news & views than the bandgap of the semiconductor. In such cases, an excited electron does not necessarily need to possess an energy greater than the semiconductor’s bandgap to be extracted, as would be the case for normal semiconducting devices. However, for efficient extraction of hot carriers from a metal, total energy is not the only condition. Once created, the hot carriers must also be moving towards the interface: that is, the hot carrier’s momentum must be primarily in the direction perpendicular to the interface, so that the carrier’s kinetic energy component in that direction is sufficient to overcome the Schottky barrier. The difficulty of achieving both the energetic and momentum requirements has been the main cause of the generally low quantum efficiency for extracting hot electrons in devices. As an example, hot carriers generated in planar plasmonic devices often have their momentum oriented in the wrong direction (Fig. 1b). This is because the hot carrier’s momentum follows that of the plasmon mode, and plasmons are typically excited by light with the electric field polarized in the plane of the interface3. Therefore, hot carriers initially propagate parallel to the interface and not towards it, resulting in a low probability of injection into the semiconductor 4,5. This problem has recently been circumvented by embedding an optical antenna within the semiconductor, so that hot carriers can be extracted throughout the interfaces at the antenna side walls and not just through the bottom interface4 (Fig. 1c). Di Fabrizio and colleagues instead use a tapered conical metallic tip6,7 that is in close proximity to a semiconductor surface. Far-field light is irradiated onto a

nanofabricated grating that is milled into the side of the tip where the light is converted to surface plasmons. These plasmons then propagate towards the apex of the tip. As they travel to smaller and smaller volumes, they are adiabatically compressed and tightly focused at the apex relaxing the momentum conservation constraint 8,9. The advantages of this approach are threefold: (1) it produces hot carriers with high efficiency and localization; (2) it can control the hot carrier momentum, which is primarily oriented along the tip apex, normal to the tip/semiconductor interface; and (3) it can confine energy in a tight space (~10 nm), making it more likely to extract the hot carriers before they have a chance to scatter 9,10 (Fig. 1d). Therefore, when this type of plasmonic tip is brought into contact with a semiconductor, the conditions are nearly ideal for the transfer of hot carriers. In this way, the researchers are able to achieve hot-electron extraction efficiencies of ~30%, a value comparable to the incident-photon-to-current conversion efficiencies previously seen only in photoelectrochemical reactions11 and many times greater than what has been achieved in other solid‑­state devices. Di Fabrizio and colleagues take advantage of this high efficiency to develop an intriguing imaging tool that tackles the critical and longstanding challenge of mapping chemical information and electronic structure with nanoscale resolution12. In their set-up, the optically excited tip is scanned over a sample while the hot-electron transfer current is measured. Because the Schottky barrier is intimately related to the electronic properties of the surface, the researchers managed to

map out the local work function and surface charge density of the semiconductor. In particular, the researchers imaged nanoscale patterns of oxidation on GaAs, showing clear material contrast with a lateral resolution below 50 nm, as well as local differences in Ga-ion concentration in implanted GaAs. These experiments demonstrate that hotelectron imaging works well even when there is very little change in topography. It is conceivable that with further optimization, this technique can be pushed to the single-defect level, therefore providing key insights into the relationship between atomic structure and electronic properties, a previously inaccessible realm within nanostructured systems. ❐ P. James Schuck is at the Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California 94720, USA. e-mail: [email protected] References 1. Chalabi, H. & Brongersma, M. L. Nature Nanotech. 8, 229–230 (2013). 2. Giugni, A. et al. Nature Nanotech. 8, 845–852 (2013). 3. Knight, M. W., Sobhani, H., Nordlander, P. & Halas, N. J. Science 332, 702–704 (2011). 4. Knight, M. W. et al. Nano Lett. 13, 1687–1692 (2013). 5. Shalaev, V. M., Douketis, C., Stuckless, J. T. & Moskovits, M. Phys. Rev. B 53, 11388–11402 (1996). 6. Babadjanyan, A. J., Margaryan, N. L. & Nerkararyan, K. V. J. Appl. Phys. 87, 3785–3788 (2000). 7. Stockman, M. I. Phys. Rev. Lett. 93, 137404 (2004). 8. Ropers, C. et al. Nano Lett. 7, 2784–2788 (2007). 9. Govorov, A. O., Zhang, H. & Gun’ko, Y. K. J. Phys. Chem. C 117, 16616–16631 (2013). 10. Scales, C. & Berini, P. IEEE J. Quantum Electron. 46, 633–643 (2010). 11. Tian, Y. & Tatsuma, T. J. Am. Chem. Soc. 127, 7632–7637 (2005). 12. Schuck, P. J. et al. Adv. Funct. Mater. 23, 2539–2553 (2013).

Published online: 20 October 2013 Corrected after print: 7 November 2013


Skyrmions singled out

Single magnetic skyrmions — topological whirls in the magnetization of certain ferromagnets — can be created and manipulated in nanostructures using electrical currents.

