Magnetic ground states and higher-order interactions beyond monolayers

物质的磁性基态是由原子磁矩之间的磁性相互作用决定的。标准自旋模型包括海森堡成对交换相互作用、Dzyaloshinskii-Moriya相互作用(DMI)和磁晶各向异性。然而，涉及两个以上位置之间交换的高阶交换相互作用可以发挥重要作用，并导致在磁性基态下有趣的三维自旋组态。此外，即使在没有DMI的情况下，诸如Skyrmions或antiSkyrmions之类的拓扑自旋结构也可以通过高阶项稳定。最近，新的高阶项，如拓扑-手征相互作用和手征-多自旋相互作用等已经被提出，尽管它们的相关性还没有得到实验验证。到目前为止，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，对于运输应用，例如在多层中，超单层系统是相关的。对于具有磁性的两层或三层体系，一个真实的原子自旋模型是缺失的，但对于进一步研究具有定制性质的材料的预测是必不可少的，例如磁性基态、相变、热力学性质、拓扑自旋结构的存在和稳定性以及自旋动力学。在这个项目中，我们的目标是深入了解模型类型的磁性薄膜系统的磁性和电学性质，这些系统超越了单晶面上的单分子膜。我们将结合密度泛函理论(DFT)、原子自旋模拟和自旋极化扫描隧道显微镜(STM)的实验来揭示哪种原子和纳米尺度的磁基态可能发生。我们将研究在不同对称性的金属表面上的几种3D过渡金属的磁性双层和三层结构，并期望发现新类型的自旋结构。为了理解这类磁性薄膜中复杂自旋结构的微观起源，我们将利用DFT得到的参数建立一个原子自旋模型，该模型包括不同类型的层内和层间相互作用。到目前为止，对于层间高阶相互作用在超单分子膜中的作用，人们基本上还没有了解。我们将揭示哪些相互作用是可能的，以及它们对磁性基态形成的作用是什么。我们还将通过密度泛函理论、扫描隧道显微镜和光谱测量的比较，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，联合的实验和理论研究对于理解不同相互作用对磁性基态的作用以及磁性对单分子膜以外系统的电子态的影响是基础的，我们的项目将导致发现新的自旋态和稳定机制。

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.