Current-induced spin-orbit torques (SOTs) are of interest for fast and energy-efficient manipulation of magnetic order in spintronic devices. To be deterministic, however, switching of perpendicularly magnetized materials by SOT requires a mechanism for in-plane symmetry breaking. Existing methods to do so involve the application of an in-plane bias magnetic field, or incorporation of in-plane structural asymmetry in the device, both of which can be difficult to implement in practical applications. Here, we report bias-field-free SOT switching in a single perpendicular CoTb layer with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and a gradient-driven Dzyaloshinskii–Moriya interaction (g-DMI), which breaks the in-plane symmetry during the switching process. Micromagnetic simulations are in agreement with experimental results, and elucidate the role of g-DMI in the deterministic switching processes. This bias-field-free switching scheme for perpendicular ferrimagnets with g-DMI provides a strategy for efficient and compact SOT device design.

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Spintronic devices use spin instead of charge to process information and are widely considered as promising candidates for next-generation electronic devices. In past decades, the main motivation in spintronics has been to discover new mechanisms and novel material systems to improve both device performance and the application prospects of spintronics. Recently, researchers have found that ferrimagnetic materials—in which sublattices are coupled antiferromagnetically—offer an emerging platform for realizing high-density, high-speed, and low-power-consumption memory and logic functions. Within such a ferrimagnetic class, vanishing magnetization and ultrafast magnetic dynamics can be achieved by adjusting chemical composition and temperature, among other parameters. Meanwhile, unlike for antiferromagnets, conventional electrical read–write methods remain suitable for ferrimagnets, which is beneficial for applications. In this review, an abundant class of ferrimagnets including oxides and alloys is surveyed, and unique magnetic dynamics and effective methods for manipulating the magnetic states of ferrimagnets are discussed. Finally, novel storage and computing devices based on ferrimagnets are considered, as there are some challenges to be addressed in future applications of ferrimagnets.

Spin–orbit torque (SOT) effect is considered as an efficient way to switch the magnetization and can inspire various high-performance spintronic devices. Recently, topological insulators (TIs) have gained extensive attention, as they are demonstrated to maintain a large effective spin Hall angle ( θ SH eff ), even at room temperature. However, molecular beam epitaxy (MBE), as a precise deposition method, is required to guarantee favorable surface states of TIs, which hinders the prospect of TIs towards industrial application. In this paper, we demonstrate that Bi2Te3 films grown by magnetron sputtering can provide a notable SOT effect in the heterostructure with perpendicular magnetic anisotropy CoTb ferrimagnetic alloy. By harmonic Hall measurement, a high SOT efficiency (8.7 ± 0.9 Oe/(109 A/m2)) and a large θ SH eff (3.3±0.3) are obtained at room temperature. Besides, we also observe an ultra-low critical switching current density (9.7×109 A/m2). Moreover, the low-power characteristic of the sputtered Bi2Te3 film is investigated by drawing a comparison with different sputtered SOT sources. Our work may provide an alternative to leverage chalcogenides as a realistic and efficient SOT source in future spintronic devices.

Spin–orbit torque induced ferromagnetic magnetization switching brought by injecting a charge current into strong spin–orbit-coupling materials is an energy-efficient writing method in emerging magnetic memories and spin logic devices. However, because of the short spin coherence length in ferromagnetic layers, the interfacial effective spin–orbit torque typically leads to high critical current density for switching thick ferromagnet, which goes against low-power and high-density requirements. Here, we experimentally demonstrate efficient bulk spin–orbit torque-driven perpendicular magnetization switching under relatively low critical current density in thick Pt/Co multilayers with gradient-induced symmetry breaking. Through tuning the thickness gradient of Pt, the spin–orbit torque efficiency and switching chirality can be highly controlled, which also indicates that net spin current arises from gradient. Meanwhile, x-ray absorption spectroscopy results reveal that the atomic intermixing can significantly enhance the spin–orbit torque efficiency through improving the strength of spin–orbit-coupling of Pt. We also establish a micromagnetic model by taking both gradient-induced and intermixing-enhanced spin–orbit torque into account to well describe the experimental observations. This work would blaze a promising avenue to develop novel spin–orbit torque devices for high-performance spintronic memory and computation systems.