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A UHV-compatible MOKE magnetometer for in situ studies operating in tandem with the PEEM at the Nanospectroscopy beamline of the Elettra synchrotron., We report on a custom-built UHV-compatible Magneto-Optical Kerr Effect (MOKE) magnetometer for applications in surface and materials sciences, operating in tandem with the PhotoEmission Electron Microscope (PEEM) endstation at the Nanospectroscopy beamline of the Elettra synchrotron. The magnetometer features a liquid-nitrogen-cooled electromagnet that is fully compatible with UHV operation and produces magnetic fields up to about 140 mT at the sample. Longitudinal and polar MOKE measurement geometries are realized. The magneto-optical detection is based on polarization analysis using a photoelastic modulator. The sample manipulation system is fully compatible with that of the PEEM, making it possible to exchange samples with the beamline endstation, where complementary X-ray imaging and spectroscopy techniques are available. The magnetometer performance is illustrated by experiments on cobalt ultra-thin films, demonstrating close to monolayer sensitivity. The advantages of combining in situ growth, X-ray Magnetic Circular Dichroism imaging (XMCD-PEEM) and MOKE magnetometry into a versatile multitechnique facility are highlighted.

Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. The linear energy dispersion and nontrivial Berry phase play a pivotal role in the electronic, optical, mechanical, and chemical properties of 2D Dirac materials. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of spin-orbit coupling (SOC). Here, we report a route to establishing robust Dirac cones in 2D materials with nonsymmorphic crystal lattice. The nonsymmorphic symmetry enforces Dirac-like band dispersions around certain high-symmetry momenta in the presence of SOC. Through μ-ARPES measurements, we observe Dirac-like band dispersions in α-bismuthene. The nonsymmorphic lattice symmetry is confirmed by μ-low-energy electron diffraction and scanning tunneling microscopy. Our first-principles simulations and theoretical topological analysis demonstrate the correspondence between nonsymmorphic symmetry and Dirac states. This mechanism can be straightforwardly generalized to other nonsymmorphic materials. The results enlighten the search of symmetry-enforced Dirac fermions in the vast uncharted world of nonsymmorphic 2D materials.

Fe and Ni compounds and their oxides offer stoichiometry-dependent magnetic properties, owing to an extremely rich variety of crystallographic phases and a complex phase diagram. They foster the fabrication of oxide/metal magnetic junctions with tunable coupling, thus avoiding the usage of costly materials such as Co and rare-earth elements. Along these lines, here, spectro-microscopy is employed to study the oxidation of FeNi alloy ultrathin films featuring laterally separated fcc and bcc phases. Under mild oxidation conditions at T < 250 °C, Fe segregates to the surface forming a thin superficial Fe-oxide layer, which eventually evolves to Fe3O4 supported on an interfacial FeNi oxide with a spinel structure. Notably, the bcc FeNi oxidizes faster than the fcc one, while the fcc/bcc phase boundary is found to be the most reactive surface region. The magnetic coupling and domain morphology of the oxide–metal heterostack, characterized by XMCD-PEEM and CEMS, are shown to depend on the crystallographic structure of the alloy film. In particular, the Fe-oxide layer on top of the fcc regions exhibits micron-sized uniform magnetic domains antiferromagnetically coupled to the metal support. In contrast, the corresponding oxide on the bcc regions displays fragmented magnetic domains at a length scale of about 100 nm, with ferromagnetic coupling to the underlying FeNi metal support. Our study identifies novel principles for engineering the magnetic coupling through the interface structure and chemistry, with potential applications in sustainable spintronics.

In stacks of two-dimensional crystals, mismatch of their lattice constants and misalignment of crystallographic axes lead to formation of moir\'{e} patterns. We show that moir\'{e} superlattice effects persist in twisted bilayer graphene (tBLG) with large twists and short moir\'{e} periods. Using angle-resolved photoemission, we observe dramatic changes in valence band topology across large regions of the Brillouin zone, including the vicinity of the saddle point at $M$ and across 3 eV from the Dirac points. In this energy range, we resolve several moir\'{e} minibands and detect signatures of secondary Dirac points in the reconstructed dispersions. For twists $\theta>21.8^{\circ}$, the low-energy minigaps are not due to cone anti-crossing as is the case at smaller twist angles but rather due to moir\'{e} scattering of electrons in one graphene layer on the potential of the other which generates intervalley coupling. Our work demonstrates robustness of mechanisms which enable engineering of electronic dispersions of stacks of two-dimensional crystals by tuning the interface twist angles. It also shows that large-angle tBLG hosts electronic minigaps and van Hove singularities of different origin which, given recent progress in extreme doping of graphene, could be explored experimentally., Comment: main text: 25 pages, 5 figures; supplement: 22 pages, 7 figures

