news & views SPINTRONICS
Nanomagnonics around the corner Two complementary strategies show how to control the spatial propagation of spin waves, thus promising complex and reconfigurable wiring in spin-wave-based circuits.
Dirk Grundler
I
n electronic devices, including computers, the information is controlled and transmitted by means of electric currents. Using propagating waves rather than the motion of electric charges would, however, be more efficient, especially in terms of losses, but also in terms of speed. One type of wave that can be used is light. The propagation of light can be controlled and directed by engineered photonic crystals1 and via graded-index glass fibres for long-distance signal transmission2. However, computers based on light waves are still a challenge. An alternative is to use spin waves, also known as magnons, in magnetic materials, which could be used to implement wave-based computation and coherent processing of data across largearea cellular networks3. Now writing in Nature Nanotechnology, two research groups report complementary ways of controlling the propagation direction of spin waves at the nanoscale, which may lead to advances in the circuitry on magnonic chips4,5. Both approaches can produce reconfigurable waveguides, thus allowing for on-demand control of spin waves, local manipulation of information, and rewiring of magnetic chips via gateable spin structures. In general, the propagation of waves can be guided by structural modification
and compositional variation of matter1,2. For spin waves, a further guiding principle is available. The equation of motion for spins, traced back to Landau and Lifshitz, contains a term for an effective field that is formed not only by an external magnetic field generated by charge currents, but also by internal fields generated by magnetic volume and surface charges6. It is already known that spin textures and domain walls give rise to distinct magnetic charge-density distributions that localize and channel spin-wave excitations7. Helmut Schultheiss and colleagues from the Institute of Ion Beam Physics and Materials Research and the Technische Universität in Dresden have used an individual domain wall to improve the nanoscale wiring of magnonic circuits4 (Fig. 1a). Exploiting magnetostatic spin waves in a movable domain wall, they re-route signals of microwave frequencies on demand. The researchers fabricated a strip of the ferromagnetic metal Ni80Fe20 on a silicon oxide substrate. The strip is wide enough to allow for a domain pattern such that a few-micrometre-long domain wall is perpendicular to an integrated microwave antenna. A microwave signal induces an oscillating magnetic field that launches propagating spin waves. At specific
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frequencies, only the narrow domain wall, not the surrounding film, supports the spin waves (Fig. 1a). Importantly, the researchers show that such a spin-wave nanochannel is gateable, because by applying a fine-tuned magnetic field its lateral position can be varied with nanometre precision (Fig. 1b). Adekunle Olusola Adeyeye and colleagues from the National University of Singapore report a complementary approach to guide and manipulate spin waves at the nanoscale5. They fabricated chains of spinwave nanoresonators (Fig. 2a), separated by only 50 nm, by nanolithography. The Ni80Fe20 nanoresonators were 25 nm thick, 260 nm wide and 600 nm long. The small separation induced coherent spin dynamics across the chain, similar to a one-dimensional magnonic crystal. The purposely designed rhomboidal nanoresonators allow different magnetic states to be programmed (Fig. 2a,b) and thereby to mould the flow of spin waves in a reconfigurable manner. They can also bend the propagation of spin waves using very low power (Fig. 2c), which has been a longstanding challenge. Both research teams used inelastic light scattering with high frequency and high spatial resolution to image the spin waves. They confirmed the confinement of
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Figure 1 | Reconfigurable magnonic conduits realized in domain walls. a, Spin wave of wavevector k propagating in a domain wall between two magnetic domains (top view). Spins precess at their given position (red arrows); electrons do not flow. The white arrows indicate magnetization vectors, M. b, A magnetic field, H, shifts the spin-wave nanochannel because a domain of magnetization, M, grows at the expense of the other domain. Magnetic volume charges are indicated by plus and minus signs. NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2016 Macmillan Publishers Limited. All rights reserved
1
news & views spin-wave channels as narrow as 40 nm could be realized with this method4. These findings further promote interest in domain wall nanoelectronics — an exciting and timely topic in ferroelectrics8. The nanoresonator chains proposed by Adeyeye and co-authors allow for gating of spin waves and logic functionality 5, making their integration with magnetic quantum-dot cellular automata particularly interesting 9. Still, both reports leave room for further exploration. The two research teams engineered non-collinear spin textures in the metallic ferromagnet Ni80Fe20. It has been previously shown that such textures give rise to additional damping in metals via intralayer spin pumping 10. Fortunately, magnons also exist in magnetic insulators, which will help overcome the damping issue. In unpatterned yttrium iron garnet, macroscopic decay lengths of up to centimetres have been demonstrated. For computation with spin waves and coherent data processing in large-area cellular networks, such low-damping magnetic insulators are of special relevance in magnonics. Unlike metals, reduction of these materials to nanoscale dimensions and their integration with silicon substrates, however, have not yet been achieved. ❐
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Dirk Grundler is at the Laboratory of Nanoscale Magnetic Materials and Magnonics, Institute of Materials and Institute of Microengineering, School of Engineering, Ecole Polytechnique Fédérale de Lausanne, Station 12, 1015 Lausanne, Switzerland. e-mail: [email protected] References
Figure 2 | Reconfigurable magnonic conduits based on nanoresonators. a,b, Strongly coupled nanomagnets of rhomboidal shape (a) give rise to reprogrammable magnonic conduits by changing the magnetization direction and magnetic surface-charge distribution at specific positions (b). c, Nanomagnet configuration for a bent magnonic conduit. Magnetic surface charges are indicated by plus and minus signs. Curved blue arrows illustrate dynamic stray fields and coupling between the spin-wave nanoresonantors. Wave-like spin precession is indicated by the red arrows.
