【作者】 王戈明；

【导师】 吴迪；

【作者基本信息】 南京大学 ， 材料物理与化学， 2013， 博士

【摘要】 以锰氧化物为代表的钙钛矿关联氧化物,由于其电子、自旋、轨道和晶格之间存在强烈的关联作用,展现出高温超导、庞磁电阻和多铁性等独特的性质。其中金属-绝缘体转变、电荷/自旋/轨道有序、相分离等物理现象具有丰富的物理内涵,一直是材料科学和凝聚态物理研究的热点之一。近年来,钙钛矿关联氧化物的异质结构引起了学术界的广泛关注。一方面,利用人工设计的异质界面对量子态进行调控,不仅可以实现材料电、磁、光等功能特性的集成,还可以衍生出不同于组分单元的新奇特性。另一方面,通过衬底与薄膜的晶格失配带来的外延应变有效的调控钙钛矿异质结构的电、磁特性。基于此,本文的工作主要集中在对几种钙钛矿锰氧化物异质结构的电、磁特性研究。本文的主要研究内容如下：1.利用配备有反射式高能电子衍射仪(RHEED)的激光脉冲沉积技术(PLD)在SrTiO3(001)单晶衬底上制备了一系列[(LaMnO3)n(SrMnO3)n]m (n=1,2,3,4,8和12)超晶格。X射线衍射(XRD)和原子力显微镜(AFM)测量表明超晶格具备尖锐的界面和光滑的表面。在长周期超晶格(n≥3)中,我们发现超晶格的磁性和输运性质强烈依赖于超晶格的周期。在LaMn(/SrMnO3界面,我们观察到由于Mn3+-O-Mn4+双交换作用而产生的界面铁磁性。长周期超晶格可以看作由反铁磁的组分单元LaMnO3层、SrMnO3层和铁磁界面层组成。基于铁磁和反铁磁相互竞争,我们很好的解释了超晶格中可能存在的自旋玻璃现象、增强的磁电阻效应和随着周期数改变而改变的输运机制。而在短周期超晶格(n=1和2)中,超晶格展现出类似电荷有序绝缘体的输运特性,其磁性也明显下降。我们将短周期超晶格中出现的磁、电特性归因于可能存在的电荷有序现象。2.采用PLD在SrTiO3(001)和LaAlO3(001)单晶衬底上制备了不同厚度的Pr0.65LA0.05Ca0.3MnO3(PLCMO)外延薄膜。用XRD研究了PLCMO薄膜的晶体结构和应变状态,结果表明薄膜的外延应变随着厚度的增加而减少。我们发现,无论是张应变还是压应变,当应变很大时,PLCMO中的电荷有序绝缘态更加稳定。随着厚度的增加,应变减小,PLCMO的电荷有序温度逐渐下降、电荷有序态失稳,伴随着居里温度和饱和磁矩增加。PLCMO薄膜中电、磁特性随膜厚的变化可以用外延应变引起的晶格畸变来解释。3.利用PLD在SrTiO3(001)单晶衬底上制备了一系列[(La0.7Ca0.3MnO3)n(Pro.7Cao.3MnO3)n]m(n=1,2,3,4和8)超晶格和Pr0.7Ca0.3MnO3、 La0.7Ca0.3MnO单层膜。RHEED强度振荡曲线和XRD谱表明了外延薄膜的层状生长模式和周期调制结构。超晶格的输运性质和磁性依赖于超晶格的周期。在短周期超晶格中,我们观察到增强的磁电阻效应、增强的饱和磁化强度。这一现象可以用周期较短时形成的“人工相分离”结构中界面处的Pr0.7Ca0.3MnO电荷有序失稳引起的铁磁自旋排列来解释。基于相均匀性和自旋耦合效应,我们解释了La0.7Ca0.3MnO和Pr0.7Ca0.3MnO的各向异性磁电阻效应随温度的变化。此外,我们发现超晶格的各向异性磁电阻效应依赖于超晶格周期,这可能和超晶格中不对称的多畴结构或MnO6八面体Jahn-Teller畸变有关。更多还原