After the discovery of graphene, many other 2D materials have been predicted theoretically and successfully prepared. In this context, single-sheet black phosphorus - phosphorene - is emerging as a viable contender in the field of (2D) semiconductors. Phosphorene offers high carrier mobility and an anisotropic structure that gives rise to a modulation of physical and chemical properties. This opens the way to many novel and fascinating applications related to field-effect transistors and optoelectronic devices. In previous studies, a single layer of blue phosphorene intermixed with Au atoms was grown using purified black phosphorus as a precursor. Starting from the observation that phosphorus vapor mainly consists of P clusters, in this work we aimed at obtaining blue phosphorus using much less expensive purified red phosphorus as an evaporant. By means of microscopy, spectroscopy and diffraction experiments, we show that black or red phosphorus deposition on Au(111) substrates yields the same blue phosphorus film.

Advanced electronic materials 2100936 (2022). doi:10.1002/aelm.202100936, Published by Wiley-VCH Verlag GmbH & Co. KG, Weinheim

The crystalline quality of the graphene lattice is a crucial parameter that not only rules the electronic and transport properties of the carbon film, but also its interaction with the substrate. Elucidating the effect of different growth pathways on the resulting graphene-substrate structural configurations and the microscopic mechanisms for their formation is, therefore, a goal of utmost importance. By using electron spectro-microscopy with high chemical and structural sensitivity, we image the structural transformation that graphene on cobalt undergoes at temperatures above 500°C, from a rotationally-incoherent, defective layer to a high quality epitaxial one. We find that the transformation takes place via the growth and propagation of mesoscopic carbidic islands. We identify the underlying mechanism for the formation of epitaxial graphene to involve the dissolution and recondensation of carbon within these regions. The activation energy of the process is estimated to be 1.84 ± 0.11 eV, indicating that the carbon detachment is the rate-limiting step. With the aid of theoretical calculations, we show that the martensitic phase transition occurring in cobalt above 420°C does not affect the graphene transformation. These findings help to establish the optimal parameters to grow high-quality graphene epilayers on Co, opening viable routes towards usage in artificially fabricated magnetic heterostructures.

The control of the spin orientation in antiferromagnets is of key importance in modern spintronics. Here, the authors demonstrate controllable antiferromagnetic spin switching in a synthetically fabricated epitaxial ferromagnet/antiferromagnet bilayer. Thanks to the precise nanoengineering of ferromagnet anisotropy and interface exchange coupling, two orthogonal spin states in the antiferromagnet can be either thermally stabilized in a field-free manner or written by a small external magnetic field.

We outline our achievements in developing electron transparent, leak-tight membranes required for environmental photoelectron spectroscopy (PES). We discuss the mechanical constraints limiting the achievable membrane size and review the development of growth protocols for the chemical vapor deposition (CVD) of single-crystalline graphene on highly (111) textured Cu foils serving as membrane material. During CVD growth, Cu tends to develop a mesoscopic staircase morphology consisting of alternating inclined surface planes, irrespective of whether the covering graphene film or the substrate are single-crystalline. This morphology remains imprinted even when converting the film into freestanding graphene, which affects its mechanical properties. Determining the number of carbon layers in freestanding graphene, we show that membranes reported to suspend over distances larger than 20 µm most likely consist of few-layer graphene. The Raman band signature often used to confirm monolayer graphene rather relates to graphene with turbostratic stacking. The vertical corrugation of freestanding graphene was shown to be almost absent for tri- and four-layer-thick graphene but substantial for bilayer and especially for monolayer graphene. The corrugation is reduced when mechanically straining the freestanding graphene through thermal expansion of the supporting frame, especially flattening membrane areas with imprinted staircase morphology. The electron signal attenuation through supported and freestanding graphene was determined as a function of the electron kinetic energy, verifying that large-area graphene-based electron windows have sufficient electron transparency required for environmental PES. Meanwhile, we managed to cover 100 µm-sized single holes by few-layer graphene up to a coverage fraction of over 99.9998%, as deduced when applying 10 mbar air on one side of the sealing membrane without detecting any measurable pressure increase on its ultrahigh vacuum side. The reported achievements will pave the way toward the development of laboratory-based environmental PES.

