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 4piM(s) with large real part mu' 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 (mu', mu) investigated in a broad frequency range up to 6 GHz. mu' and mu 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 4piM(s) and H-Ku of the bilayer the rolloff frequency associated with the FMR frequency omega(r)/2pi = gamma(4piM(S) x H-Ku)(1/2) 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 mu' = 4piM(S)/H-Ku.