Stability, Transformation and Detection of Topological Magnetic Spin Textures in Nanostructures

Book excerpt: Topology, a branch of mathematics that successfully describes several fundamental physical phenomena in the field of condensed matter physics. Understanding the role of topology plays in materials science has led to the development of several material platforms which come under the category of topological materials. The properties associated with the topological states in materials are robust against the continuous transformation and small perturbations that do not change topology. Hence, the research behind the exploration and utilization of topological materials in a goal specific application is currently advancing with a frantic pace.In magnetic materials, the marriage of topology and magnetism exists as a new form of magnetic ordering with whirlpool-like spin arrangements known as magnetic skyrmions. These topologically protected particle-like spin textures were first discovered a decade ago in non-centrosymmetric magnetic materials. Confining magnetic skyrmions in nanostructures leads to interesting fundamental insights on skyrmion stability and could provide convenient platforms for potential practical applications of skyrmions in information storage technology. In Chapter 1, I have introduced and summarized the recent advances on studying magnetic skyrmions in nanostructures of skyrmion hosting non-centrosymmetric materials (especially the cubic B20 materials) made via bottom-up synthesis or top-down fabrication methods. I further discussed various real space imaging (such as Lorentz transmission electron microscopy or electron holography) or physical property measurement (such as magneto-transport) techniques that have been used to observe and detect these exotic magnetic domains in both nanostructure and bulk samples, which have proven critical to fully understanding them. The morphology and dimensionality of skyrmion hosting materials in stabilizing isolated magnetic skyrmions in confined geometry and their benefits are critical for the implementation in magnetic memory applications. In Chapters 3 and 4, I have presented a comprehensive study of purely electrical detection methods corroborated with real space observation of magnetic skyrmions. This allows us to develop a complete toolkit to investigate stability, magnetic phase transformation and detection of magnetic skyrmion in confined geometries of nanostructures. With these investigations, we extend our understanding of the three-dimensional spin structure (known as skyrmion strings) in confined geometries of nanostructures. Magnetism mediated by symmetry breaking is ubiquitous in condensed matter systems as also depicted in case of non-centrosymmetric skyrmion hosts. Intrinsic crystal defects are often unavoidable during the growth processes of materials which break the local symmetry of the crystal in the vicinity of the defect site and could potentially influence the magnetic spin ordering. In Chapter 2, I have discussed the role of defects in controlling and manipulating the key fundamental properties in widely explored two-dimensional (2D) van der Waals (vdW) materials. Defect-tunable magnetism has not been studied in broad class of 2D magnetic van der Waals (vdW) materials. Furthermore, it might lead to an inaccurate interpretation of intrinsic magnetic ordering in 2D vdW magnets if the presence of crystal defects is not acknowledged with detailed structural characterization. In Chapter 5, I presented a study of defect-mediated ferromagnetism influenced by the presence of sulfur vacancy defects in strongly correlated 2D vdW antiferromagnet of NiPS3. These findings demonstrate the concept of tuning defect-mediated magnetic interactions to manipulate spin ordering in magnetic vdW materials. Interestingly, defects like dislocation in materials are distinguishable from other lattice imperfections as these are characterized by topological invariant known as Burgers vector and they fall under the category of topological defects. In Chapter 6, we have designed a first ever controlled vapor deposition method for the screw dislocation driven growth of layered vdW antiferromagnets, namely NiI2 and NiBr2. The presence of screw dislocation in magnetic vdW materials could allow the exploration of dislocation-spin interactions and magnetic spin structures that are governed by the principles of topology.