Precessional frequency tuning in Ni81Fe19/(Ni81Fe19)1-xEux (50 nm) bilayers

We present a study of the precessional dynamics of exchange coupled Ni₈₁Fe₁₉/Ni₈₁Fe₁₉:Eu_x bilayers, 50 nm thick. Eu codopants are of interest in soft ferromagnetic films since they can be used to enhance the gigahertz precessional frequency (f_p) in Ni₈₁Fe₁₉ thin films without contributing to damping. Underdamped precessional dynamics with f_p "tunable" by as much as roughly 0.3 GHz were observed. Dynamical behavior agrees well with a single-domain Landau-Lifshitz simulation at constant relaxation rate λ/4π=150 MHz across the series. The variation of precessional frequency with bias field shows that the enhancement of resonant frequency arises from an increase of dynamic anisotropy H_K, matched well by static H_K as measured by SQUID. An increasing contribution in g factor (2.1-2.4) is mostly balanced by a decreasing saturation magnetization M_s.