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Abstract: Topologically protected magnetic textures are promising candidates for information carriers in future memory devices, as they can be efficiently propelled at very high velocities using current-induced spin torques. These textures—nanoscale whirls in the magnetic order—include skyrmions, half-skyrmions (merons) and their antiparticles. Antiferromagnets have been shown to host versions of these textures that have high potential for terahertz dynamics, deflection-free motion and improved size scaling due to the absence of stray field. Here we show that topological spin textures, merons and antimerons, can be generated at room temperature and reversibly moved using electrical pulses in thin-film CuMnAs, a semimetallic antiferromagnet that is a testbed system for spintronic applications. The merons and antimerons are localized on 180° domain walls, and move in the direction of the current pulses. The electrical generation and manipulation of antiferromagnetic merons is a crucial step towards realizing the full potential of antiferromagnetic thin films as active components in high-density, high-speed magnetic memory devices.

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Abstract: In antiferromagnetic (AF) materials, magnetic moments align in a regular pattern such that the moments cancel perfectly in each magnetic unit cell. Hence AF materials do not show a net magnetisation and are largely inert against magnetic fields. Thus, the hidden order of antiferromagnets has only been revealed in the last century. For spintronic applications, the use of antiferromagnets promises numerous advantages compared to conventional spintronics based primarily on ferromagnetic (FM) materials. Amongst the key materials for AF spintronics research are tetragonal, antiferromagnetic CuMnAs films, because in addition to being antiferromagnetically ordered at room-temperature, tetragonal CuMnAs is one of only two conductive AF materials, for which it has been shown that the AF order can be manipulated with electrical currents. This has raised hopes for antiferromagnetic memory devices where the AF order in CuMnAs is switched electrical between two different states. The magnetic moments in CuMnAs films form ferromagnetic sheets (parallel alignment) which are stacked antiparallel along the crystallographic c-direction. The spin axis is confined within the ab-plane, but varies on a microscopic scale, which produces a variety of different AF domain structures. This thesis adresses the question: “what underlies the AF domain structures and how can they be manipulated efficiently?” Visualising antiferromagnetic domain structures remains experimentally challenging, because the domains do not show a net magnetisation. Here, it is realised by combining photoemission electron microscopy (PEEM) with x-ray magnetic linear dichroism (XMLD), which yields sensitivity to the spin axis. These measurements require x-rays with precisely tunable energy. Therefore, this work has largely been performed at a synchrotron, namely Diamond Light Source. Here, direct imaging of the response of the AF domain structure upon the application of electrical current pulses is used to study the microscopic mechanisms of electric switching in CuMnAs films. In the films studied here, the most efficient switching was found to occur via reversible AF domain wall motion induced by electrical current pulses of alternating polarity. The measurements also reveal the limiting factors of electrical switching in CuMnAs films, namely domain pinning which limits device efficiency and domain relaxation which hinders long-term memory. This illustrates that one needs to be able to precisely tune the material properties for a specific application in order to build efficient AF spintronic devices. Hence, the factors, which govern the AF spin textures in the CuMnAs films, need to be revealed. This is done by combining direct imaging of the AF domain structure with complementary techniques including electrical measurements, scanning X-ray diffraction and low-energy electron microscopy and diffraction (LEEM, LEED). The measurements reveal that the AF domain patterns are highly sensitive to the crystallographic microstructure including patterned edges and crystallographic defects. In particular, crystallographic microtwin defects are found to largely define the AF domain structure in non-patterned films. The coupling between defects and AF domains can lead to magnetostructural kinetics, where defects and AF domains grow together over weeks at room temperature and over minutes at slightly elevated temperatures of 50°C -70°C. In devices, patterned edges are found to influence the AF domains over tens of micrometers. Combining the knowledge about the effects of microtwin defects and patterned edges on the AF structure helps to understand the microscopic effects of electric current pulses and can form the basis for targeted AF domain engineering.

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Abstract: Mott transitions in real materials are first order and almost always associated with lattice distortions, both features promoting the emergence of nanotextured phases. This nanoscale self-organization creates spatially inhomogeneous regions, which can host and protect transient non-thermal electronic and lattice states triggered by light excitation. Here, we combine time-resolved X-ray microscopy with a Landau-Ginzburg functional approach for calculating the strain and electronic real-space configurations. We investigate V2O3, the archetypal Mott insulator in which nanoscale self-organization already exists in the low-temperature monoclinic phase and strongly affects the transition towards the high-temperature corundum metallic phase. Our joint experimental-theoretical approach uncovers a remarkable out-of-equilibrium phenomenon: the photo-induced stabilisation of the long sought monoclinic metal phase, which is absent at equilibrium and in homogeneous materials, but emerges as a metastable state solely when light excitation is combined with the underlying nanotexture of the monoclinic lattice.

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Abstract: The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its physical fundamentals to applications in information technologies. Here, we explore antiferromagnetic CuMnAs in which imaging by x-ray photoemission reveals the presence of magnetic textures down to nanoscale, reaching the detection limit of this established microscopy in antiferromagnets. We achieve atomic resolution by using differential phase-contrast imaging within aberration-corrected scanning transmission electron microscopy. We identify abrupt domain walls in the antiferromagnetic film corresponding to the Néel order reversal between two neighboring atomic planes. Our work stimulates research of magnetic textures at the ultimate atomic scale and sheds light on electrical and ultrafast optical antiferromagnetic devices with magnetic field–insensitive neuromorphic functionalities.

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Abstract: With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for next generation, low-cost photovoltaic technologies. Yet, the presence of nanoscale defect clusters, that form during the fabrication process, remains critical to overall device operation, including efficiency and long-term stability. To successfully deploy hybrid perovskites, we must understand the nature of the different types of defects, assess their potentially varied roles in device performance, and understand how they respond to passivation strategies. Here, by correlating photoemission and synchrotron-based scanning probe X-ray microscopies, we unveil three different types of defect clusters in state-of-the-art triple cation mixed halide perovskite thin films. Incorporating ultrafast time-resolution into our photoemission measurements, we show that defect clusters originating at grain boundaries are the most detrimental for photocarrier trapping, while lead iodide defect clusters are relatively benign. Hexagonal polytype defect clusters are only mildly detrimental individually, but can have a significant impact overall if abundant in occurrence. We also show that passivating defects with oxygen in the presence of light, a previously used approach to improve efficiency, has a varied impact on the different types of defects. Even with just mild oxygen treatment, the grain boundary defects are completely healed, while the lead iodide defects begin to show signs of chemical alteration. Our findings highlight the need for multi-pronged strategies tailored to selectively address the detrimental impact of the different defect types in hybrid perovskite solar cells.