铋系自旋失措材料高温多铁性的调控与机理研究

The spin frustrated multiferroic materials possess intrinsic coupling effect between ferromagnetic and ferroelectric orders, and there are multiple order parameters coexisting and competing. They provide new method to design magnetoelectric multi-function devices and exhibit broad prospect in application and significant value in scientific research. However, these materials have low phase transition temperature. The physical mechanisms for the coexistence of magnetic and ferroelectric properties are still unclear, and it is difficult to realize the control of magnetic property via ferroelectric property. Therefore, it becomes the hotspot in the field of multiferroic research to explore the spin frustrated materials with high temperature multiferroic property. This project focuses on the Bi-based spin frustrated multiferroic materials (such as BiMn2O5, Bi2Fe4O9) to carry out the studies: (1) Manipulate the spin interactions by doping ions on the A-site, revealing the physical mechanisms of magnetic and ferroelectric coexistence and obtaining multiferroic materials with excellent physics property; (2) Establish the physical correlations among crystal structure, magnetic order and ferroelectric order in the specimens prepared by high temperature and high pressure technique, and explore new way to enhance the multiferroic property; (3) Utilize the electric field/polarization to control the magnetic property or attempt to achieve the magnetoelectric coupling based on magnetic exchange bias effect in the selected materials with strong magnetoelectric coupling. The project would further reveal the physical mechanisms of magnetic and ferroelectric properties coexistence at high temperature and provide research foundation for realizing the application of multiferroic materials in the field of spintronic devices.

自旋失措多铁材料具有内禀的磁电耦合及多种序参量的共存和竞争，为磁电多功能器件的设计提供了新思路，展现出广阔的应用前景和重要的科学研究价值。然而这类材料的相变温度较低，磁电共存的机制尚不清楚，实现铁电性对磁性的调控比较困难。因此，寻找高温自旋失措多铁材料成为多铁领域的研究热点。本项目围绕铋系自旋失措多铁材料BiMn2O5、Bi2Fe4O9开展研究，(1)通过A位离子掺杂引入自旋涨落，调控体系中的自旋相互作用，揭示磁电共存的物理机制，合成性能优异的室温多铁材料；(2)通过高温高压对材料进行改性，建立晶体结构与磁有序和铁电有序间的物理关联，探索增强多铁材料磁电性的有效方法；(3)选取具有较强磁电耦合的材料体系，尝试基于内禀的磁电耦合或基于磁交换偏置效应，实现电场或电极化对磁性的调控。该研究将进一步揭示铋系自旋失措多铁材料高温磁电共存的物理机制，为实现多铁材料在自旋电子器件领域的应用提供研究基础。

多铁性材料同时具有磁性和铁电性，为磁电多功能器件的设计提供了新思路，具有广阔的应用前景。自旋失措多铁性材料具有本征磁电耦合效应，相对于传统的多铁性材料，在磁电性之间的相互调控方面具有显著的优势。目前发现的大多数自旋失措多铁性材料的磁电共存温度较低，磁电耦合的物理机制尚不明确，并且难以实现磁电之间的双向调控。因此，对于多铁性材料的研究仍然属于热门研究领域。本项目实施过程中选择了具有自旋失措磁结构Bi2Fe4O9为研究对象，重点关注与自旋结构关联的多铁性。在单相Bi2Fe4O9多铁性陶瓷样品中，通过对其结构、磁性、磁介电效应和热释电流进行实验研究，详细地研究了掺杂离子的自旋、价态、尺寸等与材料的结构、自旋构型和多铁性之间的关联，给出了增强多铁性的方法。此外，经过梳理不同的合成条件下样品的磁电性，发现通过控制烧结温度来减小晶粒尺寸相对其它合成手段，可以有效增强增强材料的多铁性。Bi2Fe4O9在奈尔温度以下虽具有反铁磁性，但其单向磁各向异性较弱，为了探索基于交换偏置效应的逆磁电耦合方式，我们选取了另外一种低温下能够表现出自发磁化翻转和磁交换偏置效应的稀土铬氧化物多铁性材料RCrO3进行实验研究。前期的研究集中在对补偿温度的调控及交换偏置场的增强方面，主要研究了磁性过渡金属离子或碱土金属离子掺杂对材料的结构、磁性、介电性等的调控，为今后将Bi2Fe4O9与RCrO3复合进行磁电性的调控打下了基础。