Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometre size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetization at the nanoscale. Chiral skyrmion structures have so far been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films, and under an external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures at room temperature and zero external magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral Néel internal structure, which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions.

In catalytic methanol oxidation on ultrathin vanadium oxide layers on Rh(111) (Θ_{V}≈0.2 monolayer equivalent) we observe a 2D ripening of the VO_{x} islands that is controlled by the catalytic reaction. Neighboring VO_{x} islands move under reaction conditions towards each other and coalesce. The motion and the coalescence of the islands are explained by a polymerization-depolymerization equilibrium that is sensitive to gradients in the adsorbate coverages.

SummaryThe implementation of imaging techniques with low-energy electrons at synchrotron laboratories allowed for significant advancement in the field of spectromicroscopy. The spectroscopic photoemission and low energy electron microscope, SPELEEM, is a notable example. We summarize the multitechnique capabilities of the SPELEEM instrument, reporting on the instrumental aspects and the latest developments on the technical side. We briefly review applications, which are grouped into two main scientific fields. The first one covers different aspects of graphene physics. In particular, we highlight the recent work on graphene/Ir(100). Here, SPELEEM was employed to monitor the changes in the electronic structure that occur for different film morphologies and during the intercalation of Au. The Au monolayer, which creeps under graphene from the film edges, efficiently decouples the graphene from the substrate lowering the Dirac energy from 0.42 eV to 0.1 eV. The second field combines magnetism studies at the mesoscopic length scale with self-organized systems featuring ordered nanostructures. This example highlights the possibility to monitor growth processes in real time and combine chemical characterization with X-ray magnetic circular dichroism–photoemission electron microscopy (XMCD–PEEM) magnetic imaging by using the variable photon polarization and energy available at the synchrotron source.

A major obstacle for the implementation of redox-based memristive memory or logic technology is the large cycle-to-cycle and device-to-device variability. Here, we use spectromicroscopic photoemission threshold analysis and operando XAS analysis to experimentally investigate the microscopic origin of the variability. We find that some devices exhibit variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub)filaments and that the active, current-carrying filament may change from cycle to cycle. These findings account for the observed variability in device performance and represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices.

component more critical. [ 2 ] Nevertheless, both studies revealed that ceria favors the production of methanol and olefi ns, owing to a combination of factors, including (i) sites that facilitate the adsorption of reactants (H 2 , CO 2 , and C 3 H 4 ), (ii) chemical steps that lead to the scission of adsorbate bonds (H H, C O, and C C), (iii) a hydrogenrich reducing environment, and (iv) high concentration (pressure) of reactants. Hence, we are motivated by these results to probe further the reducibility and active structure of ceria using the key reducing reactant H 2 in this process, targeting an understanding of the mechanism and the identifi cation of the essential surface states at the source of breaking the H H bond, a key step common to all hydrogenation reactions.The reactivity of ceria is intimately tied to the presence of oxygen vacancies, [ 3 ] and the nature of their formation and ordering remains a challenging research problem typically dealt with using density functional theory (DFT) [ 4 ] or static experimental measurements (e.g., by scanning probe microscopy). [ 5,6 ] Our experimental approach, utilizing state-of-the-art in situ low-energy electron microscopy and microdiffraction (LEEM/µLEED) coupled to spatially and chemically resolved X-ray spectroscopy, specifi cally elucidates the dynamic structural-chemical changes during the hydrogenation process. This approach represents a key experimental step toward correlating and understanding of catalyst structure and its function. Results and DiscussionFor our investigation, we choose a model catalyst composed of crystalline ceria in the form of microparticles supported on Ru(0001) ( Figure 1 a), a system we have characterized extensively including its growth, chemical reactivity, and surface/ interfacial structure. [ 7 , 8b ] This model catalyst is a polymorph, in between that of a single crystal and a thin fi lm surface, both of which are typically used to model the chemical behavior of ceria. [ 9 ] The advantage of such a system is that it provides a structurally well-defi ned surface and subsurface of the predominant and energetically most stable CeO 2 (111) orientation suitable for in situ surface science methods. When combined with time-resolved microscopy, this approach also enables the identifi cation of potential cooperative effects between the oxide and the metal support. Figure 1 a shows a typical LEEM image of the The interaction of molecular hydrogen with ceria is of important relevance for heterogeneous catalysis related to green chemistry and renewable energy. Here, the complex structural transformations of a well-defi ned cerium oxide model catalyst are followed in situ and in real time when exposed to a reactive H 2 environment. By using electron spectromicroscopy and diffraction with chemical and structural sensitivities, it is demonstrated that the transition from CeO 2 to crystalline Ce 2 O 3 occurs through a mixture of transient, coexisting phases on the nanoscale. The fi ndings establish a clear relationship between structure ...