The spin-orbit torque (SOT) that arises from materials with large spin-orbit coupling promises a path for ultralow power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BiSe thin films in BiSe/CoFeB heterostructures by using d.c. planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the spin torque efficiency (θ) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08 using the d.c. planar Hall and ST-FMR methods, respectively. Moreover, switching of the perpendicular CoFeB multilayers using the SOT from the BiSe was observed at room temperature with a low critical magnetization switching current density of 4.3 × 10 A cm. Quantum transport simulations using a realistic sp tight-binding model suggests that the high SOT in sputtered BiSe is due to the quantum confinement effect with a charge-to-spin conversion efficiency that enhances with reduced size and dimensionality. The demonstrated θ, ease of growth of the films on a silicon substrate and successful growth and switching of perpendicular CoFeB multilayers on BiSe films provide an avenue for the use of BiSe as a spin density generator in SOT-based memory and logic devices.

We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films -Bi 2 Se 3 and (Bi,Sb) 2 Te 3 -deposited by molecular beam epitaxy on Y 3 Fe 5 O 12 thin films. By systematically varying the Bi 2 Se 3 film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse RashbaEdelstein effect length (λ IREE ), increases dramatically as the film thickness is increased from 2 quintuple layers, saturating above 6 quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological insulator/ferromagnet heterostructures.Our conclusion is further corroborated by studying a series of Y 3 Fe 5 O 12 /(Bi,Sb) 2 Te 3 heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse RashbaEdelstein signals to determine the effective interfacial spin mixing conductance and λ IREE .

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS 2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS 2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS 2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS 2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS 2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS 2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS 2 field effect transistors exhibited an I ON / OFF of ∼10 7 and mobility of 16 cm 2 /(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

We report electrical measurements of the current-induced spin polarization of the surface current in topological insulator devices where contributions from bulk and surface conduction can be disentangled by electrical gating. The devices use a ferromagnetic tunnel junction (permalloy/Al 2 O 3 ) as a spin detector on a back-gated (Bi,Sb) 2 Te 3 channel. We observe hysteretic voltage signals as the magnetization of the detector ferromagnet is switched parallel or anti-parallel to the spin polarization of the surface current. The amplitude of the detected voltage change is linearly proportional to the applied DC bias current in the (Bi,Sb) 2 Te 3 channel. As the chemical potential is tuned from the bulk bands into the surface state band, we observe an enhancement of the spin-dependent voltages up to 300% within the range of the electrostatic gating. Using a simple model, we extract the spin polarization near charge neutrality (i.e. the Dirac point).