The Effects of Core-Shell Structure on the Magnetic Properties of Nd-Ce-Fe-B Magnets After Dy Diffusion by Micromagnetic Simulation

In this work, the influences of core-shell structure and cerium (Ce) content on magnetic properties of Nd-Ce-Fe-B permanent magnets were investigated by the finite difference micromagnetic (FDM) simulation method, and the process of magnetization reversals was systematically discussed. For the (Nd1-xCex)2Fe14B-(Dy2Fe14B) core-shell grains with different core sizes, the coercivity (Hcj) dramatically increases with the shell thickness from 0 to 2 nm and then increases slightly with shell thickness increase to 20 nm; for the grains with the same shell thickness, the Hcj decreases with the increase of core size, and it is due to the increase of demagnetization energy. Moreover, the Hcj remarkably deteriorates as the Ce content increases, which is caused by the lower magnetic crystal anisotropy field (HA). The hysteresis loops of core-shell models exhibit excellent squareness with a shell thickness of 2 nm at different Ce proportions. However, a significant two-step demagnetization feature for the model with the thicker shell sizes and higher Ce content results from the different demagnetization abilities of the shell and core. In addition, in the angular distributions of magnetization, the reversal of the core region is later than that of the outer shell, and the applied field required to inverse increases with the shell thickness. The abnormal two-step demagnetization was caused by the twice abrupt changes that occurred in the Zeeman energy (Eext) and total energy (Etot).