Topological orbital magnets at surfaces and interfaces

• Applicant: Professor Dr. Stefan Heinze
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 555842692

Because of their intriguing topological and dynamical properties non-collinear spin textures such as magnetic skyrmions - localized, whirling spin structures - are of great fundamental interest as well as promising for numerous future spintronic applications ranging from data storage to neuromorphic or quantum computing. The non-trivial spin topology of a skyrmion gives rise to an emergent magnetic field which causes the topological Hall effect. This electronic transport effect is essential for potential applications since it allows for example electrical detection of single magnetic skyrmions. Another key consequence of electron motion in the fictitious magnetic field is the topological orbital moment. Topological orbital moments can occur in non-coplanar spin structures even in the absence of spin-orbit coupling. A prominent example of such a spin structure is the triple-Q state with tetrahedron angles between adjacent spins on a hexagonal lattice. This intriguing spin state has been predicted more than 20 years ago but was only recently discovered experimentally in an ultrathin film system. Experimental evidence for the spontaneous topological Hall effect in the triple-Q state has recently also been given in a bulk system. However, so far it has been elusive to directly reveal the topological orbital moments. Topological orbital moments have also been studied only in a small number of systems to date. In this proposed project we aim to explore the formation of topological orbital moments and their order in various types of spin structures and material systems such as transition-metal interfaces, magnetic multilayers, and two-dimensional (2D) van der Waals (vdW) magnets and heterostructures. We will use first-principles electronic structure theory calculations based on density functional theory to obtain the electronic and magnetic structure of the systems and to calculate the spin and orbital moments. One goal of this project is to find new material systems with large topological orbital moments that allow manipulation by external fields. For this purpose, we will go beyond systems with a single magnetic layer to systems with multiple magnetic layers which can be realized at surfaces as well as in 2D vdW magnets. We will also address the role of spin-orbit coupling on the total orbital moment in non-coplanar spin states. Atomistic spin simulations will be used to study the formation of domain walls in non-coplanar spin states and the emergence of localized topological orbital magnetization. Within this project we will also explore possibilities to directly reveal topological orbital moments in future experiments e.g. using spin-polarized scanning tunneling microscopy.

• DFG Programme: Research Grants