单相多铁性体中磁序诱导拉曼振动的反常红外活性及其铁电极化机制

The remarkable improper ferroelectricity in CaMn7O12 is due to the unusual proper-screw spiral spin-order, which represents an important progress in the field of magnetic multiferroics. For this intriguing material, we have investigated the phase transition mechanism and the corresponding crystallographic characteristics. The most unexpected outcome of our preceding results is that the dominant ferroelectric polarization of CaMn7O12 originates from the purely Raman-type lattice distortions, which is in conflict with the ingrained concept in traditional ferroelectrics that only zone-center IR-active modes can be responsible for the appearance of spontaneous electric polarization. . In this proposal, based on the density functional theory for strongly correlated electronic systems, we prepare for performing first-principles calculations concerning complicated magnetic order, electronic structure, lattice dynamics, and ferroelectric polarization in order to catch the subtle immanent relations among them. 1) For CaMn7O12, we plan to determine the intrinsic causes of the abnormal IR-activity for the normal modes with purely Raman irreducible representation (irrep) and clarify the microscopic mechanism for ferroelectricity induced by these Raman-type lattice distortions. In addition, we will explore the possibility to further enhance the multiferroic property in CaMn7O12-related materials and search for feasible approaches to realize the reversal of ferroelectric polarization by tuning the magnetic order. 2）For other typical magnetic multiferroics with such as the cycloidal spin spiral or the E-type antiferromagnetic order, we should also elucidate whether or not the normal modes with purely Raman irrep show abnormal polarity and the underlying physics. With the purpose of obtain insights into novel avenues for magnetic-order-induced ferroelectricity, we have to focus on obtaining the general symmetry relation between the complicated magnetic structure and the induced abnormal IR-activity of purely Raman-type modes. 3）Utilizing the strategy revealed by the CaMn7O12 and related materials, we design new type of magnetic multiferroics by combing the exchange-striction mechanism and the spin-orbital coupling effect.

单相磁致多铁性体CaMn7O12显著的铁电极化来源于罕见的螺旋桨型的螺旋状自旋序，是多铁性材料领域的重要进展。在研究其相变机制和晶体学特征的前期工作中，我们意外地发现其铁电性主要来自于纯拉曼型的晶格畸变，这与传统铁电体中只有红外振动模式才可能引起自发极化的观念相矛盾。本项目拟对单相磁致多铁性材料中磁序与电子结构、晶格振动、铁电极化之间的内在关联和影响规律进行全方位的第一性原理计算研究。1）深入认识CaMn7O12中纯拉曼振动具有反常红外活性的内在原因和其产生铁电极化的微观机制，进一步提高该体系的自发极化并探索铁电磁性相互调控的有效途径。2）对于其它典型的单相磁致多铁性材料，考察拉曼振动是否呈现反常红外活性及其物理机制，阐明磁结构和拉曼振动反常红外活性之间的对称性关系和控制因素；3）利用CaMn7O12揭示的物理规律，结合磁致伸缩机制和自旋轨道耦合效应，寻找和设计新的单相磁致多铁性材料。

按照项目研究计划，选择典型的新型单相多铁性材料，对其磁序与电子结构、晶格振动、铁电极化之间的内在关联和影响规律进行了系统的第一性原理计算研究。此外，还进行了钙钛矿铁电极性表面的热力学稳定性及其对石墨烯静电掺杂的相关研究。. CaMn7O12的研究：在TN1发生R-3→R3的二级相变，主要序参量是螺旋磁序引起的Au红外活性的驱动力，Ag拉曼活性的原子受力为次要序参量。在TN2发生R3→P3的一级相变，驱动力是布里渊区边界软模引起的结构不稳定性，磁传播矢量的对称劈裂可由P3相特定的磁畴构型解释。CaMn7O12的铁电极化主要由 到R3的相变决定。根据离子极化贡献的模式分解，高达77.5%的极化来自纯拉曼型的晶格畸变，其中O2离子在ab面内的位移对沿c轴的极化起主要作用。强化Mn3和Mn2之间的SOC效应，可以通过增大Mn3的自旋取向角进一步提高磁致多铁性。. RMn2O5的研究：高温相由于几种结构能量简并具有超顺电特征，独特的电子结构使其在外部激励下可以出现<1 μC/cm2的电子铁电性。高温晶体结构和低温自旋构型不同的结合方式会导致低温GdMn2O5反常的多值铁电极化，可解释实验上GdMn2O5低温磁致多铁性对电极化路径依赖。尽管SmMn2O5的低温相也存在多值铁电性，但能量最低的结构具有P2对称性。RMn2O5磁致多铁性主要是Mn-Mn之间的交换伸缩机制，R 4f与Mn 3d自旋之间的相互作用对铁电极化影响很小。. BiMn3Cr4O12的研究：极性软模驱动的首次铁电相变将立方结构变为单斜Cm相，其自发极化~9.8μC/cm2。Mn/Cr两种亚晶格的AFM磁序诱导再次铁电相变，磁致铁电极化~1.1μC/cm2。与LaMn3Cr4O12源于SOC效应的弱磁致多铁性不同，BiMn3Cr4O12显著的磁致铁电极化来自极性Cm相中触发的交换伸缩机制。据此提出一种新的策略以寻找新型的单相多铁性材料。. 热力学稳定的钙钛矿铁电极性表面依赖于极化方向，其表面终结层既可能是化学计量的，也可能是非化学计量的。铁电极性表面的电子和自旋结构表现出对极化方向和最外原子层的强烈依赖性。利用钙钛矿铁电极性表面，可以实现对石墨烯静电掺杂和载流子类型及浓度的有效调控，有望制备出石墨烯p-n同质结和新型铁电场效应器件。