Spin excitations in ultrathin metallic films

该项目由两个子项目组成。(A)具有自发Skyrmion晶格的超薄膜中的自旋激发：在非磁性衬底上生长的超薄膜的界面上，平移对称性被破坏。在存在大的自旋-轨道耦合的情况下，平移对称性的缺失导致了非零的反对称交换相互作用，称为Dzyaloshinskii-Moriya(DM)相互作用。这种反对称交换作用可以稳定具有旋转结构的手性自旋织构，称为Skyrmion。在过去的几年里，Skyrmions因其有趣的物理特性和在自旋电子学中的潜在应用而引起了人们的极大关注。Skyrmions的一个很有前途的应用是在所谓的Skyrmion赛道存储器中，其中的想法是使用Skyrmion作为比特。对Skyrmion动力学的详细了解对于这种存储器件的进一步开发是非常必要的。磁性固体中的自旋动力学通常用基本的集体磁激发(磁子)来描述。为了更好地理解Skyrmion的动力学，磁振子-Skyrmion相互作用的实验研究具有特别重要的意义。这个项目的想法是激发在Ir(111)上生长的超薄Fe薄膜中形成的纳米Skyrmion晶格中的磁子，并研究该系统的动力学。(B)原子尺度磁子晶体的设计：在新兴的磁振子领域，中心思想是利用基本的集体磁激发(磁子)来编码或传输信息。实现这一想法的第一步是设计一种可以激发不同磁振子模式的介质。介质还应该提供一种根据需要调整磁振子频带结构的方法。磁振子覆盖相当广泛的能量(频率)谱，从几十分之一吉赫兹到数百太赫兹。磁子群的速度取决于它们的能量(能量越高，磁子的速度越快)。为了在磁振子中使用超快(太赫兹)磁子，第一步是设计一个模板，在这个模板上可以激发和操纵不同的磁振子模式。因此，迫切需要一种新的太赫兹磁振子材料设计方法。我们的想法是提出一种基于多层薄膜的太赫兹磁振子原子级磁子晶体的设计方法。这个项目的中心将是设计磁性多层膜，由铁磁性金属的交替原子层组成，在其中可以有效地激发不同的磁振子模式。在下一步，我们的目标是提供一种通过改变材料组合和原子层数来调节磁振子能带结构的方法。研究结果将对太赫兹磁振子、太赫兹电磁传感器和太赫兹微波滤波器的材料设计产生影响。

The project consists of two subprojects. (a) Spin excitations in ultrathin films with a spontaneous skyrmion lattice: At the interface of an ultrathin film grown on a nonmagnetic substrate the translational symmetry is broken. The absence of the translational symmetry in the presence of a large spin-orbit coupling leads to a non-zero antisymmetric exchange interaction, known as Dzyaloshinskii-Moriya (DM) interaction. This antisymmetric exchange interaction can stabilize a chiral spin texture with whirling structures, known as skyrmion. Skyrmions have attracted lots of attentions during the last few years because of their interesting physics and also their potential applications in spintronics. One of the promising applications of skyrmions is in the so called skyrmion racetrack memory where the idea is to use a skyrmion as a bit. A detailed knowledge on skyrmion dynamics is highly desirable for further development of such memory devices. The dynamics of spin in a magnetic solid is usually described in terms of elementary collective magnetic excitations (magnons). In other to understand the dynamics of skyrmions the experimental investigations on magnon-skyrmion interaction are of particular fundamental interest. The idea of this project is to excite magnons in a nano-skyrmionic lattice formed in an ultrathin Fe film grown on Ir(111) and investigate the dynamics of the system. (b) Design of atomic scale magnonic crystals: In the emerging field of magnonics, the central idea is to use the elementary collective magnetic excitations (magnons) for encoding or transmitting information. The first steptowards realization of this idea is to design a medium on which different magnon modes can be excited. The medium should also provide a way of tuning the magnon band structure as it is desired.Magnons cover a rather wide range of energy (frequency) spectrum, starting from a few tenth of gigahertz up to hundreds of terahertz. The magnons group velocity depends on their energy (the higher the energy, the faster the magnons). For using ultrafast (terahertz) magnons in magnonics the first step is to design a template on which different magnon modes can be excited and manipulated. Therefore a new approach of materials design for terahertz magnonics is highly demanded. Our idea is to suggest a way of designing atomic scale magnonic crystals for terahertz magnonics based on multilayer thin films. The central point of this project would be to design magnetic multilayers, composed of alternating atomic layers of ferromagnetic metals, in which different magnon modes can be efficiently excited. In the next step we aim to provide a way of tuning the magnon band structure via changing the materials combination and the number of atomic layers. The results shall have an impact on the design of materials useful for terahertz magnonics, terahertz electromagnetic sensors and terahertz microwave filters.