Enlarged Thickness Window and Maintained High Spin–Orbit Torque Efficiency for Metastable Tungsten by Increasing Amorphous Crystalline: A Path toward Low-Power MRAM

Spin–orbit torque (SOT) magnetic random access memory (MRAM) has emerged as a focal point of research due to its exceptional performance and potential for low energy consumption. However, this technology faces significant challenges, particularly low charge-to-spin conversion efficiency (spin–orbit torque efficiency, ξDL) and corresponding small integration margin issues in heavy metal/ferromagnet (HM/FM) structures. Moreover, the experimental augmentation of ξDL through doping engineering, specifically tungsten, has presented results that deviate from theoretical calculations, necessitating a clarification. Here, tungsten doped with nitrogen (single-layer WN) and multilayer ([W/WN]n) are systematically investigated. In contrast to W and single-layer WN, the [W/WN]n multilayer system exhibits both a relatively high ξDL of −0.41 and two times enlarged thickness window. The enhanced ξDL and expanded thickness window can be attributed to the increase of the amorphous-like crystalline structure in [W/WN]n, which suppresses the formation of α-phase W, and maintains the β-phase even at thicker thicknesses. Meanwhile, SOT magnetic tunnel junctions with [W/WN]n are successfully integrated on the 8 in. wafer, showing 28.8% lower switching current than that in the W-channel with the same thickness. Our study bridges the gap between the experimental observations and theoretical calculations, offering a promising way to enhance the ξDL for application in low-power MRAM devices.