Improvement of Magnetic Properties of Zn-bonded Sm-Fe-N Magnets and Investigation of Microstructural Changes at Sm₂Fe₁₇–Zn Interface

Abstract

This paper reviewed development of Zn-bonded Sm-Fe-N magnets in our group, and reported interfacial diffusion and phase changes at the Sm₂Fe₁₇–Zn interface during annealing. For increasing magnetic properties of Zn-bonded Sm-Fe-N magnets, decreasing oxygen content, improving dispersibility of Zn, and increasing relative density are necessary, therefore we prepared low oxygen content Sm-Fe-N and fine Zn powders. The powders were sintered by spark plasma sintering process for increasing density under low oxygen atmosphere resulting in obtaining high (BH)_max of 200 kJm^-3 with relatively high coercivity of 1.28 MAm^-1 and β(H_cJ) of –0.34%C^-1. In order to further improve the (BH)_max, it is necessary to control interfacial diffusion and reaction between Sm₂Fe₁₇N₃ and Zn phases. Thus, as a first step, microstructural changes and interdiffusion coefficient at the Sm₂Fe₁₇–Zn interface were investigated using Sm₂Fe₁₇–Zn diffusion couples, in this study. It was clear that interdiffusion of Zn, Fe and Sm occurred during annealing below melting temperature of Zn, and the diffusion rate of Zn was faster than those of Fe and Sm. Furthermore, with Zn diffusing into Sm₂Fe₁₇ phase, the Sm₂Fe₁₇ decompose to ThMn₁₂-Sm(Zn,Fe)₁₂ and α-FeZn phases with specific crystal orientation relationship.