Observation of field-free spin–orbit torque switching in a single crystalline (Ga,Mn)(As,P) ferromagnetic film with perpendicular anisotropy

We report the observation of field-free spin–orbit torque (SOT) magnetization switching in a single layer of (Ga,Mn)(As,P) ferromagnetic film exhibiting perpendicular magnetic anisotropy. The SOT switching phenomenon is characterized by distinct transitions between two Hall resistance (HR) states during current scans. When subjected to an in-plane bias field, the observed switching chirality in the HR hysteresis loop consistently aligns with SOT induced by spin polarization arising from Rashba- and Dresselhaus-type spin–orbit fields within the tensile-strained crystalline structure of the (Ga,Mn)(As,P) film. Remarkably, in the present experiments, we observe SOT switching even in the absence of an external bias field, and with its chirality depending on the direction of initial magnetization. We attribute this field-free switching to symmetry breaking facilitated by an internal coupling field, the orientation of which is determined by the external field experienced as the magnetization is initialized. Further evidence supporting the presence of such a coupling field includes a shift in the field-scan HR hysteresis depending on the direction of initialized magnetization. Structural analysis reveals a surface layer enriched in Mn and O, indicating the presence of oxide-based magnetic structures that are magnetically coupled to the (Ga,Mn)(As,P) film. The temperature dependence of field-free SOT switching corroborates this explanation, as the internal coupling field disappears above 40 K, consistent with the expected magnetic transition of the Mn₃O₄ structure. Our discovery of field-free SOT magnetization switching in a single-layer film represents a significant advancement, offering a novel pathway for the development of simpler and more energy-efficient spintronic devices.