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−xCo_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.