Magnetic ground states and higher-order interactions beyond monolayers

• Applicants: Dr. Kirsten von Bergmann; Professor Dr. Stefan Heinze
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 418425860

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.

• DFG Programme: Research Grants
• International Connection: Iceland
• Cooperation Partner: Professor Dr. Pavel Bessarab