稀土过渡金属氧化物晶体生长、磁性和磁电耦合特性研究

发布时间：2018-06-16 14:20

本文选题：稀土铁氧化物 + 单晶生长　；参考：《上海大学》2016年博士论文

【摘要】：同时具有铁电性、铁磁性的多铁性材料在新型信息存储器件以及多功能磁电器件等方面具有巨大潜在应用前景,近年来已成为凝聚态物理和材料科学的研究热门领域之一。铁电序和铁磁序的共存为二者之间的耦合相互作用提供了可能,即铁电有序产生的内电场可以导致电子自旋重新分布而改变系统的磁性质,反之,自旋有序涨落通过磁致伸缩或可能的电-声子相互作用导致铁电弛豫和介电异常。实验上观察到介电常数在磁相变温度处的突变异常成为本征磁电耦合存在的标志,并且进一步发现,在外加磁场的作用下,介电常数会发生明显的变化。正交钙钛矿结构的稀土铁氧化物RFeO3(R为稀土元素)所表现出来的丰富的磁特性一直是研究的热点之一,最近又发现了超快光磁效应以及多铁性等性能,并且这些新的性能和磁相变具有密切的关系,所以研究RFeO_3中磁相变附近的自旋序的变化和诱因对于探索多铁性的物理机制具有重要的意义。六角铁氧体材料中近年来发现的磁电耦合效应使得对这类材料的研究成为一个新的研究热点之一,因为其中的磁电耦合效应的发生温度较高有些甚至接近室温,在实际应用方面具有潜在的优势。所以本文选取HoFeO_3、Ho_(0.5)Pr_(0.5)FeO_3和Y型六角铁氧体Ba_(2-x)Sr_xCo_2Fe_(12-y)AlyO_(22)作为研究对象,通过元素掺杂来研究材料中的磁相变、奇异磁特性及磁电耦合效应等特性。全文共分为六章:第一章,概述了钙钛矿结构RFeO_3的晶格结构和磁结构,自旋重取向相变,以及其中的多铁性的发展概况。介绍了Y型六角铁氧体的晶体结构和磁结构,磁场诱导的电极化效应的发展概况和研究现状。第二章,主要叙述了本论文中所涉及到的实验样品的制备、表征和物性测试方法以及所使用到的实验设备。其中包括固相反应法合成多晶样品的方法,光学浮区法生长单晶的方法,以及实验设备的使用方法和工作原理。实验设备主要包括光学浮区炉、X射线衍射仪、劳厄照相机、物性测量系统、振动样品磁强计,铁电仪和高阻静电计等。测试方法主要包括磁性测量以及磁电耦合效应的测量方法。第三章,研究了HoFeO_3单晶和Ho_(0.5)Pr_(0.5)FeO_3单晶的磁特性。HoFeO_3的磁特性主要为自旋重取向,在低温下为Γ_2态,从34 K-45 K之间发生Γ_2-Γ_(12)的自旋重取向,然后在HoFeO_3单晶在50 K-60 K之间发生从Γ_2-Γ_4的磁相变。在Ho_(0.5)Pr_(0.5)FeO_3单晶中也发现了自旋重取向相变,转变温度为75 K-90 K之间,特别是在低温下a轴方向的磁化强度发生了一个跳变,跳变的大小和跳变温度对外加磁场具有很强的依赖性。经过分析Ho_(0.5)Pr_(0.5)FeO_3单晶中磁性离子磁晶格之间的相互耦合作用,认为跳变是由于Pr~(3+)离子的自旋翻转所导致的。第四章,研究了Ba位Sr掺杂Y型六角铁氧体Ba_(2-x)Sr_xCo_2Fe_(12)O_(22)(x=0.5,1,1.5)多晶样品的磁特性和磁电耦合特性。随着Sr掺入量的增加,Ba_(2-x)Sr_xCo_2Fe_(12)O_(22)样品的晶格参数逐渐降低,螺旋磁结构到线性磁结构的转变温度逐渐增加,并且样品的饱和磁化强度和矫顽力都有所提高。在Ba_(0.5)Sr_(1.5)Co_2Fe_(12)O_(22)样品中观察到磁场诱导的电极化现象,电极化出现的温度达到150 K,并且磁场能够控制电极化P的大小和符号的变化。第五章,研究了Fe位Al掺杂Y型六角铁氧体Ba_(0.5)Sr_(1.5)Co_2Fe_)(11)Al O_(22)中的磁性和磁电耦合特性。利用固相反应法合成了Ba_(0.5)Sr_(1.5)Co_2Fe_(11)AlO_(22)多晶样品,粉末X射线衍射测试(XRD)结果显示样品为单相的Y型六角铁氧体材料,磁化强度随温度的变化曲线表明,该体系在350 K以上由螺旋磁结构转变为线性磁结构,等温磁化曲线呈现亚铁磁型磁滞回线特征。在200 K以下观测到样品具有磁场诱导的电极化现象,并且能够通过磁场控制电极化的大小和符号的变化,最大电极化值达到120μA·s·m~(-2)。第六章,对本论文工作进行总结,并对研究对象的下一步的应用前景给与展望。
[Abstract]:Ferroelectric, ferromagnetic and ferromagnetic materials have great potential applications in the new information memory and multi-functional magnetic appliances. In recent years, it has become one of the hottest fields in condensed matter physics and material science. The coexistence of ferroelectric order and ferromagnetic order provides the possible coupling interaction between the two. The internal electric field produced by the ferroelectric order can cause the redistribution of the electron spin to change the magnetic properties of the system, and conversely, the spin ordered fluctuations lead to ferroelectric relaxation and dielectric anomalies by magnetostriction or possible electrophonon interaction. It is found that the dielectric constant changes obviously under the effect of applied magnetic field. The rich magnetic properties of the rare-earth iron oxide RFeO3 (R as rare earth element) in the orthogonal perovskite structure have always been one of the hot spots in the study, and the ultra fast magneto-magnetic effect and the properties of multi iron have been found most recently. And these new properties have a close relationship with the magnetic phase transition, so it is of great significance to study the change and inducement of the spin order near the magnetic phase transition in RFeO_3. The magnetoelectric coupling effect found in the six corner ferrite materials in recent years makes the study of this kind of material become a new research heat. One of them, because the temperature of the magnetoelectric coupling effect is high and even close to room temperature, it has potential advantages in practical application. So HoFeO_3, Ho_ (0.5) Pr_ (0.5) FeO_3 and Y type six corner ferrite Ba_ (2-x) Sr_xCo_2Fe_ (12-y) AlyO_ (22) are selected as the research object, and the magnetic phase in the material is studied by element doping. Variation, singular magnetic properties and magnetoelectric coupling effects. The full text