稀土鈣鈦礦鉻氧化物的磁相變機理與玻璃態(tài)動力學行為研究
發(fā)布時間：2018-07-25 09:15
【摘要】：稀土鈣鈦礦鉻氧化物RCr O3由于其獨特而豐富的磁特性和潛在的應用價值,在過去的幾十年間得到了廣泛的研究。本論文以稀土鉻氧化物Sm Cr O3及其摻雜體系為研究對象,對該系列樣品的晶格結構、表面形貌以及其在低溫區(qū)展現(xiàn)出的自旋重取向轉變、玻璃態(tài)轉變行為、負磁化現(xiàn)象等復雜磁物理行為進行了系統(tǒng)地研究。論文共分為七章,主要內容為:第一章,概述了稀土鉻氧化物和玻璃態(tài)行為的相關研究進展,著重介紹了該體系中的磁結構、低溫磁特性、自旋玻璃材料等方面的研究工作以及多鐵性和磁電材料的相關研究進展。第二章,主要敘述了實驗樣品的制備方法,包括陶瓷樣品制備方法,樣品結構和表面形貌的表征手段和方法,物性測量的基本原理和測量手段。此外還包括第一性原理的相關計算方法。第三章,通過實驗和計算相結合的方法,對Sm Cr O3在低溫區(qū)間下磁相變過程進行了深入研究。結果發(fā)現(xiàn)Cr-O-Cr晶格中的t-e軌道雜化與體系的自旋重取向轉變有極為緊密的聯(lián)系,同時伴隨著晶格畸變,最終導致了自旋重取向轉變的發(fā)生。態(tài)密度的計算結果顯示在Cr-O-Cr離子晶格之中由于交換劈裂和t-e軌道雜化,一個連續(xù)的超交換角的變化影響了O2-(px,py,pz)與Cr3+(dxy,dyz,dz2,dxz,dx2)之間的耦合作用,在不同取向上軌道雜化的產生和消失導致了弱鐵磁性的產生。根據相關理論模型,提出了一種與t-e軌道雜化有關聯(lián)的自旋重取向轉變產生的物理機制。第四章,系統(tǒng)研究了Sm Cr O3中出現(xiàn)的玻璃態(tài)動力學行為,通過對磁相變溫區(qū)的循環(huán)磁性測量與擬合計算,發(fā)現(xiàn)對于這樣的一種熱不可逆性現(xiàn)象,是與樣品本身存在的鐵磁-反鐵磁相變過程緊密相關的。在這種玻璃態(tài)轉變過程中,一旦溫度降低到了一定的超冷態(tài),部分的反鐵磁組分受到凍結,在升溫過程中就無法參與反鐵磁-鐵磁的相變,導致了場冷升溫數據階梯性降低的現(xiàn)象,引起玻璃態(tài)行為的本質因素是由于體系內部存在恒定組分的反鐵磁凍結態(tài)。在外場為50 Oe時,體系的反鐵磁凍結組分可以高達53.2%,而外磁場的改變又可以調控這種磁玻璃特性。進一步的磁化時間曲線測試結果可以通過Kohlrausch-Williams-Watt方程來有效擬合,同時利用修正的Kissinger方程初步得到了玻璃態(tài)轉變的活化能。第五章,通過不同離子的A位摻雜對Sm Cr O3磁玻璃特性的影響,來深入探索磁玻璃態(tài)行為的物理機制。通過在A位摻雜非磁性離子Ba2+,發(fā)現(xiàn)凍結組分有所降低,表明內部的反鐵磁凍結態(tài)不僅僅是由Cr-Cr磁矩貢獻,Sm-Cr磁矩也起到了不可替代的作用。而摻雜磁性離子Bi3+之后體系的磁相變發(fā)生了顯著改變,同時產生了負磁化行為和交換偏置現(xiàn)象,進一步驗證了體系內部Sm-Cr磁矩之間的強烈耦合作用。第六章,研究了Sm離子的替代對新型磁電材料Nd Cr Ti O5的電學性質和磁特性的影響,討論了其內部的反鐵磁有序的作用機制以及關聯(lián)的磁電效應產生的原因。X射線衍射結果表明在摻雜樣品中并沒有出現(xiàn)結構上的轉變,但是摻雜之后體系的反鐵磁相變溫度發(fā)生了微小的偏移,表明Cr的磁矩對反鐵磁轉變溫度有著更為重要的影響。電極化結果證實了該體系中的d-f相互作用,而Sm離子在Nd離子位置的隨機替代破壞了Nd-Cr之間原有的的3d-4f相互作用,因而致使Nd0.5Sm0.5Cr Ti O5中的鐵電相的消失。第七章,對本論文工作給予了總結和展望,針對玻璃態(tài)行為與磁相變機理的研究工作將為理解稀土鉻氧化物系統(tǒng)的物理機制提供重要參考,同時提出了本論文工作在未來后續(xù)的有價值的幾個研究方向。
[Abstract]:The rare earth perovskite chromium oxide RCr O3 has been widely studied in the past several decades due to its unique and rich magnetic properties and potential application value. This paper takes the rare earth chromium oxide Sm Cr O3 and its doping system as the research object. A total of seven chapters are divided into seven chapters. The main contents are: Chapter 1, the related research progress of rare earth chromium oxide and glass behavior are summarized, and the magnetic structure, low temperature magnetic properties and spin glass materials in this system are emphatically introduced. In the second chapter, the preparation methods of experimental samples are described, including the preparation methods of ceramic samples, the means and methods for the characterization of sample structure and surface morphology, the basic principles and measurement methods of physical properties, and the related calculations of the first principle. Methods. In the third chapter, through the combination of experiments and calculations, the magnetic phase transition process of Sm Cr O3 in the low temperature range is studied. The results show that the T-E orbital hybrids in the Cr-O-Cr lattice are closely related to the spin reorientation transformation of the system, and the lattice distortion is accompanied by the lattice distortion, which eventually leads to the spin reorientation transition. The result of the calculation of the density of state shows that in the lattice of Cr-O-Cr ions, the change of a continuous superexchange angle affects the coupling between O2- (PX, py, PZ) and Cr3+ (DXY, dyz, dZ2, dxz, DX2) due to the exchange splitting and the hybridization of T-E orbit in the lattice of ions. The generation and disappearance of