Spin–Orbit Torques Based on Topological Materials

Abstract

ConspectusWidespread applications of magnetic devices, such as magnetic random-access memory (MRAM), logic-in memory, and neuromorphic computing devices, require an efficient method to manipulate the local magnetization. One superior mechanism is the spin–orbit torque (SOT) associated with spin currents generated by charge currents in a material with strong spin–orbit coupling. A higher SOT efficiency (i.e., a higher charge-to-spin conversion efficiency) is crucial for low-power SOT device operation. Therefore, materials possessing the merit of a high SOT are urgently desired for the innovation of SOT devices. Among all of the SOT materials, topological materials such as topological insulators and topological semimetals have attracted considerable attention recently because of their nontrivial band structures with strong spin–orbit coupling and expected giant SOT. Therefore, topological materials are regarded as promising candidates for future energy-efficient device applications. However, research in the area of topological material-based spintronics is still in an early stage, and the related physical, material and device issues still need to be addressed. In this Account, we review our recent progress regarding the charge-to-spin conversion and SOT-driven magnetization switching using the emerging topological materials, with an emphasis on topological insulators and Weyl semimetals. First, we introduce the extraordinary physical features of single-layer topological insulators associated with electron transport, charge-spin interconversion, and unique spin textures and subsequently present our SOT results for topological materials mainly for Bi2Se3. This material shows pronounced SOT with the in-plane and out-of-plane component due to the spin momentum locking and hexagonal warping effect, respectively. The SOT can be efficiently modulated by varying the film crystal directions or by interface engineering between Bi2Se3 and a ferromagnet. Thereafter, room-temperature magnetization switching by the electron-mediated or magnon-mediated spin torque is demonstrated in topological insulator-based devices, and the switching current density JC is on the order of ∼105 A/cm2, which is approximately 1 to 2 orders of magnitude smaller than that in heavy metals. Second, we summarize our SOT results in Weyl semimetals. Due to the broken crystal symmetry, Weyl semimetals (e.g., WTe2) can show an additional sizable out-of-plane spin polarization, which can be detected by electrical and optical techniques. Therefore, the Weyl semimetals can possess unconventional out-of-plane damping-like SOT, which will facilitate the field-free magnetization switching. The interface Dzyaloshinskii–Moriya interaction (DMI) is observed in Weyl semimetal/ferromagnet heterostructures, which can affect the domain wall motion. We also demonstrate the energy-efficient SOT-induced magnetization switching in Weyl semimetal-based devices. Third, we discuss the advances in SOT devices with wafer-scale topological materials prepared by industry-compatible techniques such as magnetron sputtering and chemical vapor deposition. These films may have stoichiometry similar to that of single-crystalline topological materials and SOTs as large as in single-crystalline topological materials. Finally, we present our perspectives for practical applications using this emerging family of quantum materials. We anticipate that this Account will deepen the understanding of SOT on topological materials, and our insights will guide the design and applications of high-performance topological material-based SOT devices.