强磁场下室温多铁性薄膜的原位生长与物性研究

The single-phase multiferroic materials, mostly for the transition metal complex oxide of the correlated electron system, have become the focus of research in the past few years because they show simultaneously ferroelectric and magnetic ordering, exhibit multiple interaction and competition as a result of the coupling between their order parameters, and have the vital significance in the aspect of new multifunctional device development,applications as well as basic research. High magnetic field, as an important outfield condition, has a unique effect on the preparation and properties study of the correlated electron materials. Therefore, the single-phase multiferroic systems can be taken as ideal samples to be investigated on the material preparation and property regulation in a high magnetic field. In this project, the recently developed pulsed laser deposition system for the in-situ growth of thin film in a high magnetic field will be applied. BiFeO3-based materials and magnetoplumbite structure hexagonal ferrites, which have multiferroic properties at room temperature and different magnetoelectricity mechanism, will be chosen. The in-situ growth and annealing of the multiferroic thin films will be carried out in a high magnetic field. The effect of magnetic field on the microstructure and magnetic properties of thin film will be investigated, and the regulation effect and mechanism of high magnetic field will be analyzed. The characterization techniques of the Raman spectra and terahertz time-domain spectroscopy in a high magnetic field will be employed to investigate the magnetoelectric coupling mechanism and various elementary excitations of the multiferroic thin films. This research work is expected to play a significant role in obtaining the high-performance room temperature single-phase multiferroic thin films, enriching the research method of the magnetoelectric coupling in multiferroic materials, and finding the new physical effects of multiferroic materials in a high magnetic field.

单相多铁性材料同时具有铁电序和磁有序，大多为过渡金属多元氧化物关联电子体系，存在多种序参量之间的相互作用与竞争，在新型多功能器件开发应用和基础研究方面都具有重要意义，已成为多年来的研究热点。强磁场作为重要的外场条件在关联电子材料的制备与物性研究中具有独特的作用。因此，单相多铁性材料体系可以作为强磁场下材料制备与物性调控和机理研究的理想样本。本项目采用最新研制的强磁场下脉冲激光沉积薄膜生长系统，选取BiFeO3基和磁铅石结构六角型铁氧体等具有室温多铁性和不同磁电机制的材料体系，进行强磁场下的薄膜原位生长和退火处理，研究磁场对薄膜的微结构和磁电性能的影响以及调控作用与机理；结合强磁场下拉曼谱、太赫兹时域谱等表征技术，研究多铁性薄膜材料中磁电耦合机制和多种元激发。研究工作为获得高性能的室温单相多铁性薄膜材料，丰富多铁性材料磁电耦合表征方法，发现强磁场下多铁性材料中新的物理效应具有重要意义。

单相多铁性材料同时具有铁电序与磁有序，存在多种序参量之间的偶合作用，在新型多功能器件开发与应用以及基础研究方面具有重要意义，成为多年来长盛不衰的研究热点。强磁场作为重要的外场调控手段，在磁电功能材料的制备与物性研究中具有独特的作用。本项目研究选取BiFeO3（BFO）、Bi基层状Aurivillius相化合物、磁铅石结构六角型铁氧体等具有室温多铁性材料为研究对象，采用强磁场辅助脉冲激光沉积技术以及强磁场下热处理等手段，进行薄膜生长和退火处理，研究磁场对薄膜微结构和磁电性能的影响以及调控作用与机理。研究表明，BFO薄膜在不同磁场下进行晶化热处理，磁场能促使晶粒长大，降低晶界运动的活化能，并且随着外加磁场的增加，诱导出更多的Fe3+价态离子，这对薄膜的氧空位有抑制作用，从而有效降低了漏电流密度，样品的剩余极化增加、矫顽场降低，这对BFO薄膜的应用具有重要意义，表明强磁场对BFO薄膜的铁电性能改善具有独特的作用。我们在前期工作基础上选取磁电性能较好的Bi6Fe1.4Co0.6Ti3O18，在Pt-Si单晶基片上制备薄膜，并在强磁场下进行晶化热处理，随着磁场增加，薄膜样品的铁电和磁性能明显增强，这归因于磁场退火诱导薄膜沿a（b）轴取向生长和晶界连接性增强的结果，表明强磁场退火对Aurivillius相材料的多铁性具有调控作用。我们还进一步开展了Bi基层状Aurivillius相化合物以及Y型六角铁氧体的掺杂效应研究，获得了多个可能的室温多铁性材料体系。为了有效开展强磁场下太赫兹磁光谱研究，我们对多铁性Cr基尖晶石单晶样品开展了系统研究。采用太赫兹时域谱技术，在低温和强磁场下观测到了CoCr2O4单晶样品中亚晶格交换模式下磁偶极子跃迁引起的共振吸收，还在太赫兹测试频段观察到了ZnCr2Se4单晶样品与螺旋自旋结构有关的共振吸收峰。研究表明强磁场下太赫兹时域光谱技术是研究复杂磁有序体系中磁结构演化的一种有效方法。