Materials Issues in the Application of Ferromagnetic Films in Tunable High Q VHF/UHF Devices

Abstract

AbstractThin ferromagnetic films have narrow ferromagnetic resonance bandwidths, and large RF permeabilities that are attractive for tunable high Q device applications. They can also be processed at low temperatures and integrated with most RF and microwave materials. At VHF/UHF frequencies, where discrete devices can be realized, magnetic loss tangents (Q−1) less than 10−3 can be obtained, if the effects of spin resonance and eddy current losses are reduced. The material figure of merit requires a tradeoff of the permeability, and Q. The highest FOM is obtained by maximizing the magnetization, anisotropy, resistivity, and minimizing the magnetic loss. This paper discusses the interplay of these properties, and the fill factor and magnetic bias required for tunable inductors with Q>1000 in the ∼50-500 MHz range. The most promising candidates for these applications are shown to be reactively sputtered nanocrystalline films. The large resistivity, magnetization, and thickness of these films can be used to attain large film permeances, and increased shape anisotropy fields from patterned film objects. The latter eliminates the need for using a large bias magnet to achieve Q's ∼1000 at VHF/UHF operating frequencies.