Out-of-plane spin polarization in antiferromagnets for emerging spintronic devices

SOTs (Spin–Orbit Torque) have emerged as the primary pathway in achieving low power, energy-efficient, nonvolatile memory, fast write/read speed, and improved endurance in the pursuit of technological advancements in magnetic random access memory (MRAM), logic devices. Over the past few decades, considerable attention and effort have been given to switch the out-of-plane magnetization of a perpendicular magnetic anisotropy (PMA) material without an external magnetic field, primarily using the out-of-plane SOT generated mostly in lateral symmetry broken materials. SOT gives novel applications, such as magnetization switching, sustained oscillations, excitation of spin waves, and fast domain wall motion. Antiferromagnets (AFM) have the exotic property of generating out-of-plane spin polarization because of magnetic mirror symmetry breaking and this property has been utilized to realize the field-free magnetization switching of PMA materials. Due to the absence of stray fields, excellent thermal stability, and ultra-fast dynamics, AFM materials are also widely regarded as a promising choice for high-density, fast, and reliable storage solutions. However, due to the insensitivity to an applied magnetic field, both the control and the detection of the magnetization state in AFM material are very difficult and challenging. The anomalous Hall effect provides a convenient way to detect the magnetization state, thereby giving MRAM a favorable option as an alternative to ferromagnets. Most of the AFM materials have shown a giant SOT efficiency compared to those 3D heavy metals and act as an efficient source of a current-induced out-of-plane spin polarization, which is only allowed by the reduced magnetic mirror symmetry in the system. In this review, we provide an overview of the most recent advancements in unconventional SOT investigations involving AFM and demonstrate their possible reason and futuristic applications in the external field-free magnetization switching of PMA materials for MRAM and logic applications. Finally, we pinpoint outstanding challenges within the existing research and propose a few promising avenues for further exploration in dense memory storage applications using AFM materials.