Spin orbit torque-driven motion of quasi-Bloch domain wall in perpendicularly magnetized W/CoFeB/MgO structure

The motion of chiral magnetic domain walls (DWs) driven by spin–orbit torque (SOT) has been extensively studied in heavy metal/ferromagnet heterostructures with perpendicular magnetic anisotropy. This study specifically focuses on SOT-driven DWs in near Bloch-states, which we refer to as "quasi-Bloch DWs". These quasi-Bloch DWs exhibit slower motion compared to Néel-type DWs, offering potential for achieving highly controllable DW positions. Here, we investigate the characteristics of SOT-driven motion of quasi-Bloch DWs in perpendicularly magnetized ultra-thin films consisting of W/CoFeB/MgO. For analyzing the DW motion, we employ a one-dimensional model incorporating parameters derived from experimental data obtained from our samples. Our model successfully reproduces the experimental results, which reveal variations in the direction and threshold current density of DW motion among different samples. Through theoretical analysis, we unveil that the DW remains in quasi-Bloch states during motion, with SOT serving as the primary driving force rather than spin transfer torque (STT). The direction of motion is determined not only by the sign combination of Dzyaloshinskii–Moriya interaction (DMI) and spin Hall angle but also by the strength of DMI, STT, and extrinsic DW pinning. Furthermore, we provide the analytical expression for the threshold current density required for SOT-driven quasi-Bloch DW motion. These findings provide valuable insights for the design of future DW devices with specific film structures.