Effect of misalignments of individual grains' easy axis on magnetization-reversal process in granular NdFeB magnets: A finite-element micromagnetic simulation study

摘要

We studied, using finite-element micromagnetic simulations, magnetization-reversal processes of exchange-decoupled granular NdFeB magnets for a cubic model system that consists of 27 polyhedral grains of different crystallographic orientations along with non-magnetic grain boundaries. In the simulations, we varied the degree of easy-axis alignment of individual grains with different uniaxial orientations. For perfect uniaxial alignment, i.e., alpha = 1, the normalized remanence magnetization and the normalized coercivity along with the maximum energy product (BH)(max) were estimated to be 1, 0.64, and 64.71 MGOe, respectively, and were decreased to 0.59, 0.42, and 20.56 MGOe, representing reductions of 41, 34, and 68%, respectively, for a randomly oriented alignment of the grains' easy axis, alpha = 0.6. From a comparison of the magnetization-reversal processes for different easy-axis alignments, i.e., alpha = 1, 0.98, 0.94, 0.87, 0.82, 0.79, and 0.60, it was found that all of the magnetization reversals consisted of three different sub-processes: the nucleation of a reversed domain within a grain and the expansion of nucleated domains to complete the reversal in that grain via domain-wall motion; magnetization-reversal propagation to the next grain via successive nucleation in other neighboring grains; further rotation of the magnetizations of all of the grains for cases of larger misalignment of grains' easy axis. According to these reversal sub-processes, as the degree of easy-axis alignment, alpha, decreased, the nucleation of reversed domains occurred at smaller field strengths and the apparent grain-by-grain reversal propagation became slower. The lower the value of alpha, the lower the energy required for the nucleation of reversed domains and the more predominant the pinning of the apparent grain-by-grain reversal propagation. These reversal processes led to hard-magnet parameters such as normalized remanence magnetization and coercivity and the maximum energy product (BH)(max). The values of the parameters for the perfect uniaxial alignment of grains became smaller with increasing misalignment of the grain's easy axis. The results provide both a better understanding of micro-scale magnetization-reversal processes and guidelines for optimal design of hard-magnet microstructures for higher (BH)(max) values.