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Abstract: The understanding of antiferromagnetic domain walls, which are the interface between domains with different Néel order orientations, is a crucial aspect to enable the use of antiferromagnetic materials as active elements in future spintronic devices. In this work, we demonstrate that in antiferromagnetic NiO/Pt bilayers arbitrary-shaped structures can be generated by switching driven by electrical current pulses. The generated domains are T domains, separated from each other by a domain wall whose spins are pointing toward the average direction of the two T domains rather than the common axis of the two planes. Interestingly, this direction is the same for the whole domain wall indicating the absence of strong Lifshitz invariants. The domain wall can be micromagnetically modeled by strain distributions in the NiO thin film induced by the MgO substrate, deviating from the bulk anisotropy. From our measurements we determine the domain-wall width to have a full width at half maximum of Δ = 98 ± 10 nm, demonstrating strong confinement.

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Abstract: The size of the orbital moment in Fe 3 O 4 has been the subject of a long-standing and contentious debate. In this paper, we make use of ferromagnetic resonance (FMR) spectroscopy and x-ray magnetic circular dichroism (XMCD) to provide complementary determinations of the size of the orbital moment in “bulklike” epitaxial Fe 3 O 4 films grown on yttria-stabilized zirconia (111) substrates. Annealing the 100 nm as-grown films to 1100 ∘ C in a reducing atmosphere improves the stoichiometry and microstructure of the films, allowing for bulklike properties to be recovered as evidenced by x-ray diffraction and vibrating sample magnetometry. In addition, in-plane angular FMR spectra exhibit a crossover from a fourfold symmetry to the expected sixfold symmetry of the (111) surface, together with an anomalous peak in the FMR linewidth at ∼ 10 GHz; this is indicative of low Gilbert damping in combination with two-magnon scattering. For the bulklike annealed sample, a spectroscopic splitting factor g ≈ 2.18 is obtained using both FMR and XMCD techniques, providing evidence for the presence of a finite orbital moment in Fe 3 O 4 .

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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.

Open Access

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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.

Open Access

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Abstract: The authors describe and compare two complementary techniques that are habitually used to image ferromagnetic and ferroelectric materials with sub-micron spatial resolutions (typically 50 nm, at best 10 nm). The first technique is variable-temperature photoemission electron microscopy with magnetic/antiferromagnetic/polar contrast from circularly/linearly polarized incident X-rays (XPEEM). The second technique is magnetic force microscopy (MFM). Focusing mainly on the authors’ own work, but not exclusively, published/unpublished XPEEM and MFM images of ferroic domains and complex magnetic textures (involving vortices and phase separation) are presented. Highlights include the use of two XPEEM images to create 2D vector maps of in-plane (IP) magnetization, and the use of imaging to detect electrically driven local reversals of magnetization. The brief and simple descriptions of XPEEM and MFM should be useful for beginners seeking to employ these techniques in order to understand and harness ferroic materials.