By the ferromagnetic-resonance (FMR) technique, ultrathin (12-90 Angstrom)
Fe films grown on (100) GaAs substrates by the molecular-beam-epitaxy (MBE)
technique are investigated at a frequency of 9.8 GHz. The FMR linewidth of
30-50-Angstrom-thin films is about 20-30 Oe, which is close to the limit v
alues. The saturation magnetization, the constants of cubic and in-plane un
iaxial anisotropy, and the FMR linewidth are obtained as functions of the f
ilm thickness.