[en] The interplay between activation volumes and microstructure is investigated in nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ (Finemet) alloys. Experiments are performed beyond the Curie point of the amorphous matrix, where relaxation effects are relevant. Measurements are analyzed within a theoretical framework where hysteresis and relaxation phenomena are jointly described. In highly crystallized samples magnetization processes are characterized by a unique length scale. In poorly crystallized samples the system behavior is controlled by a distribution of characteristic volumes related to structural disorder. [copyright] 2001 American Institute of Physics