Magnetic domain wall motion driven by spin-orbit torque based on non-collinear spin source

Abstract

<p indent="0mm">Recently, the current-induced motion of magnetic domain walls has attracted much attention. It enables electrical control of domain wall motion without the application of an external magnetic field and thus has potential applications in high-performance magnetic racetrack memories, among others. The effective control of domain wall motion under zero magnetic field is one of the important topics in spintronics research. Current-driven magnetization switching via the spin-orbit torque effect is an important approach to realizing the field-free domain wall motion. The traditional research structure of spin-orbit torques is non-magnetic material/magnetic material (NM/FM) heterogeneous films, in which non-magnetic materials are used as the spin source layer to generate and provide spin current through charge-to-spin current conversion. Common non-magnetic materials mainly include heavy metals, such as Pt, β-Ta and β-W. However, in the system with heavy metals as spin sources, the conventional spin Hall effect requires the charge current, spin current and spin polarization directions to be perpendicular to each other, so it is necessary to apply an in-plane auxiliary magnetic field to break the symmetry, especially when driving the perpendicular magnetic moment switching, which increases extra power consumption and is not conducive to practical applications. Recently, it has been found that the spin current generated by the charge current in the outer semimetal WTe₂ is spin polarized in the <bold><italic>z</italic></bold>-direction. It has a unique in-plane symmetry breaking property, where the damping-like torque can contribute more to the domain wall motion of a ferromagnetic layer with perpendicular magnetic anisotropy. Moreover, a unique Rashba-like spin polarization has recently emerged in the noncollinear magnetic structure Mn₃GaN. A charge current is applied in the <bold><italic>x</italic></bold>-direction to generate a spin current in the <bold><italic>z</italic></bold>-direction, with spin polarization along the <bold><italic>x</italic></bold>-direction. The spin polarization along the <bold><italic>x</italic></bold>-direction produces damping-like torque (<bold><italic>τ</italic></bold>_{<italic>x</italic>}_,DL) and field-like torque (<bold><italic>τ</italic></bold>_{<italic>x</italic>}_,FL) in the adjacent in-plane and PMA ferromagnetic layers. The magnetization dynamics of the SOT mentioned above may be quite different. To realize the effectively current-induced spin-orbit torque driven domain wall motion, it is important to understand the spin source with unique physical properties of charge-to-spin conversion. In this work, we study the spin-orbit torque driven domain wall motion by micromagnetic simulations. We consider the mixture of non-collinear spin torques of Rashba-like <bold><italic>S</italic></bold><italic>_y</italic>, Dresselhaus-like <bold><italic>S</italic></bold><italic>_x</italic>, and out-of-plane <bold><italic>S</italic></bold><italic>_z</italic>. The effects of three different domain wall types: Bloch domain wall, Néel domain wall and head-to-head domain wall on motion velocity and the inclination angles of magnetic domain walls are demonstrated. The results for the same magnetic domain show that the current-induced spin polarization perpendicular to the membrane surface generates a damping-like torque perpendicular to the membrane surface, which can efficiently drive the motion of the domain wall. However, in the presence of multiple spin torques, the unconventional damping-like torques in the direction perpendicular to the film surface can be realized to drive domain wall motion at zero field. Our results may provide a new degree of freedom for understanding current-induced spin-orbit torque-driven domain wall motion from non-collinear spin sources for potential applications in spintronic devices.