Hexagonal ferrites for use in microwave notch filters and phase shifters

Recently there has been interest in small, planar, high frequency (10–50 GHz) signal processing devices which are based on metallic ferromagnets. This paper, in contrast, presents theoretical results for devices utilizing a hexagonal ferrite. The notch filter results show attenuation of frequencies in the 40–60 GHz range with applied fields in the 1–5 kOe range. For geometries similar to the ultrasmall metallic devices, the transmission loss at the notch can be from 20 to 140 dB/cm depending on the thickness of the hexagonal ferrite film and the inclusion of dielectric spacers. The phase shifter results show phase shifts of up to 360°, with losses below 2 dB/cm. These results are obtained for devices using thin films around 1 μm in thickness for the hexagonal ferrite, and for reasonable linewidths below 200 Oe.