news & views The good seal offered by the technique should also allow the measurement of subthreshold events such as excitatory and inhibitory postsynaptic potentials. However, similar to patch-clamp recording, the positioning method severely limits the scalability of the approach. Furthermore, nanowire transistors and nanoelectrodes in general will become more useful when they acquire the ability to inject stable current into
the cell. In comparison, current injection is a useful feature of the patch-clamp technique, which allows clamping of the cell membrane potential at predetermined levels to reveal the current–voltage relationship of specific types of ion channel. ❐ Ziliang Carter Lin is at the Department of Applied Physics Stanford University, Stanford, California 94305, USA. Bianxiao Cui is at the
Department of Chemistry, Stanford University, Stanford, California 94305, USA. e-mail: [email protected] References 1. Qing, Q. et al. Nature Nanotech. 9, 142–147 (2014). 2. Xie, C., Lin, Z. L., Hanson, L., Cui, Y. & Cui, B. X. Nature Nanotech. 7, 185–190 (2012). 3. Robinson, J. T. et al. Nature Nanotech. 7, 180–184 (2012). 4. Hai, A., Shappir, J. & Spira, M. E. Nature Methods 7, 200–250 (2010). 5. Tian, B. Z. et al. Science 329, 830–834 (2010).
MAGNETIC NANOSTRUCTURES
Vortices on the move
Magnetic vortices can be controllably transferred in an extended system by electrical means.
Teruo Ono
M
agnetic vortices are in-plane curling spin structures that have a nanoscale core with an out-of plane magnetization1. They are formed in ferromagnetic disks and the binary nature of the cores (whose magnetization can point up or down) combined with their thermal stability make vortices appealing for future non-volatile memory devices. Importantly for such applications, it has been shown that the cores can be switched by applying suitable electrical or magnetic field pulses2–5. A magnetic vortex is formed in the presence of a confining potential such as that provided by the geometrical confinement of a disk. However,
applications other than memory devices could be developed if magnetic vortices could be freed from such geometrical constraints. Towards this end, it has been shown that the Oersted field generated by passing an electric current into a ferromagnetic film through a metallic nanocontact can create a suitable confining potential for the formation of magnetic vortices6,7. This technique is advantageous because it allows magnetic vortices to be arbitrarily positioned in an extended film, and for the strength of the confining potential to be tuned by means of the amplitude of the electric current. Writing in Nature Nanotechnology, Thibaut Devolder
West
East SiO2 Free layer Metal Fixed layer Bottom electrode
Current
West
Current
East
Ground
Figure 1 | Schematic illustration of a device composed of two nanocontacts fabricated on top of a spin valve. The spin valve consists of a magnetically soft free-layer stacked onto a metallic spacer and a magnetically hard fixed-layer8. 96
and colleagues at IMEC, Katholieke Universiteit Leuven, CNRS and University of Paris-Sud have now shown that magnetic vortices can also be controllably transported in an extended magnetic film8. Devolder and colleagues have exploited the advantages of the nanocontact method and fabricated a device composed of two nanocontacts (east and west) on top of a spin valve structure. The spin valve is formed from a magnetically soft freelayer — that is, with a magnetization that is easy to rotate away from the equilibrium direction — stacked onto a metallic spacer and a magnetically hard fixed-layer (Fig. 1). The resistance of the spin valve depends on the relative orientation of the magnetization in the two magnetic layers. The Oersted field from the electrical current flowing through one of the nanocontacts can nucleate a magnetic vortex in the free layer. The nucleated vortex is excited by spin torque from the spin-polarized current flowing radially outward from the nanocontact in the film plane6,7,9, and undergoes circular motion in the vicinity of the nanocontact, due to an attractive force from the Zeeman energy associated with the Oersted field. This attractive force works like the gravity of a planet, with the vortex behaving like a satellite. The circular motion of the vortex around the nanocontact can be detected as a time evolution of the electrical resistance of the spin valve, which varies in dependence of the relative orientation of the magnetization in the free and pinned layers. The researchers first nucleated a vortex on the east nanocontact by injecting a current IE = 20 mA, and detected its circular motion around this contact by
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
news & views
a
Vortex
West
b
c
East
Figure 2 | The trajectories of the magnetic vortex as a function of the applied currents. a, Vortex trajectory for IE = 20 mA and IW = 0. b, Vortex trajectory for IE = 10 mA and IW = 10 mA. c, Vortex trajectory for IE = 0 and IW = 20 mA.
