Ferromagnetic grain boundary signature in die-upset Re-Fe-B magnets

Abstract

Previous nanostructural and nanocompositional studies performed on the boundaries of deformed grains in two die-upset rare earth magnets with bulk compositions Nd_13.75Fe_80.25B₆ and Pr_13.75Fe_80.25B₆ indicate that the intergranular phase in many grain boundaries is enriched in iron relative to the bulk. We present here preliminary magnetic data that provide further evidence that this grain boundary phase is indeed iron-rich, and in fact appears to be ferromagnetic. Hysteresis loops were performed at 800 K on die-upset magnets with the above compositions. Each sample showed a clear hysteresis with coercivities between 34 and 40 Oe and an average remanence 4πB_R of 6.8 G for the Nd-based sample and 10.3 G for the Pr-based sample. The ferromagnetic signals measured at high temperature in these magnets are attributed to the iron-rich grain boundary phase. The implications of this conclusion with respect to coercivity are discussed.