BiFeO3多铁薄膜的尺寸效应及其在低功耗存储器件的应用研究

BiFeO3 has attracted considerable attention in condensed matter physics and material sciences due to its room temperature multiferroism and lead-free nature, and thus has great potential for applications in low-power nonvolatile memory devices. However, until now, there have been no systematic studies on the size effects on the ferroelectric, antiferromagnetic, and magnetoelectric coupling properties in multiferroic BiFeO3 thin films. More importantly, the operating voltage of low-voltage, low-power CMOS circuits is required to be less than 1 V, how to effectively reduce the coercive voltage down to 1 V is crucial for its real applications in ultra-low-power memory devices. In this proposal, we plan to investigate the effect of BiFeO3 film thickness on the crystal and domain structure, ferroelectric, antiferromagnetic and magnetoelectric properties; combining the experimental results with theoretical modelling, the mechanism behind the size effect will be revealed. We plan to reduce the switching voltage by reducing film thickness, and in the end, we aim to achieve an operating voltage of less than 1 V to switch the polarization and control the magnetism, and effectively reduce the power consumption of the memory devices down to 100μJ∙cm^(-2). We believe that the success of this project will not only provide a better understanding of the fundamental physics of BiFeO3, but also support its real applications in ultra-low-power non-volatile memory devices.

BiFeO3因其室温多铁性和无铅特性在低功耗存储器具有巨大的应用前景，受到凝聚态物理以及材料科学家的广泛关注。然而，BiFeO3薄膜的铁电、反铁磁和磁电耦合性能的尺寸依赖关系尚未有系统的研究。更重要的是，低压、低功耗集成电路的工作电压将要求<1V，因此如何有效降低BiFeO3薄膜的矫顽电压到<1V是BiFeO3在超低功耗存储器件获得实际应用的关键所在。本项目拟研究BiFeO3外延薄膜厚度对其晶体结构、畴结构、铁电性能、反铁磁性能和磁电耦合性能的影响；结合理论计算，揭示尺寸效应的原子尺度微观物理机制；据此有效降低矫顽电压，在BiFeO3薄膜和异质结中实现用<1V的翻转电压调控铁电极化和磁性能，使存储器件的功耗减小到100μJ∙cm^(-2)以下。本项目的顺利实施不仅有助于理解BiFeO3薄膜的基本物性，也为BiFeO3在超低功耗非易失性存储器件的实际应用提供理论指导和实验依据。

项目围绕BiFeO3等铁电/多铁外延薄膜为主要研究对象，深入研究铁性薄膜厚度、界面和应变效应对薄膜晶体结构、畴结构、铁电性能和磁性能等序参量的影响机制，致力于推动铁性薄膜在低功耗元器件的应用。在项目执行期间，我们取得了多项重要研究成果：1）通过降低薄膜厚度，成功在20 nm BiFeO3外延薄膜中获得<1V的调控电压，器件功耗小于100 µJ cm−2，并发现薄膜仍具有较强的反铁磁序和与铁磁层耦合作用；进一步结合相场理论计算，研究发现薄膜随着膜厚的进一步降低，在3 nm以下因界面氧八面体旋转耦合效应，发生由菱方相向四方相的结构相变，同时伴随着压电增强，研究不仅有助于理解BiFeO3多铁薄膜的基本物性，也为 BiFeO3在超低功耗存储器件的实际应用提供理论指导和实验依据；部分成果已经发表在Acta Materialia 2020。2）针对外延薄膜中铁弹畴翻转被限制的瓶颈问题，通过设计外延应变在铁电薄膜中引入纳米尺度多畴结构共存，利用铁弹纳米畴间弹性相互作用实现协同翻转， 用仅 600 纳牛的探针力便可驱动PbTiO3铁电外延薄膜发生“多米诺骨牌”式大面积、 非局域铁弹畴翻转； 该研究有助于开发低功耗和高灵敏度微纳力传感器和驱动器(Nature Communications 2019)。3）基于在BiFeO3薄膜的应变调控（Physical Review Letters, 2019）和畴结构调控（AIP Advances, 2019，Applied Physics Express, 2019）等领域的一系列创新工作，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Nature Communications、Advanced Materials、Physical Review Letters、Nature Nanotechnology、Advanced Functional Materials和Acta Materialia等国际知名学术期刊上发表学术论文10余篇。