Abstract
We investigated the effects of grain boundary phases on magnetization reversal in permanent magnets by performing large-scale micromagnetic simulations based on Landau–Lifshitz–Gilbert equation under a periodic boundary. We considered planar grain boundary phases parallel and perpendicular to an easy axis of the permanent magnet and assumed the saturation magnetization and exchange stiffness constant of the grain boundary phase to be 10% and 1%, respectively, for Nd2Fe14B grains. The grain boundary phase parallel to the easy axis effectively inhibits propagation of magnetization reversal. In contrast, the domain wall moves across the grain boundary perpendicular to the easy axis. These properties of the domain wall motion are explained by dipole interaction, which stabilizes the antiparallel magnetic configuration in the direction perpendicular to the magnetization orientation. On the other hand, the magnetization is aligned in the same direction by the dipole interaction parallel to the magnetization orientation. This anisotropy of the effect of the grain boundary phase shows that improvement of the grain boundary phase perpendicular to the easy axis effectively enhances the coercivity of permanent magnets.
Highlights
High-performance permanent magnets are indispensable to high-power motors, which are applied in various devices such as electric vehicles and wind-power generators.[1,2] A permanent magnet consists of grains and grain boundary phases that separate the grain from each other
The dipole energy has almost the same value because the domain wall moves in the xy plane and crosses the grain boundary phases owing to the strong dipole field, which tends to align the magnetization parallel to the easy axis
We performed large-scale micromagnetic simulations to investigate the effects of the grain boundary phase on the coercivity of an anisotropic nanocrystalline permanent magnet
Summary
High-performance permanent magnets are indispensable to high-power motors, which are applied in various devices such as electric vehicles and wind-power generators.[1,2] A permanent magnet consists of grains and grain boundary phases that separate the grain from each other. Improvements in the grain boundary phase enhance the performance of the permanent magnet by increasing the coercivity. Grains interact with each other through the exchange and dipole interactions. Micromagnetic simulations based on the Landau–Lifshitz–Gilbert (LLG) equation are used to reveal the magnetization dynamics in permanent magnets.[7,8,9,10,11] In the numerical approach, it is indispensable to perform large-scale micromagnetic simulations because the grain diameters and magnetic domain wall lengths are on the submicron and nanometer scale, respectively. The dipole interaction is not inhibited by the thin grain boundary phase, in contrast to the exchange interaction, which tends to be inhibited. A grain boundary phase perpendicular to an easy axis direction of the permanent magnet enhances the coercivity effectively. The domain wall tends to move across a grain boundary parallel to the easy axis direction
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