Composition and microstructure dependent spin reorientation in nanocrystalline (Nd-Pr)-(Fe-Co)-B alloys

The effects of composition, grain size and exchange coupling on the spin-reorientation temperature (T_SR) for nanocrystalline (Nd-Pr)-(Fe-Co)-B alloys have been systematically studied. Although T_SR is usually considered to be an intrinsic magnetic property, it is nevertheless influenced by the microstructure when the grain size decreases to the nanometer level. The T_SR for all single phase nanocrystalline Nd₁₂Fe₈₂B₆ alloys were lower than the published value for a single crystal or for an aligned sintered NdFeB magnet. The results consistently demonstrate that the increasing degree of exchange coupling between the RE₂Fe₁₄B nano-grains with decreasing grain size leads to a progressive decrease in T_SR. As is the case for conventional NdFeB alloys, Pr substitution for Nd decreases T_SR linearly and rapidly with increasing Pr concentration but a spin reorientation still occurs at 75% Pr substitution (T_SR≈25 K). Co substitution for Fe also decreases T_SR, but to a lower degree than Pr for Nd. Introduction of an increasing volume fraction of soft magnetic α-(Fe-Co) phase into the nanocrystalline Nd₂Fe₁₄B alloy has no significant influence on the value of T_SR in spite of the influence of α-(Fe-Co) on the exchange coupling.