Magnetic anisotropy and crystallographic alignment in Fe and NdH2 during d-HDDR process of Nd-Fe-B-Ga-Nb powders

Abstract

Hydrogen disproportionation (HD) treatments at 820°C for 10 h with different hydrogen pressures (PHD) were performed for Nd-Fe-B-Ga-Nb magnetic powder to clarify the relationship between the structural differences and resultant magnetic anisotropy as a function of PHD during dynamic hydrogen disproportionation desorption recombination. When PHD was 30 kPa, the anisotropy was the highest after recombination, and coarsened lamellar and spherical structures comprising Fe and NdH2 were observed after HD treatment. In the coarse lamellar structures, the crystallographic orientations of adjacent Fe and NdH2 grains were the same. The diameters of such regions were approximately 1100 nm, and their total area fraction was approximately 70% of the Fe and NdH2 phases. In contrast, when PHD was 100 kPa at which the anisotropy was lower, the sample consisted of mainly spherical grains, and the total area fraction of the regions where adjacent Fe and NdH2 grains exhibited the same crystallographic orientations was small (25%). This result indicates that the presence of coarsened lamellar comprising crystallographically aligned Fe and NdH2 grains contributed significantly to the induction of anisotropy after recombination.