多铁异质结中的磁电耦合和电阻转换效应

发布时间：2018-01-11 19:02

  本文关键词：多铁异质结中的磁电耦合和电阻转换效应　出处：《中国科学技术大学》2016年博士论文　论文类型：学位论文

  更多相关文章： 多铁材料 磁电耦合效应 异质结 磁介电效应 铁电性 电阻转换效应 界面效应

【摘要】：随着自旋电子学／电子学过去数十年的迅猛发展,人们对器件的小型化、器件的读写速度以及其读写能耗都有了更高的要求。然而,现有的技术与材料已经到达其尺寸和性能的极限,并开始制约电子器件科学的发展。‘因此探索能够满足生产生活更高要求的新材料进而变革电子器件领域变得尤为迫切和重要。具有多功能的、可进行多参量调控的多铁材料受到了科学家们的广泛关注。多铁材料以其铁电极化和磁极化共存、磁电间可相互耦合调控而有望在多功能电子器件革新上发挥巨大的作用。然而,目前室温单相的多铁性材料匮乏且磁电耦合效应弱,可以进行人工设计的复合多铁性材料——多铁异质结由于其在材料选择和磁电耦合效应方面都有着巨大的优势而引起了广泛地关注。目前尽管在多铁异质结磁电耦合方面已经取得了一定的研究进展,但其中仍然存在大量的、与实际生产应用密切相关的问题,比如如何在室温甚至高温条件下实现强烈的磁电耦合效应、异质结中的磁性层对异质结整体在磁场下的介电铁电性的影响有多大,如何成功地将畴壁工程和宏观电输运性质紧密联系在一起实现更多的电阻状态等等。针对上述问题,本论文主要研究了磁性电极材料对多铁异质结中的磁电耦合效应和介电／磁介电效应的影响：探讨了多铁异质结中界面效应和铁电畴壁同时参与的三阻态阻变效应的起源,为多重态存储器件的原型设计提供了一定的思路。整篇论文分为六个章节,每一章的主要内容可概括如下：在第一章中,综述了单相多铁材料的多铁性起源以及复合多铁材料的优势,重点介绍了磁介电效应和多铁异质结中磁电耦合效应以及相关研究进展,最后简单介绍了多铁性异质结相关的电阻转换效应并对其中的机理进行了阐述。在第二章中,为了研究磁性电极电阻对外延的BiFeO3/La0.625Ca0.375MnO3 (BFO/LCMO)异质结铁电性的影响,我们利用不同的电极配置系统地测量了BFO/LCMO异质结的铁电性,发现表观矫顽电压随着LCMO电阻线性增加。通过建立分压模型,去除了来自于LCMO层的分压的非本征效应,进而得到了异质结本身的矫顽电压Vac0,同时我们发现当温度高于150 K时,Vac0同样的对磁场有明显的响应,显示出本征磁电耦合效应。实验结果证明,对于铁电／铁磁复合薄膜来说,当研究异质结本身的铁电性时,铁磁层的分压作用不可忽视；同时提供了一种有效的、定量的方式来分析LCMO层在多层异质结薄膜上的分压贡献,这对研究铁电／铁磁异质结的本征磁电耦合效应以及相关器件设计很有帮助。在第三章中,为了研究磁性电极电阻对外延BFO/LCMO异质结介电性的影响,我们利用不同的电极配置,系统地测量了Au/BFO/LCMO异质结的介电性能,发现随着LCMO底电极电阻的增加,Au/BFO/LCMO多铁异质结电容的实部有明显的降低,且体系弛豫行为所对应的频率均向高频移动。通过分析异质结介电损耗随温度的依赖关系,发现参与介电测量的底电极LCMO所占比例越多,异质结的磁介电效应也越大,但损耗tan δ的值也越大。通过研究不同电极配置下异质结电容虚部随频率的变化规律,发现对于任意的电极配置,异质结的介电弛豫行为主要可等效地反映出Mn3+和Mn4+之间的电子跃迁或转移过程。上述发现对研究铁电／铁磁异质结的磁介电效应是很有帮助的,同时可以对相关器件设计有一定的指导作用在第四章中,我们通过电感耦合等离子质谱仪(ICP-MS)判断出所生长的BFO薄膜中不存在Bi元素的缺失,这对分析半导体性质的BiFeO3的半导体类型以及在Au/BiFe03/La0.6Sr0.4Mn03 (Au/BFO/LSMO)多铁异质结中界面肖特基势垒的方向起到关键性作用；利用扫描透射电子显微镜(STEM)对异质结上下界面的晶体结构进行了细致的表征,在BFO/LSMO界面处发现了存在明显离子互扩散区域的中间过渡层,在Au/BFO界面处观察到了"dead layer ".上述发现为后续分析异质结阻变行为起源奠定了坚实的实验基础。在第五章中,我们研究了Au/BFO/LSMO多铁异质结中电磁调控的可重复、区分度较高的三阻态行为。通过改变铁电极化的方向,异质结的电阻可以在三种稳定非易失的电阻状态之间切换且不需要电形成过程。这种非易失的电阻转换行为可以被归结以下三个方面的共同作用：Au/BFO界面处的肖特基势垒高度的变化、BFO/LSMO的界面电阻大小的变化以及BFO铁电畴壁密度相关的电阻改变。使用异质结矫顽电压附近的脉冲电压极化样品后所观察到的具有最高电导率的第三态在室温下表现出明显负磁阻效应,通过宏观电测量证明了BFO铁电畴壁在室温下的负磁阻效应。以上的发现对我们深入理解多铁异质结中的界面效应以及铁电畴壁的铁磁性对非易失电阻转换效应的影响,使得将自旋电子学引入阻变存储器件成为可能。在第六章中,我们在不同单晶衬底上生长了Co薄膜,研究了其中磁各项异性,发现对于所研究的几种单晶衬底上的Co薄膜而言,均满足易磁化轴方向上的电阻大于难磁化轴方向上的电阻这一规律。但是,各项异性磁阻对外加磁场和方向的依赖关系有待进一步的研究和探索。
[Abstract]:Over the past few decades with the rapid development of spintronics / electronics, people for miniaturization of devices, devices to read and write the read and write speed and energy consumption have a higher demand. However, the ultimate technology and existing materials has reached its size and performance, and began to restrict the development of electronic science. "Therefore to explore the production of new materials to meet the higher requirements of the life and change the field of electronic devices has become particularly urgent and important. With multi functions, multiferroic materials can be multi parameter regulation is widely concerned by scientists. Many iron materials to the ferroelectric polarization and magnetic polarization coexist between the coupling and magnetoelectric is expected to play a big role in the innovation of functional electronic devices. However, the current room temperature single-phase multiferroic material shortage and magnetoelectric coupling in multiferroic composite can be weak, artificial design Materials -- iron heterojunction due to the material selection and