Magnetism, phase composition, and hyperfine fields of melt-spun Nd-Fe-B alloys with a few percent of neodymium

Abstract

It is possible that melt-spun Nd-Fe-B alloys with a few percent of neodymium can be used as a raw material for inexpensive and bonded permanent magnets. After an appropriate heat treatment, melt-spun ${\mathrm{Nd}}_4$ ${\mathrm{Fe}}_{77.5}$ ${\mathrm{B}}_{18.5}$ alloy has a coercive field of 3 kOe, a remanent magnetization of 12.5 kG, and an energy product of 13 MG Oe. However, at present, there is some debate on the origin of its hard magnetic properties. In the present work, the phase composition, magnetism, and hyperfine fields of melt-spun Nd-Fe-B alloys are studied. The effect of rare-earth element substitution for Nd, such as Pr, Gd, and Dy, on ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ magnetic hyperfine fields is also investigated. On the basis of the experimental results, the origin of the hard magnetic properties is discussed. The results indicate that (1) the melt-spun Nd-Fe-B alloys with a low Nd concentration annealed under an optimal heat treatment consist of body-centered-tetragonal ${\mathrm{Fe}}_3$B (bct-${\mathrm{Fe}}_3$B) and a few percent of α-Fe and no ${\mathrm{Nd}}_2$ ${\mathrm{Fe}}_{14}$B magnetically hard phase; (2) about 5 at. % Fe atoms in ${\mathrm{Fe}}_{\mathrm{III}}$(8g) site of bct-${\mathrm{Fe}}_3$B are replaced by Nd atoms in the samples annealed under optimal condition. NMR radio frequency (rf) enhancement effect results demonstrate that bct-${\mathrm{Fe}}_3$B containing Nd atoms has better permanent magnetic properties; (3) the substitution of some rare-earth elements, such as Pr, Gd, and Dy, does not influence the hyperfine field for ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in α-Fe; however, the hyperfine field for ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ in bct-${\mathrm{Fe}}_3$B increases with the addition of Gd or Dy. Thus, it can be concluded that the hard magnetic properties of melt-spun Nd-Fe-B with a few percent of neodymium result from bct-${\mathrm{Fe}}_3$B containing Nd atoms, and not from the presence of the ${\mathrm{Nd}}_2$ ${\mathrm{Fe}}_{14}$B magnetically hard phase.