Spin–orbit torque (SOT) provides an alternative to spin-transfer torque for electrical and flexible control of magnetization, which arouses great interest owing to its potential applications in magnetic random-access memory (MRAM) since its discovery.1,2 Besides nonvolatility and scalability of the spin-transfer torque (STT) MRAM, the SOT technique can further endow an MRAM device with lower energy consumption, higher speed, and higher endurance, which makes the SOT-MRAM an ideal candidate for embedded RAM or Cache applications.3 This prospect gives impetus to the research and development of the SOT effect and corresponding SOT devices.

Much effort has contributed to (1) exploring new materials that exhibit high charge-to-spin conversion efficiency (the SOT efficiency) and (2) designing suitable stack structures to meet requirements on magnetic anisotropy and symmetry-breaking laws for SOT switching. For the first target, besides heavy metals such as Pt, Ta, and W, some materials such as the topologic insulator BixSe1−x4 or Pt1−xAux alloys5 can show spin Hall angles >1 or high-spin Hall conductivity. For the second target, several SOT-MRAM schemes have been proposed, including magnetic tunnel junctions with in-plane6 or perpendicular magnetic anisotropy.7 Also worth noting, various schemes (such as Types Y and X) bring about distinct spin dynamics, which makes MRAM units based on these schemes have diverse performance, for example, in writing efficiency and speed.8 SOT-MRAM units of Type Y normally have smaller critical current density and slower switching speed than the counterpart of Type X if their stack structures are similar.8 More subtly, in such schemes as Types X and Z, an external or effective field is indispensable, which complicates the stack or device structure of an SOT-MRAM unit. Therefore, the real challenge of developing a practical SOT-MRAM relies on fulfilling the above requirements in all the following aspects: symmetry-breaking laws for SOT switching, high SOT efficiency, and simple and compact stack and device structures. Recently, some promising SOT-MRAM units have been reported,6,7,9 and even a 4 Kb SOT-MRAM chip10 has been demonstrated, showing the feasibility of this SOT device. Even so, the above issues are still worth intensive investigation for a more comprehensive performance.

The SOT technique can also be utilized in other promising applications, such as nonvolatile multifunctional programmable spin logic, neuromorphic spintronic devices, random number generators, and nano-oscillators. These applications make use of different characteristics of SOT-switching phenomena, such as reversibility of switching direction driven by SOT, continuous and gradual behavior in an incoherent switching process, statistical switching, and ferromagnetic resonance driven by SOT. Besides a high SOT efficiency, they also raise various demanding requirements for SOT materials, stack structures, and device architectures, which are also worthy of further deep investigations. These challenges are the motivations of this special topic: we summarize a selection of achieved results and potential in applications, and also present some new advances in the study of SOT materials, physics, and devices. Because of the abundance of work in this field, it is impossible for us to review and collect all the corresponding work within the limited length.

This special topic in Applied Physics Letters was proposed in response calling attention to this promising field. It covers some recent advances in the following aspects, as well as a review of pioneering SOT studies.11 

  1. Material engineering for high SOT efficiency.12–30 

  2. Physics to understand spin current transport and magnetization control by SOT.31–47 

  3. Methods to calibrate SOT efficiency.48–50 

  4. Novel device applications using SOT physics and techniques.51–55 

In material optimization, for instance, some 2D materials, like WTe2, have been applied as an efficient spin current source.16 Materials engineering techniques such as alloying15 and interfacial decoration30 are also used to optimize the effective SOT efficiency. In physics, spin memory loss, an important mechanism to relax spin current across an interface, is uncovered relative to bulk spin–orbit coupling of heavy metals.44 Gradual SOT switching desired for neuromorphic computing applications has been unambiguously demonstrated in ion-beam-modified samples and qualitatively modeled.38 Classic optic48 or spin-torque FMR methods40 are also improved upon to calibrate the SOT efficiency of materials more accurately. Especially, several works have been devoted to explore novel devices applications based on the SOT effect, such as nano-oscillators,51 spin-Hall magnetic sensors,53 stochastic artificial neural networks based on SOT true random number generators,54 and a field-free SOT-MRAM element,37 which really emphasize the promise of SOT techniques.

The SOT technique has been steadily enriched and improved from its infant state during the last decade not only in deepening our physical understanding to SOT phenomena and looking for optimum materials with high SOT efficiency but also in broadening potential applications of SOT techniques in the field of microelectronics. Though further investigations are still in great demand for better comprehensive performance, such applications as SOT-MRAM have already been demonstrated feasible and promising. This special topic has been organized with this as its background. Notably, we view this special topic as a smaller and lighter collection rather than an encyclopedic endeavor. Our hope is that it will ignite renewed enthusiasm from academic and industrial sectors to invest more in the technique development of practical SOT-MRAM and other devices in order to rapidly promote the maturity of a myriad of SOT studies spanning from basic physics to sophisticated technologies.

We acknowledge all authors who have contributed to this Special Topic.

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