Alloying‐Controlled Tuning of Interfacial Spin‐Orbit Interaction and Magnetic Damping in Crystalline FeCo Thin Films Grown on GaAs(001)

The discovery of intrinsic spin–orbit fields in non‐centrosymmetric ferromagnets have attracted considerable interest for both fundamental studies and technological applications. However, once such materials are synthesized, the strength of the spin‐orbit fields is difficult to tune because it is primarily a bulk property. Here, we demonstrate that the interfacial spin‐orbit interaction (SOI) in single‐crystalline Fe 1− x Co x thin films grown on GaAs(001) can be continuously tuned via alloying. Using spin‐orbit ferromagnetic resonance, we find that the Landé g ‐factor, the Gilbert damping α , and the interfacial spin‐orbit fields exhibit a common nonmonotonic dependence on Co concentration. A pronounced minimum occurs near x ≈ 0.2 where an ultra‐low damping α ≈ 0.0015 is achieved. Furthermore, we observe linear scaling between α and ( g − 2) 2 , suggesting that interfacial SOI may also contribute to magnetic relaxation. These results identify alloying as an effective knob to engineer interfacial SOI and damping in single crystalline ferromagnet/semiconductor heterostructures.