Effect of interface roughness on exchange coupling in synthetic antiferromagnetic multilayers

FeCo/Ru/FeCo synthetic antiferromagnetic (SAF) multilayers with different FeCo/Ru interface roughnesses were prepared by varying the sputtering conditions. The films were characterized by hysteresis loops and torque magnetometry. A large remanence observed in the hard direction for the SAF with interface root-mean-square (RMS) roughness of 0.83 nm suggested a large biquadratic exchange coupling, while the SAF with RMS interface roughness of 0.49 showed primarily bilinear behavior. From field dependent torque curves, assuming coherent rotation, the values of J₁ and J₂ were found to be 0.35 and 0.05 erg/cm², respectively, in the smooth SAF compared to 0.13 and 0.16 erg/cm² in the rough SAF. The results are in reasonable agreement with the values obtained from the hysteresis loops and are consistent with the model for apparent biquadratic exchange arising from ferromagnetic coupling through pinholes in the Ru spacer layer.