By using X-ray magnetic circular dichroism-photoemission electron microscopy, we study the magnetic behavior of ultra-thin Co on Re(0001) covered by a single layer of graphitic carbon. The carbon overlayer is lithographically fabricated via electron-stimulated dissociative adsorption of CO and subsequent annealing to 650 K. Graphitization is found to induce perpendicular magnetic anisotropy in cobalt at a thickness in the range 4-5 atomic layers, where out-of-plane domains separated by chiral Neel type walls are observed. The graphitic overlayer efficiently protects Co from oxidation upon exposure to ambient conditions, leaving its magnetic structure completely unaffected. The magnetic response of the Co film to an external field is imaged in situ across the hysteresis loop, allowing a quantitative analysis of the field-dependent domain area and structure. Our observations pinpoint the influence of the step morphology on the local magnetic behavior. Moreover, below the saturation field, the domains with chiral walls evolve into nanometric-sized magnetic structures resembling skyrmionic bubbles for applied fields

The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming ${\mathrm{O}}_{2}+{\mathrm{H}}_{2}$ reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in ${\mathrm{H}}_{2}$ produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the ${\mathrm{O}}_{2}+{\mathrm{H}}_{2}$ reaction involve a cyclic transformation between 3D-NiO and 2D-NiO.

We studied the symmetry of magnetic properties and the resulting magnetic textures in ultra-thin epitaxial Au0.67Pt0.33/Co/W(110), a model system exhibiting perpendicular magnetic anisotropy and interface Dzyaloshinskii–Moriya interaction (DMI). As a peculiar feature, the C2v crystal symmetry induced by the Co/W interface results in an additional uniaxial in-plane magnetic anisotropy in the cobalt layer. Photo-emission electron microscopy with magnetic sensitivity reveals the formation of self-organised magnetic stripe domains oriented parallel to the hard in-plane magnetisation axis. We attribute this behavior to the lower domain wall energy when oriented along this axis, where both the DMI and the in-plane magnetic anisotropy favor a Néel domain wall configuration. The anisotropic domain wall energy also leads to the formation of elliptical skyrmion bubbles under a weak out-of-plane magnetic field.

In two-dimensional heterostructures, crystalline atomic layers with differing lattice parameters can stack directly one on another. The resultant close proximity of atomic lattices with differing periodicity can lead to new phenomena. For umklapp processes, this opens the possibility for interlayer umklapp scattering, where interactions are mediated by the transfer of momenta to or from the lattice in the neighbouring layer. Using angle-resolved photoemission spectroscopy to study a graphene on InSe heterostructure, we present evidence that interlayer umklapp processes can cause hybridization between bands from neighbouring layers in regions of the Brillouin zone where bands from only one layer are expected, despite no evidence for moir/'e-induced replica bands. This phenomenon manifests itself as 'ghost' anti-crossings in the InSe electronic dispersion. Applied to a range of suitable 2DM pairs, this phenomenon of interlayer umklapp hybridization can be used to create strong mixing of their electronic states, giving a new tool for twist-controlled band structure engineering., Main paper: 21 pages, 4 figures. Supplementary Material: 12 pages, 7 figures

In this work we demonstrate the general characteristics of hcp and fcc stacking in low-energy electron reflectivity for transition metal surfaces, by following the restacking during homoepitaxial growth in real-time. For this purpose, the stacking of a model system, single-crystalline Ag islands during layer-by-layer growth at high temperature on O/W(110), is chosen. Multiple scattering calculations are used to model the relation between electron reflectivity and the crystal geometry. The changes in the electron reflectivity are shown to derive from the changes in the stacking sequence of the topmost surface layers. The results allow to distinguish between the hcp and fcc crystalline arrangements at a surface based on typical differences in the reflectivity curves, making the Ag results relevant for a variety of materials with hexagonal surface geometry. In particular, the multiplet structure within the first Bragg peak in the very low electron energy regime is identified with the fcc structure and thus it can be utilized as a fingerprint to determine the stacking sequence.

