Reversible and Non‐Volatile Manipulation on the Spin‐Orbit Torque in 3<i>d</i>‐5<i>d</i> System of <i>L</i>1₀‐FePt via Hydrogen Migration

Abstract Current‐induced spin‐orbit torque (SOT)‐driven magnetization switching is a cornerstone for future low‐energy‐consumption spintronics. However, achieving effective and controllable SOT remains a significant challenge. Here, a highly modulable SOT‐device is demonstrated based on the 3 d ‐5 d alloy system L 1 0 ‐FePt, where the critical current density is modulated by ≈43% through the application of a gate voltage from a solid‐state hydrogen electrolyte. The harmonic measurements reveal that the SOT efficiency of L 1 0 ‐FePt film is enhanced by over 65% under a positive gate voltage and fully recovered under a negative voltage. The underneath hydrogen ion (H + ) migration induced by the gate voltage is confirmed by X‐ray photoelectron spectroscopy (XPS) and depth‐resolved secondary ion mass spectroscopy (SIMS). Furthermore, first‐principles calculations uncover a novel spin‐current generation mechanism in the special 3 d ‐5 d alloy L 1 0 ‐FePt, where both 3 d and 5 d electrons contribute large but competing spin currents, with the net effect dominated by the 3 d electrons. The insertion of H + enhances the spin Hall conductance by 130%, primarily due to the significant enhancement of the reactive 3 d electrons. These findings pave the way for modulable, non‐volatile, and low‐power spintronic and iontronic devices.