钙钛矿锰氧化物薄膜的磁性和输运性质及磁电耦合增强的热效应

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	钙钛矿锰氧化物薄膜的磁性和输运性质及磁电耦合增强的热效应
	孟辉
学位类型	博士
导师	张志东 ; 张玉琴
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	锰氧化物薄膜轨道有序各向异性磁电阻磁热效应磁电耦合热效应 Manganite Thin Film Orbital Order Anisotropic Magneto-resistance Magneto-caloric Effect Magneto-electro Caloric Effect
摘要	"钙钛矿锰氧化物一直是磁电子学领域的研究热点。在钙钛矿锰氧化物中，电荷、轨道、自旋和晶格自由度之间复杂的耦合作用产生了丰富多彩的物理现象。由于基片与块体的晶格常数不匹配，在不同的基片上生长外延薄膜是对锰氧化物进行应力调控的一个有效手段。本文通过选择不同晶格常数的基片，生长不同厚Nd0.45Sr0.55MnO3薄膜来研究应力对体系的轨道有序及相关的输运性质和磁性的影响。研究发现，薄膜晶格常数c/a的值是影响体系物理性质的重要参数。当c/a值小于1.02时，外延薄膜同块体一样在奈尔点以下发生dx2-y2轨道的有序排列，对应的磁结构为A型的反铁磁，输运性质为二维的金属型导电性，并且dx2-y2轨道有序结构的稳定性随c/a值的减小而增强；当c/a值大于1.02时，外延薄膜则会形成d3z2-r2轨道的有序排列，对应的磁结构为C型的反铁磁，平面内的输运性质为绝缘体型。各向异性磁电阻，是指电阻值随电流与外加磁场间角度的变化。传统过渡族金属或合金中的各向异性磁电阻在居里点以下随温度降低而增大，与此不同，锰氧化物中的各向异性磁电阻在居里点附近出现一个峰值，随着温度的下降，其值逐渐减小。本文研究了dx2-y2轨道有序Nd0.45Sr0.55MnO3薄膜和无应力状态下的电荷有序Pr0.7Ca0.3MnO3薄膜的各向异性磁电阻。在Nd0.45Sr0.55MnO3薄膜中发现了依赖于温度和磁场的二次对称与四次对称的AMR(θ)及其相互转变。在Pr0.7Ca0.3MnO3薄膜中，观测到了AMR(θ)由cos2(θ)依赖关系变为sin2(θ)依赖关系的转变，而且这种转变与磁场诱导的相分离过程中，铁磁金属的逾渗效应有着密切的关系。磁热效应由于其在高效率绿色制冷领域的应用前景，一直是磁性材料研究领域的重要方向。本文系统地研究了DyB2化合物的磁性和磁热效应。研究发现，DyB2在低温区存在两个连续的磁结构转变，一个为二级的顺磁铁磁相变出现在50K处，另一个可能为自旋再取向转变出现在25K处。这两个磁结构转变使磁熵变随温度变化出现了两个极大值，这极大地提高了DyB2在该温区的制冷能力。研究表明寻找具有两个或多个连续磁结构转变的材料是提高制冷能力的有效手段。当材料中同时存在铁磁有序和铁电有序，且两者之间发生强烈的耦合时，材料在外场变化时的热效应将会出现不同于传统磁热效应和电热效应的新性质。本文用唯象的朗道相变理论对磁电耦合体系的热效应进行了研究。研究发现，材料的热效应由于铁电序和铁磁序间的耦合而得到了极大的增强。特别是当铁磁居里温度小于铁电居里温度时，可以仅通过外加电场来调控体系的磁热效应。而由于强电场较强磁场更容易获得，因此电场诱导的磁电耦合热效应是发展新型绿色制冷的一个有效途径。"
其他摘要	"Perovskite manganite oxide has been a hot topic of magnetoelectronics research since the discovery of colossal magnetoresistance. Due to complex and strong coupling among charge, orbital, spin and lattice degrees of freedom, perovskite magnanite oxides possess a wealth of physical phenomena. Tailoring the physical properties through epitaxial strain is one main research area in peroskite manganite oxide films. Due to lattice mismatch between substrate and bulk, epitaxial growth of manganite oxide films on different substrates is an effective way to control strain. We have grown Nd0.45Sr0.55MnO3films on different substrates with different thickness and investigated the effect of epitaxial strain on orbital ordering and related magnetic and transport properties. It is found that tetragonal distortion c/a ratio is indeed a key factor determining the physical properties of thin film. When c/a is smaller than 1.02, dx2-y2 orbital ordering is favored resulting in corresponding A-trype antiferromagnetism with metallic transport behavior, Moreover, dx2-y2 orbital ordering becomes more stable with decreasing c/a ratio. When c/a is larger than 1.02, d3z2-r2orbital ordering is favored resulting in corresponding C-type antiferromagnetism with insulator transport behavior. Anisotropy magnetoresistance (AMR), i.e. resistance varies with the angle between current and magnetic field, is a new direction in the research field of perovskite manganite oxide. AMR in ferromagnetic manganite is nonmonotonic temperature dependence showing a peak close to its Curie temperature. This is very different from the conventional ferromagnetic alloys where AMR increases with decreasing temperature below Curie temperature. In addition, due to complex coupling of charge, orbital, spin and lattice degrees of freedom, AMR in perovskite manganite oxide film exhibits various behavior depending on temperature, magnetic field and epitaxial strain. We have investigated the AMR effect of dx2-y2 orbital ordering Nd0.45Sr0.55MnO3 film and no strained charge ordering Pr0.7Ca0.3MnO3 film. In Nd0.45Sr0.55MnO3 film, twofold and fourfold symmetric AMR and a transition between them are observed depending on temperature and/or strength of a magnetic field. In Pr0.7Ca0.3MnO3 film, AMR sign evolution process from cos2(θ) dependence to sin2(θ)dependence is observed at intermediate fields. Moreover, such AMR sign evolutions are found to be closely related with magnetic-field-induced ferromagnetic metal percolation behavior. The magneto-caloric effect is an important subject in the magnetic materials research field due to the application prospect of magnetic refrigeration. The central task in this area is to find materials with high magnetic entropy change and large refrigeration capacity in some special working temperature. We have investigated the magnetic properties and magneto-caloric in DyB2. There are two magnetic transitions in DyB2: a spin-reorientation-like transition at 25K followed by a ferromagnetic-paramagnetic transition at 50K. These transitions greatly enlarge the magnetic refrigeration temperature window and yield a huge refrigeration capacity. The results indicate that looking for materials with two or more successive magnetic transitions is an effective routine to obtain large refrigeration capacity. When the ferromagnetic and ferroelectric order coexist and couple to each other strongly in the very same system, the caloric effect caused by external field would be different from the conventional magneto-caloric and electro-caloric effect. Using Landau's phenomenological theory of second order phase transformation, we have studied the caloric effect in magneto-electro-coupling system. It is found that the magneto-electro-caloric effect is enhanced greatly when the magneto-electro coupling is strengthened. Especially when the magnetic Curie point is lower than the ferroelectric Curie point, we can use solely the electric field to control the magneto-caloric effect, since large electric field can be obtained more easily than large magnetic field, our results shows that controlling magneto-electro-caloric effect by electric field will be a prospect way to develop new environment-friendly refrigeration technology."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64461
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	孟辉. 钙钛矿锰氧化物薄膜的磁性和输运性质及磁电耦合增强的热效应[D]. 北京. 中国科学院金属研究所,2012.