Since its proposal, the idea to vastly increase data storage density with a magnetic non-volatile 3D shift-register has sustained interest in current-induced domain wall (DW) motion. So far, experimental efforts have focused on flat nanostrips, which exhibit a wide range of noteworthy effects, yet suffer from intrinsic DW instabilities limiting their mobility. In contrast, ferromagnetic cylindrical nanowires (NWs) can host a novel type of magnetic DW, namely the Bloch-point wall (BPW), which due to its specific 3D topology should not experience the same fundamental issue. This could give rise to DW velocities over ~1000 m/s and fascinating new physics including coupling to magnetic spin waves, however, experimental evidence of DW dynamics in NWs is lacking until now. Here we report experimental results on current-induced DW motion in NWs with velocities >600 m/s, quantitatively consistent with predictions. Furthermore, our results indicate that although previously disregarded, the OErsted field induced by the current plays instead a crucial and valuable role in stabilising exclusively BPWs.

X-ray photoemission electron microscopy, one of the most successful imaging tools at synchrotrons, is known to have limitations related to the application of external fields and to the short electron mean free path. In order to overcome such issues, we adapt an existing XPEEM instrument to simultaneously perform coherent x-ray scattering measurements in reflectivity mode, thus adding a complementary method to XPEEM. Photon-in photon-out x-ray scattering provides the sensitivity to buried interfaces as well as the possibility to work under external fields, which is challenging when using charged particles for imaging. XPEEM, in turn, greatly alleviates the difficulties associated with the reconstruction methods used in coherent diffraction imaging. The combination of the two methods is demonstrated for an artifical spin-ice lattice showing both chemical and magnetic contrast.

The recent discovery of magnetic van der Waals (vdW) materials triggered a wealth of investigations in materials science and now offers genuinely new prospects for both fundamental and applied research. Although the catalog of vdW ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective X-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domain walls in micron-sized flakes of the CrTe

Pnictogens have multiple allotropic forms resulting from their ns2 np3 valence electronic configuration, making them the only elemental materials to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light group VA elements are found in the layered orthorhombic A17 phase such as black phosphorus, and can transition to the layered rhombohedral A7 phase at high pressure. On the other hand, bulk heavier elements are only stable in the A7 phase. Herein, we demonstrate that these two phases not only co-exist during the vdW growth of antimony on weakly interacting surfaces, but also undertake a spontaneous transformation from the A17 phase to the thermodynamically stable A7 phase. This metastability of the A17 phase is revealed by real-time studies unraveling its thickness-driven transition to the A7 phase and the concomitant evolution of its electronic properties. At a critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle transition from AB to AA stacked alpha-antimonene followed by a gradual relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of this intermediate phase is found to be determined by surface self-passivation and the associated competition between A7- and A17-like bonding in the bulk. These results highlight the critical role of the atomic structure and interfacial interactions in shaping the stability and electronic characteristics of vdW layered materials, thus enabling a new degree of freedom to engineer their properties using scalable processes.

Tin diselenide (SnSe2) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO2-SnSe2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe2 surface increases by 0.23 and 0.40 eV upon exposure to O2 and air, respectively, with a charge transfer reaching 0.56 e-/SnO2 between the underlying SnSe2 and the SnO2 skin. Remarkably, both pristine SnSe2 and defective SnSe2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO2-SnSe2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe2-based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules. © PID2019-109525RB-I00; Horizon 2020 Framework Programme, H2020: 730872; Ministerio de Economía y Competitividad, MINECO: CEX2018-000805-M, E12H1800010001; Ministero dell’Istruzione, dell’Università e della Ricerca, MIUR; Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0060 This work has been partially supported by the Spanish Ministerio de Ciencia e Innovación under Project PID2019-109525RB-I00. D.F. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). D.F. and A.A.T. acknowledge the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. A.P. and G.D. acknowledge the CERIC–ERIC Consortium for the access to the Nanospectroscopy facility and financial support. G.D. acknowledges funding of a Ph.D. fellowship from PON Ricerca e Innovazione 2014–2020 (Project E12H1800010001) by the Italian Ministry of University and Research (MIUR). D.W.B. acknowledges the support by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ-2020-0060).

