Improvement of hard magnetic properties of Fe90Nd7B3 alloys by two-stage crystallization treatment

The magnetic properties of a partially crystallized Fe90Nd7B3 amorphous alloy were improved by a two-stage crystallization treatment: (1) the first-stage annealing (923 K × 300 s) is employed as an optimal treatment to obtain rather good hard magnetic properties and is followed by water quenching; (2) the second-stage annealing at 723 K for 60–3600 s is followed by water quenching. The remanence (Br), coercive field (iHc) and maximum energy product ((BH)max) are 1.00 T, 200 kA m−1 and 100 kJ m−3, respectively, at the first-stage, and increase to 1.16 T, 232 kA m−1 and 135 kJ m−3, respectively, by the second annealing stage. The further increase in the hard magnetic properties is presumably due to the increase in the precipitation amount of nanoscale b.c.c.-Fe phase with high saturation magnetization (Bs) and high Curie temperature (Tc) in the retainment of the nanoscale grains of the former b.c.c.-Fe and Fe14Nd2B phases. The finding of the usefulness of the multistage crystallization treatment leading to the formation of the fully crystallized nanostructure is important because of the expectation of the further improvement of functional properties resulting from the nanoscale mixed structure.