Nonlinear ferromagnetic resonance linewidth broadening due to the interlayer exchange coupling in synthetic antiferromagnets

Abstract

We report on an unusual extrinsic contribution to the broadening of the ferromagnetic resonance (FMR) linewidth in Co/Ru/Co synthetic antiferromagnets. In samples with antiferromagnetic (AFM) interlayer exchange coupling (IEC), a nonlinear dependence of the FMR linewidth (ΔH) on the resonance frequency (fres) with a distinct peak is observed exclusively for the optical mode (OM). Notably, the sample with no IEC does not exhibit this nonlinearity, indicating that IEC plays a crucial role. The temperature dependence of ΔH for the OM exhibits an anomalous behavior, with a maximum around 50–150 K, which is similar to that expected from the slow-relaxing impurity mechanism. These results demonstrate that the IEC can act as an additional relaxation channel for magnetization dynamics in AFM-coupled multilayers, fundamentally distinct from conventional extrinsic contributions.