多铁BiFeO3超薄薄膜的精细结构、反铁磁序及其多场调控的同步辐射研究

Antiferromagnetic materials play a critical role in spin valves and in the growing field of antiferromagnetic spintronics. Discerning how to efficiently probe and manipulate the magnetic order of an antiferromagnetic material is key to realize its practical applications. Among antiferromagnets, BiFeO3 is particularly interesting because it exhibits room temperature multiferroism that allows one to use electric fields to manipulate the magnetic order at room temperature. BiFeO3 ultrathin films have attracted great attention for their intriguing physics and potential applications in ultra-low-power spintronic devices, such as multiferroic tunneling junctions and magneto-electric spin-orbit devices. Understanding and control of the antiferromagnetic order in BiFeO3 is essential for both fundamental understanding of the nature of its magnetoelectric coupling and exchange interactions with ferromagnetic layers and for creating deterministic performance. But despite these facts, there are very few studies on the fine structure and antiferromagnetic spin structure of BiFeO3 ultrathin films. Thus, in this proposal, we will perform systematic synchrotron studies on the crystal structure and antiferromagnetic order of the high-quality BiFeO3 ultrathin films synthesized by pulsed-laser deposition in PI’s lab, and to gain a comprehensive understanding of how external stimuli, such as strain, electric field, and temperature, influences the antiferromagnetic spin structure of the films. In the end, we will also fabricate prototype magnetoelectric devices based on ultrathin BiFeO3 thin films. In turn, this understanding will enable the control of magnetism using electric field for nonvolatile spintronic devices with ultra-low-power energy consumption.

反铁磁材料在自旋阀和反铁磁自旋电子学等器件中发挥着重要作用。如何有效表征和调控反铁磁序是推动其应用进程的关键科学问题。在反铁磁材料中，BiFeO3因其室温多铁特性可利用电场操控磁性而成为近十来年的研究热点。BiFeO3超薄薄膜中蕴含着丰富的物理特性，在超低功耗自旋电子器件如多铁隧道结和磁电自旋轨道逻辑器件等有着广泛的应用前景。理解和调控BiFeO3反铁磁序对揭示其磁电耦合效应和与铁磁层的交换耦合作用的物理机制，以及有效调控器件性能起着至关重要的作用。然而，目前人们对BiFeO3超薄薄膜的精细结构和反铁磁序尚未有系统的研究。本项目拟在制备高质量超薄薄膜的基础上，结合上海光源同步辐射，系统研究BiFeO3超薄薄膜精细结构和反铁磁序参量，探索应变、电场和温度场等外场对反铁磁序的调控机制，并构建相应的磁电耦合原型器件。本项目的顺利实施将有助于推动BiFeO3薄膜在超低功耗自旋电子器件的应用进程。

项目结合同步辐射X射线衍射和软X射线吸收谱技术深入研究了(1-x)BiFeO3-xBaTiO3等铁性薄膜的精细结构及反铁磁等序参量的外场调控，为理解多铁BiFeO3等铁性薄膜中应变、电场、温度、薄膜厚度和化学掺杂等对薄膜结构和性能的影响机制提供了多项重要的基本数据；这些研究有助于推动多铁BiFeO3等铁性薄膜在低功耗存储器件的应用。在项目执行期间，我们取得了多项重要研究成果：.1）研究发现当BiFeO3薄膜厚度低于5 nm时，薄膜发生由单斜相到四方相的结构相变，结合变温软X射线磁线二色谱技术发现厚度仅为2 nm的BiFeO3超薄薄膜在室温下仍然具有稳定的反铁磁序和与铁磁层耦合作用，反铁磁奈尔温度为550 K；理论计算表明该结构相变是由BiFeO3薄膜与电极SrRuO3层间的界面氧八面体旋转耦合诱导发生。部分成果已经发表在Acta Materialia 2020。.2）针对BiFeO3薄膜的漏电流大等问题，成功制备了(1-x)BiFeO3-xBaTiO3 (BFO-xBTO)高质量多铁薄膜。研究发现，相较于未掺杂BiFeO3薄膜而言，BFO-xBTO薄膜具有更高的铁电极化（饱和极化可达110 μC/cm2）、居里温度（高达850°C），更强的磁性（饱和磁矩为13emu/cc，是未掺杂BiFeO3薄膜的5倍）以及更低的漏电流（比未掺杂BiFeO3薄膜降低了近4个数量级）。利用同步辐射软X射线吸收谱技术，Bi-O-Ti之间存在较强的杂化作用，从而导致薄膜具有高的c/a比和极化强度。.3）通过同步辐射X射线倒易空间面扫描技术等研究了PbZrO3薄膜反铁电超结构的存在及温度和电场等外场调控，首次从结构和电学表征上证明了反铁电材料的反铁电弥散相变特征和反铁电-铁电序的竞争。工作发表在ACS Appl. Mater. Inter. 2022..4）应邀在 Adv. Mater.期刊上发表铁性薄膜拓扑结构领域的综述论文(Advanced Materials 2021)。项目执行期间在包括Advanced Materials、ACS Materials Letters、ACS Applied Materials & Interfaces、Applied Physics Letters和Acta Materialia等国际知名学术期刊上发表学术论文10篇，申请发明专利4项。