Nanospectroscopy highlights

Writing the magnetic state with carbon monoxide

We devised a a scalable method for writing the magnetic state by electron-stimulated molecular dissociative adsorption on ultrathin cobalt grown on Re(0001). To do this, we expose the  film to carbon monoxide while irradiating the surface with an intense, micro-focused low energy electron beam. During irradiation, the CO molecules readily dissociate. This process, which is accompanied by the desorption of oxygen, favors the accumulation of carbon at the surface up to a coverage of one atomic layer. Most notably, we find that this overlayer can be converted to graphene upon a simple thermal treatment. As demonstrated by magnetic sensitive microscopy, the irradiated (graphene-covered) and non-irradiated (clean surface) regions exhibit out-of-plane and in-plane magnetic anisotropy, respectively. Our study provides a proof-of-principle that arbitrary magnetic patterns can be

lithographically grafted in cobalt by simply depositing surface carbon. 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.

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Magnetic patterning by electron beam assisted carbon lithography;
P. Genoni, F. Genuzio, T.O. Menteş, B. Santos, A. Sala, C. Lenardi, A. Locatelli;
ACS Applied Materials & Interfaces 10(32), 27178–27187 (2018); doi: 10.1021/acsami.8b07485.

Subfilamentary Networks in Memristive Devices

Redox-based memristive devices are one of the most attractive emerging memory technologies in terms of scaling, power consumption and speed. In these devices, external electrical stimuli cause changes of the resistance of an oxide layer sandwiched between two metal electrodes. In the simplest application, the device can be set into a low resistance state (LRS) and reset into a high resistance state (HRS), which may encode a logical one and zero, respectively. The major obstacle delaying large-scale application, however, is the large cycle-to-cycle (C2C) and device-to-device (D2D) variability of both LRS and HRS resistance values. This behaviour describes the stochastic nature of the switching process within one cell, resulting in different resistances obtained for each switching cycle and different resistances obtained for different cells on the same chip. The switching process in transition metal oxides is believed to be driven by the nanoscale motion

of oxygen vacancies, which form a so-called conductive filament bridging the metal electrodes. In the present study we employ spectromicroscopic photoemission  and operando XAS on graphene/SrTiO3/Nb:SrTiO3 memristive devices to unveil the microscopic origin of variability. Our measurements could detect a change in the shape of the conductive filament as well as variations in the oxygen vacancy distribution within the filament.

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Subfilamentary Networks Cause Cycle-to-Cycle Variability in Memristive Devices;
C. Bäumer, R. Valenta, C. Schmitz, A. Locatelli, T.O. Menteş, S.P. Rogers, A. Sala, N. Raab, Sl. Nemsak, M. Shim, C.M. Schneider, S. Menzel, R. Waser, and R. Dittmann;
ACS Nano 11(7), 6921–6929 (2017);
doi: 10.1021/acsnano.7b02113.

Graphene and h-BN by a Single Molecular Precursor

We have demonstrated that a simple thermal decomposition of dimethylamine borane (DMAB) is sufficient to obtain a G-h-BN layer on Pt(111). This growth route allows an easy and controlled preparation of a continuous almost freestanding layer mostly composed by G and h-BN in the same two dimensional sheet by dehydrogenation and pyrolytic decomposition of DMAB on Pt(111). The temperature is the principal parameter to selectively grow the G-h-BN layer in competition with hybridized B-C-N layers on the clean crystal surface. We have grown and investigated the h-BNG layer on Pt(111) at the BACH beamline by high-resolution core level X-ray photoemission (XPS) and near-edge absorption spectroscopy (NEXAFS) and at the Nanospectroscopy beamline by low energy electron microscopy (LEEM) combined with X-ray photoemission electron microscopy (XPEEM), micro-spot electron energy loss spectroscopy (µ-EELS) and low energy

 electron diffraction (µ-LEED). Our findings show that dehydrogenation and pyrolytic decomposition of DMAB is an efficient and easy method for obtaining a continuous almost freestanding layer made of G, h-BN in the same two dimensional sheet on a metal substrate, such as Pt(111), paving the way for the advancement of next-generation G-like-based electronics and novel spintronic devices.

