Effect of magnetic field treatment on magnetic properties and microstructure of Fe69.5Nd7B21Nb2.5 bulk permanent magnets

In this study, the Fe69.5Nd7B21Nb2.5 permanent magnets in the form of rods were prepared by annealing the bulk amorphous precursors. The effect of magnetic field heat treatment on the magnetic properties and microstructure of the alloys was investigated. It was found that the as-cast Fe69.5Nd7B21Nb2.5 alloy showed soft magnetic properties, which became magnetically hard after normal annealing. The annealed alloys were mainly composed of soft α-Fe, hard Nd2Fe14B and nonmagnetic Nd1.1Fe4B4 phases. Furthermore, magnetic field heat treatment at the temperature close to Curie temperature (595 K) of Nd2Fe14B phase was applied to the annealed rods. The intrinsic coercivity Hci remanence Br and maximum energy product (BH)max increased from 414 kA/m to 441 kA/m, 0.60 T-0.64 T and 28.9 kJ/m3 to 35.4 kJ/m3 respectively, after the heat treatment at 573 K with 1 T magnetic field. However, the magnetic properties did not improve when the temperature was above the Curie temperature. The results of X-ray diffractively (XRD) and transmission electron microscope (TEM) indicated that the magnetic field heat treatment under the Curie temperature can not only contribute to a uniform microstructure but also be helpful in suppressing the nonmagnetic phase.