Micromagnetic Simulations of Magnetization Reversals in Nd-Fe-B Based Permanent Magnets

After getting detail structural information from Nd-Fe-B permanent magnets by multi-scale structural characterization, we employ finite element micromagnetic simulations to explain how the microstructure influences the magnetization reversals and coercivity. In this paper, the basis of the micromagnetic simulation and how the computing models are constructed are described. Then, the influence of the microstructure such as the grain size, the grain shape, the composition and the anisotropic nature of grain boundary phase on the coercivity of Nd-Fe-B permanent magnets are shown. The results showed that local demagnetization factor decreases as grain size decreases, which is attributed to a higher coercivity in fine-grained anisotropic Nd-Fe-B magnets. It was also found that the reduction of the magnetization of the grain boundary phase, particularly the grain boundaries located parallel to c-axis of Nd2Fe14B grains, leads to the coercivity enhancement of anisotropic Nd-Fe-B permanent magnets due to a stronger pinning force against magnetic domain wall motion. The coercivity of Nd-Fe-B magnets cannot be enhanced by the reduction of the grain size alone unless the grain are exchange decoupled.