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Compositionally modulated amorphous Fe70B30Ag films have been prepared by dc magnetron sputtering. The thickness of the magnetic layer (d) ranged from 3 to 90 As while the Ag layer thickness was always kept at four times as large. The multilayer structure is well established by the cross-sectional transmission electron micrograph. The magnetic properties of the system were investigated with a SQUID magnetometer. The spontaneous magnetization (Ms) at T = 8 K remains constant over the thickness range studied, indicating the absence of magnetically dead layers at the interfaces. The temperature dependence of Ms follows Bloch's law for D larger or equal 6 A.U., with the spin-wave constant inversely proportional to d. This is explained by an interface effect on spin-wave excitation. As the thickness is reduced to 4 A.U. a linear temperature dependence of Ms is observed, indicating a magnetic 3D-to-2D transition.