Spin-orbit torque driven multi-level switching in He⁺irradiated W-CoFeB-MgO Hall bars with perpendicular anisotropy

We have investigated the spin-orbit torque-driven magnetization switching in W/CoFeB/MgO Hall bars with perpendicular magnetic anisotropy. He+ ion irradiation through a mask has been used to reduce locally the effective perpendicular anisotropy at a Hall cross. Anomalous Hall effect measurements combined with Kerr microscopy indicate that the switching process is dominated by domain wall (DW) nucleation in the irradiated region followed by rapid domain propagation at a current density as low as 0.8 MA/cm2 with an assisting in-plane magnetic field. Thanks to the implemented strong pinning of the DW at the transition between the irradiated and the non-irradiated region, an intermediate Hall resistance state is induced, which is further verified by finite element simulations. Such a method to control electrically multi-level resistances using He+ ion irradiation shows great potential in realizing neuromorphic and memristor devices.