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Abstract 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⁻¹through 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. 

Unidirectional spin Hall magnetoresistance (USMR) is a magnetoresistance effect with potential applications to read two-terminal spin–orbit-torque (SOT) devices directly. In this work, we observed a large USMR value (up to 0.7 × 10 −11 per A/cm 2 , 50% larger than reported values from heavy metals) in sputtered amorphous PtSn 4 /CoFeB bilayers. Ta/CoFeB bilayers with interfacial MgO insertion layers are deposited as control samples. The control experiments show that increasing the interfacial resistance can increase the USMR value, which is the case in PtSn 4 /CoFeB bilayers. The observation of a large USMR value in an amorphous spin–orbit-torque material has provided an alternative pathway for USMR application in two-terminal SOT devices. 

Recent advancement in the switching of perpendicular magnetic tunnel junctions with an electric field has been a milestone for realizing ultra-low energy memory and computing devices. To integrate with current spin-transfer torque-magnetic tunnel junction and spin–orbit torque-magnetic tunnel junction devices, the typical linear fJ/V m range voltage controlled magnetic anisotropy (VCMA) needs to be significantly enhanced with approaches that include new materials or stack engineering. A possible bidirectional and 1.1 pJ/V m VCMA effect has been predicted by using heavily electron-depleted Fe/MgO interfaces. To improve upon existing VCMA technology, we have proposed inserting high work function materials underneath the magnetic layer. This will deplete electrons from the magnetic layer biasing the gating window into the electron-depleted regime, where the pJ/V m and bidirectional VCMA effect was predicted. We have demonstrated tunable control of the Ta/Pd(x)/Ta underlayer's work function. By varying the Pd thickness (x) from 0 to 10 nm, we have observed a tunable change in the Ta layer's work function from 4.32 to 4.90 eV. To investigate the extent of the electron depletion as a function of the Pd thickness in the underlayer, we have performed DFT calculations on supercells of Ta/Pd(x)/Ta/CoFe/MgO, which demonstrate that electron depletion will not be fully screened at the CoFe/MgO interface. Gated pillar devices with Hall cross geometries were fabricated and tested to extract the anisotropy change as a function of applied gate voltage for samples with various Pd thicknesses. The electron-depleted Pd samples show three to six times VCMA improvement compared to the electron accumulated Ta control sample.