Hard Magnetic Properties of Nanocrystalline Fe–Nd–B Alloys Containing α-Fe and Intergranular Amorphous Phase

Abstract

When an amorphous Fe₉₀Nd₇B₃ alloy subjected to annealing for 60 to 180 s at temperatures between 923 and 1023 K consists of a mixed structure of bcc-Fe, Fe₃B, Fe₁₄Nd₂B and remaining amorphous phases, the (BH)_max and remanence were found to have maximum values of 113 kJ/m³ and 1.17 T, respectively. Their crystallites which precipitated through the process of Am→Am′+bcc-Fe→Am″+bcc-Fe+Fe₃B→Am'″+bcc-Fe+Fe₃B+Fe₁₄Nd₂B have the particle sizes of 20 to 60 nm and the thickness of the intergranular amorphous layer is 5 to 30 nm. The Fe₁₄Nd₂B phase is surrounded by the bcc-Fe and remaining amorphous phase in the optimally annealed state because it precipitates from the remaining amorphous phase which surrounds the bcc-Fe particles. The further increase in annealing temperature and annealing time causes the increase in particle size of their precipitates as well as the disappearance of the residual amorphous phase, leading to the significant decrease in (BH)_max. The existence of the residual amorphous phase was also recognized for all other Fe-Nd-B alloys with maximum (BH)_max values obtained by annealing the amorphous phase. The intergranular amorphous phase as well as the bcc-Fe phase is presumed to act as an effective magnetic exchange-coupled medium. This information is extremely important for the subsequent development of permanent magnet materials by the utilization of magnetic exchange-coupled state as well as for the interpretation of the appearance of hard magnetism in the use of the rapidly solidified Fe-rich amorphous phase as a precursor.