the magnetoelectric effect have a huge advantage and has aroused wide concern. Although the magnetoelectric coupling in multiferroic heterogeneous node has made some progress. But there are still a lot of closely related with the practical application problems, such as how to realize the strong magnetoelectric effect at room temperature even under high temperature conditions, the magnetic layer heterojunction in heterojunction under magnetic field affects the overall dielectric ferroelectric is, how successfully the domain wall engineering and macro electrical transport properties closely together to achieve more resistance and so on. In view of the above the problem, this paper mainly studied the influence of magnetic electrode materials on the magnetoelectric coupling in multiferroic heterostructures and dielectric / magnetic dielectric effect: To investigate the effect of multiferroic interface and heterostructure The origin of the three resistance state resistance domain wall also participates in effect, provide some ideas for the prototype design of multiple state memory devices. The whole thesis is divided into six chapters, the main contents of each chapter are summarized as follows: in the first chapter, this paper reviews the origin of iron multiferroic materials and the advantages of composite multiferroic materials, focuses on the progress of the magnetoelectric effect magnetodielectric effect and multiferroic heterostructure and related research, finally introduces the multiferroic heterostructures the resistance switching effect and the mechanism is also discussed. In the second chapter, in order to study the magnetic resistance of the electrode on the extension of the BiFeO3/La0.625Ca0.375MnO3 (BFO/LCMO) ferroelectric heterojunction effect, we use different electrode configurations to systematically measure the ferroelectric properties of BFO/LCMO heterojunction, found that the apparent coercive voltage with resistance linear increase. By LCMO To establish the model of partial pressure, removed from the LCMO layer of the partial pressure of the extrinsic effect, then obtained the heterojunction itself coercive voltage Vac0, we also found that when the temperature is higher than 150 K, Vac0 had the same response to magnetic field shows the inherent magnetoelectric coupling effect. The experimental results show that the for the ferroelectric / ferromagnetic composite film, when the ferroelectric heterojunction itself when the effect of ferromagnetic layer pressure can not be ignored; but also provides an effective and quantitative method to analyze the LCMO layer in the multilayer heterostructure thin films on partial pressure contribution, which is very helpful to the intrinsic magnetoelectric the coupling effect of ferroelectric / ferromagnetic heterojunction devices and related design. In the third chapter, in order to study the influence of magnetic electrode resistance on the dielectric properties of the epitaxial growth of BFO/LCMO heterojunction, we use different electrode configurations, system measurement Au/BFO/LCMO heterojunction The dielectric properties, it is found that with the increase of LCMO bottom electrode resistance, real Au/BFO/LCMO iron heterojunction capacitance is obviously reduced, and the corresponding system of relaxation frequency move to high frequency. Through