Co_(2~-)基Heusler合金Co_2FeAl_(1–x)Si_x(x=0.25,x=0.5,x=0.75)的结构、电子结构及热电特性的第一性原理研究

运用基于密度泛函理论的第一性原理方法,对Co_2FeAl_(1–x)Si_x(x=0.25, 0.5, 0.75)系列Heusler合金的电子结构、四方畸变、弹性常数,声子谱以及热电特性进行了计算研究.结果显示, Co_2FeAl_(1–x)Si_x系列合金的电子结构均为半金属特性,向下自旋态(半导体性)均呈现良好的热电特性,并且随着硅原子浓度的增加功率因子随之增加.计算的声子谱不存在虚频,均满足动力学稳定性条件,弹性常数均满足玻恩稳定性条件,机械稳定性均良好.随着晶格常数c/a的比值变化,体系的能量最低点均出现在c/a=1处,即结构稳定性不随畸变度c/a的变化而变化,说明不存在马氏体相变.此系列合金薄膜的电子结构呈现较高的自旋极化率,在替代浓度x=0.75时自旋极化率达到100%,且当x=0.75时薄膜在畸变度c/a=1.2时存在马氏体相变.随着晶格畸变度的改变,总磁矩也发生变化,且主要由Fe和Co两种过渡金属原子的磁矩变化所决定.

First-principles study of structure,electronic structure and thermoelectric properties for Co2-based Heusler alloys Co2FeAl1-xSix (x = 0.25, x = 0.5, x = 0.75)

In the recent decades, the half-metallic materials have become a research hotspot because of their unique electronic structure. The 100% spin polarization at the Fermi level makes them widely used in spintronic devices. The Co-based Heusler alloys belong to an important class of magnetic material, and Co₂FeAl and Co₂FeSi have been experimentally confirmed to be half-metallic materials with 100% spin polarization at the Fermi level, and the Co₂FeSi has a high Curie temperature of 1100 K and a large magnetic moment of 6.0 μB, which is a good candidate for spintronic devices. We here choose and substitute Al atoms in Co₂FeAl with Si atoms, and then carry out the theoretical predictions of Co₂FeAl_1-xSi_x (x = 0.25, 0.5, 0.75) for both bulk and film . In this paper, using the first principles calculations based on the density functional theory (DFT) we study the electronic structure, tetragonal distortion, elastic constants, phonon spectrum and thermoelectric properties of Co₂FeAl_1-xSi_x (x = 0.25, 0.5, 0.75) series alloys. The calculation results show that the electronic structure of Co₂FeAl_1-xSi_x (x = 0.25, 0.5, 0.75) series alloys are all half-metallic with 100% spin polarization, and the down spin states (semiconducting character) all exhibit good thermoelectric properties, and the power factor increases with the substitution concentration of Si atoms increasing. The calculated phonon spectrum does not have virtual frequency, indicating its dynamic stability, and all cubic phases fulfill the mechanical stability criteria, i.e. Born criteria: C₁₁ > 0, C₄₄ > 0, C₁₁-C₁₂ > 0, C₁₁ + 2C₁₂ > 0, and C₁₂ < B < C₁₁. With the variation of lattice constant ratio c/a, the lowest energy point of the structure for Co₂FeAl_1-xSi_x (x = 0.25, 0.5, 0.75) series alloys are all at c/a = 1, showing that the stability of the structure does not change with the variation of distortion c/a, and further the martensitic transformation cannot occur. For the Co₂FeAl_1-xSi_x (x = 0.25, 0.5, 0.75) series alloy thin films, the calculated electronic structures all show a high spin polarization, and it reaches 100% at x = 0.75, and for x = 0.75, the lowest energy point of the structure is at c/a = 1.2, suggesting the martensitic transformation in this structure. With the variation of the tetragonal distortion, the total magnetic moment also changes and it is mainly determined by the changes of atomic magnetic moment of transition-metals Fe and Co.