Gallium selenide (GaSe) is a van der Waals semiconductor widely used for optoelectronic devices, whose performances are dictated by bulk properties, including band-gap energy. However, recent experimental observations that the exfoliation of GaSe into atomically thin layers enhances performances in electrochemistry and photocatalysis have opened new avenues for its applications in the fields of energy and catalysis. Here, it is demonstrated by surface-science experiments and density functional theory (DFT) that the oxidation of GaSe into Ga2O3, driven by Se vacancies and edge sites created in the exfoliation process, plays a pivotal role in catalytic processes. Specifically, both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are energetically unfavorable in pristine GaSe, due to energy barriers of 1.9 and 5.7-7.4 eV, respectively. On the contrary, energy barriers are reduced concurrently with surface oxidation. Especially, the Heyrovsky step (Hads + H+ + e- -> H2) of HER becomes energetically favorable only in sub-stoichiometric Ga2O2.97 (-0.3 eV/H+). It is also discovered that the same mechanisms occur for the case of the parental compound indium selenide (InSe), thus ensuring the validity of the model for the broad class of III-VI layered semiconductors

It is well known that intercalated species can strongly affect the graphene-substrate interaction. As repeatedly shown by experiment and theory, the intercalation of atomic species may establish a free-standing character in chemisorbed graphene systems. Here, we focus on graphene grown on a strongly interacting support, cobalt, and demonstrate that the film electronic structure and doping can be tuned via the intercalation/removal of interfacial oxygen. Importantly, cathode lens microscopy reveals the main mechanism of oxygen intercalation, and in particular how microscopic openings in the mesh enable oxygen accumulation at the graphene-cobalt interface. Our experiments show that this process can be carefully controlled through temperature, without affecting the film morphology and crystalline quality. The presence of oxygen at the interface induces an upward shift of the graphene π band, moving its crossing above the Fermi level, accompanied by an increased Fermi velocity and reduced momentum width. Control on the graphene coupling to cobalt may enable one to alter the induced spin polarization in graphene’s electronic states.

The irradiation with photons or electrons can dramatically influence the chemical stability of a molecule, either free or adsorbed on a surface, inducing its fragmentation or desorption. We revisit here the exostimulated dissociation of CO, a prototypical case, choosing hcp thin cobalt films as model support. Intense, microfocused soft X-rays or electron beams are used to locally stimulate CO dissociation. Fast-XPS gives direct access to the adsorbates' chemical state and coverage during irradiation, enabling the kinetics of the process to be monitored in real time. The energy-dependent cross sections for photon and electron stimulated molecular dissociation and desorption are estimated for a fixed initial CO coverage of 1 / 3 ML. In the soft X-ray regime, the desorption channel always prevails over dissociation and is significantly enhanced above the O K edge. The relative dissociation probability increases steadily with increasing photon energy, reaching 30% at 780 eV. Furthermore, we show that low energy electrons in the range 50 to 200 eV dissociate CO more efficiently than X-rays. The prolonged irradiation of the Co surface in CO ambient is found to produce a continuous increase of the carbon coverage, initially promoting the formation of carbides and subsequently accumulating sp 2 carbon on the surface. Far from being a detrimental effect, the CO stimulated dissociation can be exploited to lithographically graft carbon-rich microscopic patterns on Co, with resolution well into the nanometer scale. A brief thermal treatment following irradiation results in the formation of a graphitic carbon overlayer, which effectively protects Co from oxidation upon exposure to ambient conditions, preserving its out-of-plane magnetic anisotropy and domain configuration.

We report a combined study of imaging the antiferromagnetic (AFM) spin structure and measuring the spin Hall magnetoresistance (SMR) in epitaxial thin films of the insulating non-collinear antiferromagnet SmFeO3. X-ray magnetic linear dichroism photoemission electron microscopy measurements reveal that the AFM spins of the SmFeO3(1 1 0) align in the plane of the film. Angularly dependent magnetoresistance measurements show that SmFeO3/Ta bilayers exhibit a positive SMR, in contrast to the negative SMR expected in previously studied collinear AFMs. The SMR amplitude increases linearly with increasing external magnetic field at higher magnetic fields, suggesting that field-induced canting of the AFM spins plays an important role. In contrast, around the coercive field, no detectable SMR signal is observed, indicating that the SMR of the AFM and canting magnetization components cancel out. Below 50 K, the SMR amplitude increases sizably by a factor of two as compared to room temperature, which likely correlates with the long-range ordering of the Sm ions. Our results show that the SMR is a sensitive technique for non-equilibrium spin systems of non-collinear AFMs.

