnetic materials and transition metal dichalcogenides has drawn great interest in the research efforts to develop fast and ultralow power consumption devices for spintronic applications. Here, we report room temperature observations of spin-to-charge conversion arising from the interface of Ni₈₀Fe₂₀ (Py) and molybdenum disulfide (MoS₂). This phenomenon can be characterized by the inverse Edelstein effect length (λ_IEE), which is enhanced with decreasing MoS₂ thicknesses, demonstrating the dominant role of spin-orbital coupling (SOC) in MoS₂. The spin-to-charge conversion can be significantly improved by inserting a Cu interlayer between Py and MoS₂, suggesting that the Cu interlayer can prevent magnetic proximity effect from the Py layer and protect the SOC on the MoS₂ surface from exchange interactions with Py. Furthermore, the Cu-MoS₂ interface can enhance the spin current and improve electronic transport. Our results suggest that tailoring the interface of magnetic heterostructures provides an alternative strategy for the development of spintronic devices to achieve higher spin-to-charge conversion efficiencies.

The exploration initiated by the discovery of the topological insulator (Bi_xSb_1−x)₂Te₃ has extended to unlock the potential of quantum anomalous Hall effects (QAHEs), marking a revolutionary era for topological quantum devices, low-power electronics, and spintronic applications. In this study, we present the epitaxial growth of Cr-doped (Bi_0.4Sb_0.6)₂Te₃ (Cr:BST) thin films via molecular beam epitaxy, incorporating various Cr doping concentrations with varying Cr/Sb ratios (0.025, 0.05, 0.075, and 0.1). High-quality crystalline of the Cr:BST thin films deposited on a c-plane sapphire substrate has been rigorously confirmed through reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) analyses. The existence of a Cr dopant has been identified with a reduction in the lattice parameter of BST from 30.53 ± 0.05 to 30.06 ± 0.04 Å confirmed by X-ray diffraction, and the valence state of Cr verified by X-ray photoemission (XPS) at binding energies of ~573.1 and ~583.5 eV. Additionally, the influence of Cr doping on lattice vibration was qualitatively examined by Raman spectroscopy, revealing a blue shift in peaks with increased Cr concentration. Surface characteristics, crucial for the functionality of topological insulators, were explored via Atomic Force Microscopy (AFM), illustrating a sevenfold reduction in surface roughness as the Cr concentration increased from 0 to 0.1. The ferromagnetic properties of Cr:BST were examined by a superconducting quantum interference device (SQUID) with a magnetic field applied in out-of-plane and in-plane directions. The Cr:BST samples exhibited a Curie temperature (T_c) above 50 K, accompanied by increased magnetization and coercivity with increasing Cr doping levels. The introduction of the Cr dopant induces a transition from n-type ((Bi_0.4Sb_0.6)₂Te₃) to p-type (Cr:(Bi_0.4Sb_0.6)₂Te₃) carriers, demonstrating a remarkable suppression of carrier density up to one order of magnitude, concurrently enhancing carrier mobility up to a factor of 5. This pivotal outcome is poised to significantly influence the development of QAHE studies and spintronic applications.

Tungsten disulfide (WS₂) was prepared from W metal and WO₃ by ion beam sputtering and sulfurization in a different number of layers, including monolayer, bilayer, six-layer, and nine-layer. To obtain better crystallinity, the nine-layer of WS₂ was also prepared from W metal and sulfurized in a furnace at different temperatures (800, 850, 900, and 950 °C). X-ray diffraction revealed that WS₂ has a 2-H crystal structure and the crystallinity improved with increasing sulfurization temperature, while the crystallinity of WS₂ sulfurized from WO₃ (WS₂-WO₃) is better than that sulfurized from W-metal (WS₂-W). Raman spectra show that the full-width at half maximum (FWHM) of WS₂-WO₃ is narrower than that of WS₂-W. We demonstrate that high-quality monocrystalline WS₂ thin films can be prepared at wafer scale by sulfurization of WO₃. The photoluminescence of the WS₂ monolayer is strongly enhanced and centered at 1.98 eV. The transmittance of the WS₂ monolayer exceeds 80%, and the measured band gap is 1.9 eV, as shown by ultraviolet-visible-infrared spectroscopy.

