【作者】 蒋涛；

【导师】 李晓光；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2014， 博士

【摘要】 近年来,钙钛矿型锰氧化物异质结构的研究引起人们的广泛关注。这是因为：一方面,作为强关联体系中的典型材料,锰氧化物有着丰富的相图和有趣的物理现象,例如庞磁电阻效应,电荷/轨道有序以及不同的磁结构。另一方面,由于锰氧化物中存在电子、自旋、轨道和晶格等多个自由度间的强烈耦合,其物理性质极易受到如外延应力等外加作用的调控。因此,研究基于锰氧化物材料的异质外延体系中的应变-结构-性能关系对理解钙钛矿异质外延体系中界面效应和相关的多功能电子器件的实现都有着重要意义。基于这个考虑,本文主要研究了衬底的静态外延应变和压电衬底逆压电效应导致的原位动态应变对锰氧化物异质结构中电磁性质的调控。全文的安排如下：第一章介绍了类钙钛矿型锰氧化物特别是锰氧化物薄膜的研究概况,从锰氧化物研究的历史脉络开始,逐步介绍其晶体结构、基本物理机制、相分离。然后重点介绍锰氧化物外延薄膜应变调控相关的研究进展,最后再讨论锰氧化物外延薄膜中的三种各向异性效应。第二章主要讨论薄膜样品的制备方法,表征手段以及各种物性测量方法。在薄膜制备方面将介绍磁控溅射、脉冲激光沉积以及化学气相沉积三种不同的薄膜制备方法。样品结构表征上主要讨论x射线衍射包括倒易点阵的相关技术及原子力显微镜的两种测量手段。物性测量主要包括电输运测量、磁性测量以及各种仪表之间通过Labview软件编程控制的灵活联用。第三章系统研究了典型相分离的锰氧化物La0.325Pr0.3Ca0.375MnO3薄膜在各向异性应力作用下的磁各向异性、各向异性电阻以及各向异性磁阻等不同的电磁各向异性效应。研究发现在金属绝缘体转变温度附近,在30m的LPCMO薄膜中各向异性诱导的各向异性电阻可高达105%。随着薄膜厚度增加至120m,各向异性应力发生明显的弛豫,薄膜中的各向异性效应相应地发生明显的减弱,降低了将近两个数量级。为了更好理解如此巨大的各向异性电阻和磁阻和应变调控的相分离花样的关系,我们应用磁力显微镜对薄膜样品的磁畴进行了直接的观测。发现薄膜中的铁磁畴沿着张应变更大的方向取向生长,形成铁磁金属态-反铁磁绝缘态交替排列的条纹状相分离结构。从应用的角度来看,如此大幅度可调的各向异性电阻以及各向异性磁阻可能在将来的多功能器件设计上具有潜在的应用。第四章系统研究了锰氧化物薄膜/压电衬底异质结中动态应变对锰氧化物薄膜输运性质的调控。研究发现在La0.7Ca0.3Mn03/SrTi03/PMN-PT异质结中缓冲层的插入能够将应变的传递效率提高将近3倍,同时巧妙运用应变效应的易失性和铁电场效应的非易失性通过两种测量模式将这两种效应定量地进行了区分。此外在LPCMO/PMN-PT异质结中也发现了应变对相分离的调控,主要表现为极化电场对薄膜电阻-温度曲线的热滞效应的极大增强,但是对各向异性电阻的调控并不明显。这些发现对于今后的器件设计具有一定的参考意义。第五章系统研究了Cr02单晶外延薄膜的磁热效应,研究发现Cr02薄膜中存在着巨大的磁热效应。我们通过测量等温磁化曲线计算得到该薄膜在5T磁场下居里温度385K附近的磁熵变可高达8.46J·kg-1K-1,相对制冷功率则高达410J·kg-1,可与商用的磁制冷材料Gd和Gd5Si2Ge2等媲美。说明CrO2单晶薄膜可以作为高温制冷区的磁制冷材料。更多还原

【Abstract】 In recent years, the perovskite-like manganite heterostructureshave been studied extensively. This is because on one hand, as the typical material of strong correlated system, the manganites have complex phase diagram and interesting physical phenomenon such as colossal magnetoresistance, charge/orbital ordering and various magnetic structures. On the other hand, due to the interplay of the charge, spin, orbital and lattice degrees of freedom,thephysical properties in manganites can be easily tuned by external stimuli such as theepitaxial strain. Therefore, investigating the strain-structure-properties in manganite-basedheterostructures is very important for the further understanding the emergentphenomena at perovskite interfaces and design of related multi-functional electronic devices. Based on this consideration, we focus our work on the manipulation of electric and magnetic properties of manganite heterostructure by static epitaxial strain and dynamic piezo-strain. The dissertion is arranged as follows:In chapter1, The progress of manganite especially the progress of manganite films recently are reviewed. Starting from the research history of manganite, we introduce the crystal structure, basic physical mechanism, phase separation of manganites. Then the recent progress of strain effect on epitaxial manganite films are highlighted. At last, three kinds of anisotropic effects in manganite films are discussed in detail.In chapter2, some important techniques for sample fabrications, structure characterization and various measurementfor physical properties are introduced, including three popularfabrication techniquesmagnetron sputtering, pulsed laser deposition and chemical vapor deposition;X-ray diffraction and atomic force microscope, electric and magnetic measurement as well as flexible combination of different instruments via Labview.In chapter3, we systematically investigate the anisotropic strain induced anisotropic effects such as magnetic anisotropy, anisotropic resistivity and anisotropic magnetoresistance in typical phase separation manganite La0.325Pr0.3Ca0.375MnO3film. In the thinner30nm film, the anisotropic strained inducedcolossal anisotropic resistivitycan reach105%around the metal-insulator transition temperature. However, in thestrain relaxed thicker120nm film, the anisotropic resistivity decrease significantly, about two orders of magnitude smaller than the thin film. In order to eluciated the relation of the anisotropy with anisotropic strain induced phase separation,the oriented magnetic domains are directly observed by magnetic force microscopic (MFM) imagingwhich is consistent with the transport and magnetic data. From the point of application, the large magnitude and tunability of anisotropic magnetic and electronic properties caused by the strain are believed to be potential for designing artificial materials and devices.In chapter4, dynamic strain manipulation of the electric property of manganite film/piezoelectric single substrate heterostructures are systematically investigated. The strain transformation is enhanced3times by inserting a buffer layer SrTiO3in La0.7Ca0.3MnO3/SrTiO3/PMN-PT heterostructure. Moreover, in this heterosturcture, the strain effect and ferroelectric field effect are quantitatively separated first time basing on the volatility of the strain effect and the non-volatility of the field effect. In addition, the piezostrain mediated phase separation in LPCMO/PMN-PT is also observed which leads to an increasement of the thermal hysteresis in the temperature dependence of resistivity curves with the polarization field kept on the substrate. However, the anisotropic resitivity is little affected. These findings are potential for designing new electronic devices.In chapter5, The magnetocaloric properties of the CrO2single crystal films are studied. From the isothermal magnetic data, the magnetic entropy change are calculated which could reache a value of8.46J/kg·K at385K for the field variesfrom0to5T.Moreover the calculated relative cooling power values of CrO2can reach410J/kg at5T,which is comparable to that for Gd and Gd5Si2Ge2. These results suggest that CrO2film is one of the promising candidates for magnetic refrigeration applications above room temperature.更多还原