measuring the resistance (Fig. 2a). Then, by fixing the total current in the two nanocontacts (IW + IE) to 20 mA, and gradually increasing IW, the researchers observed that, when IW and IE were approximately equal, resistance changes as a function of time were observed for both nanocontacts, suggesting that the vortex was now circling around both (Fig. 2b). Further increase of IW resulted in the disappearance of the resistance change in the east nanocontact, suggesting that the vortex had been transferred from the east to the west nanocontact (Fig. 2c), similar to a satellite that is subject to a gravitational slingshot from one planet to the other.
Devolder and colleagues also demonstrated that magnetic vortices could be transferred using multiple nanocontact arrangements, including a linear chain, a diamond and a Y-shaped branching network. This suggests that, in principle, a vortex can be transferred in a film at will. The simultaneous propagation of multiple vortices in a network of arbitrary shape could also be possible, and could be used for logic operations. Because a magnetic vortex has a relatively strong stray field from its core, it could also be possible to attach an object, such as a magnetic bead, to a vortex core, and transport it using Devolder and colleagues’ technique. ❐
Teruo Ono is at the Institute for Chemical Research, Kyoto University, Gokasho Uji-city, Kyoto 611-0011, Japan. e-mail: [email protected] References 1. Shinjo, T., Okuno, T., Hassdorf, R., Shigeto, K. & Ono, T. Science 289, 930–932 (2000). 2. Van Wayeneberge, B. et al. Nature 444, 461–464 (2006). 3. Yamada, K. et al. Nature Mater. 6, 270–273 (2007). 4. Yamada, K., Kasai, S., Nakatani, Y., Kobayashi, K. & Ono, T. Appl. Phys. Lett. 93, 152502 (2008). 5. Weigand, M. et al. Phys. Rev. Lett. 102, 077201 (2009). 6. Mistral, Q. et al. Phys. Rev. Lett. 100, 257201 (2008). 7. Ruotolo, A. et al. Nature Nanotech. 4, 528–532 (2009). 8. Manfrini, M. et al. Nature Nanotech. 9, 121–125 (2014). 9. Kasai, S., Nakatani, Y., Kobayashi, K., Kohno, H. & Ono, T. Phys. Rev. Lett. 97, 107204 (2006).
QUANTUM TRANSPORT
Immune to local heating
Electronic transport in a nanodevice can be made insensitive to local heating by driving the device into strong non-equilibrium conditions.
Marc Cahay
I
n agreement with Gordon Moore’s prediction in 19651, the density of transistors in a semiconductor chip has increased in a geometric progression ever since, roughly doubling every 18 months. A projected density of 1013 transistors per cm2 is anticipated by 2017 with device dimensions in the nanoscale range. However, on-chip heatsinking technology has lagged behind in performance, which will lead to chip meltdown within a few generations of modern integrated circuits. This doomsday has been dubbed the ‘red brick wall’ by the International Technology Roadmap for Semiconductors2. The upmost challenge is to find semiconductor technologies that provide a drastic reduction in energy
dissipation during device operation. Writing in Nature Nanotechnology, Johnathan Bird and colleagues at the University at Buffalo and Sandia National Laboratories have now demonstrated a rather counterintuitive way to manage energy dissipation in nanoscale devices by driving electrons into a non-equilibrium state to tailor the electron–phonon scattering 3. The researchers carry out their experiments using a quantum point contact (QPC), which consists of a short quantum wire or constriction. The constriction is formed by depositing a pair of top gates over a two-dimensional electron gas that exists at the interface of an AlGaAs/ GaAs semiconductor heterostructure (Fig. 1a). A negative bias applied to
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
the gates depletes the two-dimensional electron gas directly under and adjacent to the gates, creating a quasi-onedimensional conduction channel between them. In the constriction, the number of quantized transverse modes — or subbands — participating in electron conduction decreases with the width of the QPC channel, which can be adjusted by appropriately biasing the gates. It was shown in 1988 that under a small constant d.c. bias applied between the source and drain contacts, ballistic transport occurs through QPCs4,5: the linear conductance of such devices as a function of the gate bias takes values that are integral multiples of a universal value, 2e2/h (where e is the magnitude of the 97
the cell. In comparison, current injection is a useful feature of the patch-clamp technique, which allows clamping of the cell membrane potential at predetermined levels to reveal the current–voltage relationship of specific types of ion channel. ❐ Ziliang Carter Lin is at the Department of Applied Physics Stanford University, Stanford, California 94305, USA. Bianxiao Cui is at the
Department of Chemistry, Stanford University, Stanford, California 94305, USA. e-mail: [email protected] References 1. Qing, Q. et al. Nature Nanotech. 9, 142–147 (2014). 2. Xie, C., Lin, Z. L., Hanson, L., Cui, Y. & Cui, B. X. Nature Nanotech. 7, 185–190 (2012). 3. Robinson, J. T. et al. Nature Nanotech. 7, 180–184 (2012). 4. Hai, A., Shappir, J. & Spira, M. E. Nature Methods 7, 200–250 (2010). 5. Tian, B. Z. et al. Science 329, 830–834 (2010).