the analysis of heterojunction dielectric loss with temperature dependence, found in the bottom electrode dielectric measurement by LCMO the proportion of the more magnetic dielectric effect of heterojunction is larger, but the loss of Tan 8 increased. Through the configuration of different electrode heterojunction imaginary capacitance changes with the frequency, for arbitrary electrode configuration, heterojunction dielectric relaxation behavior can reflect the electronic equivalent the transition or transfer process between Mn3+ and Mn4+. These findings are helpful to the study of ferroelectric / ferromagnetic heterojunction magneto dielectric effect, also can have a certain guiding role in the fourth chapter of the relevant device design, we through the inductor Coupled plasma mass spectrometry (ICP-MS) deletion of the Bi element does not exist out of the growth of the BFO thin film, the semiconductor type analysis of semiconductor properties of BiFeO3 and Au/BiFe03/La0.6Sr0.4Mn03 (Au/BFO/LSMO) - iron heterojunction interface in the direction of Schottky barrier plays a key role; by scanning transmission electron microscopy (STEM) on the crystal the structure of the interface under the heterojunction was characterized in detail, the obvious ion diffusion transition layer region is found at the interface of BFO/LSMO and Au/BFO in the interface is observed at the "dead layer". The findings for the subsequent analysis of heterojunction resistive behavior origin has laid a solid basis for experiment in Chapter fifth. Au/BFO/LSMO, we study the electromagnetic control multiferroic heterostructure can be repeated to distinguish the three resistance behavior of high degree. By changing the polarization direction of ferroelectric heterostructures, resistance In three kinds of non stable process between the resistance state of volatile switching and does not need electricity. This non volatile resistance switching behavior can be attributed to the following three aspects: the interface of Au/BFO Schottky barrier height changes, BFO/LSMO interface resistance changes in the size and resistance of BFO ferroelectric domain wall density the change of use. Near the heterojunction coercive voltage pulse voltage polarization samples observed after the third state has the highest conductivity at room temperature showed significant negative magnetoresistance effect, through the macroscopic electrical measurements demonstrated that the negative magnetoresistance effect of BFO ferroelectric domain wall at room temperature. The result of in-depth understanding of the interface effects of excess iron the heterojunction and ferromagnetic ferroelectric domain wall of nonvolatile resistive switching effect on us, which will introduce spintronics resistive memories become possible in sixth. In Chapter one, we grow Co films on different substrates, on which magnetic anisotropy, it is found that for Co single crystal substrate of several film on, meet the resistance of the easy axis direction is greater than the resistance of hard axis in the direction of the law. However, the anisotropic magnetoresistive foreign dependence the magnetic field and the direction of the need for further research and exploration.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O482.51
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本文编号：1410799