钙钛矿锰氧化物外延薄膜和异质结制备及相关物性研究

【作者】 周桃飞；

【导师】 李晓光；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2007， 博士

【摘要】 锰氧化物因其超大磁电阻效应在磁存储，磁传感器件，自旋阀，自旋晶体管等方面有着广泛的应用前景；而高质量薄膜样品的制备及其各种性质的探索和研究，对这类材料走向器件应用是至关重要的。近来，由于薄膜制备技术的提高，人们在纳米尺度调控的锰氧化物薄膜制备以及在相关原型器件研究等方面取得了一定的突破。La_0.67Sr_0.33MnO₃（LSMO）作为锰氧化物中应用最广泛的一个体系，它的高质量外延薄膜的获得显然是人们期待的。此外在锰氧化物中，一般只存在负磁电阻效应，但是由这类材料构成的异质p-n结，却不仅表现负磁电阻，还存在正磁电阻，磁电阻符号强烈依赖于温度，偏置电流，空穴浓度。本论文正是针对La_0.67Sr_0.33MnO₃薄膜的应力弛豫和磁场对La_1-xCa_xMnO₃／SNTO p-n结的调制，开展了一定的研究。在La_0.67Sr_0.33MnO₃薄膜中发现了一种由于角度弛豫而形成的面内超晶格结构调制；在异质p-n结研究方面发现锰氧化物中复杂带结构对异质结的磁电阻正负和大小起着决定性的作用，并在此研究基础上获得了锰氧化物带结构在能量尺度上随磁场的演化关系。此外通过强磁场下的超声衰减测量探讨了合作Jahn-Teller畸变在Pr_0.7Ca_0.3MnO₃铁磁绝缘相中所起的作用。各章的主要内容分别概括如下：第一章简单介绍了各种磁电阻效应；钙钛矿锰氧化物的基本性质，包括研究历史、晶体结构、电子结构、磁结构和电磁相图；锰氧化物薄膜中应力弛豫；锰氧化物在应用方面的研究进展。第二章研究了来自于菱形的La_0.67Sr_0.33MNO₃（LSMO）生长在立方衬底LSAT上的角度失配对LSMO薄膜结构的影响。通过X射线衍射实验中的摇摆曲线和倒易空间图测量，在弛豫的单层LSMO薄膜中观察到一种面内超晶格结构，这种超晶格结构的周期随着膜厚的增加而增加。不对称的（103）倒易空间图强有力地证明了这面内调制结构源于因菱形的LSMO和立方衬底LSAT之间的角度失配引起的一种剪切应力的弛豫而导致LSMO薄膜中赝立方角的恢复。第三章结合La_1-xCa_xMnO₃中与自旋有关的复杂能带结构以及p-n结中存在的电子注入的特点，详细地分析了La_1-xCa_xMnO₃／SNTO p-n结中复杂磁电阻的起源，即由于锰氧化物中晶场劈裂能、自旋劈裂能、Jahn-Teller能等能量之间微妙的相互竞争，SNTO中的电子在外电场的作用下有选择的被注入到LCMO复杂的与自旋相关的有关能带中，那些被填充到e_g^1↑和e_g^2↑带的多数自旋载流子将导致负磁电阻；被填充到t_2g^↓带的少数自旋载流子将导致正磁电阻。第四章利用La_1-xCa_xMnO₃／SNTO p-n结这一特殊异质结构研究了La_1-xCa_xMnO₃相关能隙随磁场的演化。结合p-n结反向偏置输运特点及La_1-xCa_xMnO₃能带结构，分析了p-n结中磁电阻随负偏置电压的变化曲线上的极值位置所对应的相关能隙。根据磁电阻极值的位置随磁场的依赖，获得了Jahn-Teller能和自旋劈裂能在能量尺度上随磁场的演化。第五章测量了Pr_0.7Ca_0.3MnO₃在强磁场下的超声衰减性质，发现了两个明显的超声异常：第一个在219K附近，这个异常在零场时是由电荷有序形成导致的，而在6T和14T下我们认为是由铁磁涨落导致的；第二个在150K以下，这个是由与合作Jahn-Teller畸变密切相关的长程轨道有序的建立导致的。通过对150K下声速和弹性能量损失峰P₂峰随磁场的变化的分析，阐明了合作Jahn-Teller畸变是有利于长程轨道有序和铁磁绝缘相的建立。更多还原