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Formation of a Quasi-Free-Standing Single Layer of Graphene and Hexagonal Boron Nitride on Pt(111) by a Single Molecular Precursor;
S. Nappini, I. Píš, T.O. Menteş, A. Sala, M. Cattelan, S. Agnoli, F. Bondino and E. Magnano;
Adv. Funct. Mater. 26, 1120 (2016);
doi: 10.1002/adfm.201503591.

Fabrication of 2D heterojunction in graphene

The exploitation of graphene in the next generation electronics depends on the capability of preserving and tailoring its electronic and transport properties. Substitutional implantation of exo-species into the C lattice mesh, B or N in particular, is an appealing functionalization method, as it is capable to alter the charge carrier density and even to open a bandgap. Since most devices require the fabrication of a heterojunction between a semiconducting active material and a metallic electrode, the development of lithographic tecniques for doping graphene is highly desirable. Here we report a proof-of-principle experiment demonstrating that low-energy N2+ ion irradiation through an aperture mask can be used to achieve local control on the doping in graphene and to create a 2-dimensional heterojunction between n-doped and neutral single-layer graphene on Ir(111). LEEM, XPEEM-XPS and microprobe-ARPES

measurements, conducted at the Nanospectroscopy beamline at Elettra showed that the doping pattern is resistant to annealing in UHV up to 800 °C and that the doping level can be varied as function of the increasing irradiation dose without considerable damage for the C mesh. Our results pave the way to a miniaturization of graphene heterojunctions: doping lithography at the nanometer scale would allow the creation of 2D nanocircuits, with promising performances in terms of density, efficiency and thermal dissipation.

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Fabrication of a 2D heterojunction in graphene via low energy N2+ irradiation;
A. Sala, G. Zamborlini, T.O. Menteş, A. Locatelli;
Small 11(44), 5927–5931 (2015);
doi: 10.1002/smll.201501473.

Island Ripening in a catalytic reaction

Island ripening is a fundamental process in physical chemistry in which small particles of a new phase grow in size in order to reduce interfacial energies. In the well- known Ostwald ripening, an evaporation-condensation mechanism causes large islands to grow at the expense of smaller ones. Here we report a new ripening mechanism based on a chemical reaction. This chemistry-based ripening process was seen during catalytic methanol oxidation on ultrathin vanadium oxide layers. We observe that under reaction conditions neighboring vanadium oxide islands move towards each other and coalesce. In order to elucidate the chemical and structural changes behind the island movement, microprobe XPS and microprobe LEED measurements were performed at Nanospectroscopy beamline at Elettra. The mechanism we propose here is based on an equilibrium, which is sensitive to gradients in the adsorbate coverages, between large

VOx islands on Rh(111) and isolated small VnOm clusters. With such a polymerization-depolymerization mechanism, islands do not move as a whole, but are decomposed into small parts, i.e., clusters, that easily diffuse across the surface. Elucidating the mechanism of island ripening is essential for understanding the dynamic restructuring of supported oxide catalysts under operation conditions, and for designing catalysts with a desired morphology.

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Island Ripening via a Polymerization-Depolymerization Mechanism;
M. Hesse, B. von Boehn, A. Locatelli, A. Sala, T.O. Menteş, and R. Imbihl;
Phys. Rev. Lett. 115, 136102 (2015);
doi: 10.1103/PhysRevLett.115.136102.

Nanobubbles at GPa pressure under graphene

Owing to the exceptional strength and flexibility of sp2-carbon, graphene is able to trap mesoscopic volumes of liquid or gas. Nanobubbles (NB) under graphene came in the spotlight in view of potential applications such as gas storage and high-pressure chemistry,  fostering fruition in a plethora of novel devices. To date, only few studies have addressed the basic structural properties of NB under graphene. In the present study we have investigated the morphology and spatial distribution of Ar under micron-sized graphene flakes supported on Ir(100). We provide direct evidence that irradiation of a graphene membrane on Ir with low-energy Ar ions induces formation of solid noble-gas nanobubbles. Their size can be controlled by thermal treatment, reaching tens of nanometers laterally and height of 1.5 nm upon annealing at 1080 °C. Ab initio calculations show that Ar nanobubbles are subject to pressures reaching

tens of GPa, their formation being driven by the minimization of the energy cost of film distortion and loss of adhesion. We expect that ripening of intercalated noble gases can also occur in other graphene/metal systems showing comparable adhesion strength, where it might be fruitfully exploited to strain-engineer the local chemical properties of graphene. Our study also fosters the synchrotron-based investigation of van der Waals solids under extreme pressure and temperature conditions.

