复合稀土正铁氧体晶体的生长及室温磁电耦合研究

Rare-earth orthoferrites with perovskite structure have unique magnetic properties. Some rare-earth orthoferrites have been predicated to be promising candidates as magnetoelectric materials from the viewpoint of the magnetic symmetry. However, the gigantic magnetoelectric coupling in (Dy,Gd)FeO3 appears at very low temperature. In this project, we prepare mixed rare-earth orthoferrites through substitution in A site or B site with magnetic phase transition at room temperature, then we systematically examine the anomalies of ferroelectric and dielectric around exotic magnetic transitions, looking forward to find a new multiferroic system at around room temperature. We will comprehensively study the magnetoelectric properties of mixed rare-earth orthoferrites. .The first-principle calculation will be also developed to assist the understanding the properties. The behavior of magnetic transition induced by the applied magnetic field and temperature, the dependence of magnetic transition on the crystal structure and distortion, and the relevance of ferroelectricity and magnetodielectric effect with magnetic transition will be comprehensively carried out. In addition, the substitution effect of A site and B site on the magnetic transition, ferroelectricity, and magnetodielectric effect will be investigated. Combined with XRD and Raman spectrum at various temperatures, we will try to make clear the relevance of spin-phonon coupling to the origin of ferroelectricity and magnetodielectric anomaly. Combined with the calculation, the effective substitution will be designed to improve the magnetic transition temperature. It is hoped to find new multiferroic system at room temperature and give the feasible magnetoelectric coupling mechanism.

钙钛矿结构稀土正铁氧体具有丰富而奇异的磁特性，部分稀土正铁氧体被预言是极具前景的磁电材料。本课题将从考察磁相变附近的铁电、介电异常开展系统的工作，寻找新的室温附近的多铁性体系。拟选取稀土正铁氧体为研究对象，采用光学浮区法和助熔剂法制备高质量单晶样品，以物性测量为主要手段，辅以第一性原理计算，系统深入地研究该体系的磁电性质。研究磁相变在温度和磁场诱导下的行为及其对晶体结构、晶格畸变的依赖性，阐明磁相变与铁电、磁介电异常之间的关联以及对磁电耦合的影响。探讨A 位和B 位离子掺杂对自旋重取向、负磁化等磁相变及铁电、磁介电的影响，搞清其独特磁结构中稀土与铁离子之间的交换收缩效应。结合变温XRD 和拉曼测试，澄清铁电、磁介电与自旋-声子耦合的内在关联。结合理论分析，设计有效掺杂，提高磁相变温区，寻找接近室温的多铁性体系，探索新的磁电耦合机制。

近年来，正交钙钛矿结构稀土正铁氧体RFeO3成为凝聚态和材料物理领域最新研究热点之一。在RFeO3中，源自R3 +亚晶格的4f电子和Fe3 +亚晶格的3d电子的磁自旋和晶格的耦合带来了丰富的物理现象，如自旋重定位转变，多铁性和自旋的超快光学控制。Fe的3d电子和稀土离子的4f电子也对磁电耦合有着非常重要的作用，对该体系奇异磁特性与铁电、磁介电的研究，必将推动对与自旋关联的铁电、磁介电物理机制的理解进入更深更广的层次。同时必将加速相关电子器件中的开发和应用。本课题从材料设计出发，通过A 位稀土离子的元素替代，合理组分设计，调整温度、气氛、压力等制备条件成功生长出一系列高质量多组分的复合稀土正铁氧体晶体。同时在Er0.6Dy0.4FeO3正铁氧体晶体中首次发现新的不寻常的Γ3 (Cx, Fy, Az)—Γ4 (Gx, Ay, Fz)磁相变以及交换偏置现象，有力拓展了稀土正铁氧体磁相变的研究内容。此外，在Er0.4Gd0.6FeO3正铁氧体晶体发现Γ4 (Gx, Ay, Fz)—Γ3 (Cx, Fy, Az)—Γ3 (Cx, Fy, Az)双重磁相变。多铁性能测试表明稀土离子掺杂会破坏晶体的中心对称结构，造成介电常数降低。不同频率下的介电温谱则显示Gd0.4Er0.6FeO3 和 Dy0.4Er0.6FeO3 晶体不同于 ErFeO3 晶体的热激活特性，反铁磁转变温也随着频率的增加向着高温区移动。1Hz 频率下，Gd0.4Er0.6FeO3 的饱和极化强度达到 0.18μC/cm2，且在室温下发生了铁电极化。