近邻元素掺杂YMn0.5Cr0.5O3钙钛矿结构氧化物中的亚铁磁性和B位有序研究

B-site cation ordering, which usually occurs when the differences of radius and valance state between the two cations are large, has been used with much success to expand the perovskite family and engineer new materials. Focusing on the neighboring element doped YMn0.5Cr0.5O3 oxide, in this project we will study the B-site cation ordering phenomenon implied by the ferrimagnetism of the system, as well as the details of the exchange interactions between magnetic cations. Increasing the average radius of A-site cation and decreasing the content of Jahn-Teller active cation at B-site respectively by using the element substitution method, we will investigate the evolution of ferrimagnetic ground state to clarify the influences of octahedral tilting and Jahn-Teller distortion on the B-site cation ordering. By growing single crystals and high-quality epitaxial films, we will characterize the B-site ordered microstructure in detail by using electron diffraction, high-resolution electron microscopy and other advanced techniques. We will also study the anisotropic properties, magnetoelectric coupling/multiferroics and strain effects for the epitaxial thin films. Employing the first-principles calculations on structure and magnetism, we will compare the calculated results with the experimental results to provide the theoretical guidance. The research results will be helpful to understand the complex phenomena, especially the role of lattice/orbital degree of freedom, in perovskite oxides more thoroughly. And the exploration of novel B-site cation ordered perovskite oxides will provide more solid scientific bases for the R&D of functional oxides.

钙钛矿结构氧化物中B位阳离子有序是一种新的物性调控手段，它通常发生在两种阳离子半径和价态差别较大的体系中。本项目以近邻元素掺杂的YMn0.5Cr0.5O3为对象，研究其亚铁磁性背后隐含的B位阳离子有序现象，以及磁性离子间的交换作用形式。将通过元素替位方法，分别增大A位离子半径和减少B位杨-泰勒效应离子的含量，观察亚铁磁基态的变化，澄清八面体倾斜和杨-泰勒畸变对B位阳离子有序的影响。制备单晶样品和外延薄膜，采用电子衍射、高分辨电镜等多种手段表征B位有序的微结构，并且研究样品的各向异性、磁电耦合/多铁性以及薄膜的应变效应。利用第一性原理计算方法进行理论计算研究，对比分析计算获得的结构、磁性基态与实验结果的异同，为实验研究提供参考指导。研究结果有助于深化对钙钛矿结构氧化物中复杂现象的认识，特别是晶格/轨道自由度所起的作用，并且对新型B位有序材料的探索，将为功能氧化物的设计开发提供更多的科学依据。

作为最重要的功能氧化物材料，钙钛矿结构氧化物具有丰富的物理内涵和非常广泛的应用前景，是目前凝聚态物理和材料科学的研究前沿之一。本项目以铬半掺杂YMn0.5Cr0.5O3锰氧化物为对象，深入研究其亚铁磁性背后的各种复杂磁交换作用形式以及磁致多铁性。通过La、Eu等元素对Y的替位掺杂增大A位平均离子半径，研究结果表明样品保持正交的Pbnm结构，并有向立方对称性转变的倾向，主要表现在沿c轴的Mn-O-Cr键角增大。同时亚铁磁转变温度上升源于Cr次格内部的反铁磁交换作用减弱，说明沿c轴的键角对体系的结构对称性和磁有序起到关键作用。采用磁控溅射技术在LaAlO3和SrTiO3等多种单晶基片上成功制备YMn0.5Cr0.5O3外延薄膜，发现薄膜与基片间的晶格失配度主导薄膜外延取向，改变后退火条件可以调整YMn0.5Cr0.5O3/ SrTiO3薄膜的外延取向，样品具有沿c轴的单轴磁晶各向异性。采用叉指电极结构对压缩应变YMn0.5Cr0.5O3/YAlO3外延薄膜加工各种取向的面内电容器，热释电流测量结果给出样品在磁转变温度附近出现沿a和c轴方向的各向异性自发电极化，表明其磁致多铁性。采用二次退火手段调整压缩应变薄膜的应变强度，发现应变驰豫导致电极化强度线性下降。利用第一性原理计算方法对以上实验现象进行了理论研究，计算结果表明B-位阳离子层状有序结构具有最低的总能量和亚铁磁有序基态。并且对外延薄膜的应变效应计算给出，c轴应变有利于E型反铁磁有序和自发电极化。理论计算结果与实验观察结果有较高的一致性，有助于深入认识钙钛矿结构氧化物中的复杂现象，特别是结构畸变、薄膜应变效应等对磁有序及磁致多铁性所起的作用。并且对铬半掺杂锰氧化物中磁、电极化有序基本物理机制的探索，将为功能氧化物材料的设计开发提供更多的科学依据。