Ultrathin ferromagnetic iron/silver films were epitaxially grown on GaAs substrates by molecule beam epitaxy. Sample magnetization and magnetic anisotropy were studied by magneto-optic Kerr effect experiment. The common features in the ferromagnetic resonance peak-to-peak linewidth DeltaH(pp) are identified, with the narrowest linewidth of 23 Oe. Electronically tunable wideband notch filters utilizing the flip-chip devices have been realized using Fe/Ag film GaAs waveguide layer structures. The peak absorption carrier frequency of a propagating microwave has been tuned in a range from 9.6 to 21 GHz in a modest magnetic field from 0 to 1650 Oe for single layer structure. For multilayer structures, peak absorption is intensified with tuned range from 10.7 to 27 GHz. The experimental results are in good agreement with the theoretical predictions for the case in which the magnetic field is applied along the easy axis of the Fe film.