Entropic effects counteract the topological protection of magnetic skyrmions, leading to faster decay rates than expected.

The chiral magnet Cu2OSeO3 hosts a skyrmion lattice, that may be equivalently described as a superposition of plane waves or lattice of particle-like topological objects. A thermal gradient may break up the skyrmion lattice and induce rotating domains raising the question which of these scenarios better describes the violent dynamics at the domain boundaries. Here we show that in an inhomogeneous temperature gradient caused by illumination in a Lorentz Transmission Electron Microscope different parts of the skyrmion lattice can be set into motion with different angular velocities. Tracking the time dependence we show that the constant rearrangement of domain walls is governed by dynamic 5-7 defects arranging into lines. An analysis of the associated defect density is described by Frank's equation and agrees well with classical 2D-Monte Carlo simulations. Fluctuations of boundaries show surge-like rearrangement of skyrmion clusters driven by defect rearrangement consistent with simulations treating skyrmions as point particles. Our findings underline the particle character of the skyrmion.In the last decade a non collinear topological spin texture, the skyrmion, has attracted great attention representing a new type of topological soliton in magnetic materials. Skyrmion lattices are periodic arrangements of a kind of magnetic whirls that may be found in a great variety of chiral magnets [1][2][3][4][5][6][7][8][9][10][11][12], as well as thin magnetic (multi-) layers [13][14][15][16]. The topology of skyrmions is encoded in a quantized winding number of the spin orientation. Emergent magnetic and electric fields describe the efficient coupling of the topological spin texture to electrons and magnons [17][18][19][20]. Skyrmion lattices may be described by two distinct approaches. In a wave-like picture as suggested e.g. by Small-Angle-Neutron Scattering (SANS) measurements, the skyrmionic crystal may be accounted for by a superposition of three spin helices with their propagation vector rotated by 120• with respect to each other. From this point of view, the individual (solitonic) character of single skyrmions vanishes in the collective. Note that in most materials in the skyrmion lattice phase, higher order scattering is basically absent; for MnSi it is of the order of 10 −4 suggesting a rather smooth spin texture [21]. In contrast, in a particle-like picture skyrmions are viewed as individual solitonic particles. Indeed, individual skyrmions have been observed early on [6, 13] but the non-linear character as well as the degree of the particlecharacter in the skyrmion phase remained unresolved. In fact, recent studies reveal strong deformation of the precise shape of skyrmions under large strain [22].The validity of either approach may be tested critically in studies of imperfect skyrmion lattices, alluding to similarities with well-known atomic lattices which also allows to verify particle conservation. In fact, the existence of defects and domains in skyrmion lattices has been reported [23][24][25][26], however,...

The stereoselective hydrogenation of alkynes constitutes one of the key approaches for the construction of stereodefined alkenes. The majority of conventional methods utilize noble and toxic metal catalysts. This study concerns a simple catalyst comprised of the commercial chemicals iron(II) acetylacetonate and diisobutylaluminum hydride, which enables the Z‐selective semihydrogenation of alkynes under near ambient conditions (1–3 bar H2, 30 °C, 5 mol % [Fe]). Neither an elaborate catalyst preparation nor addition of ligands is required. Mechanistic studies (kinetic poisoning, X‐ray absorption spectroscopy, TEM) strongly indicate the operation of small iron clusters and particle catalysts.

Cubic chiral magnets, such as Cu 2 OSeO 3 , exhibit a variety of non-collinear spin textures, including a trigonal lattice of spin whirls, so-called skyrmions. Using magnetic resonant elastic x-ray scattering (REXS) on a crystalline Bragg peak and its magnetic satellites while exciting the sample with magnetic fields at GHz frequencies, we probe the ferromagnetic resonance modes of these spin textures by means of the scattered intensity. Most notably, the three eigenmodes of the skyrmion lattice are detected with large sensitivity. As this novel technique, which we label REXS-FMR, is carried out at distinct positions in reciprocal space, it allows to distinguish contributions originating from different magnetic states, providing information on the precise character, weight and mode mixing as a prerequisite of tailored excitations for applications. arXiv:1909.08293v2 [cond-mat.mtrl-sci]

The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr with LiEt BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol % cat., 2-10 bar H , 20-80 °C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism.