[en] Using the magneto-optical Kerr effect, magnetic hysteresis loops are measured on annealed amorphous Co₆₆Fe₄B₁₅Si₁₅ samples, to characterize the magnetostatic properties of the heterogeneous crystalline/amorphous phases. A gradual change, on microscale inhomogeneous, change of the magnetic properties with respect to thickness is revealed on HCP-Co, FCC-Co crystalline phases near the surface. The inner amorphous phase exhibits an irregular variation of the local magnetic properties, presumably due to the occurrence and distribution of microcrystallites. The effective field, exerted by the crystalline layer on the amorphous phase, is opposite to the surface magnetization, indicating that there is an antiferromagnetic coupling between surface and inner amorphous phases