ABSTRACT

It has been empirically known that the coercivity of rare-earth permanent magnets depends on the size and shape of fine particles of the main phase in the system. Also, recent experimental observations have suggested that the atomic-scale structures around the grain-boundaries of the fine particles play a crucial role to determine their switching fields. In this article, we review a theoretical attempt to describe the finite temperature magnetic properties and to evaluate the reduction of the switching fields of fine particles of several rare-earth permanent magnetic materials based on an atomistic spin model that is constructed using first-principles calculations. It is shown that, over a wide temperature range, the spin model gives a good description of the magnetization curves of rare-earth intermetallic compounds such as R₂Fe₁₄B (R= Dy, Ho, Pr, Nd, Sm) and SmFe₁₂. The atomistic spin model approach is also used to describe the local magnetic anisotropy around the surfaces of the fine particles, and predicts that the rare-earth ions may exhibit planar magnetic anisotropy when they are on the crystalline-structure surfaces of the particles. The dynamical simulation of the atomistic spin model and the corresponding micromagnetic simulation show that the planar surface magnetic anisotropy causes a reduction in the switching field of fine particles by approximately 20–30%, which may be relevant to the atomic-scale surface effects found in the experimental studies.

摘要

经验表明，稀土永磁体的矫顽力取决于系统中主相细颗粒的大小和形状。此外，最近的实验观察表明，细颗粒晶界周围的原子尺度结构在确定它们的转换场方面起着至关重要的作用。在本文中，我们回顾了描述有限温度磁特性的理论尝试，并基于使用第一性原理计算构建的原子自旋模型来评估几种稀土永磁材料的细颗粒的开关场的减少. 结果表明，在很宽的温度范围内，自旋模型很好地描述了稀土金属间化合物的磁化曲线，例如R ₂ Fe ₁₄ B（R = Dy、Ho、Pr、Nd、Sm）和 SmFe ₁₂。原子自旋模型方法也用于描述细颗粒表面周围的局部磁各向异性，并预测稀土离子在颗粒的晶体结构表面上可能表现出平面磁各向异性。原子自旋模型的动力学模拟和相应的微磁模拟表明，平面表面磁各向异性导致细粒子的开关场减少约 20-30%，这可能与原子尺度表面效应有关。实验研究。