MAGNETIC NANOSTRUCTURES
Vortices on the move
Magnetic vortices can be controllably transferred in an extended system by electrical means.
Teruo Ono
M
agnetic vortices are in-plane curling spin structures that have a nanoscale core with an out-of plane magnetization1. They are formed in ferromagnetic disks and the binary nature of the cores (whose magnetization can point up or down) combined with their thermal stability make vortices appealing for future non-volatile memory devices. Importantly for such applications, it has been shown that the cores can be switched by applying suitable electrical or magnetic field pulses2–5. A magnetic vortex is formed in the presence of a confining potential such as that provided by the geometrical confinement of a disk. However,
applications other than memory devices could be developed if magnetic vortices could be freed from such geometrical constraints. Towards this end, it has been shown that the Oersted field generated by passing an electric current into a ferromagnetic film through a metallic nanocontact can create a suitable confining potential for the formation of magnetic vortices6,7. This technique is advantageous because it allows magnetic vortices to be arbitrarily positioned in an extended film, and for the strength of the confining potential to be tuned by means of the amplitude of the electric current. Writing in Nature Nanotechnology, Thibaut Devolder
West
East SiO2 Free layer Metal Fixed layer Bottom electrode
Current
West
Current
East
Ground
Figure 1 | Schematic illustration of a device composed of two nanocontacts fabricated on top of a spin valve. The spin valve consists of a magnetically soft free-layer stacked onto a metallic spacer and a magnetically hard fixed-layer8. 96
and colleagues at IMEC, Katholieke Universiteit Leuven, CNRS and University of Paris-Sud have now shown that magnetic vortices can also be controllably transported in an extended magnetic film8. Devolder and colleagues have exploited the advantages of the nanocontact method and fabricated a device composed of two nanocontacts (east and west) on top of a spin valve structure. The spin valve is formed from a magnetically soft freelayer — that is, with a magnetization that is easy to rotate away from the equilibrium direction — stacked onto a metallic spacer and a magnetically hard fixed-layer (Fig. 1). The resistance of the spin valve depends on the relative orientation of the magnetization in the two magnetic layers. The Oersted field from the electrical current flowing through one of the nanocontacts can nucleate a magnetic vortex in the free layer. The nucleated vortex is excited by spin torque from the spin-polarized current flowing radially outward from the nanocontact in the film plane6,7,9, and undergoes circular motion in the vicinity of the nanocontact, due to an attractive force from the Zeeman energy associated with the Oersted field. This attractive force works like the gravity of a planet, with the vortex behaving like a satellite. The circular motion of the vortex around the nanocontact can be detected as a time evolution of the electrical resistance of the spin valve, which varies in dependence of the relative orientation of the magnetization in the free and pinned layers. The researchers first nucleated a vortex on the east nanocontact by injecting a current IE = 20 mA, and detected its circular motion around this contact by
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
news & views
a
Vortex
West
b
c
East
Figure 2 | The trajectories of the magnetic vortex as a function of the applied currents. a, Vortex trajectory for IE = 20 mA and IW = 0. b, Vortex trajectory for IE = 10 mA and IW = 10 mA. c, Vortex trajectory for IE = 0 and IW = 20 mA.
measuring the resistance (Fig. 2a). Then, by fixing the total current in the two nanocontacts (IW + IE) to 20 mA, and gradually increasing IW, the researchers observed that, when IW and IE were approximately equal, resistance changes as a function of time were observed for both nanocontacts, suggesting that the vortex was now circling around both (Fig. 2b). Further increase of IW resulted in the disappearance of the resistance change in the east nanocontact, suggesting that the vortex had been transferred from the east to the west nanocontact (Fig. 2c), similar to a satellite that is subject to a gravitational slingshot from one planet to the other.
