Alpha T. N’Diaye

Structural coherency of graphene on Ir(111).

Low-pressure chemical vapor deposition allows one to grow high structural quality monolayer graphene on Ir(111). Using scanning tunneling microscopy, we show that graphene prepared this way exhibits remarkably large-scale continuity of its carbon rows over terraces and step edges. The graphene layer contains only a very low density of defects. These are zero-dimensional defects, edge dislocation cores consisting of heptagon-pentagon pairs of carbon atom rings, which we relate to small-angle in-plane tilt boundaries in the graphene. We quantitatively examined the bending of graphene across Ir step edges. The corresponding radius of curvature compares to typical radii of thin single-wall carbon nanotubes.

Tailoring the chirality of magnetic domain walls by interface engineering.

Contacting ferromagnetic films with normal metals changes how magnetic textures respond to electric currents, enabling surprisingly fast domain wall motions and spin texture-dependent propagation direction. These effects are attributed to domain wall chirality induced by the Dzyaloshinskii-Moriya interaction at interfaces, which suggests rich possibilities to influence domain wall dynamics if the Dzyaloshinskii-Moriya interaction can be adjusted. Chiral magnetism was seen in several film structures on appropriately chosen substrates where interfacial spin-orbit-coupling effects are strong. Here we use real-space imaging to visualize chiral domain walls in cobalt/nickel multilayers in contact with platinum and iridium. We show that the Dzyaloshinskii-Moriya interaction can be adjusted to stabilize either left-handed or right-handed Néel walls, or non-chiral Bloch walls by adjusting an interfacial spacer layer between the multilayers and the substrate. Our findings introduce domain wall chirality as a new degree of freedom, which may open up new opportunities for spintronics device designs.

Growth temperature dependent graphene alignment on Ir(111)

The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moire spots reflect graphene in a single orientation epitaxial incommensurate phase.

Spin-current probe for phase transition in an insulator

Spin fluctuation and transition have always been one of the central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron-scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and therefore it can bring spin into a sample without being disturbed by electric energy. However, large facilities such as a nuclear reactor are necessary. Here we show that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop microprobe for spin transition; spin current is a flux of spin without an electric charge and its transport reflects spin excitation. We demonstrate detection of antiferromagnetic transition in ultra-thin CoO films via frequency-dependent spin-current transmission measurements, which provides a versatile probe for phase transition in an electric manner in minute devices.

Unlocking Bloch-type chirality in ultrathin magnets through uniaxial strain

Chiral magnetic domain walls are of great interest because lifting the energetic degeneracy of left- and right-handed spin textures in magnetic domain walls enables fast current-driven domain wall propagation. Although two types of magnetic domain walls are known to exist in magnetic thin films, Bloch- and Néel-walls, up to now the stabilization of homochirality was restricted to Néel-type domain walls. Since the driving mechanism of thin-film magnetic chirality, the interfacial Dzyaloshinskii-Moriya interaction, is thought to vanish in Bloch-type walls, homochiral Bloch walls have remained elusive. Here we use real-space imaging of the spin texture in iron/nickel bilayers on tungsten to show that chiral domain walls of mixed Bloch-type and Néel-type can indeed be stabilized by adding uniaxial strain in the presence of interfacial Dzyaloshinskii-Moriya interaction. Our findings introduce Bloch-type chirality as a new spin texture, which may open up new opportunities to design spin-orbitronics devices.

Unconventional magnetisation texture in graphene/cobalt hybrids

Magnetic domain structure and spin-dependent reflectivity measurements on cobalt thin films intercalated at the graphene/Ir(111) interface are investigated using spin-polarised low-energy electron microscopy. We find that graphene-covered cobalt films have surprising magnetic properties. Vectorial imaging of magnetic domains reveals an unusually gradual thickness-dependent spin reorientation transition, in which magnetisation rotates from out-of-the-film plane to the in-plane direction by less than 10° per cobalt monolayer. During this transition, cobalt films have a meandering spin texture, characterised by a complex, three-dimensional, wavy magnetisation pattern. In addition, spectroscopy measurements suggest that the electronic band structure of the unoccupied states is essentially spin-independent already a few electron-Volts above the vacuum level. These properties strikingly differ from those of pristine cobalt films and could open new prospects in surface magnetism.

Real-space imaging of the Verwey transition at the (100) surface of magnetite

Effects of the Verwey transition on the (100) surface of magnetite were studied using scanning tunelling microscopy and spin polarized low-energy electron microsccopy. On cooling through the transition temperature Tv, the initially flat surface undergoes a roof-like distortion with a periodicity of ~0.5 um due to ferroelastic twinning within monoclinic domains of the low-temperature monoclinic structure. The monoclinic c axis orients in the surface plane, along the [001]c directions. At the atomic scale, the charge-ordered sqrt2xsqrt2R45 reconstruction of the (100) surface is unperturbed by the bulk transition, and is continuous over the twin boundaries. Time resolved low-energy electron microscopy movies reveal the structural transition to be first-order at the surface, indicating that the bulk transition is not an extension of the Verwey-like sqrt2xsqrt2R45 reconstruction. Although conceptually similar, the charge-ordered phases of the (100) surface and sub-Tv bulk of magnetite are unrelated phenomena.

Low energy electron microscopy and Auger electron spectroscopy studies of Cs-O activation layer on p-type GaAs photocathode

Work function, photoemission yield, and Auger electron spectra were measured on (001) p-type GaAs during negative electron affinity (NEA) surface preparation, surface degradation, and heating processes. The emission current sensitively depends on work function change and its dependence allows us to determine that the shape of the vacuum barrier was close to double triangular. Regarding the NEA surface degradation during photoemission, we discuss the importance of residual gas components the oxygen and hydrogen. We also found that gentle annealing (≤100 °C) of aged photocathodes results in a lower work function and may offer a patch to reverse the performance degradation.

Tuning Perpendicular Magnetic Anisotropy by Oxygen Octahedral Rotations in (La1–xSrxMnO3)/(SrIrO3) Superlattices

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La_{1-x}Sr_{x}MnO_{3} and SrIrO_{3}, we find that all superlattices (0≤x≤1) exhibit ferromagnetism despite the fact that La_{1-x}Sr_{x}MnO_{3} is antiferromagnetic for x>0.5. PMA as high as 4×10^{6}  erg/cm^{3} is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.

Out-of-plane chiral domain wall spin-structures in ultrathin in-plane magnets.

Chiral spin textures in ultrathin films, such as skyrmions or chiral domain walls, are believed to offer large performance advantages in the development of novel spintronics technologies. While in-plane magnetized films have been studied extensively as media for current- and field-driven domain wall dynamics with applications in memory or logic devices, the stabilization of chiral spin textures in in-plane magnetized films has remained rare. Here we report a phase of spin structures in an in-plane magnetized ultrathin film system where out-of-plane spin orientations within domain walls are stable. Moreover, while domain walls in in-plane films are generally expected to be non-chiral, we show that right-handed spin rotations are strongly favoured in this system, due to the presence of the interfacial Dzyaloshinskii–Moriya interaction. These results constitute a platform to explore unconventional spin dynamics and topological phenomena that may enable high-performance in-plane spin-orbitronics devices.