is divided into six chapters. In Chapter 1, the lattice structure and magnetic structure of perovskite structure RFeO_3, the spin reorientation phase transition, and the development of the ferromagnetism are summarized. The crystal structure and magnetic structure of the Y type ferrite are introduced, and the development of the magnetic field induced electrode effect is introduced. General situation and research status. The second chapter mainly describes the preparation of experimental samples, characterization and physical testing methods and the experimental equipment used in this paper, including the method for the synthesis of polycrystalline samples by solid state reaction method, the method of growing single crystal by optical floating zone method, and the method and working principle of the experimental equipment. The experimental equipment mainly includes optical floating zone furnace, X ray diffractometer, Laue camera, physical measurement system, vibrating sample magnetometer, ferroelectric meter and high resistivity electrometer. The test method mainly includes magnetic measurement and magnetoelectric coupling effect measurement method. The third chapter studies the magnetic properties.HoF of HoFeO_3 single crystal and Ho_ (0.5) Pr_ (0.5) FeO_3 single crystal. The magnetic properties of eO_3 are mainly spin reorientation, the gamma _2 state at low temperature, the spin heavy orientation of gamma _2- gamma (12) from 34 K-45 K, and the magnetic phase transition from gamma _2- _4 to HoFeO_3 single crystal between 50 K-60 K. The magnetization of the a axis at low temperature has a jump, the jump size and the jump temperature are strongly dependent on the external magnetic field. After analyzing the interaction between magnetic ions magnetic lattices in Ho_ (0.5) Pr_ (0.5) FeO_3 single crystals, the jump is attributed to the spin reversal of Pr ~ (3+) ions. The fourth chapter is the study. The magnetic and magnetoelectric coupling properties of Ba bit Sr doped Y six corner ferrite Ba_ (2-x) Sr_xCo_2Fe_ (12) O_ (22) (22) (x=0.5,1,1.5) polycrystalline samples are obtained. With the increase of Sr doping amount, the lattice parameters of the Ba_ (2-x) Sr_xCo_2Fe_ (12) Sr_xCo_2Fe_ (22) samples gradually decrease, the transition temperature of the spiral magnetic structure to the linear magnetic structure increases gradually, and the saturated magnetic field of the sample is increased. The intensities and coercive forces were improved. The magnetic field induced electrode was observed in Ba_ (0.5) Sr_ (1.5) Co_2Fe_ (12) O_ (22) samples, the temperature of the electric polarization reached 150 K, and the magnetic field could control the size of P and the change of the symbol. The fifth chapter studied the Fe bit Al doped Y type six corner ferrite Ba_ (0.5) Sr_ (1.5) Co_2Fe_) (11). The magnetic and magnetoelectric coupling characteristics in Al O_ (22). The Ba_ (0.5) Sr_ (1.5) Co_2Fe_ (11) AlO_ (22) polycrystal samples were synthesized by solid phase reaction. The powder X ray diffraction (XRD) results showed that the sample was a single-phase Y type six corner ferrite material, and the magnetization intensity with the temperature variation curve showed that the system was changed by the spiral magnetic structure above 350 K. For linear magnetic structure, the isothermal magnetization curve is characterized by ferromagnetic hysteresis loop. Under 200 K, the sample has a magnetic field induced electrode phenomenon, and the maximum electrode value can reach 120 mu A. S. M~ (-2). The maximum electrode value can be controlled by magnetic field. The work of this paper is summarized and studied. The next step of the application prospects for the future.
【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】：O482.5

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