【Abstract】 The perovskite-type manganese oxides show a wide range of applications potential in magnetic storage, magnetic sensors, spin valve, spin transistors etc. because of colossal magnetoresistance （CMR） effect. Since most applications require thin film, it is of great importance to be able to prepare high quality single-crystal epitaxial films and have a full knowledge of all kinds of properties in them. Recently, due to the progress in preparation technique, the growth optimization of manganite thin films in nano-scale has become possible and some breakthroughs in the fabrication of related prototype devices have been obtained. La_0.67Sr_0.33MnO₃ （LSMO） is one of the most widely systems used in devices, and its high-quality epitaxial films are obviously expected. In addition, there is only negative magnetoresistance effect in manganese oxides in general, however, it is intriguing that not only negative MR but also positive MR, even a bias-dependent MR was observed in manganite-based heterojunctions.This dissertation mainly focuses on the strain relaxation in LSMO film and the modulation of La_1-xCa_xMnO₃/SNTO p-n junction in magnetic fields. In LSMO films, an in-plane modulated superlattice originating from shear strain relaxation was found; in La_1-xCa_xMnO₃/SNTO p-n junction, our results show that the complicated band structure of La_1-xCa_xMnO₃ plays a decisive role in the sign and magnitude of magnetoresistance, and on the basis of this study, the band structure evolution of La_1-xCa_xMnO₃ with magnetic field in energy scale is obtained. Moreover, the role of cooperative Jahn-Teller distortion in the ferromagnetic insulating behavior of Pr_0.7Ca_0.3MnO₃ is also studied by the ultrasonic attenuation measurement in magnetic field.This dissertation consists of five chapters. The arrangement of the chapters and the main contents in each chapter are presented as follows:In chapter 1, we make a brief introduction to all kinds of magnetoresistance effects; the basic properties of the perovskite-type manganese oxides, including research history, crystal structure, electronic structure, magnetic structure, and the electronic and magnetic phase diagrams; the strain relaxation of manganite thin films; and the research progress in application.In chapter 2, the strain relaxation and its effect on the in-plane superlattice formation of epitaxial LSMO thin films on the （LaAlO₃）_0.3（Sr₂AlTaO₆）_0.7 （001） [LSAT （001）] substrates were investigated as a function of film thickness by X-ray diffraction. Rocking curves and reciprocal space mappings （RSM） around （002） and （003）, （103） reflections reveal that the film with a thickness above 27 nm is almost fully relaxed and has a modulated superlattice structure. The recovery of the pseudocubic angle of LSMO resulting from the relaxation of the shear strain introduced by the growth of rhombohedral LSMO on cubic LSAT was suggested to be the origin of the superlattice structure.In chapter 3, the origin of the complex magnetoresistance in La_1-xCa_xMnO₃/SNTO p-n junction was studied by combining the complicated band structure of La_1-xCa_xMnO₃ with charge injection characteristics in p-n junction. Due to the subtle interplay among the Hund’s coupling, JT effect and crystal field of La_1-xCa_xMnO₃, the filling of the electrons from SNTO in the different subbands of LCMO at the LCMO-SNTO interface plays a key ingredient in understanding the remarkable MR variations for the heterojunction. Our results also suggest that MASC and MISC are responsible for the negative and positive MR of the system, respectively.In chapter 4, the shift of the maximum （or minimum） position in unusual magnetoresistance （MR） vs V curves of the heterojunctions composed of La_1-xCa_xMnO₃ （LCMO, x=0.18 and 0.3） and 0.5 wt% Nb-doped SrTiO₃ （SNTO） was used to investigate the change of band gaps of LCMO with magnetic field.In chapter 5, the ultrasonic properties of Pr_0.7Ca_0.3MnO₃ under magnetic field were investigated and two sound velocity anomalies were observed. The first one is around 219 K, which is ascribed to the formation of charge ordering in 0T and to the ferromagentic spin fluctuation in 6T and 14T, respectively. The latter one takes place at the temperature lower than 150 K, which is ascribed to long-range orbital-ordering （OO）. With increasing magnetic field, the magnitude of the relative change of sound velocity becomes smaller and elastic energy loss peak P₂ shifts towards lower temperature. These results illuminate that the cooperative JT distortion favors the establishment of the long-range OO and the FMI phase.更多还原