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Nanobubbles at GPa pressure under graphene;
G. Zamborlini, M. Imam , L.L. Patera , T.O. Menteş , N. Stojić , C.Africh , A. Sala , N. Binggeli , G. Comelli and A.Locatelli;
Nano Lett. 15(9), pp 6162–6169 (2015);
doi: 10.1021/acs.nanolett.5b02475.

Edge specific graphene nanoribbons

In this work, we begin the groundwork needed to address the origin of sidewall ribbons high mobilities by providing detailed structural and electronic information on the differences between armchair (AC) and zigzag (ZZ) edge sidewall graphene ribbons.
Using a combination of low energy electron microscopy (LEEM), microspot-low energy electron diffraction (μ-LEED), X-ray photoemission microscopy (XPEEM), and microprobe angle resolved photoemission spectroscopy (μ-ARPES), we havecompared the growth and electronic structure of AC and ZZ graphene ribbon arrays. We demonstrate that ZZ ribbons with linear π-bands do not grow on the sidewalls like AC ribbons but instead grow along a narrow strip on the (0001) (Si-face) surface near the step edges that ar

perpendicular to the ⟨1120⟩ direction. On the other hand, AC edge graphene grows up and over the SiC (110n) sidewall facets and is decoupled from a functionalized sidewall ribbon below that is bonded to the SiC facet wall.

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The Bottom-up Growth of Edge Specific Graphene Nanoribbons;
M. S. Nevius, F. Wang, C. Mathieu, N. Barrett, A. Sala, T. O. Menteş, A. Locatelli, and E. H. Conrad;
Nano Lett. 14(11), 6080–6086 (2014);
doi: 10.1021/nl502942z.

Imaging the way molecules desorb from catalysts

Unlike surface catalytic reactions, desorption has been thought as a relatively simple process consisting of a series of statistically independent events randomly and uniformly occurring over the surface: in this picture adsorbates take off as soon as their thermal energy exceeds the binding energy. Following Irving Langmuir’s mean-field treatment, the rate of recombinative desorption of adsorbed particles is thought to be proportional to the square of the coverage. However, several measurements contradicted this model. As a matter of fact, temperature-programmed desorption (TPD), the standard method for determining desorption kinetics, hardly ever shows pure Langmuir behavior. The exponent m in the desorption rate law, referred to as desorption order, is often a fractional number rather than an integer (m = 1 or 2); further, the desorption peaks in TPD exhibit unexpected widths or

symmetries or are split into multiple peaks. In such cases, the desorption mechanism cannot be uniquely understood from TPD data. Instead, a detailed characterization of the complex microscopic mechanisms occurring into the adlayer is instead needed. In order to shed light on the microscopic origin of desorption, we have investigated oxygen on Ag(111) combining structure sensitive electron microscopy with TPD.

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Desorption kinetics from a surface derived from direct imaging of the adsorbate layer;
S. Günther, T.O. Menteş, M.A. Nino, A. Locatelli, S. Böcklein, and J. Wintterlin;
Nat. Comm. 5, 3853 (2014);
doi: 10.1038/ncomms4853.

Towards the perfect graphene membrane

We investigated the limiting factors affecting the crystal quality of graphene grown on polycrystalline Cu foils, a promising new system for applications in large-scale graphene production. The first important result of this study is that a high temperature hydrogen pretreatment of the Cu foil lowers considerably the nucleation density of graphene crystals, favoring the growth of large single crystalline graphene flakes. Second, a characterization of the morphology and structure of the Cu support was performed. The faceting of the substrate involves ordered sequences of (100)- and (410)-facets, interrupted by further highly inclined planes, consisting of (n11) - type facets. The alignment of the graphene layer has no influence on the restructuring of the Cu foil. Despite the fact that the interaction of graphene on the Cu substrate is weak, the graphene layer was found to be a replica of the former morphology of the faceted Cu

substrate. This structure even persists if suspended graphene membranes are formed when carefully removing locally the Cu support underneath the monolayer graphene. As a result, graphene based membranes or transferred graphene layers, which have been grown on Cu foils, are intrinsically corrugated.

