A study of the magnetic properties and high-frequency characteristics of Fe-Co/FeCoB-based bilayer films

The magnetic properties and high-frequency characteristics of sputtered Fe-Co/FeCoB-based bilayer films have been studied. For the Fe55Co45 top layer sputtered on 180 nm (Fe0.55Co0.45)(90)B-10 films, with increasing the thickness t of the top layer from 0 to 40 nm, the magnetic amisotropy field, (H-k) is improved from 330 to 367 Oe, whereas the saturation magnetization (4 pi M-s) is increased from 17.2 to 17.9 kG. On the other hand, for films with Fe65Co35 as the top layer, the 4 pi M-s of the bilayer film is enhanced by 4.1% for t = 40 nm, while H-k is increased to 349 Oe for t = 10 nm at first and is decreased to 333 Oe for t = 40 nm. Furthermore, the H-c of the bilayer films could be reduced by doping Ti and Nb in the (Fe0.55Co0.45)(90)B-10 layer. In this study, the Fe65Co35 (20 nm)/(Fe0.55Co0.45)(90)B6Ti2Nb2 (180 nm) bilayer film shows the optimum properties of 4 pi M-s = 16.4 kG, H-ce = 14.8 Oe, H-ch = 12.6 Oe, H-k = 197 Oe, rho = 217 mu Omega cm and f(FRM, cal.) = 5.1 GHz. The permeability of the film is as large as 180 in the frequency range of 1-3 GHz.