稀土钙钛矿铬氧化物的磁相变机理与玻璃态动力学行为研究

发布时间：2018-07-25 09:15

【摘要】：稀土钙钛矿铬氧化物RCr O3由于其独特而丰富的磁特性和潜在的应用价值,在过去的几十年间得到了广泛的研究。本论文以稀土铬氧化物Sm Cr O3及其掺杂体系为研究对象,对该系列样品的晶格结构、表面形貌以及其在低温区展现出的自旋重取向转变、玻璃态转变行为、负磁化现象等复杂磁物理行为进行了系统地研究。论文共分为七章,主要内容为:第一章,概述了稀土铬氧化物和玻璃态行为的相关研究进展,着重介绍了该体系中的磁结构、低温磁特性、自旋玻璃材料等方面的研究工作以及多铁性和磁电材料的相关研究进展。第二章,主要叙述了实验样品的制备方法,包括陶瓷样品制备方法,样品结构和表面形貌的表征手段和方法,物性测量的基本原理和测量手段。此外还包括第一性原理的相关计算方法。第三章,通过实验和计算相结合的方法,对Sm Cr O3在低温区间下磁相变过程进行了深入研究。结果发现Cr-O-Cr晶格中的t-e轨道杂化与体系的自旋重取向转变有极为紧密的联系,同时伴随着晶格畸变,最终导致了自旋重取向转变的发生。态密度的计算结果显示在Cr-O-Cr离子晶格之中由于交换劈裂和t-e轨道杂化,一个连续的超交换角的变化影响了O2-(px,py,pz)与Cr3+(dxy,dyz,dz2,dxz,dx2)之间的耦合作用,在不同取向上轨道杂化的产生和消失导致了弱铁磁性的产生。根据相关理论模型,提出了一种与t-e轨道杂化有关联的自旋重取向转变产生的物理机制。第四章,系统研究了Sm Cr O3中出现的玻璃态动力学行为,通过对磁相变温区的循环磁性测量与拟合计算,发现对于这样的一种热不可逆性现象,是与样品本身存在的铁磁-反铁磁相变过程紧密相关的。在这种玻璃态转变过程中,一旦温度降低到了一定的超冷态,部分的反铁磁组分受到冻结,在升温过程中就无法参与反铁磁-铁磁的相变,导致了场冷升温数据阶梯性降低的现象,引起玻璃态行为的本质因素是由于体系内部存在恒定组分的反铁磁冻结态。在外场为50 Oe时,体系的反铁磁冻结组分可以高达53.2%,而外磁场的改变又可以调控这种磁玻璃特性。进一步的磁化时间曲线测试结果可以通过Kohlrausch-Williams-Watt方程来有效拟合,同时利用修正的Kissinger方程初步得到了玻璃态转变的活化能。第五章,通过不同离子的A位掺杂对Sm Cr O3磁玻璃特性的影响,来深入探索磁玻璃态行为的物理机制。通过在A位掺杂非磁性离子Ba2+,发现冻结组分有所降低,表明内部的反铁磁冻结态不仅仅是由Cr-Cr磁矩贡献,Sm-Cr磁矩也起到了不可替代的作用。而掺杂磁性离子Bi3+之后体系的磁相变发生了显著改变,同时产生了负磁化行为和交换偏置现象,进一步验证了体系内部Sm-Cr磁矩之间的强烈耦合作用。第六章,研究了Sm离子的替代对新型磁电材料Nd Cr Ti O5的电学性质和磁特性的影响,讨论了其内部的反铁磁有序的作用机制以及关联的磁电效应产生的原因。X射线衍射结果表明在掺杂样品中并没有出现结构上的转变,但是掺杂之后体系的反铁磁相变温度发生了微小的偏移,表明Cr的磁矩对反铁磁转变温度有着更为重要的影响。电极化结果证实了该体系中的d-f相互作用,而Sm离子在Nd离子位置的随机替代破坏了Nd-Cr之间原有的的3d-4f相互作用,因而致使Nd0.5Sm0.5Cr Ti O5中的铁电相的消失。第七章,对本论文工作给予了总结和展望,针对玻璃态行为与磁相变机理的研究工作将为理解稀土铬氧化物系统的物理机制提供重要参考,同时提出了本论文工作在未来后续的有价值的几个研究方向。
[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.
【学位授予单位】：上海大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O482

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