Iron-based microstrip band-stop filters at higher microwave frequency range: Design optimization using shape anisotropy

Microwave filters that use thin films of ferromagnetic metals are now being established as an option compared to YIG based filters due to their higher frequency response at very small magnetic fields. The aim of the present investigation is to significantly boost the operating frequency of notch filters in very low applied magnetic fields. To do this, we fabricated a series of notch filters with Fe films of different thicknesses and shapes. The stop-band response of each of these filters cover four waveguide bands (X, KU, K and Ka) ranging from 8 to 40 GHz with an applied magnetic field up to 4.5 kOe. The frequency characteristics of these filters at zero field can be significantly changed by changing the geometry of the magnetic element. For example, an Fe film which is 7.5 mu m wide, 100 nm thick, and 3 mm long has a stop band centered at 9.5 GHz, but this is increased to 22 GHz for an 800 nm thick Fe film. For narrower signal lines (4.5 mu m wide), we observed further increases in the frequencies: 11 GHz for a 100 nm Fe film increased to 25 GHz for an 800 nm thick Fe film. This upshift in frequency is due to the shape anisotropy of the Fe film. In addition, we investigated the temperature variations in the properties of the devices from room temperature to +85 degrees C (C) 2006 American Institute of Physics.