Giant magnetic hardening of a Fe-Zr-B-Cu amorphous alloy during the first stages of nanocrystallization

The coercive field of ${\mathrm{Fe}}_{87.2}$ ${\mathrm{Zr}}_{7.4}$ ${\mathrm{B}}_{4.3}$ ${\mathrm{Cu}}_{1.1}$ amorphous ribbons obtained by melt spinning becomes 1 A ${\mathrm{m}}^{\mathrm{-}1}$, in the relaxed state, which is achieved after annealing at 410 °C during 1 h. After annealing for the same time at 480 °C the coercive force abruptly increases up to 100 A ${\mathrm{m}}^{\mathrm{-}1}$. For increasing annealing temperatures the coercivity drops again to 10 A ${\mathrm{m}}^{\mathrm{-}1}$. X-ray diffraction and Mössbauer spectroscopy show that the magnetic hardening is due to the appearance of a few nanocrystals of Fe which are separated a distance that in average is longer than the exchange correlation length of the amorphous matrix. As the number of Fe nanocrystals increase, the intergranular distances decrease and the grains become exchange coupled giving rise to the subsequent magnetic softening.