the orbital hybridization in different orientations lead to the production of weak ferromagnetism. The theoretical model has proposed a physical mechanism produced by the spin reorientation transformation associated with the T-E orbital hybridization. In the fourth chapter, the dynamic behavior of the glass state in the Sm Cr O3 is systematically studied. The thermal irreversibility of this kind is found to be the same as the sample itself. The existence of ferromagnetic antiferromagnetic phase transition is closely related. In the process of glass transition, once the temperature is reduced to a certain supercooling state, some of the antiferromagnetic components are frozen, and the phase transition of the antiferromagnetic ferromagnetic field can not be involved in the heating process, which leads to the phenomenon of the staircase reduction of the field cold heating data, which causes the behavior of glass state. The essential factor is the antiferromagnetic freezing state of the constant component in the system. When the field is 50 Oe, the antiferromagnetic freezing component of the system can be as high as 53.2%, and the change of the external magnetic field can regulate the properties of the magnetic glass. The further magnetization time curve test results can be effectively simulated by the Kohlrausch-Williams-Watt equation. At the same time, the activation energy of glass transition is preliminarily obtained by using the modified Kissinger equation. The fifth chapter, through the effect of A bit doping on the properties of Sm Cr O3 magnetic glass, explores the physical mechanism of the magnetic glass state behavior. By doping non magnetic ion Ba2+ in A bit, it is found that the freezing component is reduced, indicating the internal reaction. The ferromagnetic freezing state not only contributes to the Cr-Cr magnetic moment, but also plays an irreplaceable role in the Sm-Cr magnetic moment. The magnetic phase transition of the system after the doping of magnetic ion Bi3+ has been significantly changed, and the negative magnetization and the exchange bias are produced, and the strong coupling effect between the Sm-Cr magnetic moments in the system is further verified. The sixth chapter, research The effect of the substitution of Sm ions on the electrical properties and magnetic properties of the new magnetoelectric material, Nd Cr Ti O5, is discussed. The mechanism of the internal antiferromagnetic order and the causes of the associated magnetoelectric effect are discussed. The results of the.X ray diffraction show that there is no structural change in the doped sample, but the antiferromagnetism after the doping system is found. The phase transition temperature has a slight shift, indicating that the magnetic moment of Cr has a more important influence on the antiferromagnetic transition temperature. The results of the electrical polarization confirm the interaction of d-f in the system, and the random substitution of Sm ions in the position of Nd ions destroys the original 3d-4f interaction between Nd-Cr and thus leads to the ferroelectric phase in Nd0.5Sm0.5Cr Ti O5. The seventh chapter gives a summary and prospect for the work of this paper. The research work on the glass behavior and the mechanism of magnetic phase transition will provide important reference for understanding the physical mechanism of the rare earth chromium oxide system. At the same time, some valuable research directions in the future will be put forward in this paper.
