铁磁/铁电异质外延结构的界面磁电耦合和自旋电子输运的微观机制

Multiferroic magnetoelectric composites have potential applications in novel multifunctional devices due to the coexistence of large ferromagnetism and ferroelectricity above room-temperature, as well as the strong coupling interaction between the two ferroic orders. The epitaxial ferromagnetic/ferroelectric (FM/FE) heterostructures have recently attracted considerable interest in the fields of materials science and condensed matters physics. In particular, the underling mechanisms for interfacial magnetoelectric coupling and spin-dependent tunneling deserve intensive investigations. In this proposal, based on the density functional theory and the quantum ballistic transport approaches, we prepare for performing first-principles calculations concerning interfacial electronic structure, magnetoelectric coupling, and spin-dependent transport for the epitaxial FM/FE heterostructures and FM/FE/FM multiferroic tunnel junctions. 1) We plan to determine the dependences of interfacial electronic reconstruction and magnetoelectric coupling mechanisms on the different materials systems and interface atomic configurations. In order to design FM/FE superlattices with more efficient magnetoelectric effects by combining different coupling mechanisms, we should also elucidate the influences of the interfacial magnetic structure and the type of magnetic interaction on the cross-coupling between magnetism and ferroelectricity. 2) For FM/FE/FM multiferroic tunnel junctions, we focus on the spin-dependent transport and the related tunneling magnetoresistance and electroresistance effects. With the purpose of obtaining insights into the underlying mechanisms, we have to illustrate the effects of interfacial chemical bonding, screening of ferroelectric polarization bound charges, and interfacial magnetoelectric coupling on the transmission distributions for each spin-channel. Utilizing the effect of atomic-scale spin-valve, we design novel four-state resistance devices based on FM/FE/FM multiferroic tunnel junctions by combining the tunneling magnetoresistance and electroresistance effects.

复相多铁性材料可在室温下具有大的磁化和电极化强度以及强的磁电耦合效应，具有可预期的广阔应用前景。人工铁磁/铁电外延异质结构和多铁性隧道结是材料科学和凝聚态物理中的备受关注的新领域，其界面磁电耦合效应和自旋电子输运性质是非常值得研究的课题。 本项目拟利用密度泛函理论和量子弹道输运方法，对铁磁/铁电外延异质结构和多铁性隧道结进行界面电子结构、磁电耦合效应、及自旋电子输运等物理性质进行全方位的第一性原理计算研究。1）确定材料体系和界面结构对界面电子重构和磁电耦合机制的影响规律，阐明界面磁结构和磁相互作用类型对铁电磁性相互调控的物理机制，设计具有更强磁电耦合效应的人工铁磁/铁电超晶格；2）研究铁磁/铁电/铁磁多铁性隧道结中界面化学键合、铁电极化屏蔽机制、磁电耦合效应对自旋电子输运性质的影响规律，利用界面原子层的自旋阀效应，结合隧穿磁致电阻和隧穿电致电阻效应，探索具有实用前景的多态电阻存储器件。

人工铁磁/铁电外延异质结构和多铁性隧道结是材料科学和凝聚态物理中的备受关注的新领域，其界面磁电耦合效应和自旋电子输运性质是非常值得研究的课题。基于典型的ABO3铁电材料（PbTiO3和KNbO3）和铁磁金属M（Fe，Co，Ni），对M/ABO3的界面原子和电子重构、界面磁电耦合效应、以及M/ABO3/M多铁性隧道结的自旋电子输运进行了系统的第一性原理计算研究。在完成项目计划书中的各项指标的基础上，还对单相多铁性体BiFeO3(0001)极性表面的热力学稳定和CaMn7O12铁电相变的微观机制进行了研究。. M/ABO3的界面磁电耦合机制：通过研究M/ABO3的界面化学键合和轨道杂化，确定了铁电极化束缚电荷的界面屏蔽和磁电耦合机制。以Fe/PbTiO3/Fe为例，理论预言在铁电层单侧界面插入异种单原子金属层可显著提高其界面磁电耦合效应。以具有非对称界面的Ni/KNbO3/Ni为例，研究表明，铁电极化反转引起的界面磁重构是提高界面磁电耦合系数的另一种有效途径。. M/ABO3/M多铁性隧道结的自旋电子输运：通过计算PbTiO3和KNbO3的复能带结构，确定了隧穿渐失态的对称性。结合铁电势垒层衰减速率在二维布里渊区的分布和铁磁电极M在费米能级处的电子结构，阐明了自旋极化电子与势垒层渐失态的耦合情况。以M/PbTiO3/M多铁性隧道结为例，研究表明，由铁电极化原位屏蔽驱动的界面电子重构和隧穿势垒特殊的复能带结构在自旋极化电子隧穿中起决定性的作用。具有非对称界面的M/ABO3/M多铁性隧道结，由于TER和TMR效应的共存而出现四种阻态。以Ni/KNbO3/Ni为例，研究结果表明，由铁电极化反转引起的界面磁重构因其自旋过滤效应而引起显著的TER效应。. 关于单相多铁性体的研究：对type-I单相多铁性体BiFeO3(0001)极性表面，研究表明，其热力学稳定的表面化学组成以及表面物理/化学性质都显著依赖于铁电极化方向，这一结果对基于BiFeO3的磁电耦合和自旋电子器件有实际的指导意义。对于type-II单相多铁性体CaMn7O12, 在阐明其铁电相变的微观机制和晶体学特征的基础上，发现其铁电极化主要来自于晶体学意义上的纯拉曼型晶格畸变，对探索新的磁致多铁性体提供了新的思路。