Revisiting the demagnetization curves of Dy-diffused Nd-Fe-B sintered magnets

The grain boundary diffusion process (GBDP) is now widely used to increase coercivity in Nd-Fe-B sintered magnets with a more efficient use of heavy rare earth elements (Dy, Tb). This process leads to a typical core-shell structure for the grains consisting of (Nd,Dy)(2)Fe14B shells at the outer grain regions and Nd2Fm4B cores. The thickness of the (Nd,Dy)(2)Fe14B shells decreases from the diffusion surface to the magnet core. This inhomogeneous distribution in Dy content gives rise to a coercivity gradient within the magnet and leads therefore to a reduced squareness of the demagnetization curve. The purpose of this work is to provide a quantitative understanding of the influence of composition profiles after GBDP on the shape of the demagnetization curve of NdFe-B sintered magnets diffused with the Dy63Co37 (at. %) intermetallic compound. SEM/X-EDS analyses along the Fisher diffusion model allow the estimation of the Dy concentration in grains and at different depths. Then, after ascribing to the grains some critical values for the switching field that are related to the local Dy content, a macroscopic finite element model is implemented to provide a better understanding of the grain reversal sequence in the graded magnets tested in closed-circuit. Grain reversal patterns show that demagnetization starts from the less coercive grains in the magnet core, remains constricted in this zone thanks to a shielding effect from the external surface, and then propagates towards outer layers via magnetostatic interactions. When the coercivity gradient is large, the coercivity of the whole magnet measured in closed-circuit could be 100-200 kA/m lower than the value expected without considering magnetostatic interactions, suggesting that the shielding effect from the diffusion affected layers could be limited and counterbalanced by magnetostatic interactions.