稀土过渡金属氧化物磁性、磁热及磁电效应研究

发布时间：2018-05-30 07:32

本文选题：多铁性材料 + 自旋重取向　；参考：《上海大学》2016年博士论文

【摘要】：磁性材料、磁制冷材料以及多铁性材料作为功能材料,分别在磁存储、低温磁制冷系统以及新一代的快速读写方面具有巨大的应用前景,一直是凝聚态物理和材料科学领域探索和研究的热点材料体系。RCrO3(R=rare earth)体系丰富的磁相变、RFeO3(R=rare earth)中的各向异性磁熵变和A4B2O9(A=Co、Mn,B=Nb、Ta)体系报道的磁电相互调控,使得RFeO3、RFeO3和A4B2O9等氧化物材料体系成为众多研究者关注的焦点。本论文工作对CeCrO3多晶样品的合成和磁化翻转、TbFeO3单晶中的各向异性磁热效应以及A4B2O9体系单晶的制备和多铁性与磁电耦合效应进行了研究。全文共分为六章,主要内容如下:第一章为绪论,综述了RCrO3体系磁性、RFeO3体系磁热效应以及A4B2O9体系的磁电效应的研究进展。并阐述了本论文的研究目的和意义,以及主要的研究内容。第二章,介绍了与本论文工作相关的主要实验原理和方法,包括样品制备、结构分析(包括中子衍射、晶体Laue衍射,以及X射线粉末衍射的变温测试与分析)、铁电性相关测试以及物性测量系统PPMS-9T的交/直流磁性测量方法等。第三章,研究了CeCrO3多晶的合成与复杂的磁相变。我们首次采用电弧法制备了纯相的CeCrO3多晶样品,通过对CeCrO3多晶样品进行系统的磁性测量,确认了CeCrO3的Ce3+与Cr3+磁矩之间的反铁磁相互作用,并系统地研究了该样品温度和磁场诱导的磁相变,在该样品中观测到对温度和磁场非常敏感的自旋翻转效应,该效应在磁存储和磁开关等方面有着潜在的应用。第四章,研究了TbFeO3单晶样品的制备与各向异性磁制冷效应。采用光学浮区法成功制备了单相的TbFeO3单晶,对TbFeO3单晶的磁热效应进行了系统的测试,发现TbFeO3单晶在ab面和c轴之间存在明显的各向异性磁熵变,这种各向异性磁熵变和较大的制冷量,有望通过旋转样品来实现磁制冷,对于各向异性磁熵变的实际应用具有重要意义。第五章,研究了A4B2O9体系中的Co4Nb2O9、Mn4Nb2O9和Co4Ta2O9等单晶样品的制备与磁电效应。该系列单晶样品均采用光学浮区法制备,其中Mn4Nb2O9比较特殊,由于多晶原料很难通过固相反应法合成,我们发明了一种基于光学浮区法的一步法直接生长其单晶样品的方法。对于Co4Nb2O9单晶,我们在其a轴方向观测到了磁场调控的电极化和电场调控的磁化现象,并通过中子衍射实验和精修给出了其磁结构。我们发现Co4Nb2O9单晶具有较大的磁电耦合系数,并且是非线性磁电耦合效应,这种强的磁电耦合效应在实际的器件设计中具有重要意义。对于Mn4Nb2O9单晶,我们发现其磁热曲线与Co4Nb2O9不同,其反铁磁相变发生在c轴方向,并且没有观察到自旋反转相变。通过变温XRD测试,我们发现Mn4Nb2O9在奈尔温度附近出现结构相变,该结构相变温度点与其奈尔温度一致,因此我们认为其反铁磁相变是由该结构相变导致的。Co4Ta2O9单晶与Co4Nb2O9单晶显示出相似的磁电耦合性质。尽管Co4Ta2O9的奈尔温度略低于Co4Nb2O9,但更加容易发生自旋翻转,有利于在实际应用中进行操控。第六章,归纳总结了本论文的研究工作,并对CeCrO3、TbFeO3单晶和A4B2O9体系单晶的研究前景进行了展望。
[Abstract]:Magnetic materials, magnetic refrigerating materials and multi iron materials as functional materials have great application prospects in magnetic storage, low temperature magnetic refrigeration system and the new generation of fast reading and writing. It has been a hot material system.RCrO3 (R=rare earth) system of condensed state physics and material science, which has rich magnetic phase transition, R The anisotropic magnetic entropy change in FeO3 (R=rare earth) and the mutual regulation of magnetic and electricity reported by A4B2O9 (A=Co, Mn, B=Nb, Ta) system have made the system of RFeO3, RFeO3 and A4B2O9 oxide materials become the focus of many researchers. This paper works on the synthesis and magnetization reversal of the samples of CeCrO3 polycrystalline, and the anisotropic magnetothermal effect in the single crystal The preparation of single crystal and the effect of magnetic and magnetic coupling on the A4B2O9 system are studied. The full text is divided into six chapters. The main contents are as follows: the first chapter is the introduction, summarizing the research progress of the magnetic properties of the RCrO3 system, the magnetothermal effect of the RFeO3 system and the magnetoelectric effect of the A4B2O9 system. The second chapter introduces the main experimental principles and methods related to the work of this paper, including sample preparation, structural analysis (including neutron diffraction, crystal Laue diffraction, and temperature variation test and analysis of X ray powder diffraction), ferroelectric correlation test and PPMS-9T alternating / direct magnetic measurement of physical measurement system, and so on. Third In this chapter, we studied the synthesis and complex magnetic phase transition of CeCrO3 polycrystals. We first prepared pure phase CeCrO3 polycrystalline samples