We report on the proof of principle of a scalable method for writing the magnetic state by electron-stimulated molecular dissociative adsorption on ultrathin Co on Re(0001). Intense microfocused low-energy electron beams are used to promote the formation of surface carbides and graphitic carbon through the fragmentation of carbon monoxide. Upon annealing at the CO desorption temperature, carbon persists in the irradiated areas, whereas the clean surface is recovered elsewhere, giving origin to chemical patterns with nanometer-sharp edges. The accumulation of carbon is found to induce an in-plane to out-of-plane spin reorientation transition in Co, manifested by the appearance of striped magnetic domains. Irradiation at doses in excess of 1000 L of CO followed by ultrahigh vacuum annealing at 380 °C determines the formation of a graphitic overlayer in the irradiated areas, under which Co exhibits out-of-plane magnetic anisotropy. Domains with opposite magnetization are separated here by chiral Neél walls. Our fabrication protocol adds lateral control to spin reorientation transitions, permitting to tune the magnetic anisotropy within arbitrary regions of mesoscopic size. We envisage applications in the nano-engineering of graphene-spaced stacks exhibiting the desired magnetic state and properties.

Lateral quantum confinement is of great interest in tuning the electronic properties of graphene-based nanostructures, making them suitable for technological applications. In principle, these properties might be controlled through the edge topology: for example, zigzag nanoribbons are predicted to have spin-polarized edge states. The practical realization of these structures is of utmost importance in fully harnessing the electronic properties of graphene. Here, the formation of regular, 1.4 nm wide ribbon-like graphene structures with zigzag edges are reported, showing 1D electronic states. It is found that these “pseudo-ribbons” embedded in single-layer graphene supported on Ni(100) can spontaneously form upon carbon segregation underneath 1D graphene moiré domains, extending hundreds of nanometers in length. On the basis of both microscopy/spectroscopy/diffraction experiments and theoretical simulations, it is shown that these structures, even though seamlessly incorporated in a matrix of strongly interacting graphene, exhibit electronic properties closely resembling those of zigzag nanoribbons.

While the usual approach to tailor the behavior of condensed matter and nanosized systems is the choice of material or finite-size or interfacial effects, topology alone may be the key. In the context of the motion of magnetic domain walls (DWs), known to suffer from dynamic instabilities with low mobilities, we report unprecedented velocities $g600\text{ }\text{ }\mathrm{m}/\mathrm{s}$ for DWs driven by spin-transfer torques in cylindrical nanowires made of a standard ferromagnetic material. The reason is the robust stabilization of a DW type with a specific topology by the \OE{}rsted field associated with the current. This opens the route to the realization of predicted new physics, such as the strong coupling of DWs with spin waves above $g600\text{ }\text{ }\mathrm{m}/\mathrm{s}$.

We report a combined study of imaging the antiferromagnetic (AFM) spin structure and measuring the spin Hall magnetoresistance (SMR) in epitaxial thin films of the insulating non-collinear antiferromagnet SmFeO$_3$. X-ray magnetic linear dichroism photoemission electron microscopy measurements reveal that the AFM spins of the SmFeO$_3$(110) align in the plane of the film. Angularly dependent magnetoresistance measurements show that SmFeO$_3$/Ta bilayers exhibit a positive SMR, in contrast to the negative SMR expected in previously studied collinear AFMs. The SMR amplitude increases linearly with increasing external magnetic field at higher magnetic field, suggesting that field-induced canting of the AFM spins plays an important role. In contrast, around the coercive field, no detectable SMR signal is observed, indicating that SMR of AFM and canting magnetization components cancel out. Below 50~K, the SMR amplitude increases sizably by a factor of two as compared to room temperature, which likely correlates with the long-range ordering of the Sm ions. Our results show that the SMR is a sensitive technique for non-equilibrium spin system of non-collinear AFM systems., Comment: Accepted for publication in Journal of Physics: Condensed Matter. This is the version of the article before peer review or editing, as submitted by an author to Journal of Physics: Condensed Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it

We show that in a uniform thickness NiO(111)/Fe(110) epitaxial bilayer system, at given temperature near 300 K, two magnetic states with orthogonal spin orientations can be stabilized in antiferromagnetic NiO. Field-free, reversible switching between these two antiferromagnetic states is demonstrated. The observed phenomena arise from the unique combination of precisely tuned interface magnetic anisotropy, thermal hysteresis of spin reorientation transition and interfacial ferromagnet/antiferromagnet exchange coupling. The possibility of field-free switching between two magnetic states in an antiferromagnet is fundamentally interesting and can lead to new ideas in heat assisted magnetic recording technology.

