Robust Zero-Field Skyrmion Formation in FeGe Epitaxial Thin Films.

B20 phase magnetic materials have been of significant interest because they enable magnetic Skyrmions. One major effort in this emerging field is the stabilization of Skyrmions at room temperature and zero magnetic field. We grow phase-pure, high crystalline quality FeGe epitaxial films on Si(111). Hall effect measurements reveal a strong topological Hall effect after subtracting the ordinary and anomalous Hall effects, demonstrating the formation of high density Skyrmions in FeGe films between 5 and 275 K. In particular, a substantial topological Hall effect was observed at a zero magnetic field, showing a robust Skyrmion phase without the need of an external magnetic field.

On the origin of extraordinary cyclic strengthening of the austenitic stainless steel Sanicro 25 during fatigue at 700°C.

The origin of the extraordinary strengthening of the highly-alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700°C was investigated by use of advanced scanning transmission electron microscopy (STEM). Along with substantial change of dislocation structure, nucleation of two distinct populations of nanoparticles was revealed. Fully coherent Cu-rich nanoparticles were observed homogeneously dispersed with high density along with nanometer-sized incoherent NbC carbides precipitating on dislocations during cyclic loading. Probe-corrected HAADF STEM imaging was used to characterize the atomic structure of nanoparticles. Compositional analysis was conducted using both EELS and high spatial resolution EDS. High temperature exposure induced precipitation of a high density of coherent Cu-rich nanoparticles while strain-induced nucleation of incoherent NbC nanoparticles leads to retardation of dislocation movement. The pinning effects and associated obstacles to dislocation motion prevent recovery and formation of the localized low-energy cellular structures. As a consequence, the alloy exhibits remarkable cyclic hardening at elevated temperature.

Phase transformation strengthening of high-temperature superalloys.

Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new 'phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ' to η phase along stacking faults in γ' precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.

Quantitative STEM Imaging of Order-Disorder Phenomena in Double Perovskite Thin Films.

Using aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), we investigate ordering phenomena in epitaxial thin films of the double perovskite Sr_{2}CrReO_{6}. Experimental and simulated imaging and diffraction are used to identify antiphase domains in the films. Image simulation provides insight into the effects of atomic-scale ordering along the beam direction on HAADF-STEM intensity. We show that probe channeling results in ±20% variation in intensity for a given composition, allowing 3D ordering information to be probed using quantitative STEM.

Epitaxial growth of iridate pyrochlore Nd2Ir2O7 films.

Epitaxial films of the pyrochlore Nd2Ir2O7 have been grown on (111)-oriented yttria-stabilized zirconia (YSZ) substrates by off-axis sputtering followed by post-growth annealing. X-ray diffraction (XRD) results demonstrate phase-pure epitaxial growth of the pyrochlore films on YSZ. Scanning transmission electron microscopy (STEM) investigation of an Nd2Ir2O7 film with a short post-annealing provides insight into the mechanism for crystallization of Nd2Ir2O7 during the post-annealing process. STEM images reveal clear pyrochlore ordering of Nd and Ir in the films. The epitaxial relationship between the YSZ and Nd2Ir2O7 is observed clearly while some interfacial regions show a thin region with polycrystalline Ir nanocrystals.