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  • 學位論文

以第一原理計算研究雙鈣鈦礦中鉍基底的磁性半金屬

Half-metallic materials study of the double perovskites Bi2BB′O6 (B,B′=3d transition metal) via First-Principle Calculations

指導教授 : 陳永芳
共同指導教授 : 王銀國

摘要


在這篇論文中,我們以鉍基底的雙鈣鈦(Bi-based Double Perovskites)結構Bi2BB′O6(B,B’=過渡金屬),以第一原理計算找尋可能存在的半金屬。我們使用的計算程式為VASP根據密度泛函(DFT)理論來計算材料的結構最佳化,從最初的四種磁相態出發:鐵磁(FM)、亞鐵磁(FiM)、反鐵磁(AFM)與無磁性(NM),其中使用廣義梯度近似(GGA)以及考慮庫倫排斥效應(GGA+U)。 在第一章中,我們簡單介紹磁性半金屬過去的研究發展,以及我們找到那些可能的磁性磁半金屬候選者。 在第二章中,我們簡單介紹相關的理論及計算方法,包括Born-Oppenheimer 近似、密度泛函理論(DFT)。其中包括Hohangberg-Kohn理論、Kohn-Sham方程式、交換關連效應、侷域密度近似(LDA)與廣義梯度近似(GGA)。使用的計算程式為VASP,其使用擴增平面波方式來計算。並且最後介紹庫倫電子關聯效應(LDA/GGA+U)。 在第三章中,我們簡單介紹磁性半金屬的特性,並且詳細介紹雙鈣鈦結構以及初始的四種磁相態。最後,我們介紹整個研究的計算流程與計算的設定參數以及造成半金屬的重要物理機制-雙交換作用(double exchange)。 在第四章中,將會詳細介紹在鉍基底的雙鈣鈦(Bi-based Double Perovskites)結構Bi2BB′O6(B,B’=過渡金屬)中,我們找尋到的半金屬候選人,透過廣義梯度近似(GGA)計算,最後我們發現了7個可能的半金屬材料: 3個FM-HM(鐵磁半金屬) 材料 Bi2CrCoO6, Bi2CrNiO6,與Bi2FeNiO6—與 4個 FiM-HM(亞鐵磁半金屬) 材料—Bi2CrCuO6, Bi2FeZnO6, Bi2CrZnO6 與 Bi2CoZnO6. 當考慮庫倫排斥效應(GGA+U)時,則有3個可能的半金屬材料:Bi2CrNiO6, Bi2CrZnO6 與 Bi2ScZnO6. 最後,我們總結所有理論預測結果並且重述造成半金屬的物理機制。我們希望這篇論文可以在尋找半金屬材料方面的研究提供一些有用的訊息,希望對於未來合成半金屬材料的實驗能有所幫助。

並列摘要


In this thesis, we thoroughly investigated three possible candidates series of half-metallic (HM) in the double perovskites structure Bi-based double perovskites Bi2BB′O6 (BB′=transition metal ions). The calculation is based on the density functional theory (DFT) with full-structure optimization by generalized gradient approximation (GGA) and consideration of the strong correlation effect (GGA+U) and started with 4 types of initial magnetic states, i.e. ferromagnetic (FM), ferrimagnetic (FiM), antiferromagnetics (AF) and nonmagnetic (NM) using full-potential projector augmented wave (PAW) method within conjugate-gradient (CG) method implemented in VASP package (code). In the first chapter, we briefly introduced the previous researches of HM compounds and what series investigation that we had done. In chapter 2, we introduced the Born-Oppenheimer approximation, DFT (including Hohangberg-Kohn theorems, Kohn-Sham equations, exchange-correlation functional, local (spin) density approximation (L(S)DA) and generalized-gradient approximation (GGA)), as well as computational methods we used, including Projector Augmented Wave (PAW) method in VASP code and electron correlation effect (+U calculation). In chapter 3, we introduced its characteristics and properties of HM materials with reviewing the different structures that had been discovered. The detail of double perovskites structure and the initial magnetic states configurations are also introduced in this chapter. In the end, the calculation procedure with detailed setting parameter and double exchange mechanism of causing HM characteristics are schematic diagramed. In chapter 4, for Bi-based double perovskites Bi2BB′O6 (BB′=transition metal ions), we classified the possible HM compound. With the consideration of 4 types of magnetic states, i.e. ferromagnetic (FM), ferromagnetic (FiM), antimagnetic (AF) and nonmagnetic (NM), after the GGA scheme, we found 7 promising candidates for half-metallic (HM) materials: 3 FM-HM materials Bi2CrCoO6, Bi2CrNiO6,and Bi2FeNiO6—and 4 FiM-HM materials—Bi2CrCuO6, Bi2FeZnO6, Bi2CrZnO6 and Bi2CoZnO6. When the Coulomb interaction correction (GGA+U) is considered, there are 3 half-metallic (HM) materials: Bi2CrNiO6, Bi2CrZnO6 and Bi2ScZnO6 In the last chapter, we will make a summary of our work including the research method, the main results and the mechanism of causing HM compounds. We hope that this thesis on the searching HM compounds in double perovskites structures are useful for experimental research and bring up the research upsurge of HM materials.

參考文獻


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