Rembert Duine


pintronic devices based on the current-driven manipulation of magnetization in nanoscale ferromagnets could be used as future memory elements, which will combine low cost, high performance and non-volatility. Examples of such devices include magnetic random access memory 1,2, the magnetic racetrack memory 3, and those based on 800

the manipulation of magnetic skyrmions. Magnetic skyrmions (Fig. 1) are textures of magnetization that cannot be continuously deformed into the uniform ferromagnetic state without causing a singularity, and are therefore topologically protected. They were recently observed in certain ferromagnetic materials4,5, and their stability, together with their small size and the fact that

they can be moved by currents of very small densities6, makes them attractive as information carriers in memory technology. So far, experimental results on current-driven motion have concerned lattices of skyrmions. However, it is likely that any future technology will be based on the manipulation of individual skyrmions. Writing in Nature Nanotechnology, two


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Magnus force Drag force and current


Repulsive force

Figure 1 | Skyrmions in nanostructures. a, A magnetic skyrmion. b, The magnetization of the skyrmion in (a) follows a left-hand rule, with the spins at the centre pointing down (along the thumb) and the spin around the skyrmion core swirling in the direction of the fingers. c, The left-hand rule is obeyed both by the skyrmion and the entire nanostructure. Because the magnetization in the nanowire is pointing up (except at the position of the skyrmion) the magnetization swirls clockwise around its edge, leading to repulsive forces that tend to push the skyrmions away from the edge. The current flows along the wire and results in Magnus and drag forces on the skyrmion, which are perpendicular and parallel to the current direction, respectively. The Magnus force is balanced by the repulsive force away from the edge, leaving the drag force as the main driving mechanism.

independent research teams7,8 have now used numerical simulations to show that single skyrmions can be created or destroyed with current pulses, and that skyrmions can be efficiently pushed along nanowires using a current. The intrinsic stability of skyrmions is at odds with the need to easily create and destroy them. Because of their topological character, the creation of a skyrmion in the ferromagnetic state is the magnetic equivalent of drilling a hole through an object, and requires a significant perturbation. In physical terms, the perturbation must overcome the strong exchange coupling that forces neighbouring spins to be aligned. Nagaosa and colleagues7 at University of Tokyo, Aoyama Gakuin University and the RIKEN Center for Emergent Matter Science demonstrate that a skyrmion can be nucleated when a relatively large current is driven along a nanowire that has a notch along one of its edges. The notch generates inhomogeneities in the magnetization, which act as a seed for skyrmion nucleation. This method has the attractive

feature of selectively nucleating skyrmions depending on the direction of the current. Alternatively, Cros and colleagues8 at the Université Paris Sud consider injecting spin currents into a small region of the ferromagnet. This locally reverses the magnetization, resulting in an excitation that typically evolves into a skyrmion. In fact, creation of skyrmions by injection of spin currents from the ferromagnetic tip of a scanning tunnelling microscope has been recently observed in skyrmion lattices at low temperatures9. Both research teams examined the current-driven motion of the individual skyrmions. For skyrmion lattices that occur in large systems, the current densities required to move skyrmions are much smaller than those typically needed for current-driven domain-wall dynamics. This is due to the efficient coupling of current to skyrmions, and because skyrmion lattices are relatively unaffected by disorder 6,10. The two teams find that single skyrmions can be moved efficiently through nanostructures. Similar to the case of skyrmion lattices, this is in part


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due to the efficient coupling to the current. Furthermore, the efficient current-driven motion results from how the skyrmion interacts with the edges of a magnetic nanostructure. The interaction is ultimately the result of the Dzyaloshinskii–Moriya coupling, which occurs in ferromagnets without inversion symmetry, and allows the magnetic system to lower its energy by tilting neighbouring spins. The Dzyaloshinskii–Moriya coupling also gives a preferred chirality or ‘handedness’ to the skyrmions, which can be understood by noting that the spins around the skyrmion core swirl in a preferred direction with the spins at the core pointing down (Fig. 1). The spins at the edge of the nanostructure are tilted in such a way that the entire magnetic system also obeys the left-hand rule. As a result, the magnetization that swirls around the skyrmion core points opposite to the magnetization swirling around the sample edge, which leads to an effective repulsive force pushing the skyrmions away from the edge. Like a moving and spinning ball, which has a handedness determined by its 801

news & views sense of rotation, the skyrmion experiences Magnus forces that are perpendicular to the current direction. These forces drive the skyrmion to the edge of the nanowire and are cancelled by the repulsive force from the edge. The motion along the nanowire and current direction is then determined by the balance between friction and current-induced forces along the wire, which, because friction is typically small, leads to large velocities. For large currents, the force on the skyrmion can overcome the repulsive force from the edge, and the skyrmion is annihilated. For skyrmion lattices, the position and velocity of the skyrmions could be electrically detected by means of the emergent electromagnetic fields arising from the topological character of skyrmions11. The next step towards skyrmion-based magnetic memories is the electrical detection of a single skyrmion. Possibly, this could be achieved by similar magnetoresistive effects.

