Micromagnetic study of sphericity effect in bulk permanent magnets

The sphericity, κ, is introduced to describe the morphology of the magnetic grains. The effect of κ on the magnetic properties of rare earth-transition metal (RE-TM) permanent magnets was investigated using the micromagnetic simulation, where randomly aligned, irregular-shaped grains with ferromagnetic/non-ferromagnetic grain boundary (GB) phases were considered. For the simulated RE-TM systems, the coercivity increases significantly with the increase in κ. In particular, with 10 nm thick non-ferromagnetic GB phase, the coercivity, 1.74 MA/m, for к = 0.9 increases by 22% compared to 1.43 MA/m for к = 0.6 in the Nd2Fe14B system. Among the simulated materials, the SmCo5 system displays the largest increment in coercivity, increasing by 0.82 MA/m from 7.61 MA/m for к = 0.6 to 8.43 MA/m for к = 0.9. Magnetization reversal patterns under different external fields indicate that the dependence of coercivity on κ is attributed to the formation of local reversals at different regions and mechanisms of reversal propagation. Meanwhile the effect of κ cannot be neglected even at elevated temperatures. The simulated results of (Dy0.47Nd0.53)2Fe14B systems with the non-ferromagnetic GB layer of 4 nm at a temperature of 450 K show that the coercivity for к = 0.9 (1.88 MA/m) increased by 23% compared with that of к = 0.6 (1.52 MA/m). Our study demonstrates that the sphericity can be used as a key parameter to design high-coercivity RE-TM magnets.