Devolder and colleagues also demonstrated that magnetic vortices could be transferred using multiple nanocontact arrangements, including a linear chain, a diamond and a Y-shaped branching network. This suggests that, in principle, a vortex can be transferred in a film at will. The simultaneous propagation of multiple vortices in a network of arbitrary shape could also be possible, and could be used for logic operations. Because a magnetic vortex has a relatively strong stray field from its core, it could also be possible to attach an object, such as a magnetic bead, to a vortex core, and transport it using Devolder and colleagues’ technique. ❐
Teruo Ono is at the Institute for Chemical Research, Kyoto University, Gokasho Uji-city, Kyoto 611-0011, Japan. e-mail: [email protected] References 1. Shinjo, T., Okuno, T., Hassdorf, R., Shigeto, K. & Ono, T. Science 289, 930–932 (2000). 2. Van Wayeneberge, B. et al. Nature 444, 461–464 (2006). 3. Yamada, K. et al. Nature Mater. 6, 270–273 (2007). 4. Yamada, K., Kasai, S., Nakatani, Y., Kobayashi, K. & Ono, T. Appl. Phys. Lett. 93, 152502 (2008). 5. Weigand, M. et al. Phys. Rev. Lett. 102, 077201 (2009). 6. Mistral, Q. et al. Phys. Rev. Lett. 100, 257201 (2008). 7. Ruotolo, A. et al. Nature Nanotech. 4, 528–532 (2009). 8. Manfrini, M. et al. Nature Nanotech. 9, 121–125 (2014). 9. Kasai, S., Nakatani, Y., Kobayashi, K., Kohno, H. & Ono, T. Phys. Rev. Lett. 97, 107204 (2006).
QUANTUM TRANSPORT
Immune to local heating
Electronic transport in a nanodevice can be made insensitive to local heating by driving the device into strong non-equilibrium conditions.
Marc Cahay
I
n agreement with Gordon Moore’s prediction in 19651, the density of transistors in a semiconductor chip has increased in a geometric progression ever since, roughly doubling every 18 months. A projected density of 1013 transistors per cm2 is anticipated by 2017 with device dimensions in the nanoscale range. However, on-chip heatsinking technology has lagged behind in performance, which will lead to chip meltdown within a few generations of modern integrated circuits. This doomsday has been dubbed the ‘red brick wall’ by the International Technology Roadmap for Semiconductors2. The upmost challenge is to find semiconductor technologies that provide a drastic reduction in energy
dissipation during device operation. Writing in Nature Nanotechnology, Johnathan Bird and colleagues at the University at Buffalo and Sandia National Laboratories have now demonstrated a rather counterintuitive way to manage energy dissipation in nanoscale devices by driving electrons into a non-equilibrium state to tailor the electron–phonon scattering 3. The researchers carry out their experiments using a quantum point contact (QPC), which consists of a short quantum wire or constriction. The constriction is formed by depositing a pair of top gates over a two-dimensional electron gas that exists at the interface of an AlGaAs/ GaAs semiconductor heterostructure (Fig. 1a). A negative bias applied to
NATURE NANOTECHNOLOGY | VOL 9 | FEBRUARY 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
the gates depletes the two-dimensional electron gas directly under and adjacent to the gates, creating a quasi-onedimensional conduction channel between them. In the constriction, the number of quantized transverse modes — or subbands — participating in electron conduction decreases with the width of the QPC channel, which can be adjusted by appropriately biasing the gates. It was shown in 1988 that under a small constant d.c. bias applied between the source and drain contacts, ballistic transport occurs through QPCs4,5: the linear conductance of such devices as a function of the gate bias takes values that are integral multiples of a universal value, 2e2/h (where e is the magnitude of the 97
