Ferromagnetic resonance study of structure and relaxation of magnetization in NiFe/Ru superlattices

The structural properties and relaxation processes of magnetization in [Ni81Fe19(t1)/Ru(t2)]N superlattices (N=number of bilayers) were analyzed by ferromagnetic resonance (FMR) with a fixed microwave frequency. One series of samples was deposited with constant NiFe layer thickness (t1) and variable Ru layer thickness (t2); the other series, with constant t2 and variable t1. A single FMR mode was observed for t2<15 Å and t1>75 Å and it has been attributed to the resonance of the exchange-coupled NiFe layers across the Ru interlayers. For the other values of t1 and t2, several FMR modes appeared and they were associated to non-coupled magnetic phases with different effective magnetization formed during the multilayer growth. The FMR linewidths were analyzed as a function of the magnetic layer thickness and a strong dependence on t1−2 was observed. It was attributed to the contribution of the two-magnon scattering mechanism for the linewidth.