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Towards the perfect graphene membrane? - Improvement and limits during formation of high quality graphene grown on Cu-foils;
J. Kraus, S.Böcklein, R. Reichelt, S. Günther, B. Santos, T.O. Menteş, A. Locatelli;
Carbon 64, 377–390 (2013);
doi: 10.1016/j.carbon.2013.07.090.

Rippling of graphene on Ir(100)

The complex interaction between graphene and a support with non-threefold symmetry may result in a peculiar film morphology and structure. On the square Ir(100) we discovered flat and buckled graphene phases that coexist at room temperature; they form stripe-shaped domains which relieve the strain accumulated after cooling the film below growth temperature. In the buckled phase, a small fraction of the carbon atoms chemisorbs to the substrate, originating a textured structure with exceptionally large one-dimensional ripples of nm periodicity. The two graphene phases exhibit distinctively different electronic structure. By imaging emission from graphene's π-band at the reciprocal space K point, photoemission microscopy enabled a direct comparison of the local density of states (DOS) of flat and buckled graphene. The low intensity observed in

the latter points to the disruption of the Dirac cones. This result was confirmed by theory, which also revealed that the metallic-like character of the buckled phase does not originate from strain or rippling, but rather from the chemisorption to the substrate. The novelty highlighted in our study is that the change in the graphene DOS at the Dirac point is due to the chemisorption of just a small fraction of the atoms in the unit cell.

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Rippling and bonding of a graphene superlattice on Ir(100);
A. Locatelli, C. Wang, C. Africh, N. Stojić, T.O. Menteş, G. Comelli, N. Binggeli;
ACS Nano 7, 6955–6963 (2013);
doi: 10.1021/nn402178u.

Thinnest loadstone ever

Despite being the oldest known magnetic material, magnetite is attracting renewed scientific and technological interest in view of spintronics applications. For the realization of practical devices, it is of utmost importance to control the material's magnetic properties in the case the reduced dimensionality limit is approached, as it occurs in thin films or nanostructures, aiming at obtaining stable anisotropy and magnetization. Although magnetite thin films are widely studied, the minimum thickness at which ferrimagnetic behavior can be observed is still under debate. In fact, most thin film studies are performed by separate growth and characterization experiments, which are typically carried out using laterally averaging techniques. To solve this fundamental problem we have exploited the current capabilities of photoelectron spectromicroscopy, and characterized the structural, chemical and magnetic properties of selected magnetite nanocrystals. Through the combined use of low energy electron microscopy,

X-ray photoemission microscopy, and X-ray circular dichroism photoemission electron microscopy we successfully monitored the growth of a single magnetite nanocrystal in real time while characterizing its crystal structure, surface stoichiometry and magnetic behavior. In this way, we have been able to find out that magnetite is ferromagnetic at room temperature at a thickness of only one nanometer, in what may be the thinnest lodestone ever.

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Magnetism in nanometer-thick magnetite;
M. Monti, B. Santos, A. Mascaraque, O.R. de la Fuente, M.A. Niño, T.O. Menteş, A. Locatelli, K.F. McCarty, J.F. Marco, J. de la Figuera;
Phys. Rev. B 85, 020404(R) (2012);
doi: 10.1103/PhysRevB.85.020404.

Thermal stability of Graphene on Re(0001)

This study reports on a novel approach to determine the relationship between the corrugation and the thermal stability of epitaxial graphene grown on a strongly interacting substrate. According to density functional theory calculations, the C single layer grown on Re(0001) is strongly corrugated, with a buckling of 1.6 Å, yielding a simulated C 1s core level spectrum which is in excellent agreement with the experimental one. We found that corrugation is closely knit with the thermal stability of the C network: C-C bond breaking is favored in the strongly buckled regions of the moiré cell, though it requires the presence of diffusing graphene layer vacancies. Our data shows that there is a close relationship between graphene corrugation and

graphene corrugation and its thermal stability, a key achievement in sight of the potential high temperature applications of supported graphene.

