铁基合金纳米薄膜表面磁电耦合效应与结构相变的第一性原理研究

This project plans to study the surface magnetoelectric coupling and electric field driven structural and magnetic phase transition in ultrathin Fe-based binary alloy films (Nano Island). By using the density functional theory, the phase stability and magnetic order of the Fe-based alloys thin films with different crystallographic structures ( bcc(110), fcc(111) and hcp(0001) ) are to be investigated systematically. The influence of alloy composition, in-plane stress, surface segregation, substrate material etc. on the phase stability of the alloy thin films is to be studied. And try to find out a class of alloy thin film/substrate structures with crystallographic (magnetic order) bistable states. The effect of external electric field on different stable crystallographic phases as well as the dynamics of electrically driven phase transition between two stable phases of this class of alloy films is to be studied. The dependence of surface electronic structures on the external electric field, in plane strain, alloying elements and alloy composition, surface segregation is to be analyzed extensively. Accordingly look for alloy thin film/substrate systems showing large surface magnetoelectric coupling. Within this research, basic material database is to be established for the application of electrically driven structural phase transition of Fe-based alloys in high density data storage device.

本立项拟研究Fe基合金超薄膜的表面磁电耦合效应和电场调控的结构与磁性相变。运用基于密度泛函理论的第一性原理计算，系统研究合金组分、面内应力、表面偏析、衬底材料等可控因素对合金薄膜材料形成不同密堆结构(bcc(110), fcc(111)和hcp(0001))时的相对稳定性和磁有序性的影响，探寻具有结构（磁有序性）双重稳态的合金薄膜（纳米岛）材料和相应的衬底。研究电场对此类合金薄膜（纳米岛）不同结构稳态的影响、稳态之间的转变与相变动力学；深入分析表面电子结构与外电场、面内应力、合金化元素及其组分、表面偏析等因素的关系，探索具有大的表面磁电耦合效应的合金薄膜/衬底结构，为设计电场驱动的高密度相变存储元件提供基础性材料数据。

本立项围绕Fe基合金及化合物的结构、磁性、热力学性质和磁性及结构相变方面开展了研究工作，取得了以下成果：（1）研究了Fe基二元合金体相的bcc和fcc结构的对稳定性和磁性、Cu衬底上Fe纳米岛的bcc相和fcc相的相对稳定性、磁性和边缘效应，阐明了3d过渡金属以及间隙原子C和N对Fe的fcc和bcc相稳定性的影响规律，解释了Fe纳米岛在Cu(111) 衬底上的fcc相至bcc相转变的电子结构和原子结构机理。（2）研究了Fe2P基化合物的磁弹性相变、热力学性质，解释了磁弹性相变过程中FeMnPT(T=Si, Ge) 体系声子模式软化来自于绝热电子-声子相互作用，阐明Mn原子的声子模式软化的重要贡献；理论预测了Ga原子替代P可获得具有高磁熵变和良好力学性质的磁热工质材料FeMnPGa化合物。（3）研究建立了描述纳米薄膜力学行为的理论模型，阐明了纳米薄膜的对称性降低和与之伴随的新弹性常数的引入对纳米体系力学性质的重要性，首次合理解释了W(110)衬底上Fe超薄膜的负磁致伸缩行为, 并阐明了负表面应力对纳米线体系屈曲和失稳的重要作用。