by arc method. By systematic magnetic measurements of CeCrO3 polycrystalline samples, the antiferromagnetic interaction between the Ce3+ and Cr3+ magnetic moments of CeCrO3 was confirmed, and the temperature and magnetic field induced magnetism of the sample were systematically studied. The spin flip effect, which is very sensitive to temperature and magnetic field, is observed in the sample. This effect has potential applications in magnetic storage and magnetic switching. In the fourth chapter, the preparation of TbFeO3 single crystal samples and the effect of anisotropic magnetic refrigeration are studied. The single phase TbFeO3 single crystal is prepared by the optical floating area method, and the single crystal of TbFeO3 is prepared. The magnetocaloric effect is systematically tested. It is found that there is an obvious anisotropic magnetic entropy change between the AB surface and the c axis of the TbFeO3 single crystal. This anisotropic magnetic entropy change and the larger refrigeration capacity are expected to realize magnetic refrigeration by rotating samples. The fifth chapter is of great significance to the actual use of the anisotropic magnetic entropy change. The fifth chapter studies the A4B2O9 body. The preparation and magnetoelectric effects of single crystal samples, such as Co4Nb2O9, Mn4Nb2O9 and Co4Ta2O9 in the series, are prepared by optical floating zone method. The Mn4Nb2O9 is very special, because polycrystalline materials are difficult to be synthesized by the solid state reaction method. A single step method based on the optical floating region method has been invented to develop the single crystal sample directly. For the Co4Nb2O9 single crystal, we observed the magnetization of the magnetic field controlled by the magnetic field and the control of the electric field in the direction of the a axis. The magnetic structure was given by the neutron diffraction experiment and the refinement. We found that the Co4Nb2O9 single crystal has a large magnetoelectric coupling coefficient and a nonlinear magnetoelectric coupling effect. This strong magnetoelectric coupling effect is found. It is of great significance in practical device design. For Mn4Nb2O9 single crystal, we find that the magneto thermal curve is different from that of Co4Nb2O9. Its antiferromagnetic phase transition occurs in the direction of C axis, and the spin reversal phase transition is not observed. Through the temperature variable XRD test, we found that the structure phase transition of Mn4Nb2O9 at Nair temperature is near, and the phase transition temperature point of this structure is also found. Nair has the same temperature, so we think that the antiferromagnetic phase transition is a similar magnetoelectric coupling property between.Co4Ta2O9 single crystal and Co4Nb2O9 single crystal caused by the structure phase transition. Although the Nair temperature of Co4Ta2O9 is slightly lower than Co4Nb2O9, it is more easy to have spin flip, which is beneficial to manipulate in practical application. The sixth chapter is summed up. The research work in this paper and prospects for the research of CeCrO3, TbFeO3 single crystal and A4B2O9 single crystal are prospected.
【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】：O441.6

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