microwave signals to deep-subwavelength length scales. In the work of Schultheiss and co-workers, the channel width is smaller than the wavelength in free space by a factor of about 10–6 (ref. 4). The extraordinary squeezing of an electromagnetic wave is a key feature of a magnonics-based technology.
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The two reports represent a turning point. They imply that prototypical nanomagnonic devices, such as an integrated spin-wave interferometer, can be implemented and interconnected. The concept of Schultheiss and colleagues promises, in particular, reprogrammable magnonic circuitry. Micromagnetic simulations suggest that
1. Joannopoulos, J. D., Johnson, S. G., Winn, J. N. & Meade, R. D. Photonic Crystals, Molding the Flow of Light (Princeton Univ. Press, 2008). 2. Kao, C. K. Sand from centuries past: send future voices fast (Nobel lecture, 2009); http://go.nature.com/FJPJQx 3. Khitun, A., Bao, M. & Wang, K. L. J. Phys. D 43, 264005 (2010). 4. Wagner, K. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2015.339 (2016). 5. Haldar, A., Kumar, D. & Adeyeye, A. O. Nature Nanotech. http://dx.doi.org/10.1038/nnano.2015.332 (2016). 6. Gurevich, A. G. & Melkov, G. A. Magnetization Oscillations and Waves (CRC Press, 1996). 7. Perzlmaier, K. et al. Phys. Rev. Lett. 94, 057202 (2005). 8. McGilly, L. J., Yudin, P., Feigl, L., Tagsantsev, A. K. & Setter, N. Nature Nanotech. 10, 145–150 (2015). 9. Imre, A., Csaba, G., Ji, L., Orlov, A. & Porod, W. Science 311, 205–208 (2006). 10. Nembach, H. T., Shaw, J. M., Boone, C. T. & Silva, T. J. Phys. Rev. Lett. 110, 117201 (2013).
Published online: 1 February 2016
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2016 Macmillan Publishers Limited. All rights reserved
Nanomagnonics around the corner Two complementary strategies show how to control the spatial propagation of spin waves, thus promising complex and reconfigurable wiring in spin-wave-based circuits.
Dirk Grundler
I
n electronic devices, including computers, the information is controlled and transmitted by means of electric currents. Using propagating waves rather than the motion of electric charges would, however, be more efficient, especially in terms of losses, but also in terms of speed. One type of wave that can be used is light. The propagation of light can be controlled and directed by engineered photonic crystals1 and via graded-index glass fibres for long-distance signal transmission2. However, computers based on light waves are still a challenge. An alternative is to use spin waves, also known as magnons, in magnetic materials, which could be used to implement wave-based computation and coherent processing of data across largearea cellular networks3. Now writing in Nature Nanotechnology, two research groups report complementary ways of controlling the propagation direction of spin waves at the nanoscale, which may lead to advances in the circuitry on magnonic chips4,5. Both approaches can produce reconfigurable waveguides, thus allowing for on-demand control of spin waves, local manipulation of information, and rewiring of magnetic chips via gateable spin structures. In general, the propagation of waves can be guided by structural modification
and compositional variation of matter1,2. For spin waves, a further guiding principle is available. The equation of motion for spins, traced back to Landau and Lifshitz, contains a term for an effective field that is formed not only by an external magnetic field generated by charge currents, but also by internal fields generated by magnetic volume and surface charges6. It is already known that spin textures and domain walls give rise to distinct magnetic charge-density distributions that localize and channel spin-wave excitations7. Helmut Schultheiss and colleagues from the Institute of Ion Beam Physics and Materials Research and the Technische Universität in Dresden have used an individual domain wall to improve the nanoscale wiring of magnonic circuits4 (Fig. 1a). Exploiting magnetostatic spin waves in a movable domain wall, they re-route signals of microwave frequencies on demand. The researchers fabricated a strip of the ferromagnetic metal Ni80Fe20 on a silicon oxide substrate. The strip is wide enough to allow for a domain pattern such that a few-micrometre-long domain wall is perpendicular to an integrated microwave antenna. A microwave signal induces an oscillating magnetic field that launches propagating spin waves. At specific
a
frequencies, only the narrow domain wall, not the surrounding film, supports the spin waves (Fig. 1a). Importantly, the researchers show that such a spin-wave nanochannel is gateable, because by applying a fine-tuned magnetic field its lateral position can be varied with nanometre precision (Fig. 1b). Adekunle Olusola Adeyeye and colleagues from the National University of Singapore report a complementary approach to guide and manipulate spin waves at the nanoscale5. They fabricated chains of spinwave nanoresonators (Fig. 2a), separated by only 50 nm, by nanolithography. The Ni80Fe20 nanoresonators were 25 nm thick, 260 nm wide and 600 nm long. The small separation induced coherent spin dynamics across the chain, similar to a one-dimensional magnonic crystal. The purposely designed rhomboidal nanoresonators allow different magnetic states to be programmed (Fig. 2a,b) and thereby to mould the flow of spin waves in a reconfigurable manner. They can also bend the propagation of spin waves using very low power (Fig. 2c), which has been a longstanding challenge. Both research teams used inelastic light scattering with high frequency and high spatial resolution to image the spin waves. They confirmed the confinement of
