蒙特卡洛模拟Fe-Cr合金的磁团簇展开

发布时间：2019-03-02 21:26

【摘要】：相变是指物质系统从一种状态转变为另一种状态的过程。铁磁体在低温下的内部自旋,相互作用表现出有序的排列。当温度超过临界值时,有序旋转布置被破坏,这个时候铁磁体被转换成顺磁性。计算机技术的快速发展,铁磁相变模型不断完善。磁团簇扩展模型考虑到原子的磁矩和原子在合金中的位置,可应用于所有大小的团簇,是一种研究铁磁相变的有效方法。本文以二元Fe-Cr合金为研究对象,通过磁簇扩展模型对Fe-Cr二元合金磁性相变进行系统研究。从微观角度理解二元合金铁磁相变的本质原理,本文的主要内容和研究结果如下:(1)利用基于密度泛函数理论的平面波赝势方法的VASP程序拟合得出磁团簇模型的各个参数值,将模拟结果与实验数据进行比较,验证了参数的准确性。(2)用磁团簇模型研究无序状态下Cr含量与居里点的关系:在Cr含量较低时(0-10%),FeCr的居里温度先上升,在Cr含量为6%左右达到最大值,后随着Cr含量的增加,而逐渐降低。(3)用磁团簇模型研究Fe-Cr合金中Cr含量与体系焓的关系:在低成分时,混合焓是一个负值,此时Fe-Cr合金结构稳定;混合焓随着Cr的增加,先降低后上升,在Cr6%时到达一个最小值,此时结构最为稳定;当成分到达大概12%时,混合焓符号为正,从而出现相分离的趋势。(4)用磁团簇模型研究有序状态下Cr含量与居里点的关系,并将有序和无序对比,得到在同一组分的样品中,有序结构的居里温度高于无序样品,特别是在6%样品中的差别最大。与有序结构结构相比,无序态下Cr原子形成团簇的几率增加,Cr-Cr之间反铁磁相互作用能够部分抵消磁矩,从而降低了相变温度。(5)采用基于磁团簇模型的相图计算方法,计算α-γ和γ-δ的结构相变,从而得到相图。
[Abstract]:Phase transition refers to the transition of material system from one state to another. The internal spin of ferromagnet at low temperature shows an orderly arrangement of the interaction. When the temperature exceeds the critical value, the ordered rotation arrangement is destroyed, and the ferromagnet is converted to paramagnetism. With the rapid development of computer technology, the ferromagnetic phase transition model has been improved. Considering the magnetic moment and the position of atoms in the alloy, the magnetic cluster expansion model can be applied to all kinds of clusters. It is an effective method to study ferromagnetic phase transition. In this paper, the magnetic transformation of binary Fe-Cr alloy is systematically studied by means of cluster expansion model. To understand the essential principle of ferromagnetic transformation in binary alloys from a microscopic point of view. The main contents and results of this paper are as follows: (1) using the VASP program of plane wave pseudopotential method based on the density function theory, the parameters of the magnetic cluster model are obtained, and the simulation results are compared with the experimental data. The accuracy of the parameters was verified. (2) the relationship between Cr content and Curie point in disordered state was studied by using magnetic cluster model: when the content of Cr was low (Curie temperature of 0 / 10%), FeCr), the Curie temperature increased first and reached the maximum value at about 6% of Cr content. (3) the relationship between the content of Cr and the enthalpy of the system in Fe-Cr alloy was studied by using magnetic cluster model: the enthalpy of mixing was a negative value when the composition of Cr was low, and the structure of Fe-Cr alloy was stable; The mixing enthalpy decreases first and then increases with the increase of Cr, and reaches a minimum value at Cr 6%, and the structure is most stable at this time. When the composition reaches about 12%, the symbol of enthalpy of mixing is positive, which leads to the trend of phase separation. (4) the relationship between Cr content and Curie point in ordered state is studied by using magnetic cluster model, and the order and disorder are compared. It was found that the Curie temperature of the ordered structure was higher than that of the disordered sample in the same component, especially in the 6% sample. Compared with the ordered structure, the probability of forming clusters of Cr atoms in out-of-order states is increased, and the antiferromagnetic interaction between Cr-Cr can partially counteract the magnetic moments. Thus the phase transition temperature is reduced. (5) the phase diagram based on magnetic cluster model is used to calculate the phase transitions of 伪-纬 and 纬-未, thus the phase diagram is obtained.
【学位授予单位】：南京邮电大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG132.27

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