Roadmap of spin-orbit torques (2104.11459v2)

Abstract: Spin-orbit torque (SOT) is an emerging technology that enables the efficient manipulation of spintronic devices. The initial processes of interest in SOTs involved electric fields, spin-orbit coupling, conduction electron spins and magnetization. More recently interest has grown to include a variety of other processes that include phonons, magnons, or heat. Over the past decade, many materials have been explored to achieve a larger SOT efficiency. Recently, holistic design to maximize the performance of SOT devices has extended material research from a nonmagnetic layer to a magnetic layer. The rapid development of SOT has spurred a variety of SOT-based applications. In this Roadmap paper, we first review the theories of SOTs by introducing the various mechanisms thought to generate or control SOTs, such as the spin Hall effect, the Rashba-Edelstein effect, the orbital Hall effect, thermal gradients, magnons, and strain effects. Then, we discuss the materials that enable these effects, including metals, metallic alloys, topological insulators, two-dimensional materials, and complex oxides. We also discuss the important roles in SOT devices of different types of magnetic layers. Afterward, we discuss device applications utilizing SOTs. We discuss and compare three-terminal and two-terminal SOT-magnetoresistive random-access memories (MRAMs); we mention various schemes to eliminate the need for an external field. We provide technological application considerations for SOT-MRAM and give perspectives on SOT-based neuromorphic devices and circuits. In addition to SOT-MRAM, we present SOT-based spintronic terahertz generators, nano-oscillators, and domain wall and skyrmion racetrack memories. This paper aims to achieve a comprehensive review of SOT theory, materials, and applications, guiding future SOT development in both the academic and industrial sectors.

Comprehensive Analysis of Spin-Orbit Torques and Their Technological Implications

The manuscript provides an in-depth survey of spin-orbit torques (SOTs), offering a critical evaluation of their theoretical foundations, material requirements, and potential applications in spintronic devices. Spintronic technologies leverage electron spin to achieve functionalities beyond those permissible by classical charge-based systems, and SOTs present a novel mechanism for spin manipulation, transcending traditional methods like the spin-transfer torque (STT).

Theoretical Foundations of Spin-Orbit Torques

The paper elucidates on various theoretical underpinnings of SOTs, such as the spin Hall effect and Rashba-Edelstein effect, both of which are predicated on spin-orbit coupling. The former generates a transverse spin current, while the latter results in spin accumulation at interfaces. A comprehensive understanding of these effects provides a foundation for investigating SOTs, which are further illuminated by first-principles calculations. The prospect of utilizing orbital Hall effects, which could either complement or oppose conventional spin currents, introduces a new dimension to reducing power consumption and optimizing device efficiency.

Material Considerations for Spin-Orbit Torques

Selection of materials is pivotal for optimizing SOT efficiency. The paper categorizes materials into metals, metallic alloys, topological insulators, 2D materials, oxides, antiferromagnets, and ferrimagnets. Each class is evaluated for its potential to enhance SOTs through intrinsic properties or structural design, such as inducing large spin Hall angles or low damping coefficients. For instance, heavy metals like Pt and Ta are renowned for their robust SOT efficiencies, while topological insulators offer intriguing surfaces states with potential for high-efficiency applications.

Device Applications and Future Prospects

A substantial portion of the survey is devoted to the colonization of SOTs into various device architectures. SOT-MRAM (Magnetic Random-Access Memory) is critically reviewed for its potential to supersede STT-MRAM due to its favorable endurance and speed profiles. The discussion extends to two-terminal configurations, neuromorphic devices, magnetization switching technologies, and terahertz generation, all exemplifying the versatility of SOTs in modern technology.

The discussion further explores advanced SOT applications, including domain wall and skyrmion manipulation, critical for emerging non-volatile and logic applications. The implications of field-free switching mechanisms, arising from symmetry considerations and effective magnetic anisotropy manipulations, highlight ongoing efforts to integrate SOTs into scalable memory systems. Additionally, the paper addresses fundamental technological hurdles toward industrial implementation, emphasizing the necessity for improved write efficiency and layer processing techniques.

Implications for Future Developments in Spintronics

The prospects of SOTs in spintronic applications hold profound theoretical and practical ramifications. The potential to achieve a comprehensive understanding of SOT physics could lead to substantial advancements in memory, logic, and sensing technologies. On the theoretical front, SOTs challenge existing paradigms of angular momentum transfer, pushing the boundaries of magnetic switching dynamics under energy-efficient conditions.

The manuscript implicitly underscores the ongoing need for interdisciplinary collaboration to enhance material synthesis, fabrication processes, and device integration. The ambitious vision of SOT-based applications for extending Moore's Law suggests significant future contributions to energy-efficient, high-density magnetic memories.

In conclusion, this extensive roadmap consolidates the foundation of SOTs while mapping out a realistic trajectory towards the integration of spintronics into next-generation technologies. The potential developments in this domain promise transformative impacts on computational and memory solutions across a multitude of sectors.