The spin-to-charge conversion in Permalloy (Py)/Cu/Bi₂Se₃ is tunable by changing the Cu layer thickness. The conversion rate was studied using the spin pumping technique. The inverse Edelstein effect (IEE) length λ_IEE is found to increase up to ~2.7 nm when a 7 nm Cu layer is introduced. Interestingly, the maximized λ_IEE is obtained when the effective spin-mixing conductance (and thus J_s) is decreased due to Cu insertion. The monotonic increase in λ_IEE with decreasing J_s suggests that the IEE relaxation time (τ) is enhanced due to the additional tunnelling barrier (Cu layer) that limits the interfacial transmission rate. The results demonstrate the importance of interface engineering in the magnetic heterostructure of Py/topological insulators (TIs), the key factor in optimizing spin-to-charge conversion efficiency.

The intrinsic magnetic topological insulator MnBi₂Te₄ has attracted much attention due to its special magnetic and topological properties. To date, most reports have focused on bulk or flake samples. For material integration and device applications, the epitaxial growth of MnBi₂Te₄ film in nanoscale is more important but challenging. Here, we report the growth of self-regulated MnBi₂Te₄ films by the molecular beam epitaxy. By tuning the substrate temperature to the optimal temperature for the growth surface, the stoichiometry of MnBi₂Te₄ becomes sensitive to the Mn/Bi flux ratio. Excessive and deficient Mn resulted in the formation of a MnTe and Bi₂Te₃ phase, respectively. The magnetic measurement of the 7 SL MnBi₂Te₄ film probed by the superconducting quantum interference device (SQUID) shows that the antiferromagnetic order occurring at the Néel temperature 22 K is accompanied by an anomalous magnetic hysteresis loop along the c-axis. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) at 80 K reveals a Dirac-like surface state, which indicates that MnBi₂Te₄ has topological insulator properties in the paramagnetic phase. Our work demonstrates the key growth parameters for the design and optimization of the synthesis of nanoscale MnBi₂Te₄ films, which are of great significance for fundamental research and device applications involving antiferromagnetic topological insulators.

Antimonene is a promising two-dimensional (2D) material that is calculated to have a significant fundamental bandgap usable for advanced applications such as field-effect transistors, photoelectric devices, and the quantum-spin Hall (QSH) state. Herein, we demonstrate a phenomenon termed topological proximity effect, which occurs between a 2D material and a three-dimensional (3D) topological insulator (TI). We provide strong evidence derived from hydrogen etching on Sb₂Te₃ that large-area and well-ordered antimonene presents a 2D topological state. Delicate analysis with a scanning tunneling microscope of the evolutionary intermediates reveals that hydrogen etching on Sb₂Te₃ resulted in the formation of a large area of antimonene with a buckled structure. A topological state formed in the antimonene/Sb₂Te₃ heterostructure was confirmed with angle-resolved photoemission spectra and density-functional theory calculations; in particular, the Dirac point was located almost at the Fermi level. The results reveal that Dirac fermions are indeed realized at the interface of a 2D normal insulator (NI) and a 3D TI as a result of strong hybridization between antimonene and Sb₂Te₃. Our work demonstrates that the position of the Dirac point and the shape of the Dirac surface state can be tuned by varying the energy position of the NI valence band, which modifies the direction of the spin texture of Sb-BL/Sb₂Te₃via varying the Fermi level. This topological phase in 2D-material engineering has generated a paradigm in that the topological proximity effect at the NI/TI interface has been realized, which demonstrates a way to create QSH systems in 2D-material TI heterostructures.