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 4/spl pi/M/sub s/ with a large real part /spl 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 (/spl mu/', /spl mu/") investigated in a broad frequency range up to 6 GHz. /spl mu/' and /spl mu/" of the Ni/sub 80/Fe/sub 20/ layer are strongly sensitive to the Co and CoFe layers even or mainly at very small thickness starting from t/sub Co,CoFe/=0.5 nm. Due to an increase with t/sub Co/ and t/sub CoFe/ of the effective 4/spl pi/M/sub s/ and H/sub Ku/ of the bilayer the rolloff frequency associated with the FMR frequency /spl omega//sub r//2/spl pi/=/spl gamma/(4/spl pi/M/sub S//spl times/H/sub Ku/)/sup 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 /spl mu/'=4/spl pi/M/sub S//H/sub Ku/.