Dimensional Engineering of Co/W Superlattices Enables Out-of-Plane Spin Polarization with 90% Field-Free Switching in Perpendicular CoFeB.

Deterministic spin-orbit torque (SOT) switching of perpendicular magnets without external magnetic fields remains a central challenge for scalable SOT-MRAM. Here, we demonstrate that dimensional engineering of a Co/W superlattice spin-orbit layer enables >90% field-free switching of perpendicular CoFeB. Harmonic Hall measurements reveal an unconventional out-of-plane torque, while first-principles calculations reveal reconstruction of the spin-resolved electronic structure and increased W-projected out-of-plane spin character, together supporting the modification of torque symmetry through Co/W superlattice engineering. The optimized superlattice achieves a critical current density of 23.3 MA cm-2 and a thermal stability factor Δ ≈ 45. Nanoscale devices (100 nm) preserve deterministic field-free reversal with comparable efficiency, confirming scalability toward magnetic tunnel junction integration. Micromagnetic analysis shows that out-of-plane spin-polarized torque together with canted anisotropy enables intrinsic symmetry breaking, highlighting superlattice engineering as a route to tailor torque symmetry for effective field-free SOT devices.