High frequency magnetic properties of FeCoBSi/SiO/sub 2/ and (FeCo/CoB)/SiO/sub 2/ multilayer thin films

Soft magnetic, magnetostrictive (Fe/sub 90/Co/sub 10/)/sub 78/Si/sub 12/B/sub 10/ thin films and Fe/sub 50/Co/sub 50//Co/sub 80/B/sub 20/ multilayers are used in high frequency applications like stress sensors or transmission-line structures. The devices are based on strip-lines with incorporated magnetic thin films, in order to increase the inductance of the strip. However, relatively thick (/spl ap/1 /spl mu/m) magnetic layers are necessary to reach a high inductance. In both investigated applications high losses (imaginary part of permeability /spl mu//sub ac/") are deteriorating the high frequency properties and thus applicability of the devices. One source of losses are eddy currents in the magnetic layers. A possible way to reduce these eddy currents is to fabricate magnetic/dielectric multilayers. Theory predicts important changes in permeability, when comparing the multilayers with a corresponding single layer magnetic film. Fig.1. shows a calculation for a 2/spl mu/m FeCoSiB and a 25/spl times/80 nm/50 nm FeCoSiB/SiO/sub 2/ multilayer. /spl mu/\´ is reduced by a factor of 1.6 whereas /spl mu/" is decreased by a factor of 6.2 at 0.1 GHz. The resonance peak in /spl mu/" is shifted to higher frequencies.