  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.