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Figure 1 | Reconfigurable magnonic conduits realized in domain walls. a, Spin wave of wavevector k propagating in a domain wall between two magnetic domains (top view). Spins precess at their given position (red arrows); electrons do not flow. The white arrows indicate magnetization vectors, M. b, A magnetic field, H, shifts the spin-wave nanochannel because a domain of magnetization, M, grows at the expense of the other domain. Magnetic volume charges are indicated by plus and minus signs. NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2016 Macmillan Publishers Limited. All rights reserved
1
news & views spin-wave channels as narrow as 40 nm could be realized with this method4. These findings further promote interest in domain wall nanoelectronics — an exciting and timely topic in ferroelectrics8. The nanoresonator chains proposed by Adeyeye and co-authors allow for gating of spin waves and logic functionality 5, making their integration with magnetic quantum-dot cellular automata particularly interesting 9. Still, both reports leave room for further exploration. The two research teams engineered non-collinear spin textures in the metallic ferromagnet Ni80Fe20. It has been previously shown that such textures give rise to additional damping in metals via intralayer spin pumping 10. Fortunately, magnons also exist in magnetic insulators, which will help overcome the damping issue. In unpatterned yttrium iron garnet, macroscopic decay lengths of up to centimetres have been demonstrated. For computation with spin waves and coherent data processing in large-area cellular networks, such low-damping magnetic insulators are of special relevance in magnonics. Unlike metals, reduction of these materials to nanoscale dimensions and their integration with silicon substrates, however, have not yet been achieved. ❐
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Dirk Grundler is at the Laboratory of Nanoscale Magnetic Materials and Magnonics, Institute of Materials and Institute of Microengineering, School of Engineering, Ecole Polytechnique Fédérale de Lausanne, Station 12, 1015 Lausanne, Switzerland. e-mail: [email protected] References
Figure 2 | Reconfigurable magnonic conduits based on nanoresonators. a,b, Strongly coupled nanomagnets of rhomboidal shape (a) give rise to reprogrammable magnonic conduits by changing the magnetization direction and magnetic surface-charge distribution at specific positions (b). c, Nanomagnet configuration for a bent magnonic conduit. Magnetic surface charges are indicated by plus and minus signs. Curved blue arrows illustrate dynamic stray fields and coupling between the spin-wave nanoresonantors. Wave-like spin precession is indicated by the red arrows.
microwave signals to deep-subwavelength length scales. In the work of Schultheiss and co-workers, the channel width is smaller than the wavelength in free space by a factor of about 10–6 (ref. 4). The extraordinary squeezing of an electromagnetic wave is a key feature of a magnonics-based technology.
2
The two reports represent a turning point. They imply that prototypical nanomagnonic devices, such as an integrated spin-wave interferometer, can be implemented and interconnected. The concept of Schultheiss and colleagues promises, in particular, reprogrammable magnonic circuitry. Micromagnetic simulations suggest that
1. Joannopoulos, J. D., Johnson, S. G., Winn, J. N. & Meade, R. D. Photonic Crystals, Molding the Flow of Light (Princeton Univ. Press, 2008). 2. Kao, C. K. Sand from centuries past: send future voices fast (Nobel lecture, 2009); http://go.nature.com/FJPJQx 3. Khitun, A., Bao, M. & Wang, K. L. J. Phys. D 43, 264005 (2010). 4. Wagner, K. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2015.339 (2016). 5. Haldar, A., Kumar, D. & Adeyeye, A. O. Nature Nanotech. http://dx.doi.org/10.1038/nnano.2015.332 (2016). 6. Gurevich, A. G. & Melkov, G. A. Magnetization Oscillations and Waves (CRC Press, 1996). 7. Perzlmaier, K. et al. Phys. Rev. Lett. 94, 057202 (2005). 8. McGilly, L. J., Yudin, P., Feigl, L., Tagsantsev, A. K. & Setter, N. Nature Nanotech. 10, 145–150 (2015). 9. Imre, A., Csaba, G., Ji, L., Orlov, A. & Porod, W. Science 311, 205–208 (2006). 10. Nembach, H. T., Shaw, J. M., Boone, C. T. & Silva, T. J. Phys. Rev. Lett. 110, 117201 (2013).
Published online: 1 February 2016
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2016 Macmillan Publishers Limited. All rights reserved