All previous experimental work with magnetic skyrmions has been carried out at low temperatures (up to ~270 K; ref. 12). From a materials science point of view, there is no obvious choice for the most promising material for achieving currentdriven control of skyrmions at room temperature. Of particular interest are magnetic multilayers with perpendicular magnetic anisotropy, in which recent experiments on domain walls have elucidated the role of interface-induced Dzyaloshinskii–Moriya interactions13,14. These interactions may be strong enough to energetically favour skyrmions. However, the coupling between current and magnetization in these materials may be different from that in the skyrmionlattice materials previously studied in experiments6 because of strong intrinsic spin–orbit coupling. Nevertheless, it is clear that there are many challenges ahead before skyrmion-based memories could become a reality, but there are also numerous

opportunities to explore the fascinating physics of these magnetic nanostructures.❐ Rembert Duine is at the Institute for Theoretical Physics, Leuvenlaan 4, 3584 CE Utrecht, The Netherlands. e-mail: [email protected] References 1. Slonczewski, J. C. J. Mag. Mag. Mater. 159, L1–L7 (1996). 2. Berger, L. Phys. Rev. B 54, 9353–9358 (1996). 3. Parkin, S. S. P., Hayashi, M. & Thomas, L. Science 320, 190–194 (2008). 4. Mühlbauer, S. et al. Science 323, 915–919 (2009). 5. Yu, X. Z. et al. Nature 465, 901–904 (2010). 6. Jonietz, F. et al. Science 330, 1648–1651 (2010). 7. Iwasaki, J., Mochizuki, M. & Nagaosa, N. Nature Nanotech. 8, 742–747 (2013). 8. Sampaio, J., Cros, V., Rohart, S., Thiaville, A. & Fert, A. Nature Nanotech. 8, 839–844 (2013). 9. Romming, N. et al. Science 341, 636–639 (2013). 10. Iwasaki, J., Mochizuki, M. & Nagaosa, N. Nature Commun. 4, 1463 (2013). 11. Schulz, T. et al. Nature Phys. 8, 301–304 (2012). 12. Yu, X. Z. et al. Nature Commun. 3, 988 (2012). 13. Emori, S., Bauer, U., Ahn, S.‑M., Martinez, E. & Beach, G. S. D. Nature Mater. 12, 611–616 (2013). 14. Ryu, K.‑S., Thomas, L., Yang S.‑H. & Parkin, S. S. P. Nature Nanotech. 8, 527–533 (2013).


A probe for graphene electronics The local electronic properties of graphene grain boundaries can be obtained by deciphering the interference patterns produced by surface plasmons.

Rémi Carminati


urface plasmon polaritons are electromagnetic waves that are formed by coupling light and free-electron oscillations at the surface of a metal or a semiconductor. The dual identity of these polaritons — half optical wave, half electronic oscillation — offers the possibility of simultaneously carrying out optical imaging and probing the electronic properties of the surface of a material. Writing in Nature Nanotechnology, Dimitri Basov of the University of California, San Diego and colleagues in the US, Singapore, Germany and Spain now report that they have used nanoscale plasmon interferometry to optically image grain boundaries on graphene samples grown by chemical vapour deposition (CVD)1. The interferometric signal leads to a direct measurement of the charge-carrier density and the scattering rate at the grain boundaries, and the results can explain why static electronic conductivity is greatly reduced on CVD-grown graphene compared with pristine graphene. 802

From an electromagnetic wave standpoint, a surface plasmon polariton propagates along a flat interface with a wavevector qp and decays exponentially away from the surface. The wavevector defines the plasmon wavelength λp and the damping rate γp, which describes the wave attenuation due to ohmic losses in the material. On graphene, λp is reduced by a factor of 40 compared with the infrared wavelength (λIR = 10 μm) used to excite the surface plasmons2. On a flat surface, the excitation and detection of a surface plasmon polariton cannot be performed using standard optical microscopy because of the evanescent nature of the wave. In their experiment, Basov and colleagues use the sharp tip of a scattering-type scanning near-field optical microscope as an optical antenna, which works both as an emitter and a receiver when positioned a few nanometres from the graphene surface (Fig. 1). This mode of operation offers a unique ability to excite and detect surface plasmons in the infrared range2–4.

Under illumination by an external beam at λIR, scattering at the tip apex produces a bright spot that locally excites surface plasmons. When a surface plasmon reaches a grain boundary, it encounters a discontinuity in the electronic characteristics of the supporting medium, which generates a reflected plasmon wave. Incident and reflected plasmons produce an interference pattern, whose intensity is locally detected by the tip and transmitted to a far-field detector. An image is formed by scanning the tip along the surface and recording the electromagnetic field intensity versus tip position. One of the advantages of near-field imaging is that it is not diffraction limited, and does not require any specific preparation of the graphene surface, such as chemical functionalization for optical microscopy 5 or transfer onto a grid substrate for transmission electron microscopy 6. According to the model developed by Basov and colleagues, which fits the experimental data nicely, reflection by grain boundaries locally change


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Spintronics: Skyrmions singled out.

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