本文選題：BiFeO_3 + 鐵電性�。� 參考：《華東師范大學》2014年博士論文

【摘要】：多鐵性材料是一類同時具有兩種或兩種以上鐵性特征(鐵電性、鐵磁性、鐵彈性或鐵渦性)的多功能材料,而其中各種鐵性之間存在耦合關系使得多鐵性材料具有潛在的應用前景。作為多鐵性材料的研究先鋒,BiFeO3(BFO)是少數(shù)在室溫條件下同時具有反鐵磁性和鐵電性的單相多鐵性材料,其磁電耦合效應和電控磁性受到研究者的廣泛關注。而作為自旋電子學的一個重要分支,電控磁性成為目前信息科學的研究熱點之一,并且在基礎研究與信息存儲領域有著重要的意義。 目前普遍的觀點認為BFO的鐵電性起源于Bi-6s孤對電子,但這很難解釋四方相BFO的鐵電極化強度以及鐵電極化方向和磁化易軸存在的耦合關系。本論文采用第一性原理計算方法,結合我們發(fā)展的軌道選擇性外場(Orbital Selective External Potential, OSEP)方法,闡明了BFO的鐵電起源,并揭示了相關電控磁性的物理機制。主要內容和創(chuàng)新點如下： 1和南京大學萬賢綱教授合作發(fā)展了OSEP方法。該方法可以精確地控制特定原子軌道的移動,有助于研究原子軌道對材料物理和化學性質的影響。OSEP方法可以同時在不同原子軌道上施加外勢場,這給研究體系的某些現(xiàn)象或是起源問題提供了多種選擇性。 2在間接磁交換體系中發(fā)現(xiàn)一種全新的鐵電形成機制,磁性相互作用有利于形成鐵電相,若磁性能具備克服彈性能的條件,體系會呈現(xiàn)鐵電相。以經典的反鐵磁材料MnO為例,計算得到雙軸壓應力可以增加磁性能,降低彈性能,從而導致鐵電相的出現(xiàn)。 3研究了BFO的鐵電起源。計算發(fā)現(xiàn)除了Bi-6s孤對電子對鐵電性有影響之外,Fe-3d態(tài)對鐵電性也有貢獻,包括間接超交換作用和Fe-O成鍵。尤其是在四方相BFO中,由于Fe-O-Fe超交換作用的增強,使得Fe-3d態(tài)對鐵電性的影響更加明顯,鐵電極化強度也隨之增強。 4研究了類四方相BFO的電控磁有序和金屬-絕緣相變。結果發(fā)現(xiàn)[001]極化態(tài)的BFO在晶格常數(shù)為3.91A附近其反鐵磁序出現(xiàn)從C1型向G型的轉變,而這種改變可以用海森堡交換積分常數(shù)Jc和J2c的競爭來解釋。同時,由于[111]極化態(tài)一直保持在G型反鐵磁序,因此在一定條件下外加電場使得鐵電極化從[001]轉到[111]方向,相應的反鐵磁序從C1型轉變?yōu)镚型,從而實現(xiàn)了電控磁有序。而通過計算類四方相BFO的線性和非線性光學性質,發(fā)現(xiàn)在低頻處二次諧波系數(shù)χzzz(2)對C1型反鐵磁序的響應要比G型反鐵磁序強很多,因此可以通過探測χzzz(2)來區(qū)別C1型和G型反鐵磁序。應力和鐵電極化方向不僅能改變磁有序,也能誘導金屬-絕緣相變,當晶格常數(shù)逐漸增加時,[001]極化態(tài)從絕緣態(tài)變?yōu)榻饘賾B(tài),而[110]極化態(tài)卻從金屬態(tài)變?yōu)榻^緣態(tài),因此在一定的晶格常數(shù)下外加電場可以改變鐵電極化方向來引起金屬-絕緣相變的出現(xiàn)。 5研究了BFO的磁晶各向異性能和凈磁矩。計算發(fā)現(xiàn)當鐵電極化從[111]轉到[001]方向時,其磁晶各向異性能將會增加一個數(shù)量級,這增強了體系中Dzyaloshinskii-Moriya相互作用。通過考慮BFO的非共線磁序,計算得到磁矩偏角以及凈磁矩都會出現(xiàn)明顯的增加,從而體現(xiàn)出一種電控凈磁矩的效應。
[Abstract]:Multiferroic material is a kind of multi - functional material with two or more ferroic characteristics ( ferroelectric , ferromagnetic , iron elastic or iron eddy ) , and there is a coupling relation between the various iron . As a research pioneer of multiferroic materials , BiFeO3 ( BFO ) is a few single - phase multiferroic materials with antiferromagnetic and ferroelectric properties at room temperature .

It is very difficult to explain the ferroelectric polarization intensity of BFO and the coupling relation between the polarization direction and the easy axis of magnetization . The paper uses the first principle to calculate the ferroelectric origin of BFO and reveals the physical mechanism of the related electric control magnetism . The main contents and innovations are as follows :

The OSEP method can accurately control the movement of the specific atomic orbit , which can help to study the influence of the atomic orbit on the physical and chemical properties of the material .

In the indirect magnetic exchange system , a new mechanism of ferroelectric formation is found , and the magnetic interaction is beneficial to the formation of ferroelectric phase . If the magnetic property has the condition of overcoming the elastic energy , the system will exhibit the ferroelectric phase . In the classical antiferromagnetic material MnO as an example , the biaxial compressive stress can be calculated to increase the magnetic energy and reduce the elastic energy , thus leading to the appearance of the ferroelectric phase .

It is found that the Fe - 3d states contribute to the ferroelectric properties , including indirect superswitching and Fe - O bonding . Especially in the tetragonal phase BFO , the influence of Fe - O - Fe super - exchange is more obvious , and the ferroelectric polarization intensity is enhanced .

In this paper , we find that the antiferroic order of the quasi - tetragonal phase BFO is changed from the C1 to G type in the vicinity of the lattice constant of 3.91A , and this change can be explained by the competition of the integral constants Jc and J2c . In the meantime , because of the linear and nonlinear optical properties of the quasi - tetragonal phase BFO , it is found that the second harmonic coefficient 蠂 zzz ( 2 ) changes from the insulating state to the state of the state .

The magnetic anisotropy energy and net magnetic moment of BFO have been studied . It is found that the magnetic anisotropy energy will increase by an order of magnitude when the polarization of iron is transferred from 111 擄 to ~ 001 擄 direction , which enhances the interaction of Dzyaloshinskii in the system . By taking into account the non - collinear magnetic ordering of BFO , the magnetic moment deflection angle and net magnetic moment can be increased obviously , thus reflecting the effect of electric control net magnetic moment .

【學位授予單位】：華東師范大學
【學位級別】：博士
【學位授予年份】：2014
【分類號】：TM221

【參考文獻】

相關期刊論文 前3條

1 施科;何泓材;王寧;;多鐵性磁電材料應用于存儲技術的研究現(xiàn)狀[J];硅酸鹽學報;2011年11期

2 施展;王翠萍;劉興軍;南策文;;基于磁電復合材料的四態(tài)存儲器[J];科學通報;2008年10期

3 段純剛;;磁電效應研究進展[J];物理學進展;2009年03期

，

本文編號：1793688