Topology of magnetism and superconductivity in non-collinear spin textures

Abstract

The concept of topology in condensed matter physics has received a lot of attention in recent decades, due to its possible utilization in several applications. In the field of unconventional computing, non-collinear spin textures such as skyrmions and merons possess a topological protection making them to promising candidates for commercial spintronic devices. An approach in the realization of quantum computing is to utilize topologically robust edge modes at the boundary of topological non-trivial bulk systems, e. g. in topological superconductors. To obtain topological superconductivity, so-called two-dimensional magnet-superconductor hybrids are promising candidate systems for the realization of topology-based quantum technologies and superconducting spintronics. Magnet-superconductor systems hosting spin textures with collinear spin alignment are already well established, however, the experimental investigation of non-collinear spin textures in the vicinity of an s-wave superconductor is still pending. In this thesis, the first observation of nanoscale skyrmions on a square symmetric lattice in one monolayer of Mn on W(001) is presented. By using low-temperature spin-polarized scanning tunneling microscopy, an interesting mixed state is found, consisting of a spin spiral texture background with a magnetic period of P = 2.2 nm and several magnetic skyrmions on the square symmetric crystal lattice surface of W(001). Further, the realization of the first two-dimensional non-collinear magnet-superconductor-hybrid system in the monolayer of Fe on Ta(110) is presented. Fe/Ta(110) exhibits a spin spiral texture with a magnetic period of approximately P = 6 nm. As for the system of Mn/W(001), the spin spiral state of Fe/Ta(110) is transitioning to a skyrmionic phase at high magnetic fields of a few Teslas. In contrast to Mn/W(001), the skyrmions are elongated due to the two-fold lattice symmetry and they occur in a ferromagnetic background. In Fe/Ta(110), depending on the morphology of the monolayer, the system offers beside the spin spiral texture also an in-plane ferromagnetic state with non-collinear domain wall configurations. One domain wall orientation is characterized by a meron-antimeron texture along the propagation direction of the spin spiral. To study the interaction between the spin spiral texture and superconductivity, the samples with a spin spiral as the magnetic ground state were measured at a temperature of T=1.3 K. The spectroscopic analysis shows in-gap states modulations in the bulk area of the Fe monolayer, while at the boundaries of the Fe, chiral edge states are identified at straight edges along the [001]-direction. Furthermore, tight-binding calculations validate nodal-point superconductivity for the system and identify the different signal intensities along edges with the same crystallographic orientation as an effect of the local spin orientation at the edge onto the chiral edge state dispersion.