Symmetry-breaking and topology in antiferromagnet-based heterostructures

Abstract

In recent decades spintronic devices such as magnetic random-access memory (MRAM), which make use of the electron spin to carry information, have emerged as an alternative to traditional charge-based technologies. Spintronic devices have the potential to be significantly more energy efficient as well as to offer higher information storage densities. This may for instance be achieved by storing information using magnetic quasi-particles such as skyrmions in racetrack memory. So far, spintronics research has mostly focused on ferromagnetic (FM) materials; however, in recent years investigations into antiferromagnetic (AFM) materials have increased. Antiferromagnets exhibit faster dynamics, are more robust against stray fields, and due to their net-zero magnetization exhibit no stray fields themselves. To develop next-generation AFM-based spintronic devices a deep understanding of the parameters governing AFM properties is required. Additionally, nano-scale fabrication methods, such as self-assembly, are needed. In this thesis different phenomena arising in ultra-thin AFM films are investigated using spin-polarized scanning tunneling microscopy (SP-STM).

In the first experimental part of this thesis, the role of magnetism in processes like adatom diffusion and nanostructure growth is investigated, as this may have implications for the design of AFM devices. These processes are often studied above the magnetic ordering temperature, so magnetic effects do not have to be considered. Even below the magnetic ordering temperature, where magnetic effects may exist, their strength is usually considered negligible compared to binding energies and diffusion barriers. In this thesis the motion of adatoms is investigated on a single fcc-stacked Mn layer on a Re(0001) crystal, hosting a row-wise AFM (RW-AFM) state. We observe one-dimensional adatom motion on the hexagonal Mn layer at 4.2 K, dictated by the underlying uniaxial magnetic state. Furthermore, a comparison of the growth of Co monolayer (ML) islands on Mn/Re(0001) above and below the magnetic ordering temperature reveals that the magnetic state causes the formation of 1D Co-nanostructures. For the latter case these findings demonstrate the existence of a reciprocal effect between structure and magnetism.

Also in the context of topological quasi-particles, AFM spin textures are believed to be superior to FM spin textures. FM skyrmions are topological particle-like knots in the magnetization that can be driven by current, although with additional transversal motion (skyrmion-Hall-effect). AFM skyrmions hosting ferromagnetic skyrmions on two sublattices are predicted to show no transversal motion when driven by current. The system of the Mn ML and Mn DL on Ta(110) is demonstrated to be a model-type system, hosting the first experimentally observed AFM skyrmions in a single magnetic layer. Magnetic frustration at the interface between the Mn ML and DL, hosting a non-collinear AFM spin-spiral and a collinear AFM state, respectively, is employed to stabilize the AFM skyrmions.

AFM on superconductors (SC) form magnet-superconductor-hybrid (MSH) systems, that can host topological superconducting phases. At the boundary to different topological phases zero-energy edge modes emerge. Control over the emergence of these edge modes and their characteristics is desirable for future spintronic applications. In this thesis, the edge modes between two different MSH systems, the Mn ML and Mn DL on Ta(110), are investigated, and the edge mode at the [001] edge is found to be spinpolarized. Together with collaborative tight-binding calculations, both MSH systems are shown to host topological nodal point superconducting (TNPSC) phases. Furthermore, it is demonstrated that TNPSCs can be a platform to tune the emergence and properties of edge modes.