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

The lack of large-area synthesis processes on substrates compatible with industry requirements has been one of the major hurdles facing the integration of 2D materials in mainstream technologies. This is particularly the case for the recently discovered monoelemental group V 2D materials which can only be produced by exfoliation or growth on exotic substrates. Herein, to overcome this limitation, we demonstrate a scalable method to synthesize antimonene on germanium substrates using solid-source molecular beam epitaxy. This emerging 2D material has been attracting a great deal of attention due to its high environmental stability and its outstanding optical and electronic properties. In situ low energy electron microscopy allowed the real time investigation and optimization of the 2D growth. Theoretical calculations combined with atomic-scale microscopic and spectroscopic measurements demonstrated that the grown antimonene sheets are of high crystalline quality, interact weakly with germanium, exhibit semimetallic characteristics, and remain stable under ambient conditions. This achievement paves the way for the integration of antimonene in innovative nanoscale and quantum technologies compatible with the current semiconductor manufacturing.

The novel organic semiconductor dinaphthothienothiophene (DNTT) has gained considerable interest because its large charge carrier mobility and distinct chemical robustness enable the fabrication of organic field effect transistors with remarkable long-term stability under ambient conditions. Structural aspects of DNTT films and their control, however, remain so far largely unexplored. Interestingly, the crystalline structure of DNTT is rather similar to that of the prototypical pentacene, for which the molecular orientation in crystalline thin films can be controlled by means of interface-mediated growth. Combining atomic force microscopy, near-edge X-ray absorption fine structure, photoelectron emission microscopy, and X-ray diffraction, we compare substrate-mediated control of molecular orientation, morphology, and wetting behavior of DNTT films on the prototypical substrates SiO2 and graphene as well as technologically relevant dielectric surfaces (SiO2 and metal oxides that were pretreated with self-a...

The reversible transformations of thin magnetite (Fe3O4) and hematite (α-Fe2O3) films grown on Pt(111) and Ag(111) single crystals as support have been investigated by a combined low energy electro...

Combining low energy electron microscopy (LEEM) with low energy electron diffraction (LEED) and x-ray photoemission electron microscopy (XPEEM), we studied the phase transformations between Fe3O4, γ-Fe2O3, and α-Fe2O3, grown as 10 nm thin oxide films on Pt(111) and Ag(111) single crystals. These transformations occur as moving reaction fronts in most cases, the shapes and velocities of which show strong dependences on temperature and oxygen pressure, but also on defects like step bunches of the supporting substrate and domain boundaries in the initial oxide film. While the non-uniform moving fronts make quantitative analysis difficult, we have extracted approximate values for the average front velocities. We discuss these as well as the qualitative information on the non-uniform fronts in terms of the known geometric situations and the likely motional steps.

The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS2 bilayer forms at the boundaries of single-layer single-domain MoS2 islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems.

Magnetic skyrmions are chiral spin textures that hold great promise as nanoscale information carriers. Since their first observation at room temperature, progress has been made in their current-induced manipulation, with fast motion reported in stray-field-coupled multilayers. However, the complex spin textures with hybrid chiralities and large power dissipation in these multilayers limit their practical implementation and the fundamental understanding of their dynamics. Here, we report on the current-driven motion of N\'eel skyrmions with diameters in the 100-nm range in an ultrathin Pt/Co/MgO trilayer. We find that these skyrmions can be driven at a speed of 100 m/s and exhibit a drive-dependent skyrmion Hall effect, which is accounted for by the effect of pinning. Our experiments are well substantiated by an analytical model of the skyrmion dynamics as well as by micromagnetic simulations including material inhomogeneities. This good agreement is enabled by the simple skyrmion spin structure in our system and a thorough characterization of its static and dynamical properties., Comment: 9 pages, 4 figures + Supplementary Information (12 pages, 9 figures)

Resistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure.