Linear Enhancement of Spin-Orbit Torque and Absence of Bulk Rashba Spin Splitting in Perpendicularly Magnetized Pt/Co/W_n Superlattices

The development of magnetic heterostructures with strong perpendicular magnetic anisotropy (PMA), strong spin-orbit torques (SOTs), low impedance, and good integration compatibility at the same time is central for high-performance spintronic memory and computing applications. Here, we report the development of the PMA superlattice Pt/Co/W_n that can be sputtered-deposited on commercial oxidized silicon substrates and have giant SOTs, strong uniaxial PMA of ≈9.2 Merg/cm³, and rigid macrospin performance. The dampinglike and fieldlike SOTs of the Pt/Co/W_n superlattices exhibit a linear increase with the repeat number n and reach the giant values of 225% and -33% (two orders of magnitude greater than that in clean-limit Pt) at n = 12, respectively. The dampinglike SOT is also of the opposite sign and much greater in magnitude than the fieldlike SOT, regardless of the number of n. These results clarify that the spin current that generates SOTs in the Pt/Co/W_n superlattices arises predominantly from the spin Hall effect rather than bulk Rashba spin splitting, providing a unified understanding of the SOTs in the superlattices. We also demonstrate deterministic switching in thicker-than-50-nm PMA Pt/Co/W₁₂ superlattices at a low current density. This work establishes the Pt/Co/W_n superlattice as a compelling material candidate for ultra-fast, low-power, long-retention nonvolatile spintronic memory and computing technologies.