Influence of exchange bias on spin torque ferromagnetic resonance for quantification of spin–orbit torque efficiency

Antiferromagnet (AFM)/ferromagnet (FM) heterostructure is a popular system for studying the spin–orbit torque (SOT) of AFMs. However, the interfacial exchange bias field induces that the magnetization in FM layer is noncollinear to the external magnetic field, namely the magnetic moment drag effect, which further influences the characteristic of SOT efficiency. In this work, we study the SOT efficiencies of IrMn/NiFe bilayers with strong interfacial exchange bias by using spin-torque ferromagnetic resonance (ST-FMR) method. A full analysis on the AFM/FM systems with exchange bias is performed, and the angular dependence of magnetization on external magnetic field is determined through the minimum rule of free energy. The ST-FMR results can be well fitted by this model. We obtained the relative accurate SOT efficiency ξ_DL = 0.058 for the IrMn film. This work provides a useful method to analyze the angular dependence of ST-FMR results and facilitates the accurate measurement of SOT efficiency for the AFM/FM heterostructures with strong exchange bias.