Improvements in the Coercivity of Iron-Based Nanocrystalline Low-Rare-Earth Fe3B-Nd Permanent Magnets

Fe
₃
B-based Nd-Fe-B permanent magnet materials containing Co and M as additives (M=Al, Si, Cu, Ga, Ag, Au) were prepared in an extremely low rare-earth concentration range centered on 4 at%. The hard magnetic properties develop as a result of heat treatment of rapidly solidified amorphous ribbons, which produces a metastable nanocrystalline structure consisting of Fe
₃
B and Nd
₂
Fe
₁₄
B. Use of the additives considerably reduces the crystallite sizes, and consequently the alloys exhibit higher coercivity than that seen in alloys without additives. Compaction-molded resin-bonded magnets produced from such Nd-Fe-B-Co-M materials have the following magnetic properties: H
_cj
=300 kA/m to 470 kA/m; B
_r
=0.7 T to 0.9 T; (BH)
_max
=50 kJ/m
³
to 73 kJ/m
³
. Typical values for the temperature coefficients of B
_r
and H
_cj
of bonded magnets are -0.05%/K and -0.35 %/K, respectively, in the range from 297K to 413K.