Magnetically tunable microwave circuits using iron films

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Microwave/millimeter wave transmission characteristics of microstrip lines with a single and multilayered iron films are analyzed to examine the bandstop characteristics caused by applying a dc bias magnetic field along the line. The propagating microwave signal is attenuated around the ferromagnetic resonance frequency of the iron, so the line shows bandstop characteristics. The center frequency of the stop band is tunable over the range of 10 to 40GHz by the bias magnetic field up to 8000 Oersted.First, the microstrip line is modeled as a two-dimensional waveguide, and the dispersion relation is derived by the hybrid analysis after introducing the permeability tensor of iron. Numerical calculations have been done for various parameters and their combinations, such as the thicknesses of the iron films and the substrate, and the saturation magnetizations of the iron. Focus is put on broadening the bandwidth of the stopband. Some discussions are given on the field distributions linked to the attenuation characteristics.Next, the propagation characteristics considering the effect of the strip width are calculated using the spectral domain approach after introducing the effective permeability of the iron which can be derived from the tensor permeability. The attenuation characteristics around the stopband for the three-dimensional waveguide are compared with the former two-dimensional case. It is confirmed that the wider strip width gives the less peak absorption. The current distributions on the strip conductor are also shown.