【作者】 钱萍；

【导师】 陈难先；

【作者基本信息】 北京科技大学 ， 凝聚态物理， 2005， 博士

【摘要】 原子间相互作用势的研究,是材料设计学的一个重要方向。但由于稀土金属间化合物结构复杂,而有关的原子间相互作用势也难以获得,致使此类材料的原子级模拟受到较大的限制,尤其是有关稀土与过渡族金属之间的相互作用势的研究还很少有人涉及。针对我国稀土资源丰富的国情,运用原子间相互作用势加强稀土过渡族金属间化合物永磁材料的研究意义重大。本工作的研究方法是利用简单虚拟结构的第一性原理计算,通过自主创新的Chen-M?bius晶格反演获得的稀土-过渡金属间化合物的原子间相互作用势,在稀土金属间化合物领域开展系列性的研究工作。本文系列性的研究了1:12钴基、锰基、铁基,2:17钴基、铁基以及间隙化合物的结构确定、相稳定性、择优代位、居里温度、磁晶各向异性以及晶格振动。涉及了160多种原子间相互作用势及300余种稀土金属间化合物。主要成果是:(1)首先使用晶格反演的原子间相互作用对势,通过能量最小化结构弛豫,得到了稀土永磁材料1:12,2:17体系的结构参数,与实验报道符合很好,误差在3.0 %范围以内;(2)用反演原子间对势确定了第三组元在1:12,2:17体系中的择优代位。结果表明,在1:12 Co基、Fe基体系中第三组元择优占据8i晶位,1:12 Mn基体系中Fe, Co, Ni择优占据8f晶位,在2:17体系中第三组元择优占据6c或4f晶位。对于1:12 Co基化合物,第三组元在择优占位的基础上还存在择优组态问题;(3)在择优代位的基础上,计算了一些特殊晶位原子间的键长,了解在不同组分的含量下反铁磁耦合的Fe-Fe磁矩及铁磁耦合的Co-Co磁矩的分布情况,研究了在不同稀土金属间化合物中居里温度随第三组元及间隙原子浓度的变化的机制;(4)磁晶各向异性与第三组元的择优占位关系最密切,不同晶位在磁晶各向异性中扮演着不同的角色,替位原子可以强化或者削弱被占据晶位的磁晶各向异性贡献,利用第三组元择优占位的计算,探讨了磁晶各向异性对组分的依赖关系;(5)利用晶格反演的原子间对势,计算了三元体系的声子态密度,同时从原子尺度分析了不同原子对晶格振动的贡献,并预测了这些化合物的比热、振动熵和德拜温度等热力学性质。(6)利用第一性原理对晶格反演势作用下弛豫后的晶体结构进行了态密度及磁性计算,对结果进行了定性分析。本工作进行了晶格反演势的原子级系列模拟,预测了稀土金属间化合物的结构性质、热力学性质以及磁学性质,力求能够在探索材料性能优化方面提供新的方法与借鉴。更多还原

【Abstract】 The research of interatomic potentials is an important part in material design. However, the atomistic simulations of the rare-earth intermetallics are limited, because the structures of these alloys are complicated and the effective interatomic potentials are difficult to be acquired, especially for the potentials of rare-earth and transition-metal elements. As the situation that our country is rich in rare-earth resource, studies by using interatomic potentials to strengthen permanent magnetic materials are of great significance.Based on the interatomic potentials related to the rare earth and transition metals, which are obtained from the first-principle cohesive energy curves of simple virtual structures and Chen-M?bius inversion method, a large amount of serial researches are made systematically in the field of the rare-earth intermetallics.In present work, the phase stability, site preference, Curie temperature, magnetocrystalline anisotropy and lattice vibrations of 1:12 and 2:17 systems are studied by the Chen’s inverted potentials, which related to more than 160 interatomic potentials and more than 300 rare-earth intermetallics. There are some results are worth mentioned in the paper. The lattice constants of rare earth compounds are calculated by the inverted potentials and they are all close to the experimental reports. The differences of lattice constants between the calculated and the experimental values are less than 3.0 %. The site preference of ternary element is studied. The ternary elements prefer the 8i sites in 1:12 Co-based systems, and the 8f sites in 1:12 Mn-based systems, and the 6c or 4f sites in Th2Zn17 or Th2Ni17 structure respectively. Especially, it is worth noting that when the ternary elements preferentially occupy the 8i sites, they probably present the configuration preference. Based upon site preference, with the bond lengths between special sites being calculated, and with the distribution of antiferromagnetic coupling of Fe-Fe magnetic moment and ferromagnetic coupling of Co-Co magnetic moment with different content, we have studied the mechanism for Curie temperature of different rare earth intermetallic compounds which varies with ternary element and interstitial atom concentration. There is a close relationship between magnetocrystalline anisotropy and site preference of ternary element, with different site playing a different role in magnetocrystalline anisotropy, and with doping atoms being able to strengthen or weaken magnetocrystalline anisotropy. We have studied the relationship between magnetocrystalline anisotropy and component by using the site preference of ternary element. We have presented the phonon spectra of ternary alloys by the inverted potentials. A qualitative analysis is carried out to discuss the contributions of distinct atoms to the vibrational modes on the atomic scale. Furthermore, the properties related to the lattice dynamics, such as the vibrational entropy, specific heat, Debye temperature are also evaluated. By using ab initio method, the densities of states and magnetic properties are calculated for the relaxed crystal structures.The present work finally reveals that the inverted potentials can be effectively applied to study the structural, thermodynamic and magnetic properties of the rare earth intermetallic compounds with complex structures. The present work is trying to play a guiding or referring part on optimizing material properties.更多还原