六角LuFeO3外延薄膜中铁磁性能的声子调制与增强研究

As one of the multi-ferroic materials, hexagonal LuFeO3 (LFO) not only shows the spontaneous electric and magnetic polarizations coexist above room temperature, but also the antiferromagnetic-ferromagnetic transition in the low temperature. If the transition from antiferromagnetism to ferromagnetism can be pushed to a higher temperature in hLFO by tuning its spin superexchange interaction, it will give rise to the possibility of coexisting spontaneous electric and magnetic polarizations and their couplings above room temperature! In this research project, we will study the mechanism of the antiferromagnetic-ferromagnetic transition in hexagonal LFO from the view of the phonon effect on the spin exchange. From the microscopic theory of spin–phonon coupling to a simple phenomenological theory, the dependence of spin exchange interaction on the three phonon modes that are responsible for the structural distortion of hexagonal LFO will be investigated. The phonon mode which has the greatest effect on the spin exchange strength will be determined.Theorectically, by combining first-principles calculations, group-theoretic techniques and microscopic spin models, the phenomenological theory will established to analyze the spin-phonon coupling in hexagonal LFO expitaxial thin films. The dependence of spin exchange interaction on each of three phonon modes will be studied detailed. Experimentally, the temperature dependence of Raman scattering spectra, infrared absorption spectra and Extended X ray absorption fine structure will be used to study the phonon characteristics on the diffrenet temperatures. Combining the temperature dependence of magnetic properties of hexagonal LFO, the relation between the phonon modes and the ferromagnetism will be studied experimentally. Based the above theoretical and experimental studies, by epitaxial strain engineering, the phonon mode which has a great effect on ferromagnetism will be tunned to push the transition from antiferromagnetism to ferromagnetism to a higher temperature in hexagonal LFO epitaxial thin films.

六角LuFeO3(LFO)作为一种重要的多铁材料不仅同时具有室温铁电和反铁磁性，而且在低温下存在反铁磁-铁磁相变。了解六角LFO反铁磁-铁磁相变的物理本质，调制提高其相变温度到室温以上，将可能获得具有室温自发电极化、磁极化和电磁耦合的多铁材料。本申请项目拟从声子角度探索六角LFO中反铁磁-铁磁相变机理，建立自旋-声子耦合理论和分析模型，获得自旋-声子耦合中起决定作用的特征声子模，进而通过外延应变增强该特征声子模使得六角LFO外延薄膜发生高温反铁磁-铁磁相变。理论上，结合第一性原理计算、群论分析法和宏观自旋模型建立自旋-声子耦合理论,分析研究六角LFO中不同声子模对其铁磁性的影响程度。实验上，利用变温拉曼、红外和X射线吸收光谱研究六角LFO的声子和结构特征，结合变温磁性能测量分析，获得声子和铁磁性的关联作用。在此基础上，通过研究外延应变-声子耦合效应，利用外延应变来增强六角LFO的铁磁性能。

本项目紧紧围绕六角LuFeO3(LFO)多铁材料进行了一系列研究。理论上，基于第一性原理计算探讨了六角LFO外延薄膜的本征自旋电子结构与特定光学性质的强关联作用，以此建立了六角LFO外延薄膜自旋-声子耦合的宏观自旋理论模型，研究了薄膜中不同声子模对其铁磁性的影响。实验上，通过固相反应烧结法制备了成分单一、致密度高的LFO陶瓷靶材，利用脉冲激光沉积方法在不同单晶衬底上通过晶格失配应变调控获得了高质量的六角LFO外延薄膜，利用超导量子干涉仪对六角LFO外延薄膜的磁学性质进行了研究。结合变温拉曼散射光谱、红外吸收光谱和扩展X射线吸收光谱研究了六角LFO外延薄膜的声子模和晶体精细结构，从而探讨了外延应变-声子耦合效应，以此确定了六角LFO外延薄膜中声子和铁磁性的关联，并提出了利用外延应变来增强六角LFO外延薄膜铁磁性能的方法。这些研究结果可以为理解外延应变增强六角LFO外延薄膜铁磁性的机理和实现外延应变增强其铁磁性提供一定的指导和依据。