We have studied the magnetization of amorphous Co–Dy–B and Fe–Dy–B alloys at 1.5 K under very high fields up to 35 T. Above a critical field the antiferromagnetic coupling breaks as indicated by the strong increase in the magnetization. This is explained by a model. The analysis of this behavior enables us to obtain some important parameters of fundamental interest, such as, the molecular field coefficient, the exchange integral, etc. ©1996 American Institute of Physics.