Enhanced Voltage-Controlled Magnetic Anisotropy and Field-Free Magnetization Switching Achieved with High Work Function and Opposite Spin Hall Angles in W/Pt/W SOT Tri-Layers

Voltage-Gated Spin-Orbit-Torque (VGSOT) Magnetic Random-Access Memory (MRAM) is a promising candidate for reducing writing energy and improving writing speed in emerging memory and in-memory computing applications. However, conventional Voltage Controlled Magnetic Anisotropy (VCMA) approaches are often inefficient due to the low VCMA coefficient at the CoFeB/MgO interface. Additionally, traditional heavy metal/perpendicular magnetic anisotropy (PMA) ferromagnet bilayers require an external magnetic field to overcome symmetry constraints and achieve deterministic SOT switching. Here, a novel and industry-compatible SOT underlayer for next-generation VGSOT MRAM by employing a composite heavy metal tri-layer with a high work function is presented. This approach achieves a VCMA coefficient exceeding 100 fJ V⁻¹m⁻¹ through electron depletion effects, which is ten times larger than that observed with a pure W underlayer. Furthermore, it is demonstrated that this composite heavy metal SOT underlayer facilitates the integration of VCMA with opposite spin Hall angles, enabling field-free SOT switching in industry-compatible PMA CoFeB/MgO systems.