CoFeSiO/SiO $_{2}$ Multilayer Granular Films With Very Narrow Ferromagnetic Resonant Linewidth

In order to fabricate GHz-band micro-magnetic devices, we have investigated the high-frequency permeability of CoFeSiO/SiO₂ multilayer granular films comprising alternate stacks of granular and insulator layers, which can enable us to control the magnetic grain size and intergrain spacing along the film thickness direction independent of each other. A very narrow ferromagnetic resonance (FMR) linewidth was obtained under specific film conditions. A sharp FMR peak was obtained for granular layer and insulator layer thicknesses of 6 and 1 nm, respectively. The CoFe/SiO₂ volume ratio of the granular layer has a strong influence on the FMR linewidth. A high CoFe/SiO₂ volume ratio resulted in a very narrow FMR linewidth. HR-TEM observation of a film with a high CoFe/SiO₂ volume ratio revealed a well-defined multilayer granular structure and a homogeneous CoFe grain size, which seem to be necessary for obtaining a very narrow FMR linewidth. The narrowest FMR linewidth observed in this study is 420 MHz, for an FMR frequency of 2.4 GHz, a permeability of 380, and a damping factor alpha of 0.007.