Magnetic ground states and higher-order interactions beyond monolayers

材料的磁基态由原子磁矩之间的磁相互作用决定。标准的自旋模型包括海森堡成对交换作用、Dzyaloshinskiii-Moriya相互作用（英语：Dzyaloshinskii-Moriya interaction）和磁晶各向异性。然而，涉及两个以上站点之间的交换的高阶交换相互作用可以发挥重要作用，并导致有趣的三维磁基态自旋配置。此外，拓扑自旋结构，如skyrmions或antiskyrmions，可以稳定的高阶项，即使在没有的情况下。最近，一些新的高阶项如拓扑手征相互作用和手征多自旋相互作用被提出，尽管它们的相关性尚未得到实验验证。迄今为止，实验和理论研究几乎完全集中在单原子层的磁性系统上。然而，对于传输应用，例如在多层中，超单层系统是相关的。一个现实的原子自旋模型，这种系统的磁性双层或三层是失踪，但不可或缺的进一步研究与定制的性质，如磁基态，相变，结晶性能，存在和稳定性的拓扑自旋结构的材料的预测，自旋动力学。在这个项目中，我们的目标是深入了解模型型系统的磁性薄膜超越单晶表面上的单层的磁性和电子特性。我们将结合联合收割机密度泛函理论（DFT），原子自旋模拟，并与自旋极化扫描隧道显微镜（STM）的实验，以解开哪种原子和纳米尺度的磁基态可以发生。几个磁性的双层和三层的3d过渡金属在不同对称性的金属表面将进行研究，我们希望发现新类型的自旋结构。为了了解在这种磁性薄膜中复杂的自旋结构的微观起源，我们将开发一个原子自旋模型，其中包括不同类型的内部和层间相互作用，使用从DFT获得的参数。到目前为止，基本上没有了解层间高阶相互作用在超单层膜中的作用。我们将揭示哪些相互作用是可能的，以及它们对磁基态形成的作用是什么。我们还将通过DFT与STM和光谱测量的比较，研究磁性织构的对称性和与输运性质相关的电子性质之间的相互作用。我们相信，实验和理论的联合研究对于理解不同相互作用对磁基态的作用以及磁对单层以外系统中电子态的影响至关重要，并且我们的项目将导致发现新的自旋状态和稳定机制。

The magnetic ground state of a material is governed by the magnetic interactions between atomic magnetic moments. The standard spin model includes the Heisenberg pairwise exchange interaction, the Dzyaloshinskii-Moriya interaction (DMI), and the magnetocrystalline anisotropy. However, higher-order exchange interactions involving exchange between more than two sites can play an important role and lead to intriguing three-dimensional spin configurations in magnetic ground states. In addition, topological spin structures such as skyrmions or antiskyrmions can be stabilized by higher-order terms even in the absence of DMI. Recently, new higher-order terms such as the topological-chiral and chiral-multi spin interactions have been proposed albeit their relevance has not yet been verified experimentally. Experimental and theoretical studies have so far focused almost exclusively on magnetic systems of single atomic layers. For transport applications, e.g. in multilayers, however, beyond-monolayer systems are relevant. A realistic atomistic spin model for such systems with magnetic bi- or trilayers is missing but indispensable for further studies regarding predictions for materials with tailored properties, such as magnetic ground state, phase transitions, thermodynamical properties, existence and stability of topological spin structures, and spin dynamics. In this project we aim to obtain insight into the magnetic and electronic properties of model-type systems of magnetic films beyond monolayers on single crystal surfaces. We will combine density functional theory (DFT), atomistic spin simulations, and experiments with spin-polarized scanning tunneling microscopy (STM) to unravel which kind of atomic- and nano-scale magnetic ground states can occur. Several magnetic bi- and trilayer of 3d transition metals on metallic surfaces of different symmetries will be studied and we expect to discover novel types of spin structures. To understand the microscopic origin of complex spin structures in such magnetic films we will develop an atomistic spin model which includes different types of intra- and interlayer interactions, using parameters obtained from DFT. To date, there is basically no understanding concerning the role of interlayer higher-order interactions in beyond-monolayer films. We will reveal which interactions are possible and what their role for the magnetic ground state formation is. We will also study the mutual interactions between the symmetry of the magnetic texture and the electronic properties, relevant for the transport properties, by comparison of DFT with STM and spectroscopy measurement. We believe that a joint experimental and theoretical study is fundamental for the understanding of the role of different interactions onto the magnetic ground states and the impact of magnetism on the electronic states in systems beyond monolayers and that our project will lead to the discovery of novel spin states and stabilization mechanisms.