Magnetic nanostructure of Fe73.5Si15.5B7Nb3Cu1 at elevated temperatures studied by SANS

Rapidly quenched Fe73.5Si15.5B7Nb3Cu1 metallic glass has been transformed by annealing at 823 K for 1 h into a nanocrystalline state consisting of an amorphous matrix and alpha-Fe(Si) crystallites with 5 nm radius in average. The magnetic contrast of the two phases has been measured as a function of temperature and magnetic field. It is on maximum at the Curie temperature T-c(am) = 650 K of the amorphous matrix. The scattering function has been derived for the general case of partly aligned moments and compared with experiment. Above T-c(am) the alignment of ferromagnetic single domain particles follow the Langevin statistics of superparamagnetism. In a slightly modified form the same formalism has been successfully applied to the data below T-c(am). This suggests that at elevated temperatures the crystallites interact by magnetostatic forces only and not by exchange coupling via the amorphous matrix.