Surface modes in spin wave resonance in thin amorphous films of (Gd1-xCox)1-yMoy alloys were studied. The samples were obtained with rf sputtering technique and a bias voltage was applied. Technological conditions were carefully pre-determined for which surface modes were excited in the resonance experiment. One surface mode was present for samples just after deposition and two modes could be observed in some cases for the same samples kept at room temperature for two or three months. The surface anisotropy constant Ks was determined from the surface inhomogeneity (SI) model with symmetrical or non-symmetrical boundary conditions for one or two surface modes, respectively. The fitted Ks values agree with theoretically predicted ones and they are also compatible with numbers found by experimentalists for monocrystalline, polycrystalline or amorphous films.
For all samples we also determined the critical angles ?c's between the external magnetic field and the normal to the film plane for which the position of the surface mode coincides with the position of the first volume mode. The corresponding critical angles c's for the magnetization differ from ?/4 which suggests the presence of surface inhomogeneities of the magnetization distribution.