Abstract The origin of the effective anisotropy field H K in (Gd x Co 1− x ) 1− y Ar y thin films was studied. The films were obtained by dc sputtering with a bias voltage. The samples were annealed in a vacuum of 10 -5 Torr. The effective anisotropy field H K was determined from perpendicular and parallel ferromagnetic resonance. The dependence of H K on the annealing temperature T a reveals several mechanisms responsible for the net anisotropy of amorphous (Gd x Co 1− x ) 1− y Ar y films.