【學位授予單位】：上海大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：O482
本文編號：2143348
[Abstract]:The rare earth perovskite chromium oxide RCr O3 has been widely studied in the past several decades due to its unique and rich magnetic properties and potential application value. This paper takes the rare earth chromium oxide Sm Cr O3 and its doping system as the research object. A total of seven chapters are divided into seven chapters. The main contents are: Chapter 1, the related research progress of rare earth chromium oxide and glass behavior are summarized, and the magnetic structure, low temperature magnetic properties and spin glass materials in this system are emphatically introduced. In the second chapter, the preparation methods of experimental samples are described, including the preparation methods of ceramic samples, the means and methods for the characterization of sample structure and surface morphology, the basic principles and measurement methods of physical properties, and the related calculations of the first principle. Methods. In the third chapter, through the combination of experiments and calculations, the magnetic phase transition process of Sm Cr O3 in the low temperature range is studied. The results show that the T-E orbital hybrids in the Cr-O-Cr lattice are closely related to the spin reorientation transformation of the system, and the lattice distortion is accompanied by the lattice distortion, which eventually leads to the spin reorientation transition. The result of the calculation of the density of state shows that in the lattice of Cr-O-Cr ions, the change of a continuous superexchange angle affects the coupling between O2- (PX, py, PZ) and Cr3+ (DXY, dyz, dZ2, dxz, DX2) due to the exchange splitting and the hybridization of T-E orbit in the lattice of ions. The generation and disappearance of the orbital hybridization in different orientations lead to the production of weak ferromagnetism. The theoretical model has proposed a physical mechanism produced by the spin reorientation transformation associated with the T-E orbital hybridization. In the fourth chapter, the dynamic behavior of the glass state in the Sm Cr O3 is systematically studied. The thermal irreversibility of this kind is found to be the same as the sample itself. The existence of ferromagnetic antiferromagnetic phase transition is closely related. In the process of glass transition, once the temperature is reduced to a certain supercooling state, some of the antiferromagnetic components are frozen, and the phase transition of the antiferromagnetic ferromagnetic field can not be involved in the heating process, which leads to the phenomenon of the staircase reduction of the field cold heating data, which causes the behavior of glass state. The essential factor is the antiferromagnetic freezing state of the constant component in the system. When the field is 50 Oe, the antiferromagnetic freezing component of the system can be as high as 53.2%, and the change of the external magnetic field can regulate the properties of the magnetic glass. The further magnetization time curve test results can be effectively simulated by the Kohlrausch-Williams-Watt equation. At the same time, the activation energy of glass transition is preliminarily obtained by using the modified Kissinger equation. The fifth chapter, through the effect of A bit doping on the properties of Sm Cr O3 magnetic glass, explores the physical mechanism of the magnetic glass state behavior. By doping non magnetic ion Ba2+ in A bit, it is found that the freezing component is reduced, indicating the internal reaction. The ferromagnetic freezing state not only contributes to the Cr-Cr magnetic moment, but also plays an irreplaceable role in the Sm-Cr magnetic moment. The magnetic phase transition of the system after the doping of magnetic ion Bi3+ has been significantly changed, and the negative magnetization and the exchange bias are produced, and the strong coupling effect between the Sm-Cr magnetic moments in the system is further verified. The sixth chapter, research The effect of the substitution of Sm ions on the electrical properties and magnetic properties of the new magnetoelectric material, Nd Cr Ti O5, is discussed. The mechanism of the internal antiferromagnetic order and the causes of the associated magnetoelectric effect are discussed. The results of the.X ray diffraction show that there is no structural change in the doped sample, but the antiferromagnetism after the doping system is found. The phase transition temperature has a slight shift, indicating that the magnetic moment of Cr has a more important influence on the antiferromagnetic transition temperature. The results of the electrical polarization confirm the interaction of d-f in the system, and the random substitution of Sm ions in the position of Nd ions destroys the original 3d-4f interaction between Nd-Cr and thus leads to the ferroelectric phase in Nd0.5Sm0.5Cr Ti O5. The seventh chapter gives a summary and prospect for the work of this paper. The research work on the glass behavior and the mechanism of magnetic phase transition will provide important reference for understanding the physical mechanism of the rare earth chromium oxide system. At the same time, some valuable research directions in the future will be put forward in this paper.
【學位授予單位】：上海大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：O482
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