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Thermal Stability of corrugated epitaxial graphene grown on Re(0001);
E. Miniussi, M. Pozzo, A. Baraldi, E. Vesselli, R. R. Zhan, G. Comelli, T.O. Menteş, M.Á. Niño, A. Locatelli, S. Lizzit, and D. Alfè;
Phys. Rev. Lett. 106, 216101 (2011);
doi: 10.1103/PhysRevLett.106.216101.

Stress Engineering at the Nanometer Scale

Spontaneous formation of periodic patterns is an example of nature's tendency towards order. A class of such structures is induced by surface stress, and has been widely observed on single crystal surfaces. It is well known that the formation of these equilibrium patterns is driven by a competition between interactions at different length scales. The forces in action, due to short-range near-neighbour and long-range dipolar interactions, are of the most general type resulting in very similar phenomena occurring also in magnetic and electrostatic systems. Here, we explore the possibility of controlling the thermal fluctuations by adding a second adspecies to an already stripe-forming system. The idea is based on the slow dynamics in a high-density

binary lattice gas, which leads to a tendency towards glassy behaviour. In particular, we implement this by dosing small amounts of oxygen on submonolayer Pd/W(110).

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Stress engineering at the nanometer scale: Two-component adlayer stripes;
T.O. Menteş, N. Stojic, A. Locatelli, L. Aballe, N. Binggeli, M.Á. Niño, M. Kiskinova, E. Bauer; EPL (Europhysics Letters), 94 (3), 38003 (2011).
doi: 10.1209/0295-5075/94/38003.

Image blur in XPEEM

Space charge artifacts in PEEM have been often observed in PEEM instruments employing ultra-bright laser sources. In this work, we study artifacts in XPEEM imaging and spectroscopy using the high intensity synchrotron radiation produced by an undulator source and  report on image blur and energy broadening effects at extremely high flux densities. We have recently demonstrated that Coulomb interactions between photoelectrons along the PEEM optics result in the degradation of both the microscope lateral and energy resolution, due to the combined action of the Loeffler and Boersch effects. At a flux of 2×1013 photons/s, the lateral resolution in XPEEM imaging with either core level or secondary electrons deteriorates to more than 50 nm. Broadening of the

Fermi level up to several hundred meV and spectral shift to higher kinetic energies are observed at similar photon fluxes, which correspond to peak electron photocurrents of a few μA in our estimates. These effects might have severe implications for the emerging generation of aberration corrected PEEM instruments, as they impose a physical limitation on the best lateral and energy resolution that can be achieved in XPEEM

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Image blur and energy broadening effects in XPEEM;
A. Locatelli, T.O. Menteş, M.Á. Niño, and E. Bauer;
Ultramicroscopy, 111, 1447-1454 (2011);
doi: 10.1016/j.ultramic.2010.12.020.

AFM domain imaging using LEEM

Imaging magnetic domains is vital to research aimed at understanding magnetism and harnessing its properties towards deliverable technological goals. Most efforts to date have been focused on a rich variety of ferromagnetic (FM) systems and the processes thereby occurring. Despite the development of several imaging methods that allow us to image magnetism on thin films and surfaces, the elusive nature of net magnetic moment in antiferromagnic (AFM) materials still poses serious experimental difficulties. Here, we show the power of a laboratory-based approach for imaging surface AFM domains, based on using coherently exchange-scattered unpolarized electrons in a standard low energy electron microscope (LEEM) . Our results are discussed in comparison with one of the most frequently employed magnetic surface imaging techniques, X-ray Magnetic Linear Dichroism

Photoemission Electron Microscopy (XMLD-PEEM). This method promises to widen the application of AFM imaging to a broader class of thin films and nanostructures, providing the advantage of enhanced surface sensitivity. Most importantly, it demonstrates to grant access to important details of the antiferromagnetic structure such as domain walls in ultra thin films, of which many aspects are still unexplored.

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Surface antiferromagnetic domain imaging using low-energy unpolarized electrons;
K.S.R. Menon, S. Mandal, J. Das, T.O. Menteş, M.Á. Niño, A. Locatelli, and R. Belkhou;
Phys. Rev. B 84, 132402 (2011);
doi: 10.1103/PhysRevB.84.132402.

