Dynamics over 4 GHz of spring-magnet type NiFe-CoFe bilayers with high permeability

Bilayers of spring-magnet type with soft and hard layers such as NiFe-Co and NiFe-CoFe for pinned and free layers are a key structure of GMR spin-valves transducers. Advanced magnetics for integrated micro-inductors and transformers or for magnetic write heads required for ultrahigh density recording imply soft high saturation magnetization 4πM_s with a large real part μ\´ at high frequency up to or higher than 1 GHz. In this paper, we studied the dynamical behavior of such bilayers from their complex permeability spectra (μ\´, μ") investigated in a broad frequency range up to 6 GHz. μ\´ and μ" of the Ni₈₀Fe₂₀ layer are strongly sensitive to the Co and CoFe layers even or mainly at very small thickness starting from t_Co,CoFe=0.5 nm. Due to an increase with t_Co and t_CoFe of the effective 4πM_s and H_Ku of the bilayer the rolloff frequency associated with the FMR frequency ω_r/2π=γ(4πM_S×H_Ku)¹2/ of the NiFe layer can be increased from 0.65 GHz by a factor 6 (4 GHz) without dramatic reduction of the real part of the susceptibility μ\´=4πM_S/H_Ku.