【Abstract】 Correlated perovskite oxides, represented by manganites, displays unique properties such as high-temperature superconductivity, colossal magnetoresistivity and multiferroicity, due to strong correlations between spin, charge, orbital and lattice degrees of freedom. Phenomena such as metal-insulator transition, charge/spin/orbital ordering and phase separation have long been hot topics in the field of materials science and condensed matter physics. In recent years, heterostructures composed of different correlated perovskites have attracted much attention. On the one hand, the heterostrcutures provide opportunities to manipulate the competition of various quantum states via artificially designed interfaces, to generate novel functionalities that cannot be found the respective component units. On the other hand, epitaxial strain resulting from the mismatch between the substrate and film lattice offers an additional chance to effectively modulate the magnetic and electrical characteristics of the heterostructures. In this thesis, we discuss deposition of several correlated perovskite heterostructures and their electric and magnetic properties. Major achievements of this thesis include:1. A series of [(LaMnO3)n(SrMnO3)n]m (n=1,2,3,4,8and12) superlattices have been deposited on SrTiO3(001) single crystal substrates by pulsed laser depositon, monitored in situ by reflective high-energy electron diffraction. X-ray diffraction and atomic force microscopy studies reveal the sharp interfaces and smooth surface in the superlattices. In long-period superlattices (n≥3), magnetic and transport characteristics are observed to depend strongly on the period of the superlattice. We observs an interfacial ferromagnetism due to Mn3+-O-Mn4+double exchange across the LaMnO3/SrMnO3interface. The long-period superlattices can be regarded as a combination of antiferromagnetic LaMnO3/SrMnO3layers and ferromagnetic interface layers. The spin-glass-like behaviors, the large magnetoresistance and the evolution of the transport mechanism with increasing period in the superlattices are discussed in terms of the competition between the ferromagnetic and antiferromagnetic interactions. However, the short-period superlattices (n=1and2) exhibits reduced magnetization and their transport characteritics are similar to that of an charge ordered insultor. We ascribe this unusual magnetic and electrical properties to the possible charge ordering.2. Pr0.65La0.05Ca0.3MnO3(PLCMO) thin films with various thickness have been epitaxially deposited on SrTiO3(001) and LaAlO3(001) single crystal substrates by pulsed laser depositon. Epitaxial strain in films relaxes with increasing film thickness, as evidenced from x-ray diffraction. It is observed that the charge ordered insulating phase is stabilized in strained PLCMO films, no matter it is compressive or tensile. The charge ordering temperature decreases with the decrease of residual strain in PLCMO films. The curie temperature and the saturated magnetic moment in films increase as the strain relaxes with increasing film thickness. The evolutions of the transport and magnetic properties with the change of the film thickness can be understood in terms of the lattice distortion induced by epitaxial strain.3. A series of [(La0.7Ca0.3MnO3)n/(Pro.7Cao.3MnO3)n]m (n=1,2,3,4and8) superlattices, Pr0.7Ca0.3MnO3and La0.7Ca0.3MnO3thin films has been deposited on SrTiO3(001) single crystal substrates by pulsed laser depositon. The RHEED intensity oscillation and X-ray diffraction confims the layer-by-layer growth and periodically modulated heterostructures. The transport and magnetic characteristics are observed to depend strongly on the period of the superlattice. It is observed that the magnetoresistivity and saturated magnetization are enhanced in short-period superlattices. This can be ascribed to the artificial phase separation structure in the short-period superlattices, where charge ordered insulating state in Pr0.7Ca0.3MnO3 interface is made unstable by the ferromagnetic La0.7Ca0.3MnO3layer. The evolution of angular dependent magnetoresistance (AMR) effect as a function of temperature in La0.7Ca0.3MnO3and Pr0.7Ca0.3MnO3thin films are explained based on phase homogeneity and spin coupling effects. It is observed that the AMR effect in superlattices also depends on the superlattice period, which may be related to the unsymmetrical multidomain structure or the Jahn-Teller distortion in MnO6octahedron.更多还原