ARPES on corrugated graphene

Exfoliated graphene crystals are not perfectly flat but can deform out-of-plane due to intrinsic and extrinsic factors. Ripples and distortions are known to be the most important sources of electron scattering in graphene, greatly affecting its transport properties. Such corrugations are also a serious obstacle to carry out angle resolved photoemission (ARPES) for probing the material's electronic structure, since this technique demands atomically flat surfaces. Combining ARPES with microprobe low energy electron diffraction makes it possible to circumvent such limitations. By measuring independently the short range roughness of corrugated suspended graphene sheets, we can distinguish corrugation effects from intrinsic lifetime broadening in ARPES,

showing that the quasiparticle lifetime scales inversely with energy. This approach is expected to be useful for probing the band structure of a variety of corrugated 2D system.

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Making angle-resolved photoemission measurements on corrugated monolayer crystals: Suspended exfoliated single-crystal graphene;
K. R. Knox, A.Locatelli, M.B. Yilmaz, D. Cvetko, T.O. Menteş, M.Á. Niño, P. Kim, A. Morgante, and
R.M. Osgood, Jr.;
Phys. Rev. B 84, 115401 (2011).
doi: 10.1103/PhysRevB.84.115401

Corrugation in Exfoliated Graphene: An Electron Microscopy and Diffraction Study

Corrugations, lattice distortions and charge transfer from adsorbates are the most important sources of electron scattering in graphene. Here, we investigate corrugations in graphene exploiting the multi-technique capabilities offered by a low energy electron microscope (LEEM): real space imaging of the sample morphology over large surface areas (up to several tens micron), with lateral resolution of 10 nm and atomic depth sensitivity, micro-probe low energy electron diffraction (μ-LEED). The short-range roughness of graphene at length scales below 20 nm is quantified by diffraction line-shape analysis, depending on film thickness and interaction with the SiO2 support.  Due to its reduced stiffness, single-layer graphene shows larger roughness than multi-layers. Because of the absence of an interacting support, suspended graphene displays a

 smoother texture than supported graphene, resulting in a notable narrowing of diffraction spots. Our LEED data suggests that the corrugation in suspended graphene films is influenced by both extrinsic and intrinsic factors, and in particular by adsorbate load and temperature.

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Corrugation in Exfoliated Graphene: An Electron Microscopy and Diffraction Study;
A. Locatelli, K. R. Knox, D. Cvetko, T.O. Menteş, M.Á. Niño, S. Wang, M. B. Yilmaz, P. Kim, R. M. Osgood Jr., and A. Morgante;
ACS Nano, 4, 4879-4889 (2010);
doi: 10.1021/nn101116n.

Domain-Wall Depinning Assisted by Pure Spin Currents

The depinning of domain walls in a nonlocal spin valve structure based on two ferromagnetic Permalloy elements with copper as the nonmagnetic spin conduit is found to be efficiently assisted by pure diffusive spin currents. These results open now possibilities for future low-power spintronic devices.One of the newly devised non-volatile memories is the so-called racetrack memory which is based on magnetic domain wall motion. For this purpose, various approaches have been put forward. Using pure diffusive spin currents, where the electrons diffuse without an associated net charge current, is a possible alternative approach. Its main advantage is that the generation of spin currents, that involves energy dissipation, can occur at a distant location from the device, which can thus be kept cool and still manipulated by the absorbed diffusive spin currents. In line with this 

approach, non-local spin valve geometries have been developed, where pure spin currents are generated across ferromagnetic - non-magnetic contacts.Here, we report the  first observation of domain wall depinning assisted by pure spin currents.

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Domain-Wall Depinning Assisted by Pure Spin Currents;
D. Ilgaz, J. Nievendick, L. Heyne, D. Backes, J. Rhensius, T.A. Moore, M.Á. Niño, A. Locatelli, T.O. Menteş, A. V. Schmidsfeld, A. V. Bieren, S. Krzyk, L. J. Heyderman, and M. Kläui;
Phys. Rev. Lett. 105, 076601 (2010);
doi: 10.1103/PhysRevLett.105.076601.

Ultima modifica il Martedì, 21 Maggio 2019 09:41