Synergistic effects of the surface grain coarsening and the grain boundary phase magnetism on the magnetic properties of Re-Fe-B (Re: Rare earth elements) magnets after the grain boundary diffusion

Abstract

The grain boundary diffusion process (GBDP) is one of the most popular methods to enhance the coercivity of Re-Fe-B (Re: Rare earth elements) magnets. The surface grain coarsening is the common characteristic for the sintered Re-Fe-B magnets after GBDP. However, the effect of the coarse surface grains induced by GBDP on the magnetic properties has not been thoroughly studied. In this work, sintered Nd-Ce-Fe-B magnets were subjected to GBDP with Pr70Cu30 alloy and Pr60Tb10Cu30 alloy, respectively. The size of the coarse surface grains on the Pr70Cu30 alloy diffused magnet surface is far larger than that on the Pr60Tb10Cu30 alloy diffused magnet surface. The diffusion depth decreases with the size increase of the coarsened surface grains. Micromagnetic simulation reveals that for the magnet with the non-ferromagnetic grain boundary phase, increasing the polar surface grain size decreases the coercivity. However, the coercivity increases with the coarsening of the polar surface grain size for the magnet with a ferromagnetic grain boundary phase. Magnetic properties of the magnets after GBDP are synergistically affected by the polar surface grain size and the grain boundary phase magnetism.